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Supersaturation in the peripheral retina Jamie K. Opper,1,* Nathaniel D. Douda,1 Vicki J. Volbrecht,1 and Janice L. Nerger2 2
1 Department of Psychology, Colorado State University, Fort Collins, Colorado 80523-1876, USA College of Natural Sciences, Colorado State University, Fort Collins, Colorado 80523-1801, USA *Corresponding author: [email protected]
Received September 26, 2013; revised December 19, 2013; accepted December 23, 2013; posted December 24, 2013 (Doc. ID 198354); published January 31, 2014 Foveal and peripheral hue-scaling data were obtained for a 1° foveal stimulus and a 3° stimulus presented at 10° retinal eccentricity under both bleach (reducing rod input) and no-bleach (permitting rod input) conditions. Uniform appearance diagrams (UADs) were generated from the data. Peripheral stimuli appeared more saturated than foveal stimuli (i.e., supersaturated), especially in the green–yellow region of the UADs. This effect was particularly pronounced for the peripheral bleach condition. The range of wavelengths perceived as green–yellow in the peripheral retina was expanded as compared to the fovea, while the range of wavelengths experienced as blue–green was compressed. This indicates that there are shifts in the unique hue loci with retinal location. While several factors can be ruled out as potential causes for these perceptual differences, the underlying mechanism of this supersaturation effect in the peripheral retina is unknown. © 2014 Optical Society of America OCIS codes: (330.1690) Color; (330.1720) Color vision; (330.5510) Psychophysics; (330.7310) Vision. http://dx.doi.org/10.1364/JOSAA.31.00A148
1. INTRODUCTION Saturation refers to the magnitude of a perceptual color experience, often defined as the degree to which a color contains a chromatic component relative to an achromatic component [1]. Thus, the greater the achromatic component, the less saturated a color appears. For example, while a red stimulus may appear very saturated, by adding a white component to the red stimulus, the stimulus will appear pink. Traditionally, two methods have been used to measure saturation functions across the visible spectrum in the fovea. In the first method, observers view a bipartite field with each half composed of a broadband “white” light. Observers gradually increase the amount of chromatic light in one half field until they detect a just-noticeable color difference between the two half fields [2]. In the second method, each half of the bipartite field is filled with the same wavelength of light (chromatic component), and “white” light is added to one of the fields until a just-noticeable color difference is detected between the two half fields [3]. In general, results from these studies show that middle-wavelength stimuli (e.g., 570 nm) appear less saturated than stimuli from the short- and long-wavelength portions of the visible spectrum, i.e., the saturation function has an overall v-shape with the nadir near 570 nm [1,4–6; cf. 3]. Hue-scaling procedures can also be used to derive saturation functions. In some procedures, observers directly scale a stimulus for saturation by assigning a percentage ranging from 0% (completely achromatic) to 100% (completely chromatic) [7], or the experimenter can derive percent values from saturation rating scales [8]. Saturation functions obtained in the fovea with this procedure resemble those foveal functions measured by the more traditional psychophysical methods [9], i.e., percent saturation is lower for the middle wavelengths and higher for the spectral extremes. The added advantage of the hue-scaling method is that it is easier to measure saturation functions in the peripheral retina 1084-7529/14/04A148-11$15.00/0
than the more traditional psychophysical procedures using a bipartite field. Hue-scaling studies have shown that as a stimulus is moved from the fovea to the peripheral retina, it generally becomes less saturated [e.g., 7,10]. With few exceptions, asymmetric color-matching procedures have also generally confirmed the loss of saturation as a stimulus is moved from the fovea to the periphery [11,12]. This loss in saturation has been predominantly attributed to the presence of rods in the peripheral retina [10,12], and can be compensated for with an increase in stimulus size [7,10,11]. As stimulus size increases and the perceptive fields are filled for the four elemental hues, peripheral stimuli appear more fovea-like in both hue [7; cf. 12] and saturation [7,11], although the peripheral stimuli are still less saturated. Results from these studies imply that the magnitude of the chromatic experience associated with peripheral stimuli cannot be greater than that in the fovea, but there have been some exceptions to this tenet noted in the color vision literature from asymmetric color-matching experiments. In these experiments, a monochromatic stimulus is presented in the peripheral retina and the three primaries from a colorimeter are presented to the fovea [13,14]. Unlike typical colormatching experiments, though, negative coordinates cannot be generated, i.e., one of the primaries from the fovea cannot be moved to the monochromatic stimulus in the peripheral retina [15]. Stabell and Stabell [14] noted that after a rod bleach, stimuli from 500 to 580 nm presented 7.5° temporally could not be completely matched by the three primaries in the foveal stimulus. The monochromatic peripheral stimuli appeared too saturated. Similarly, Moreland and Cruz [13] reported that a 10° nasal test stimulus appeared too saturated to be matched by the three primaries of the colorimeter, and subsequently no measurements were obtained at that eccentricity under dark adaptation conditions. Furthermore, saturation functions derived from asymmetric matching data © 2014 Optical Society of America
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showed that long-wavelength stimuli appeared more saturated at 10° inferior and 15° nasal retinal eccentricities than in the fovea; and for one observer, the 570 nm stimulus was more saturated at 25° inferior eccentricity and 25° and 30° nasal eccentricities [13]. In some of these instances, the stimuli from the above studies were smaller than the perceptive field sizes measured from hue-scaling studies [7,16–18], and the number of cones underlying the stimulus in the peripheral retina is assumed to be fewer than those underlying the foveal stimulus, based on physiological data indicating cones are more sparsely distributed in the peripheral retina than in the fovea [19]. Because of these findings from the asymmetric matching studies, our laboratory reexamined hue-scaling data that had been used previously to derive perceptive field sizes at 10° temporal, nasal, superior, and inferior eccentricities [18], and to determine the similarity in color appearance at 10° retinal eccentricity among the nasal, temporal, superior, and inferior retinas [20]. Foveal hue-scaling data, however, were not included in either of the previous studies, so differences in saturation between the fovea and peripheral retina were not examined. In this study, we compared the perception of saturation in the fovea to the perception of saturation in the peripheral retina for monochromatic stimuli filling the perceptive fields of the four elemental hues (blue, green, yellow, red), to determine if stimuli appear more saturated in the peripheral retina than in the fovea.
2. METHOD An overview of the methods is provided in this section; more details regarding observers, apparatus, stimuli, and procedures are given in Volbrecht et al. [18]. A. Observers Three color-normal females (28, 41, and 45 years), as assessed by panel tests (Farnsworth–Munsell 100, D-15, desaturated D-15), plate tests (Dvorine pseudoisochromatic, F-2 tritan), and the Neitz anomaloscope, served as observers; two of the observers were coauthors of this paper. Each observer received practice with the “4 1” hue-scaling task and was unaware of the outcome of their data until the completion of data collection. All observers served as experimenters. B. Apparatus A three-channel Maxwellian-view optical system with a 300 W (5500 K) xenon arc lamp (Oriel, model 66065) regulated at 290 W by a dc power supply (Oriel, model 68811) was used for this experiment. Channel 1 determined the wavelength, intensity, size, and duration of the test stimulus. This was accomplished through the use of a grating monochromator (Instruments SA, Inc., Model H20, with a 4 nm half-amplitude bandpass), a two log-unit circular neutral-density wedge (Ealing Electro-Optics) controlled by a computer, neutraldensity filters (Ealing Electro-Optics), a driver-controlled shutter (Uniblitz, model T132), and a field stop. Channel 2 produced the fixation point array, which was generated by a field stop placed in a collimated portion of the pathway. Channel 3 provided the broadband (5500 K) bleaching stimulus, which was used to minimize rod activity. The size of the bleaching field was determined with a field stop placed in collimated light. For both channels 2 and 3, neutral density filters placed
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in collimated portions of the pathways were used to define the intensity of the fixation points and bleaching field. A beam-splitter combined light from all three channels. The light then passed through an artificial pupil, which determined the size of the final Maxwellian image (1.8 mm), and a final lens focused the light on the observer’s right pupil. Each observer aligned to the optical system by means of a dentalimpression bite-bar assembly, which permitted movement in three orthogonal directions. C. Stimuli Foveal data were obtained with a 1° stimulus. Data from the 3° stimulus presented at 10° temporal, nasal, superior, and inferior eccentricities are used for our analysis in this paper. The 3° stimulus filled the perceptive fields for the four elemental hues under both bleach and no-bleach conditions [17,18]. The monochromatic test stimuli were photopically equated to 1.3 log td, and ranged from 440 to 660 nm in 10 nm steps. The test stimulus was presented for 500 ms with an 18 s interstimulus interval (ISI) between presentations. The fixation point array consisted of three pinhole-sized fixation points. Two points were vertically displaced approximately 5° apart, and stimuli were centered between these two points. For peripheral measures, a third fixation point was positioned 10° laterally from the center of the first two points. The array was rotated to achieve 10° presentations along the vertical and horizontal meridians. The intensity of the fixation points was set so that each observer could just detect them, in order to minimize adaptation effects [21]. To minimize rod input, an 18°, 6.74 log Td, broadband (5500 K) bleaching field was centered at 10° retinal eccentricity. Viewing this field for 10 s bleached approximately 99.8% of the rod photopigment [22,23]. D. Calibrations The neutral-density wedge and filters in channel 1 were calibrated with a radiometer (EG&G Gamma Scientific Radiometer, Model DR-1500A). Photometric measurements (Minolta Chroma Meter, Model CS-100) were made in channel 1 at a reference wavelength of 550 nm to enable the conversion of radiometric measurements to photometric measurements and to equate our stimuli photopically. Photometric measurements were also taken in channel 3 for the broadband bleaching field (5500 K), and neutral-density filters used in channel 3 were calibrated with the photometer. Troland values were then calculated using Westheimer’s method [24]. The monochromator was calibrated so that the maximal emission of a He–Ne laser (Spectra Physics; 632.8 nm) occurred at 633 nm. E. Procedure In the no-bleach condition, observers adapted to the dark for 30 min before making peripheral judgments, and for 10 min before making foveal judgments. For the bleach condition, observers adapted to the dark for 10 min and then viewed the bleaching stimulus for 10 s, which was then followed by 4.5 min of dark adaptation. Stimuli were then presented for 4–5 min following the 4.5 min period of dark adaptation (i.e., the time period associated with the cone plateau of the dark adaptation function). It was not feasible to collect all judgments during a single post-bleach time period, so the bleaching process was repeated immediately at the end of the
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4–5 min data collection period. Two bleaches were usually sufficient to obtain hue responses for all wavelengths at one stimulus size. It had been found previously from pilot work in the laboratory that hue and saturation responses remain relatively stable during this 4–5 min data collection period. Observers used the “4 1” hue-scaling procedure [7] to make hue and saturation judgments during the 18 s ISI. With this procedure, observers described hue by assigning percentages from 0% to 100% to no more than two of the four elemental hue terms (blue, green, yellow, and red), with the constraint that the total hue percentage(s) equaled 100. Observers also assigned a percent value to reflect the saturation of the stimulus. The percentage of saturation ranged from 0% (completely achromatic) to 100% (completely chromatic). Data were collected for all experimental conditions from one retinal location before obtaining data from the next location. The retinal location and adaptation condition orders were pseudorandom, while wavelength presentation within an experimental session was random. For a 3° peripheral stimulus, three hue judgments were obtained per wavelength, each in a separate experimental session. For the foveal stimuli, two hue judgments were obtained per wavelength, again in separate experimental sessions.
3. RESULTS Prior to the computation of mean percentages, each individual hue and saturation percent underwent an arcsine transformation to equalize variance across wavelength. The transformed hue percentages at each wavelength for each session and observer were scaled to saturation such that the total hue percent equaled the total percent of saturation [7,9]. Mean hue and saturation values for each experimental condition were then calculated across sessions for each observer and wavelength. If an observer used more than two hue terms across sessions to describe a given wavelength, the smallest hue percent was reapportioned into the other two percentages [20,25]. Traditionally, mean hue and saturation percentages are plotted as a function of wavelength for each experimental condition, with the color appearance of each wavelength described by two to three data points (one or two hue percentages and the saturation percentage). Abramov et al. [25] have proposed representing hue-scaling data in a Uniform
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appearance diagram (UAD). In a UAD, the percentage of red or green is plotted as a function of the percentage of yellow or blue; and thus, the color appearance of each wavelength is specified by one data point with distance from the origin representing the degree of saturation. The mean percent hues in a UAD undergo a smoothing procedure whereby the hue percentages at each wavelength are weighed by the mean hue percentages of the immediately adjacent wavelengths. For example, at 500 nm, a new mean is calculated with the mean percent at 500 nm and 50% of the mean values at 490 and 510 nm. A cubic spline function is then fitted to these “smoothed” points and defines the outer limits of perceptual space for a particular experimental condition, but the relation between the perceptual experience and the physical stimulus (i.e., wavelength) is lost [20,25]. It is these cubic spline fits that are presented to assess if stimuli appear more saturated in the peripheral retina than in the fovea. Figure 1 provides an example comparing the traditional saturation function (left panel) to saturation represented in a UAD (right panel). In the left panel, percent saturation is plotted as a function of wavelength for the foveal (open circles) and the temporalbleach (solid circles) conditions of one observer. The foveal function resembles what has been reported previously in the literature [1,4–6,9], with saturation higher at the spectral extremes and lower in the middle-wavelength region of the visible spectrum. This reduction in saturation in the middlewavelength region is no longer present in the temporal retina after a rod bleach; thus, stimuli appear more saturated at these wavelengths of light in the peripheral retina. One point to note is, even if saturation is perceived to be the same for both the fovea and temporal retina at a particular wavelength (e.g., 500 nm), the hue is not. In the fovea, the 500 nm stimulus is perceived to be bluish green, while in the periphery, it is perceived to be yellowish green. The right panel presents the cubic spline fits of the same data from the left panel in a UAD (fovea, solid lines; temporal bleach, dashed lines). In this figure, percent red or green (y axis) is plotted as a function of percent blue or yellow (x axis). The percent red and blue values have arbitrarily been set to negative values while percent green and yellow values are arbitrarily defined as positive values [25]. The vector projecting from the origin of the UAD represents one hue ratio (0.35 Yellow: 1 Green), but the wavelength of the stimulus eliciting that hue ratio is
Fig. 1. The left panel presents a traditional saturation function with data from the fovea (open circles) and the temporal-bleach condition (filled circles) for one observer. Error bars are 1 standard deviation. The right panel presents data from the same observer and retinal locations in a UAD.
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substantially different in the fovea (550 nm) from that in the peripheral retina (515 nm). Furthermore, even though the hue ratio is the same, saturation is different. At this hue ratio, the peripheral stimulus is also perceived to be more saturated than the foveal stimulus, i.e., the distance from the origin is greater for the peripheral stimulus. Figures 2, 3, and 4 compare UAD functions obtained for the four peripheral retinal locations (temporal, nasal, superior, inferior) at 10° retinal eccentricity under no-bleach (dotted line) and bleach conditions (dashed line) to the foveal condition (solid line) for observers JN, VV, and CA, respectively. As illustrated in Figs. 2–4, under some conditions, stimuli in the peripheral retina are perceived as equally saturated or more saturated than stimuli presented to the fovea, i.e., the dotted and/or dashed lines are farther from the origin than the solid lines. For observers JN and VV, this effect is particularly pronounced in the portion of the UAD corresponding to the perceptual experience of green–yellow (upper right quadrant of the figure), and is strongest for the peripheral bleach condition (dashed line). JN also perceived yellow–red stimuli from the bleach condition (lower right quadrant of the figure) in the peripheral retina to be more saturated than yellow–red stimuli in the fovea. Observer CA exhibits a weaker effect that is mainly limited to the green and yellow axes of the UAD, the areas associated with unique green and unique yellow. Additionally, for all observers, under some conditions, stimuli in the peripheral no-bleach condition (dotted line) were also perceived as more saturated than stimuli in the fovea. For observer JN, this can be seen primarily in the blue–green and green–yellow quadrants (upper left and right
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quadrants, respectively) for the inferior, nasal, and superior locations. Observer VV also perceived peripheral no-bleach stimuli in the green–yellow UAD quadrant as more saturated than in the fovea for the nasal, superior, and temporal retinal locations; in the temporal retina, peripheral no-bleach stimuli around the yellow UAD axis were also reported as more saturated than the peripheral bleach stimuli. Observer CA perceived peripheral no-bleach stimuli near the yellow UAD axis as more saturated than the fovea for all four peripheral locations, and as more saturated than the peripheral bleach condition for the inferior, nasal, and temporal locations. Although not depicted in Figs. 2–4, there was a shift in the wavelengths serving as a boundary between color experiences (e.g., the wavelength at which a stimulus ceases to be perceived as reddish blue and begins to be perceived as greenish blue). Figure 5 presents the changes in the unique hues or boundaries of color experiences for all observers under the different viewing conditions. The wavelengths associated with unique blue (top panel), unique green (middle panel), and unique yellow (bottom panel) are plotted as a function of retinal location for the bleach (left panels) and the no-bleach (right panels) conditions. (The foveal bleach data will be discussed below.) Different data points denote the three observers: JN (squares), VV (diamonds), CA (circles). In general, except for JN’s inferior no-bleach data, the loci of the three unique hues shifted toward shorter wavelengths in the peripheral conditions as compared to the no-bleach foveal condition. Without observer JN’s inferior data, the mean shift to shorter wavelengths in the peripheral retina was greatest for unique green, 35 nm (range: 13–52 nm) in the bleach
Fig. 2. Comparison of UADs from the peripheral bleach (dashed line), peripheral no-bleach (dotted line), and foveal (solid line) conditions for observer JN.
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Fig. 3. Same as Fig. 2, except for observer VV.
Fig. 4. Same as Fig. 2, except for observer CA.
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condition and 32 nm (range: 4–48 nm) in the no-bleach condition. Likewise, the mean shift to shorter wavelengths for unique blue (unique yellow) was 11 nm (9 nm) for the bleach condition and 12 nm (7 nm) for the no-bleach condition. Because the magnitude of the shift of unique hue loci to shorter wavelengths in the peripheral conditions varied with a particular unique hue, the range of wavelengths associated with red–blue, blue–green, green–yellow, and yellow–red changed with retinal eccentricity. The greatest changes were observed for blue–green and green–yellow. For example, the range of wavelengths appearing blue–green and green–yellow in the fovea was 44–71 nm and 27–51 nm, respectively, while the range of wavelengths appearing blue–green and green– yellow in the peripheral bleach (no-bleach) condition was 26–48 nm (20–89 nm) and 57–72 nm (38–75 nm), respectively.
4. DISCUSSION Our results show that peripheral stimuli often appear more saturated than stimuli viewed in the fovea, particularly for stimuli that appear green–yellow. This is not limited to conditions minimizing rod input (bleach condition) as, at some peripheral locations and for some observers, stimuli viewed in the no-bleach condition also appear more saturated than when presented in the fovea (see Figs. 2–4). Researchers [13,14] conducting asymmetric color-matching experiments have also reported that stimuli can appear more saturated in the peripheral retina than in the fovea (see Introduction) under both dark adaptation [13] and bleach conditions [14]. Although Abramov et al. [7] did not report supersaturation in their hue-scaling study, it should be noted that their experimental conditions were very similar to those of our no-bleach condition, where the supersaturation was often smaller or lacking (see Figs. 2–4). While not all observers in our study showed the same saturation effect, it is not unusual for intersubject variability to be greater for the perception of saturation than for the perception of hue. For example, Boynton et al. [8] noted that perceived saturation measured in a hue-scaling study varied more among observers than perceived hue. Likewise, in an asymmetric color matching study, greater variability has been reported among observers for saturation measures than hue measures [10]. This may be due to physiological differences but could also be because observers are less familiar with making saturation judgments. Results from our study also showed that boundaries between color categories differed between the foveal and peripheral locations (see Fig. 5); in particular, unique hue loci are at shorter wavelengths in the peripheral retina than in the fovea. In contrast, Abramov and Gordon [26], who also derived unique hue loci from UADs, found that the loci of unique blue and unique yellow at 10° nasal eccentricity were similar to those derived in the fovea, while the locus of unique green was at shorter wavelengths in the peripheral retina compared to the fovea. One difference between the studies is that the unique hues in the Abramov and Gordon [26] study were derived from UADs of mean data across six observers, whereas we derived unique hue loci from UADs of each individual observer, although this is unlikely to account for the differences between the two studies. Studies that have directly measured unique hue loci have shown results similar to those of this study. Researchers
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showed that unique green loci measured after a rod bleach in the peripheral retina were shorter than those measured in the fovea [27; cf. 28]. Likewise, Buck et al. [28] have also shown that unique blue and yellow loci are at shorter wavelengths in the peripheral retina than the fovea for both bleach and no-bleach conditions [cf. 29]. Ultimately, this shift in hue boundaries results in stimuli appearing to have more yellow in the peripheral retina, a hue shift similarly shown in asymmetric color-matching studies [12,13] and attributed to a shift in the predominance of the yellow/blue opponent process over the red/green opponent process with retinal eccentricity [12]. A. Saturation Differences and the Achromatic System The reason for stimuli appearing more saturated in the peripheral retina than the fovea is a bit perplexing. For two of the observers (JN & VV, see Figs. 2 and 3), green–yellow stimuli in the peripheral bleach condition consistently appeared more saturated than the green–yellow stimuli in the fovea. Since saturation reflects the output of the chromatic system relative to the summed response of both the chromatic and achromatic systems [30,31], perhaps the bleaching field adapted the achromatic system, thereby reducing the contribution of an achromatic component to hue perception. The top panels of Fig. 6 present hue-scaling measurements obtained in the fovea for two observers (CA and VV) with a 1° stimulus after exposure to a bleaching field and during the time period associated with the cone plateau of the dark adaptation function (dashed line), and compare them to the foveal no-bleach measurements (solid line) from Figs. 3 (VV) and 4 (CA). For CA, the bleaching field in general has no effect except along the yellow axis, where stimuli viewed after a bleach appear to be more saturated than stimuli viewed following 10 min of dark adaptation. This pattern is similar to that observed in Fig. 4, where the peripheral bleach stimuli appear to be more saturated along the yellow axis; however, it should be noted that the peripheral no-bleach stimuli also appear to be more saturated along this axis for CA. CA also shows that unique hue loci from the foveal bleach condition are shifted to slightly longer wavelengths than those from the foveal no-bleach condition (Fig. 5). For VV, stimuli viewed following the bleaching condition (dashed line) are more saturated than stimuli viewed after 10 min of dark adaptation (solid line), and the unique hue loci for the foveal bleach condition are similar to those obtained for the foveal no-bleach condition (Fig. 5), with the greatest difference for unique green (a 10 nm shift to a shorter wavelength with the bleach condition). To see if this change in saturation with the bleach could account for the difference in saturation between the peripheral retina and fovea, VV’s foveal bleach UAD (solid line) from the upper right panel was compared to the peripheral bleach (bottom, left panel) and no-bleach (bottom, right panel) UADs from Fig. 3. As Fig. 6 shows, while the foveal data gathered after exposure to a bleaching field are more similar to the peripheral data, the pattern of results remains: peripheral stimuli in the green–yellow region appear to be more saturated. The magnocellular (M) pathway is often implicated as the neural mechanism underlying the achromatic system. As others [11,32] have noted, equating stimuli photopically should produce a constant output from the achromatic system at each wavelength, i.e., any differences in saturation cannot be attributed to the achromatic system but instead reflect
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Fig. 5. Comparison of unique blue (top row), unique green (middle row), and unique yellow (bottom row) loci from the bleach (left column) and no-bleach (right column) conditions for the different retinal locations (F, fovea; T, temporal; N, nasal; S, superior; I, inferior) and different observers (JN, squares; VV, diamonds; and CA, circles).
differences in the chromatic systems between the fovea and the peripheral retina. The problem in this study was that stimuli were photopically equated for the fovea and not the peripheral retina; thus, differences in macular pigment density between the fovea and the peripheral retina are not accounted for in the retinal illuminance of the peripheral stimuli. Since macular pigment density at 10° retinal eccentricity would be minimal, the retinal illuminances at the wavelengths associated with macular pigment density (approximately 400–530 nm, peak 460 nm) are greater in the peripheral retina than in the fovea [33]. It seems unlikely that this can explain the greater saturation perceived for green–yellow stimuli in the peripheral retina, given the wavelengths of the macular pigment differences; however, to verify that macular pigment density differences were not contributing to differences in saturation, two observers repeated the hue-scaling experiment
with a 2° stimulus in the fovea. In one condition (Condition 1), the stimuli were photopically equated to a 2° standard observer [33], which accounts for the presence of lens and macular pigment density in the central retina, and in the other condition (Condition 2), the retinal illuminance was increased by the density of the macular pigment at each wavelength (440–660 nm, 10 nm steps) [33]. The retinal illuminance of stimuli in this later condition mimicked the retinal illuminance of stimuli in the peripheral retina; i.e., retinal illuminance is higher at the wavelengths associated with macular pigment. All measurements were obtained after 10 min of dark adaptation. The top panels of Fig. 7 present the results from CA and VV for these two foveal conditions along with the 1° no-bleach foveal data from Figs. 3 (VV) and 4 (CA). Neither observer shows a difference between the two 2° conditions, indicating that the difference in cone input to the achromatic system
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Fig. 6. Top row: comparison of UADs from the foveal no-bleach condition (solid line) to the foveal bleach condition (dashed line) for observers CA (left panel) and VV (right panel). Bottom row: comparison of UADs from the foveal bleach condition to the peripheral bleach (left panel) and no-bleach (right panel) conditions for observer VV.
Fig. 7. Top row: comparison of foveal UADs from the 1° no-bleach (dashed line) condition and the two 2° conditions (condition 1, solid line; condition 2, dotted line) for observers CA (left panel) and VV (right panel). Bottom row: comparison of UADs from Condition 1 with the 2° foveal stimulus (see text) to the peripheral bleach (left panel) and no-bleach (right panel) conditions for observer VV.
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between the fovea and peripheral retina is not a factor that can explain the saturation differences. It does appear, however, that for VV (upper right panel), the 2° foveal stimuli are more saturated than the 1° foveal stimuli. The bottom two panels, therefore, present the 2° foveal data from Condition 1 for observer VV and compare it to her peripheral data from the bleach (left panel) and the no-bleach peripheral conditions (right panel) in Fig. 3. For this observer, the 2° foveal stimuli appear as saturated as, if not more saturated than, the peripheral stimuli. In particular, the supersaturation effect has disappeared for many of the peripheral stimuli in the no-bleach condition, and for stimuli in the nasal and superior locations of the bleach condition. This is a different pattern than shown with the 1° foveal stimulus. One difference between the 1° and 2° stimuli is that the 2° stimulus most likely stimulates more rods and many more S-cones than the 1° stimulus [19,34]. While saturation may be comparable between the 2° foveal data and the peripheral data, the 2° data do differ from the peripheral data in regards to crossover points. For observer VV, unique blue [466 nm (Conditions 1 and 2)] and unique green [508 nm (Condition 1) and 514 nm (Condition 2)] loci for the 2° stimuli lie between those of the 1° foveal and peripheral stimuli (see Fig. 5), whereas unique yellow loci [575 nm (Condition 1) and 574 nm (Condition 2)] for the 2° stimuli are shifted toward longer wavelengths compared to the 1° foveal stimulus condition. The 2° foveal unique blue loci [467 nm (Conditions 1 and 2)] for CA are similar to the 1° foveal locus (see Fig. 5), while the unique green [541 nm (Condition 1) and 532 nm (Condition 2)] and unique yellow [576 nm (Condition 1) and 575 nm (Condition 2)] loci are shifted to longer wavelengths compared to the 1° foveal condition. B. Saturation Differences and Rods Rods have long been implicated in the desaturation of peripheral stimuli [20,35–37], and physiological studies have shown that rod input is relatively strong at mesopic levels (10–40 Td) in the M pathway [38–40]. Accordingly, the achromatic signal via the M pathway can be enhanced by rod input, potentially increasing the desaturation of a stimulus. But, as researchers [e.g., 10,41] have noted, there is a paradox with rod input and desaturation effects. As stimulus size increases in the peripheral retina, saturation increases, even though there are more rods underlying the stimulus and presumably more rod contribution to the achromatic system. As shown in Figs. 2–4, overall, there were few differences between the bleach and no-bleach conditions for saturation, and in some cases stimuli in the no-bleach condition appeared more saturated. One suggestion proffered is that a larger stimulus adapts the rods, thereby reducing the rod signal to the achromatic pathway and thus reducing the desaturating effect of rods [41]. It seems more likely, however, due to fewer cones in the peripheral retina [19], that an increase in the stimulus size results in more cone stimulation. Once the chromatic signal is strong enough, it dominates and suppresses rod input to the achromatic system [20,41], possibly due to response latency differences between the rods and cones [42–44]. This cone inhibition of rod activity would account for the similarity in saturation between the bleach and no-bleach peripheral conditions for some of the observers, but it cannot explain the saturation differences between the fovea and peripheral retina.
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C. Saturation Differences and the Chromatic System McKeefry et al. [11] demonstrated that changes in the L–M pathway or parvocellular pathway could explain changes in saturation. As stimulus size decreased in the peripheral retina or as retinal eccentricity increased, saturation decreased. From their modeling, this loss in saturation was mostly attributed to a weakening in the response of the L–M pathway and perceptually manifested in the selective desaturation of green stimuli. If it is assumed that the midget ganglion cells underlie the L–M pathway, and possibly the red–green system, it seems unlikely that the L–M signal would be weaker in the fovea than at 10° retinal eccentricity since the number of midget ganglion cells receiving information from the fovea is much greater than the number of midget ganglion cells activated at 10° retinal eccentricity [45]. Also, the perceptive fields for the four elemental hues are filled in both the peripheral and foveal locations, so the processing of chromatic information should be somewhat comparable, a larger stimulus in the periphery compensating for the potentially weaker L–M system mediating color perception in the peripheral retina. Studies investigating the Stiles–Crawford effect of the second kind (SCII) have reported that stimuli from approximately 500 to 530 nm appear more saturated in the fovea when pupil entry of the stimulus is displaced from the center [46,47]. These studies also show a change in hue perception, such that when the observer matches stimuli entering the pupil obliquely to stimuli entering the pupil perpendicularly, the wavelength of the monochromatic oblique stimulus needs to be decreased to match perpendicular stimuli shorter than approximately 500 nm and longer than approximately 560 nm. For example, a longer wavelength stimulus presented obliquely in the fovea appears redder than a stimulus presented perpendicularly in the fovea. Middle-wavelength stimuli presented obliquely either require little change in wavelength to match the perpendicular stimuli or the wavelength needs to be made longer to match the perpendicular stimuli [46–48]. One mechanism used to explain this difference in hue perception between the two stimuli is a difference in photopigment optical density, i.e., the beam of light entering the pupil obliquely transverses less of the outer segment than the beam of light entering the pupil perpendicularly [48,49]. As others have shown, photopigment optical density decreases with retinal eccentricity [50,51]. Given that stimuli not only appeared more saturated in the green–yellow region of UADs, but hue perception was also altered (see Fig. 5), it might be possible that a SCII due to decreased photopigment optical density in the peripheral retina may partially explain the results from this study. To test this, hue ratios (B/Y:R/G) were computed for each wavelength from the nasal and foveal (1° stimulus) UADs for observers VV and JN, the two observers showing the greatest differences in saturation. The wavelengths in the peripheral retina that generated the same hue ratios as in the fovea from 440 to 660 nm in 10 nm steps were determined. In Fig. 8, the difference between the fovea and peripheral wavelengths yielding the same hue ratios are specified as a function of the foveal wavelength. A positive (negative) value indicates that a peripheral stimulus at a shorter (longer) wavelength matched the foveal stimulus. Different symbols denote the bleach (open squares) and no-bleach (open circles) conditions, and each panel represents a different observer. SCII
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Fig. 8. The change in a peripheral wavelength at 10° nasal retinal eccentricity required to match a foveal wavelength is plotted as a function of foveal wavelength for observers VV (left panel) and JN (right panel). Open squares (circles) denote values derived from the bleach (no-bleach) condition in the hue-scaling study. These values are compared to data (solid diamonds and X’s) from two SCII studies (see text).
data are included from Stiles [46] for observer WSS (solid diamonds) and from Walraven and Bouman [48] for observer PW (Xs). Note that a positive (negative) difference for SCII indicates that the oblique stimulus needed to decrease (increase) in wavelength to match the center stimulus. Figure 8 shows that both JN and VV deviate from the SCII effect at the middle wavelengths, showing a change in the opposite direction. The change in hue ratio is also greater than the SCII effect at the shorter and longer wavelengths for both observers, with JN showing an effect in the opposite direction at the longer wavelengths. Since the peripheral matching stimuli from this hue-scaling study do not show the same pattern as that observed with SCII, it is unlikely that differences in photopigment optical density with retinal eccentricity can explain the saturation and hue changes reported here between the fovea and peripheral retina. D. Summary In conclusion, there are conditions under which stimuli viewed peripherally may be experienced as more saturated than stimuli presented to the fovea, even for conditions in which rod input has not been minimized. This supersaturation effect is particularly pronounced for stimuli perceived as green–yellow. The cause of this supersaturation is not entirely clear.
ACKNOWLEDGMENTS This research was supported by NSF grant 1127711 to V. J. V. and J. L. N.
REFERENCES 1. 2. 3. 4.
D. Jameson and L. M. Hurvich, “Some quantitative aspects of an opponent-colors theory. I. Chromatic responses and spectral saturation,” J. Opt. Soc. Am. 45, 546–552 (1955). I. G. Priest and F. G. Brickwedde, “The minimum perceptible colorimetric purity as a function of dominant wave-length,” J. Opt. Soc. Am. 28, 133–139 (1938). W. D. Wright and F. H. G. Pitt, “The colour-vision characteristics of two trichromats,” Proc. Phys. Soc. 47, 205–217 (1935). P. K. Kaiser, J. P. Comerford, and D. M. Bodinger, “Saturation of spectral lights,” J. Opt. Soc. Am. 66, 818–826 (1976).
5. L. C. Martin, F. L. Warburton, and W. J. Morgan, “Determination of the sensitiveness of the eye to differences in the saturation of colours,” Report No. XIII (His Majesty’s Stationery Office, 1933). 6. W. D. Wright and F. H. G. Pitt, “The saturation discrimination of two trichromats,” Proc. Phys. Soc. 49, 329–331 (1937). 7. I. Abramov, J. Gordon, and H. Chan, “Color appearance in the peripheral retina: effects of stimulus size,” J. Opt. Soc. Am. A 8, 404–414 (1991). 8. R. M. Boynton, W. Schafer, and M. E. Neun, “Hue-wavelength relation measured by color-naming method for three retinal locations,” Science 146, 666–668 (1964). 9. J. Gordon, I. Abramov, and H. Chan, “Describing color appearance: hue and saturation scaling,” Percept. Psychophys. 56, 27–41 (1994). 10. J. Gordon and I. Abramov, “Color vision in the peripheral retina. II. Hue and saturation,” J. Opt. Soc. Am. 67, 202–207 (1977). 11. D. J. McKeefry, I. J. Murray, and N. R. A. Parry, “Perceived shifts in saturation and hue of chromatic stimuli in the near peripheral retina,” J. Opt. Soc. Am. A 24, 3168–3179 (2007). 12. N. R. A. Parry, D. J. McKeefry, and I. J. Murray, “Variant and invariant color perception in the near peripheral retina,” J. Opt. Soc. Am. A 23, 1586–1597 (2006). 13. J. D. Moreland and A. Cruz, “Colour perception with the peripheral retina,” Opt. Acta 6, 117–151 (1959). 14. B. Stabell and U. Stabell, “Rod and cone contributions to peripheral colour vision,” Vis. Res. 16, 1099–1104 (1976). 15. B. Stabell and U. Stabell, “Peripheral colour vision: effects of rod intrusion at different eccentricities,” Vis. Res. 36, 3407–3414 (1996). 16. I. Abramov, J. Gordon, and H. Chan, “Color appearance across the retina: effects of a white surround,” J. Opt. Soc. Am. A 9, 195–202 (1992). 17. L. J. Troup, M. A. Pitts, V. J. Volbrecht, and J. L. Nerger, “Effect of stimulus intensity on the sizes of chromatic perceptive fields,” J. Opt. Soc. Am. A 22, 2137–2142 (2005). 18. V. J. Volbrecht, C. L. Clark, J. L. Nerger, and C. E. Randell, “Chromatic perceptive field sizes measured at 10° eccentricity along the horizontal and vertical meridians,” J. Opt. Soc. Am. A 26, 1167–1177 (2009). 19. C. A. Curcio, K. R. Sloan, R. E. Kalina, and A. E. Hendrickson, “Human photoreceptor topography,” J. Comp. Neurol. 292, 497–523 (1990). 20. V. J. Volbrecht and J. L. Nerger, “Color appearance at ±10° along the vertical and horizontal meridians,” J. Opt. Soc. Am. A 29, A44–A51 (2012). 21. D. Jameson and L. M. Hurvich, “Fixation-light bias: an unwanted by-product of fixation control,” Vis. Res. 7, 805–809 (1967). 22. M. Alpern, “Effect of a bright light flash on dark adaptation of human rods,” Nature 230, 394–396 (1971).
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J. Opt. Soc. Am. A / Vol. 31, No. 4 / April 2014
23. W. A. H. Rushton and D. S. Powell, “The rhodopsin content and the visual threshold of human rods,” Vis. Res. 12, 1073–1081 (1972). 24. G. Westheimer, “The Maxwellian view,” Vis. Res. 6, 669–682 (1966). 25. I. Abramov, J. Gordon, and H. Chan, “Color appearance: properties of the uniform appearance diagram derived from hue and saturation scaling,” Atten. Percept. Psychophys. 71, 632–643 (2009). 26. I. Abramov and J. Gordon, “Seeing unique hues,” J. Opt. Soc. Am. A 22, 2143–2153 (2005). 27. J. L. Nerger, V. J. Volbrecht, and C. J. Ayde, “Unique hue judgments as a function of test size in the fovea and at 20-deg temporal eccentricity,” J. Opt. Soc. Am. A 12, 1225–1232 (1995). 28. S. L. Buck, R. F. Knight, and J. Bechtold, “Opponent-color models and the influence of rod signals on the loci of unique hues,” Vis. Res. 40, 3333–3344 (2000). 29. B. Drum, “Hue signals from short- and middle-wavelengthsensitive cones,” J. Opt. Soc. Am. A 6, 153–157 (1989). 30. K. Fuld, “The contribution of chromatic and achromatic valence to spectral saturation,” Vis. Res. 31, 237–246 (1991). 31. L. M. Hurvich and D. Jameson, “An opponent-process theory of color vision,” Psychol. Rev. 64, 384–404 (1957). 32. J. Gordon and I. Abramov, “Color vision,” in Blackwell Handbook of Sensation and Perception, B. E. Goldstein, ed. (Blackwell, 2001), pp. 92–127. 33. J. J. Vos, “Literature review of human macular absorption in the visible and its consequences for the cone receptor primaries,” Report No. IZF 1972-17 (Institute for Perception RVO-TNO, 1972). 34. C. A. Curcio, K. A. Allen, K. R. Sloan, C. L. Lerea, J. B. Hurley, I. B. Klock, and A. H. Milam, “Distribution and morphology of human cone photoreceptors stained with anti-blue opsin,” J. Comp. Neurol. 312, 610–624 (1991). 35. B. A. Ambler, “Hue discrimination in peripheral vision under conditions of dark and light adaptation,” Percept. Psychophys. 15, 586–590 (1974). 36. R. Knight and S. L. Buck, “Time-dependent changes of rod influence on hue perception,” Vis. Res. 42, 1651–1662 (2002).
Opper et al. 37. P. W. Trezona, “Aspects of peripheral colour vision,” Mod. Probl. Ophthalmol. 17, 52–70 (1976). 38. V. Virsu and B. B. Lee, “Light adaptation in cells of macaque lateral geniculate nucleus and its relation to human light adaptation,” J. Neurophysiol. 50, 864–878 (1983). 39. V. Virsu, B. B. Lee, and O. D. Creutzfeldt, “Mesopic spectral responses and the Purkinje shift of macaque lateral geniculate nucleus cells,” Vis. Res. 27, 191–200 (1987). 40. B. B. Lee, V. C. Smith, J. Pokorny, and J. Kremers, “Rod inputs to macaque ganglion cells,” Vis. Res. 37, 2813–2828 (1997). 41. U. Stabell and B. Stabell, “Rod-cone color mixture: effect of size and exposure time,” J. Opt. Soc. Am. A 16, 2638–2642 (1999). 42. P. Gouras and K. Link, “Rod and cone interaction in darkadapted monkey ganglion cells,” J. Physiol. 184, 499–510 (1966). 43. P. Gouras, “Precortical physiology of colour vision,” in The Perception of Colour, P. Gouras, ed. (CRC Press, 1991), pp. 163–178. 44. D. M. Schneeweis and J. L. Schnapf, “Photovoltage of rods and cones in the macaque retina,” Science 268, 1053–1056 (1995). 45. C. A. Curcio and K. A. Allen, “Topography of ganglion cells in human retina,” J. Comp. Neurol. 300, 5–25 (1990). 46. W. S. Stiles, “The luminous efficiency of monochromatic rays entering the eye pupil at different points and a new colour effect,” Proc. R. Soc. B 123, 90–118 (1937). 47. J. M. Enoch and W. S. Stiles, “The colour change of monochromatic light with retinal angle of incidence,” Optom. Wkly. 8, 329–358 (1961). 48. P. L. Walraven and M. A. Bouman, “Relation between directional sensitivity and spectral response curves in human cone vision,” J. Opt. Soc. Am. 50, 780–784 (1960). 49. G. S. Brindley, “The effects on colour vision of adaptation to very bright lights,” J. Physiol. 122, 332–350 (1953). 50. A. E. Elsner, S. A. Burns, and R. H. Webb, “Mapping cone photopigment optical density,” J. Opt. Soc. Am. A 10, 52–58 (1993). 51. A. B. Renner, H. Knau, M. Neitz, J. Neitz, and J. S. Werner, “Photopigment optical density of the human foveola and a paradoxical senescent increase outside the fovea,” Vis. Neurosci. 21, 827–834 (2004).
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Supersaturation in the peripheral retina Jamie K. Opper,1,* Nathaniel D. Douda,1 Vicki J. Volbrecht,1 and Janice L. Nerger2 2
1 Department of Psychology, Colorado State University, Fort Collins, Colorado 80523-1876, USA College of Natural Sciences, Colorado State University, Fort Collins, Colorado 80523-1801, USA *Corresponding author: [email protected]
Received September 26, 2013; revised December 19, 2013; accepted December 23, 2013; posted December 24, 2013 (Doc. ID 198354); published January 31, 2014 Foveal and peripheral hue-scaling data were obtained for a 1° foveal stimulus and a 3° stimulus presented at 10° retinal eccentricity under both bleach (reducing rod input) and no-bleach (permitting rod input) conditions. Uniform appearance diagrams (UADs) were generated from the data. Peripheral stimuli appeared more saturated than foveal stimuli (i.e., supersaturated), especially in the green–yellow region of the UADs. This effect was particularly pronounced for the peripheral bleach condition. The range of wavelengths perceived as green–yellow in the peripheral retina was expanded as compared to the fovea, while the range of wavelengths experienced as blue–green was compressed. This indicates that there are shifts in the unique hue loci with retinal location. While several factors can be ruled out as potential causes for these perceptual differences, the underlying mechanism of this supersaturation effect in the peripheral retina is unknown. © 2014 Optical Society of America OCIS codes: (330.1690) Color; (330.1720) Color vision; (330.5510) Psychophysics; (330.7310) Vision. http://dx.doi.org/10.1364/JOSAA.31.00A148
1. INTRODUCTION Saturation refers to the magnitude of a perceptual color experience, often defined as the degree to which a color contains a chromatic component relative to an achromatic component [1]. Thus, the greater the achromatic component, the less saturated a color appears. For example, while a red stimulus may appear very saturated, by adding a white component to the red stimulus, the stimulus will appear pink. Traditionally, two methods have been used to measure saturation functions across the visible spectrum in the fovea. In the first method, observers view a bipartite field with each half composed of a broadband “white” light. Observers gradually increase the amount of chromatic light in one half field until they detect a just-noticeable color difference between the two half fields [2]. In the second method, each half of the bipartite field is filled with the same wavelength of light (chromatic component), and “white” light is added to one of the fields until a just-noticeable color difference is detected between the two half fields [3]. In general, results from these studies show that middle-wavelength stimuli (e.g., 570 nm) appear less saturated than stimuli from the short- and long-wavelength portions of the visible spectrum, i.e., the saturation function has an overall v-shape with the nadir near 570 nm [1,4–6; cf. 3]. Hue-scaling procedures can also be used to derive saturation functions. In some procedures, observers directly scale a stimulus for saturation by assigning a percentage ranging from 0% (completely achromatic) to 100% (completely chromatic) [7], or the experimenter can derive percent values from saturation rating scales [8]. Saturation functions obtained in the fovea with this procedure resemble those foveal functions measured by the more traditional psychophysical methods [9], i.e., percent saturation is lower for the middle wavelengths and higher for the spectral extremes. The added advantage of the hue-scaling method is that it is easier to measure saturation functions in the peripheral retina 1084-7529/14/04A148-11$15.00/0
than the more traditional psychophysical procedures using a bipartite field. Hue-scaling studies have shown that as a stimulus is moved from the fovea to the peripheral retina, it generally becomes less saturated [e.g., 7,10]. With few exceptions, asymmetric color-matching procedures have also generally confirmed the loss of saturation as a stimulus is moved from the fovea to the periphery [11,12]. This loss in saturation has been predominantly attributed to the presence of rods in the peripheral retina [10,12], and can be compensated for with an increase in stimulus size [7,10,11]. As stimulus size increases and the perceptive fields are filled for the four elemental hues, peripheral stimuli appear more fovea-like in both hue [7; cf. 12] and saturation [7,11], although the peripheral stimuli are still less saturated. Results from these studies imply that the magnitude of the chromatic experience associated with peripheral stimuli cannot be greater than that in the fovea, but there have been some exceptions to this tenet noted in the color vision literature from asymmetric color-matching experiments. In these experiments, a monochromatic stimulus is presented in the peripheral retina and the three primaries from a colorimeter are presented to the fovea [13,14]. Unlike typical colormatching experiments, though, negative coordinates cannot be generated, i.e., one of the primaries from the fovea cannot be moved to the monochromatic stimulus in the peripheral retina [15]. Stabell and Stabell [14] noted that after a rod bleach, stimuli from 500 to 580 nm presented 7.5° temporally could not be completely matched by the three primaries in the foveal stimulus. The monochromatic peripheral stimuli appeared too saturated. Similarly, Moreland and Cruz [13] reported that a 10° nasal test stimulus appeared too saturated to be matched by the three primaries of the colorimeter, and subsequently no measurements were obtained at that eccentricity under dark adaptation conditions. Furthermore, saturation functions derived from asymmetric matching data © 2014 Optical Society of America
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showed that long-wavelength stimuli appeared more saturated at 10° inferior and 15° nasal retinal eccentricities than in the fovea; and for one observer, the 570 nm stimulus was more saturated at 25° inferior eccentricity and 25° and 30° nasal eccentricities [13]. In some of these instances, the stimuli from the above studies were smaller than the perceptive field sizes measured from hue-scaling studies [7,16–18], and the number of cones underlying the stimulus in the peripheral retina is assumed to be fewer than those underlying the foveal stimulus, based on physiological data indicating cones are more sparsely distributed in the peripheral retina than in the fovea [19]. Because of these findings from the asymmetric matching studies, our laboratory reexamined hue-scaling data that had been used previously to derive perceptive field sizes at 10° temporal, nasal, superior, and inferior eccentricities [18], and to determine the similarity in color appearance at 10° retinal eccentricity among the nasal, temporal, superior, and inferior retinas [20]. Foveal hue-scaling data, however, were not included in either of the previous studies, so differences in saturation between the fovea and peripheral retina were not examined. In this study, we compared the perception of saturation in the fovea to the perception of saturation in the peripheral retina for monochromatic stimuli filling the perceptive fields of the four elemental hues (blue, green, yellow, red), to determine if stimuli appear more saturated in the peripheral retina than in the fovea.
2. METHOD An overview of the methods is provided in this section; more details regarding observers, apparatus, stimuli, and procedures are given in Volbrecht et al. [18]. A. Observers Three color-normal females (28, 41, and 45 years), as assessed by panel tests (Farnsworth–Munsell 100, D-15, desaturated D-15), plate tests (Dvorine pseudoisochromatic, F-2 tritan), and the Neitz anomaloscope, served as observers; two of the observers were coauthors of this paper. Each observer received practice with the “4 1” hue-scaling task and was unaware of the outcome of their data until the completion of data collection. All observers served as experimenters. B. Apparatus A three-channel Maxwellian-view optical system with a 300 W (5500 K) xenon arc lamp (Oriel, model 66065) regulated at 290 W by a dc power supply (Oriel, model 68811) was used for this experiment. Channel 1 determined the wavelength, intensity, size, and duration of the test stimulus. This was accomplished through the use of a grating monochromator (Instruments SA, Inc., Model H20, with a 4 nm half-amplitude bandpass), a two log-unit circular neutral-density wedge (Ealing Electro-Optics) controlled by a computer, neutraldensity filters (Ealing Electro-Optics), a driver-controlled shutter (Uniblitz, model T132), and a field stop. Channel 2 produced the fixation point array, which was generated by a field stop placed in a collimated portion of the pathway. Channel 3 provided the broadband (5500 K) bleaching stimulus, which was used to minimize rod activity. The size of the bleaching field was determined with a field stop placed in collimated light. For both channels 2 and 3, neutral density filters placed
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in collimated portions of the pathways were used to define the intensity of the fixation points and bleaching field. A beam-splitter combined light from all three channels. The light then passed through an artificial pupil, which determined the size of the final Maxwellian image (1.8 mm), and a final lens focused the light on the observer’s right pupil. Each observer aligned to the optical system by means of a dentalimpression bite-bar assembly, which permitted movement in three orthogonal directions. C. Stimuli Foveal data were obtained with a 1° stimulus. Data from the 3° stimulus presented at 10° temporal, nasal, superior, and inferior eccentricities are used for our analysis in this paper. The 3° stimulus filled the perceptive fields for the four elemental hues under both bleach and no-bleach conditions [17,18]. The monochromatic test stimuli were photopically equated to 1.3 log td, and ranged from 440 to 660 nm in 10 nm steps. The test stimulus was presented for 500 ms with an 18 s interstimulus interval (ISI) between presentations. The fixation point array consisted of three pinhole-sized fixation points. Two points were vertically displaced approximately 5° apart, and stimuli were centered between these two points. For peripheral measures, a third fixation point was positioned 10° laterally from the center of the first two points. The array was rotated to achieve 10° presentations along the vertical and horizontal meridians. The intensity of the fixation points was set so that each observer could just detect them, in order to minimize adaptation effects [21]. To minimize rod input, an 18°, 6.74 log Td, broadband (5500 K) bleaching field was centered at 10° retinal eccentricity. Viewing this field for 10 s bleached approximately 99.8% of the rod photopigment [22,23]. D. Calibrations The neutral-density wedge and filters in channel 1 were calibrated with a radiometer (EG&G Gamma Scientific Radiometer, Model DR-1500A). Photometric measurements (Minolta Chroma Meter, Model CS-100) were made in channel 1 at a reference wavelength of 550 nm to enable the conversion of radiometric measurements to photometric measurements and to equate our stimuli photopically. Photometric measurements were also taken in channel 3 for the broadband bleaching field (5500 K), and neutral-density filters used in channel 3 were calibrated with the photometer. Troland values were then calculated using Westheimer’s method [24]. The monochromator was calibrated so that the maximal emission of a He–Ne laser (Spectra Physics; 632.8 nm) occurred at 633 nm. E. Procedure In the no-bleach condition, observers adapted to the dark for 30 min before making peripheral judgments, and for 10 min before making foveal judgments. For the bleach condition, observers adapted to the dark for 10 min and then viewed the bleaching stimulus for 10 s, which was then followed by 4.5 min of dark adaptation. Stimuli were then presented for 4–5 min following the 4.5 min period of dark adaptation (i.e., the time period associated with the cone plateau of the dark adaptation function). It was not feasible to collect all judgments during a single post-bleach time period, so the bleaching process was repeated immediately at the end of the
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4–5 min data collection period. Two bleaches were usually sufficient to obtain hue responses for all wavelengths at one stimulus size. It had been found previously from pilot work in the laboratory that hue and saturation responses remain relatively stable during this 4–5 min data collection period. Observers used the “4 1” hue-scaling procedure [7] to make hue and saturation judgments during the 18 s ISI. With this procedure, observers described hue by assigning percentages from 0% to 100% to no more than two of the four elemental hue terms (blue, green, yellow, and red), with the constraint that the total hue percentage(s) equaled 100. Observers also assigned a percent value to reflect the saturation of the stimulus. The percentage of saturation ranged from 0% (completely achromatic) to 100% (completely chromatic). Data were collected for all experimental conditions from one retinal location before obtaining data from the next location. The retinal location and adaptation condition orders were pseudorandom, while wavelength presentation within an experimental session was random. For a 3° peripheral stimulus, three hue judgments were obtained per wavelength, each in a separate experimental session. For the foveal stimuli, two hue judgments were obtained per wavelength, again in separate experimental sessions.
3. RESULTS Prior to the computation of mean percentages, each individual hue and saturation percent underwent an arcsine transformation to equalize variance across wavelength. The transformed hue percentages at each wavelength for each session and observer were scaled to saturation such that the total hue percent equaled the total percent of saturation [7,9]. Mean hue and saturation values for each experimental condition were then calculated across sessions for each observer and wavelength. If an observer used more than two hue terms across sessions to describe a given wavelength, the smallest hue percent was reapportioned into the other two percentages [20,25]. Traditionally, mean hue and saturation percentages are plotted as a function of wavelength for each experimental condition, with the color appearance of each wavelength described by two to three data points (one or two hue percentages and the saturation percentage). Abramov et al. [25] have proposed representing hue-scaling data in a Uniform
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appearance diagram (UAD). In a UAD, the percentage of red or green is plotted as a function of the percentage of yellow or blue; and thus, the color appearance of each wavelength is specified by one data point with distance from the origin representing the degree of saturation. The mean percent hues in a UAD undergo a smoothing procedure whereby the hue percentages at each wavelength are weighed by the mean hue percentages of the immediately adjacent wavelengths. For example, at 500 nm, a new mean is calculated with the mean percent at 500 nm and 50% of the mean values at 490 and 510 nm. A cubic spline function is then fitted to these “smoothed” points and defines the outer limits of perceptual space for a particular experimental condition, but the relation between the perceptual experience and the physical stimulus (i.e., wavelength) is lost [20,25]. It is these cubic spline fits that are presented to assess if stimuli appear more saturated in the peripheral retina than in the fovea. Figure 1 provides an example comparing the traditional saturation function (left panel) to saturation represented in a UAD (right panel). In the left panel, percent saturation is plotted as a function of wavelength for the foveal (open circles) and the temporalbleach (solid circles) conditions of one observer. The foveal function resembles what has been reported previously in the literature [1,4–6,9], with saturation higher at the spectral extremes and lower in the middle-wavelength region of the visible spectrum. This reduction in saturation in the middlewavelength region is no longer present in the temporal retina after a rod bleach; thus, stimuli appear more saturated at these wavelengths of light in the peripheral retina. One point to note is, even if saturation is perceived to be the same for both the fovea and temporal retina at a particular wavelength (e.g., 500 nm), the hue is not. In the fovea, the 500 nm stimulus is perceived to be bluish green, while in the periphery, it is perceived to be yellowish green. The right panel presents the cubic spline fits of the same data from the left panel in a UAD (fovea, solid lines; temporal bleach, dashed lines). In this figure, percent red or green (y axis) is plotted as a function of percent blue or yellow (x axis). The percent red and blue values have arbitrarily been set to negative values while percent green and yellow values are arbitrarily defined as positive values [25]. The vector projecting from the origin of the UAD represents one hue ratio (0.35 Yellow: 1 Green), but the wavelength of the stimulus eliciting that hue ratio is
Fig. 1. The left panel presents a traditional saturation function with data from the fovea (open circles) and the temporal-bleach condition (filled circles) for one observer. Error bars are 1 standard deviation. The right panel presents data from the same observer and retinal locations in a UAD.
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substantially different in the fovea (550 nm) from that in the peripheral retina (515 nm). Furthermore, even though the hue ratio is the same, saturation is different. At this hue ratio, the peripheral stimulus is also perceived to be more saturated than the foveal stimulus, i.e., the distance from the origin is greater for the peripheral stimulus. Figures 2, 3, and 4 compare UAD functions obtained for the four peripheral retinal locations (temporal, nasal, superior, inferior) at 10° retinal eccentricity under no-bleach (dotted line) and bleach conditions (dashed line) to the foveal condition (solid line) for observers JN, VV, and CA, respectively. As illustrated in Figs. 2–4, under some conditions, stimuli in the peripheral retina are perceived as equally saturated or more saturated than stimuli presented to the fovea, i.e., the dotted and/or dashed lines are farther from the origin than the solid lines. For observers JN and VV, this effect is particularly pronounced in the portion of the UAD corresponding to the perceptual experience of green–yellow (upper right quadrant of the figure), and is strongest for the peripheral bleach condition (dashed line). JN also perceived yellow–red stimuli from the bleach condition (lower right quadrant of the figure) in the peripheral retina to be more saturated than yellow–red stimuli in the fovea. Observer CA exhibits a weaker effect that is mainly limited to the green and yellow axes of the UAD, the areas associated with unique green and unique yellow. Additionally, for all observers, under some conditions, stimuli in the peripheral no-bleach condition (dotted line) were also perceived as more saturated than stimuli in the fovea. For observer JN, this can be seen primarily in the blue–green and green–yellow quadrants (upper left and right
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quadrants, respectively) for the inferior, nasal, and superior locations. Observer VV also perceived peripheral no-bleach stimuli in the green–yellow UAD quadrant as more saturated than in the fovea for the nasal, superior, and temporal retinal locations; in the temporal retina, peripheral no-bleach stimuli around the yellow UAD axis were also reported as more saturated than the peripheral bleach stimuli. Observer CA perceived peripheral no-bleach stimuli near the yellow UAD axis as more saturated than the fovea for all four peripheral locations, and as more saturated than the peripheral bleach condition for the inferior, nasal, and temporal locations. Although not depicted in Figs. 2–4, there was a shift in the wavelengths serving as a boundary between color experiences (e.g., the wavelength at which a stimulus ceases to be perceived as reddish blue and begins to be perceived as greenish blue). Figure 5 presents the changes in the unique hues or boundaries of color experiences for all observers under the different viewing conditions. The wavelengths associated with unique blue (top panel), unique green (middle panel), and unique yellow (bottom panel) are plotted as a function of retinal location for the bleach (left panels) and the no-bleach (right panels) conditions. (The foveal bleach data will be discussed below.) Different data points denote the three observers: JN (squares), VV (diamonds), CA (circles). In general, except for JN’s inferior no-bleach data, the loci of the three unique hues shifted toward shorter wavelengths in the peripheral conditions as compared to the no-bleach foveal condition. Without observer JN’s inferior data, the mean shift to shorter wavelengths in the peripheral retina was greatest for unique green, 35 nm (range: 13–52 nm) in the bleach
Fig. 2. Comparison of UADs from the peripheral bleach (dashed line), peripheral no-bleach (dotted line), and foveal (solid line) conditions for observer JN.
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Fig. 3. Same as Fig. 2, except for observer VV.
Fig. 4. Same as Fig. 2, except for observer CA.
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condition and 32 nm (range: 4–48 nm) in the no-bleach condition. Likewise, the mean shift to shorter wavelengths for unique blue (unique yellow) was 11 nm (9 nm) for the bleach condition and 12 nm (7 nm) for the no-bleach condition. Because the magnitude of the shift of unique hue loci to shorter wavelengths in the peripheral conditions varied with a particular unique hue, the range of wavelengths associated with red–blue, blue–green, green–yellow, and yellow–red changed with retinal eccentricity. The greatest changes were observed for blue–green and green–yellow. For example, the range of wavelengths appearing blue–green and green–yellow in the fovea was 44–71 nm and 27–51 nm, respectively, while the range of wavelengths appearing blue–green and green– yellow in the peripheral bleach (no-bleach) condition was 26–48 nm (20–89 nm) and 57–72 nm (38–75 nm), respectively.
4. DISCUSSION Our results show that peripheral stimuli often appear more saturated than stimuli viewed in the fovea, particularly for stimuli that appear green–yellow. This is not limited to conditions minimizing rod input (bleach condition) as, at some peripheral locations and for some observers, stimuli viewed in the no-bleach condition also appear more saturated than when presented in the fovea (see Figs. 2–4). Researchers [13,14] conducting asymmetric color-matching experiments have also reported that stimuli can appear more saturated in the peripheral retina than in the fovea (see Introduction) under both dark adaptation [13] and bleach conditions [14]. Although Abramov et al. [7] did not report supersaturation in their hue-scaling study, it should be noted that their experimental conditions were very similar to those of our no-bleach condition, where the supersaturation was often smaller or lacking (see Figs. 2–4). While not all observers in our study showed the same saturation effect, it is not unusual for intersubject variability to be greater for the perception of saturation than for the perception of hue. For example, Boynton et al. [8] noted that perceived saturation measured in a hue-scaling study varied more among observers than perceived hue. Likewise, in an asymmetric color matching study, greater variability has been reported among observers for saturation measures than hue measures [10]. This may be due to physiological differences but could also be because observers are less familiar with making saturation judgments. Results from our study also showed that boundaries between color categories differed between the foveal and peripheral locations (see Fig. 5); in particular, unique hue loci are at shorter wavelengths in the peripheral retina than in the fovea. In contrast, Abramov and Gordon [26], who also derived unique hue loci from UADs, found that the loci of unique blue and unique yellow at 10° nasal eccentricity were similar to those derived in the fovea, while the locus of unique green was at shorter wavelengths in the peripheral retina compared to the fovea. One difference between the studies is that the unique hues in the Abramov and Gordon [26] study were derived from UADs of mean data across six observers, whereas we derived unique hue loci from UADs of each individual observer, although this is unlikely to account for the differences between the two studies. Studies that have directly measured unique hue loci have shown results similar to those of this study. Researchers
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showed that unique green loci measured after a rod bleach in the peripheral retina were shorter than those measured in the fovea [27; cf. 28]. Likewise, Buck et al. [28] have also shown that unique blue and yellow loci are at shorter wavelengths in the peripheral retina than the fovea for both bleach and no-bleach conditions [cf. 29]. Ultimately, this shift in hue boundaries results in stimuli appearing to have more yellow in the peripheral retina, a hue shift similarly shown in asymmetric color-matching studies [12,13] and attributed to a shift in the predominance of the yellow/blue opponent process over the red/green opponent process with retinal eccentricity [12]. A. Saturation Differences and the Achromatic System The reason for stimuli appearing more saturated in the peripheral retina than the fovea is a bit perplexing. For two of the observers (JN & VV, see Figs. 2 and 3), green–yellow stimuli in the peripheral bleach condition consistently appeared more saturated than the green–yellow stimuli in the fovea. Since saturation reflects the output of the chromatic system relative to the summed response of both the chromatic and achromatic systems [30,31], perhaps the bleaching field adapted the achromatic system, thereby reducing the contribution of an achromatic component to hue perception. The top panels of Fig. 6 present hue-scaling measurements obtained in the fovea for two observers (CA and VV) with a 1° stimulus after exposure to a bleaching field and during the time period associated with the cone plateau of the dark adaptation function (dashed line), and compare them to the foveal no-bleach measurements (solid line) from Figs. 3 (VV) and 4 (CA). For CA, the bleaching field in general has no effect except along the yellow axis, where stimuli viewed after a bleach appear to be more saturated than stimuli viewed following 10 min of dark adaptation. This pattern is similar to that observed in Fig. 4, where the peripheral bleach stimuli appear to be more saturated along the yellow axis; however, it should be noted that the peripheral no-bleach stimuli also appear to be more saturated along this axis for CA. CA also shows that unique hue loci from the foveal bleach condition are shifted to slightly longer wavelengths than those from the foveal no-bleach condition (Fig. 5). For VV, stimuli viewed following the bleaching condition (dashed line) are more saturated than stimuli viewed after 10 min of dark adaptation (solid line), and the unique hue loci for the foveal bleach condition are similar to those obtained for the foveal no-bleach condition (Fig. 5), with the greatest difference for unique green (a 10 nm shift to a shorter wavelength with the bleach condition). To see if this change in saturation with the bleach could account for the difference in saturation between the peripheral retina and fovea, VV’s foveal bleach UAD (solid line) from the upper right panel was compared to the peripheral bleach (bottom, left panel) and no-bleach (bottom, right panel) UADs from Fig. 3. As Fig. 6 shows, while the foveal data gathered after exposure to a bleaching field are more similar to the peripheral data, the pattern of results remains: peripheral stimuli in the green–yellow region appear to be more saturated. The magnocellular (M) pathway is often implicated as the neural mechanism underlying the achromatic system. As others [11,32] have noted, equating stimuli photopically should produce a constant output from the achromatic system at each wavelength, i.e., any differences in saturation cannot be attributed to the achromatic system but instead reflect
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Fig. 5. Comparison of unique blue (top row), unique green (middle row), and unique yellow (bottom row) loci from the bleach (left column) and no-bleach (right column) conditions for the different retinal locations (F, fovea; T, temporal; N, nasal; S, superior; I, inferior) and different observers (JN, squares; VV, diamonds; and CA, circles).
differences in the chromatic systems between the fovea and the peripheral retina. The problem in this study was that stimuli were photopically equated for the fovea and not the peripheral retina; thus, differences in macular pigment density between the fovea and the peripheral retina are not accounted for in the retinal illuminance of the peripheral stimuli. Since macular pigment density at 10° retinal eccentricity would be minimal, the retinal illuminances at the wavelengths associated with macular pigment density (approximately 400–530 nm, peak 460 nm) are greater in the peripheral retina than in the fovea [33]. It seems unlikely that this can explain the greater saturation perceived for green–yellow stimuli in the peripheral retina, given the wavelengths of the macular pigment differences; however, to verify that macular pigment density differences were not contributing to differences in saturation, two observers repeated the hue-scaling experiment
with a 2° stimulus in the fovea. In one condition (Condition 1), the stimuli were photopically equated to a 2° standard observer [33], which accounts for the presence of lens and macular pigment density in the central retina, and in the other condition (Condition 2), the retinal illuminance was increased by the density of the macular pigment at each wavelength (440–660 nm, 10 nm steps) [33]. The retinal illuminance of stimuli in this later condition mimicked the retinal illuminance of stimuli in the peripheral retina; i.e., retinal illuminance is higher at the wavelengths associated with macular pigment. All measurements were obtained after 10 min of dark adaptation. The top panels of Fig. 7 present the results from CA and VV for these two foveal conditions along with the 1° no-bleach foveal data from Figs. 3 (VV) and 4 (CA). Neither observer shows a difference between the two 2° conditions, indicating that the difference in cone input to the achromatic system
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Fig. 6. Top row: comparison of UADs from the foveal no-bleach condition (solid line) to the foveal bleach condition (dashed line) for observers CA (left panel) and VV (right panel). Bottom row: comparison of UADs from the foveal bleach condition to the peripheral bleach (left panel) and no-bleach (right panel) conditions for observer VV.
Fig. 7. Top row: comparison of foveal UADs from the 1° no-bleach (dashed line) condition and the two 2° conditions (condition 1, solid line; condition 2, dotted line) for observers CA (left panel) and VV (right panel). Bottom row: comparison of UADs from Condition 1 with the 2° foveal stimulus (see text) to the peripheral bleach (left panel) and no-bleach (right panel) conditions for observer VV.
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between the fovea and peripheral retina is not a factor that can explain the saturation differences. It does appear, however, that for VV (upper right panel), the 2° foveal stimuli are more saturated than the 1° foveal stimuli. The bottom two panels, therefore, present the 2° foveal data from Condition 1 for observer VV and compare it to her peripheral data from the bleach (left panel) and the no-bleach peripheral conditions (right panel) in Fig. 3. For this observer, the 2° foveal stimuli appear as saturated as, if not more saturated than, the peripheral stimuli. In particular, the supersaturation effect has disappeared for many of the peripheral stimuli in the no-bleach condition, and for stimuli in the nasal and superior locations of the bleach condition. This is a different pattern than shown with the 1° foveal stimulus. One difference between the 1° and 2° stimuli is that the 2° stimulus most likely stimulates more rods and many more S-cones than the 1° stimulus [19,34]. While saturation may be comparable between the 2° foveal data and the peripheral data, the 2° data do differ from the peripheral data in regards to crossover points. For observer VV, unique blue [466 nm (Conditions 1 and 2)] and unique green [508 nm (Condition 1) and 514 nm (Condition 2)] loci for the 2° stimuli lie between those of the 1° foveal and peripheral stimuli (see Fig. 5), whereas unique yellow loci [575 nm (Condition 1) and 574 nm (Condition 2)] for the 2° stimuli are shifted toward longer wavelengths compared to the 1° foveal stimulus condition. The 2° foveal unique blue loci [467 nm (Conditions 1 and 2)] for CA are similar to the 1° foveal locus (see Fig. 5), while the unique green [541 nm (Condition 1) and 532 nm (Condition 2)] and unique yellow [576 nm (Condition 1) and 575 nm (Condition 2)] loci are shifted to longer wavelengths compared to the 1° foveal condition. B. Saturation Differences and Rods Rods have long been implicated in the desaturation of peripheral stimuli [20,35–37], and physiological studies have shown that rod input is relatively strong at mesopic levels (10–40 Td) in the M pathway [38–40]. Accordingly, the achromatic signal via the M pathway can be enhanced by rod input, potentially increasing the desaturation of a stimulus. But, as researchers [e.g., 10,41] have noted, there is a paradox with rod input and desaturation effects. As stimulus size increases in the peripheral retina, saturation increases, even though there are more rods underlying the stimulus and presumably more rod contribution to the achromatic system. As shown in Figs. 2–4, overall, there were few differences between the bleach and no-bleach conditions for saturation, and in some cases stimuli in the no-bleach condition appeared more saturated. One suggestion proffered is that a larger stimulus adapts the rods, thereby reducing the rod signal to the achromatic pathway and thus reducing the desaturating effect of rods [41]. It seems more likely, however, due to fewer cones in the peripheral retina [19], that an increase in the stimulus size results in more cone stimulation. Once the chromatic signal is strong enough, it dominates and suppresses rod input to the achromatic system [20,41], possibly due to response latency differences between the rods and cones [42–44]. This cone inhibition of rod activity would account for the similarity in saturation between the bleach and no-bleach peripheral conditions for some of the observers, but it cannot explain the saturation differences between the fovea and peripheral retina.
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C. Saturation Differences and the Chromatic System McKeefry et al. [11] demonstrated that changes in the L–M pathway or parvocellular pathway could explain changes in saturation. As stimulus size decreased in the peripheral retina or as retinal eccentricity increased, saturation decreased. From their modeling, this loss in saturation was mostly attributed to a weakening in the response of the L–M pathway and perceptually manifested in the selective desaturation of green stimuli. If it is assumed that the midget ganglion cells underlie the L–M pathway, and possibly the red–green system, it seems unlikely that the L–M signal would be weaker in the fovea than at 10° retinal eccentricity since the number of midget ganglion cells receiving information from the fovea is much greater than the number of midget ganglion cells activated at 10° retinal eccentricity [45]. Also, the perceptive fields for the four elemental hues are filled in both the peripheral and foveal locations, so the processing of chromatic information should be somewhat comparable, a larger stimulus in the periphery compensating for the potentially weaker L–M system mediating color perception in the peripheral retina. Studies investigating the Stiles–Crawford effect of the second kind (SCII) have reported that stimuli from approximately 500 to 530 nm appear more saturated in the fovea when pupil entry of the stimulus is displaced from the center [46,47]. These studies also show a change in hue perception, such that when the observer matches stimuli entering the pupil obliquely to stimuli entering the pupil perpendicularly, the wavelength of the monochromatic oblique stimulus needs to be decreased to match perpendicular stimuli shorter than approximately 500 nm and longer than approximately 560 nm. For example, a longer wavelength stimulus presented obliquely in the fovea appears redder than a stimulus presented perpendicularly in the fovea. Middle-wavelength stimuli presented obliquely either require little change in wavelength to match the perpendicular stimuli or the wavelength needs to be made longer to match the perpendicular stimuli [46–48]. One mechanism used to explain this difference in hue perception between the two stimuli is a difference in photopigment optical density, i.e., the beam of light entering the pupil obliquely transverses less of the outer segment than the beam of light entering the pupil perpendicularly [48,49]. As others have shown, photopigment optical density decreases with retinal eccentricity [50,51]. Given that stimuli not only appeared more saturated in the green–yellow region of UADs, but hue perception was also altered (see Fig. 5), it might be possible that a SCII due to decreased photopigment optical density in the peripheral retina may partially explain the results from this study. To test this, hue ratios (B/Y:R/G) were computed for each wavelength from the nasal and foveal (1° stimulus) UADs for observers VV and JN, the two observers showing the greatest differences in saturation. The wavelengths in the peripheral retina that generated the same hue ratios as in the fovea from 440 to 660 nm in 10 nm steps were determined. In Fig. 8, the difference between the fovea and peripheral wavelengths yielding the same hue ratios are specified as a function of the foveal wavelength. A positive (negative) value indicates that a peripheral stimulus at a shorter (longer) wavelength matched the foveal stimulus. Different symbols denote the bleach (open squares) and no-bleach (open circles) conditions, and each panel represents a different observer. SCII
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Fig. 8. The change in a peripheral wavelength at 10° nasal retinal eccentricity required to match a foveal wavelength is plotted as a function of foveal wavelength for observers VV (left panel) and JN (right panel). Open squares (circles) denote values derived from the bleach (no-bleach) condition in the hue-scaling study. These values are compared to data (solid diamonds and X’s) from two SCII studies (see text).
data are included from Stiles [46] for observer WSS (solid diamonds) and from Walraven and Bouman [48] for observer PW (Xs). Note that a positive (negative) difference for SCII indicates that the oblique stimulus needed to decrease (increase) in wavelength to match the center stimulus. Figure 8 shows that both JN and VV deviate from the SCII effect at the middle wavelengths, showing a change in the opposite direction. The change in hue ratio is also greater than the SCII effect at the shorter and longer wavelengths for both observers, with JN showing an effect in the opposite direction at the longer wavelengths. Since the peripheral matching stimuli from this hue-scaling study do not show the same pattern as that observed with SCII, it is unlikely that differences in photopigment optical density with retinal eccentricity can explain the saturation and hue changes reported here between the fovea and peripheral retina. D. Summary In conclusion, there are conditions under which stimuli viewed peripherally may be experienced as more saturated than stimuli presented to the fovea, even for conditions in which rod input has not been minimized. This supersaturation effect is particularly pronounced for stimuli perceived as green–yellow. The cause of this supersaturation is not entirely clear.
ACKNOWLEDGMENTS This research was supported by NSF grant 1127711 to V. J. V. and J. L. N.
REFERENCES 1. 2. 3. 4.
D. Jameson and L. M. Hurvich, “Some quantitative aspects of an opponent-colors theory. I. Chromatic responses and spectral saturation,” J. Opt. Soc. Am. 45, 546–552 (1955). I. G. Priest and F. G. Brickwedde, “The minimum perceptible colorimetric purity as a function of dominant wave-length,” J. Opt. Soc. Am. 28, 133–139 (1938). W. D. Wright and F. H. G. Pitt, “The colour-vision characteristics of two trichromats,” Proc. Phys. Soc. 47, 205–217 (1935). P. K. Kaiser, J. P. Comerford, and D. M. Bodinger, “Saturation of spectral lights,” J. Opt. Soc. Am. 66, 818–826 (1976).
5. L. C. Martin, F. L. Warburton, and W. J. Morgan, “Determination of the sensitiveness of the eye to differences in the saturation of colours,” Report No. XIII (His Majesty’s Stationery Office, 1933). 6. W. D. Wright and F. H. G. Pitt, “The saturation discrimination of two trichromats,” Proc. Phys. Soc. 49, 329–331 (1937). 7. I. Abramov, J. Gordon, and H. Chan, “Color appearance in the peripheral retina: effects of stimulus size,” J. Opt. Soc. Am. A 8, 404–414 (1991). 8. R. M. Boynton, W. Schafer, and M. E. Neun, “Hue-wavelength relation measured by color-naming method for three retinal locations,” Science 146, 666–668 (1964). 9. J. Gordon, I. Abramov, and H. Chan, “Describing color appearance: hue and saturation scaling,” Percept. Psychophys. 56, 27–41 (1994). 10. J. Gordon and I. Abramov, “Color vision in the peripheral retina. II. Hue and saturation,” J. Opt. Soc. Am. 67, 202–207 (1977). 11. D. J. McKeefry, I. J. Murray, and N. R. A. Parry, “Perceived shifts in saturation and hue of chromatic stimuli in the near peripheral retina,” J. Opt. Soc. Am. A 24, 3168–3179 (2007). 12. N. R. A. Parry, D. J. McKeefry, and I. J. Murray, “Variant and invariant color perception in the near peripheral retina,” J. Opt. Soc. Am. A 23, 1586–1597 (2006). 13. J. D. Moreland and A. Cruz, “Colour perception with the peripheral retina,” Opt. Acta 6, 117–151 (1959). 14. B. Stabell and U. Stabell, “Rod and cone contributions to peripheral colour vision,” Vis. Res. 16, 1099–1104 (1976). 15. B. Stabell and U. Stabell, “Peripheral colour vision: effects of rod intrusion at different eccentricities,” Vis. Res. 36, 3407–3414 (1996). 16. I. Abramov, J. Gordon, and H. Chan, “Color appearance across the retina: effects of a white surround,” J. Opt. Soc. Am. A 9, 195–202 (1992). 17. L. J. Troup, M. A. Pitts, V. J. Volbrecht, and J. L. Nerger, “Effect of stimulus intensity on the sizes of chromatic perceptive fields,” J. Opt. Soc. Am. A 22, 2137–2142 (2005). 18. V. J. Volbrecht, C. L. Clark, J. L. Nerger, and C. E. Randell, “Chromatic perceptive field sizes measured at 10° eccentricity along the horizontal and vertical meridians,” J. Opt. Soc. Am. A 26, 1167–1177 (2009). 19. C. A. Curcio, K. R. Sloan, R. E. Kalina, and A. E. Hendrickson, “Human photoreceptor topography,” J. Comp. Neurol. 292, 497–523 (1990). 20. V. J. Volbrecht and J. L. Nerger, “Color appearance at ±10° along the vertical and horizontal meridians,” J. Opt. Soc. Am. A 29, A44–A51 (2012). 21. D. Jameson and L. M. Hurvich, “Fixation-light bias: an unwanted by-product of fixation control,” Vis. Res. 7, 805–809 (1967). 22. M. Alpern, “Effect of a bright light flash on dark adaptation of human rods,” Nature 230, 394–396 (1971).
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J. Opt. Soc. Am. A / Vol. 31, No. 4 / April 2014
23. W. A. H. Rushton and D. S. Powell, “The rhodopsin content and the visual threshold of human rods,” Vis. Res. 12, 1073–1081 (1972). 24. G. Westheimer, “The Maxwellian view,” Vis. Res. 6, 669–682 (1966). 25. I. Abramov, J. Gordon, and H. Chan, “Color appearance: properties of the uniform appearance diagram derived from hue and saturation scaling,” Atten. Percept. Psychophys. 71, 632–643 (2009). 26. I. Abramov and J. Gordon, “Seeing unique hues,” J. Opt. Soc. Am. A 22, 2143–2153 (2005). 27. J. L. Nerger, V. J. Volbrecht, and C. J. Ayde, “Unique hue judgments as a function of test size in the fovea and at 20-deg temporal eccentricity,” J. Opt. Soc. Am. A 12, 1225–1232 (1995). 28. S. L. Buck, R. F. Knight, and J. Bechtold, “Opponent-color models and the influence of rod signals on the loci of unique hues,” Vis. Res. 40, 3333–3344 (2000). 29. B. Drum, “Hue signals from short- and middle-wavelengthsensitive cones,” J. Opt. Soc. Am. A 6, 153–157 (1989). 30. K. Fuld, “The contribution of chromatic and achromatic valence to spectral saturation,” Vis. Res. 31, 237–246 (1991). 31. L. M. Hurvich and D. Jameson, “An opponent-process theory of color vision,” Psychol. Rev. 64, 384–404 (1957). 32. J. Gordon and I. Abramov, “Color vision,” in Blackwell Handbook of Sensation and Perception, B. E. Goldstein, ed. (Blackwell, 2001), pp. 92–127. 33. J. J. Vos, “Literature review of human macular absorption in the visible and its consequences for the cone receptor primaries,” Report No. IZF 1972-17 (Institute for Perception RVO-TNO, 1972). 34. C. A. Curcio, K. A. Allen, K. R. Sloan, C. L. Lerea, J. B. Hurley, I. B. Klock, and A. H. Milam, “Distribution and morphology of human cone photoreceptors stained with anti-blue opsin,” J. Comp. Neurol. 312, 610–624 (1991). 35. B. A. Ambler, “Hue discrimination in peripheral vision under conditions of dark and light adaptation,” Percept. Psychophys. 15, 586–590 (1974). 36. R. Knight and S. L. Buck, “Time-dependent changes of rod influence on hue perception,” Vis. Res. 42, 1651–1662 (2002).
Opper et al. 37. P. W. Trezona, “Aspects of peripheral colour vision,” Mod. Probl. Ophthalmol. 17, 52–70 (1976). 38. V. Virsu and B. B. Lee, “Light adaptation in cells of macaque lateral geniculate nucleus and its relation to human light adaptation,” J. Neurophysiol. 50, 864–878 (1983). 39. V. Virsu, B. B. Lee, and O. D. Creutzfeldt, “Mesopic spectral responses and the Purkinje shift of macaque lateral geniculate nucleus cells,” Vis. Res. 27, 191–200 (1987). 40. B. B. Lee, V. C. Smith, J. Pokorny, and J. Kremers, “Rod inputs to macaque ganglion cells,” Vis. Res. 37, 2813–2828 (1997). 41. U. Stabell and B. Stabell, “Rod-cone color mixture: effect of size and exposure time,” J. Opt. Soc. Am. A 16, 2638–2642 (1999). 42. P. Gouras and K. Link, “Rod and cone interaction in darkadapted monkey ganglion cells,” J. Physiol. 184, 499–510 (1966). 43. P. Gouras, “Precortical physiology of colour vision,” in The Perception of Colour, P. Gouras, ed. (CRC Press, 1991), pp. 163–178. 44. D. M. Schneeweis and J. L. Schnapf, “Photovoltage of rods and cones in the macaque retina,” Science 268, 1053–1056 (1995). 45. C. A. Curcio and K. A. Allen, “Topography of ganglion cells in human retina,” J. Comp. Neurol. 300, 5–25 (1990). 46. W. S. Stiles, “The luminous efficiency of monochromatic rays entering the eye pupil at different points and a new colour effect,” Proc. R. Soc. B 123, 90–118 (1937). 47. J. M. Enoch and W. S. Stiles, “The colour change of monochromatic light with retinal angle of incidence,” Optom. Wkly. 8, 329–358 (1961). 48. P. L. Walraven and M. A. Bouman, “Relation between directional sensitivity and spectral response curves in human cone vision,” J. Opt. Soc. Am. 50, 780–784 (1960). 49. G. S. Brindley, “The effects on colour vision of adaptation to very bright lights,” J. Physiol. 122, 332–350 (1953). 50. A. E. Elsner, S. A. Burns, and R. H. Webb, “Mapping cone photopigment optical density,” J. Opt. Soc. Am. A 10, 52–58 (1993). 51. A. B. Renner, H. Knau, M. Neitz, J. Neitz, and J. S. Werner, “Photopigment optical density of the human foveola and a paradoxical senescent increase outside the fovea,” Vis. Neurosci. 21, 827–834 (2004).
