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Contrast–contrast asynchronies Alex Rose-Henig1,2 and Arthur G. Shapiro1,2,* 1
Department of Psychology, American University, 4400 Massachusetts Avenue, NW, Washington, DC 20016, USA 2 Center for Behavioral Neuroscience, American University, 4400 Massachusetts Avenue, NW, Washington, DC 20016, USA *Corresponding author: [email protected] Received September 30, 2013; revised January 7, 2014; accepted January 10, 2014; posted January 13, 2014 (Doc. ID 198552); published February 12, 2014 We introduce the “contrast–contrast asynchrony,” a dynamic stimulus configuration that combines elements of the Shapiro contrast asynchrony with elements of the Chubb contrast–contrast illusion. In the contrast–contrast asynchrony, static textured fields surround two textured fields; one surround has high-contrast texture, and the other has low-contrast texture. The contrasts of the center fields modulate in phase with each other at 1 Hz, and as a consequence, the difference between the contrast of the centers and the contrast of the respective surround modulates in antiphase. Most observers report an antiphase appearance for high-contrast, fine-grained centers. These observers therefore respond to the difference between the center contrast and surround contrast. We also document three observers who do not see the asynchrony for high-contrast modulations of the center, suggesting possibly interesting individual differences. © 2014 Optical Society of America OCIS codes: (330.1720) Color vision; (330.5020) Perception psychology; (330.1800) Vision - contrast sensitivity; (330.6100) Spatial discrimination. http://dx.doi.org/10.1364/JOSAA.31.00A232
1. INTRODUCTION Generations of scientists have studied the effect of context on perception with the idea that such displays will give some indication of how the brain transforms light into appearance [1–4]. For instance, in a standard simultaneous contrast display, test patches with the same midluminance pixel values are placed in different spatial contexts: one disk is on a dark background, while the other is on a bright background. The disk on the bright background appears darker than the disk placed on the bright background, so the perceptual appearance differs even though the disks are physically identical. Here, we investigate what happens when we combine two variations of spatial context on perception: the contrast– contrast illusion [5] and the contrast asynchrony [6–8]. A. Contrast Asynchrony Contrast asynchronies are not standard simultaneous contrast displays; rather, the contrast asynchrony is a configuration that creates two competing perceptions by juxtaposing the luminance of identical disks with the contrast between the disks and their surrounds. Consider Fig. 1(a), which shows a series of identical disks placed on a gradient background. Each disk has two sources of information: one source of information corresponds to the luminance of the disks (i.e., the disks could all be called “white,” albeit slightly different achromatic shades); another source of information corresponds to the contrast between the disks and the gradient background (i.e., the disks on the left appear in high contrast with the background, while the disks on the right appear in low contrast with the background). By temporally modulating the luminance of the disks, contrast asynchronies separate the visual response to luminance from the visual response to contrast. 1084-7529/14/04A232-07$15.00/0
To illustrate this point, consider Fig. 1(b), which shows four frames of the contrast asynchrony, in which the surrounding ring on the left is dark while the ring on the right is bright. The luminance levels of the center disks are always identical to each other and modulate sinusoidally in time, and the contrast of the disks relative to the rings modulates in antiphase [see the plot in Fig. 1(c)]. So, when the luminance levels of the disks are high, both disks appear bright, but the left disk/ring pair has high contrast while the right disk/ring pair has low contrast, and when the luminance levels of the disks are low, the disks appear dark and the contrast levels are reversed. At low temporal frequencies, observers have the paradoxical effect of seeing the disks modulate out of phase (in line with the contrast modulation); yet, with attention, observers can also perceive that the disks are becoming bright and dark at the same time (in line with the luminance modulation). It needs to be emphasized that contrast asynchronies are different from dynamic simultaneous contrast, in which the modulation of the surround changes the appearance of the center [9–11]. The experiments measure the effects on the center typically through a nulling technique. In contrast asynchronies, however, the center disks’ difference from each other is not the main concern; rather, the question is which type of perceptual information predominates, the luminance of the center or the contrast between the center and the surround. Contrast asynchronies are useful because they are capable of separating the perception of color from the perception of color contrast. The contrast asynchrony has been used to study color [6,12], the effects of short-range and long-range motion [8], interactions of spatial changes in contrast [13], paradoxical effects of edges and contrast [14], and contrast in monocular and binocular configurations [15]. The appearance of the asynchrony is strongest when modulation is © 2014 Optical Society of America
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Fig. 1. Contrast asynchrony [5–7]. (a) Physically identical disks on a gradient background. The disk on the left has higher contrast relative to the background than the disk on the right. (b) Four frames from the contrast asynchrony which presents two physically identical disks, one with a dark surround and the other with a light surround. In each frame, the luminance levels of the disks are always the same; the contrast between each disk and surround modulates in antiphase. (c) Plots of the changes in luminance and contrast levels over time. The luminance levels of the disks are in phase; the contrast levels are in antiphase.
between 3 and 5 Hz, and when lights and background are in similar color directions and have higher modulation amplitude. B. Contrast–Contrast Illusion The contrast–contrast illusion [5], also referred to as the Chubb illusion, is a type of simultaneous contrast but with a textured patch. The effect can be seen by examining the textured disks in Fig. 2(a). The textures that fill the center disks have the same physical contrasts, yet the disks on the left are perceived to have a higher contrast texture than the disks on the right. Contrast–contrast illusions are important partly because they suggest the presence of a gain control that operates on texture [5,16]. This gain control is strongest when the texture of the surround matches that of the center, suggesting the presence of tuning curves for neural encoding of texture. Contrast–contrast illusions have also generated much attention because they seem to show individual differences and differences between some clinical and nonclinical populations. For instance, there has been documentation of an observer for whom the direction of the contrast– contrast effect was dramatically reversed [17]; and patients with schizophrenia have been reported to have weak or absent responses [18,19]. However, some caution regarding
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Fig. 2. Contrast–contrast asynchrony. (a) Version of the static contrast–contrast illusion. The disks all have the same internal contrast and are placed on a contrast gradient. The disk on the left appears to have lower internal contrast than the disk on the right. The paper makes a distinction between the physical contrast inside each disk (C1center ) and the difference between the contrast of the disk and the contrast of the surround (C2). (b) Four frames from the contrast– contrast asynchrony. The C1centers of the left and right disks are always the same; the C2 on the left is not always the same as the C2 on the right. (c) Plots of the changes in luminance, contrast, and contrast–contrast over time in the contrast–contrast asynchrony; the luminance levels of disks are always constant. The C1center of the left and right disks modulates in phase; the contrast–contrast (C2) modulates in antiphase.
conclusions about individual difference is advisable because other studies did not find a significant difference between normal observers and people with schizophrenia or schizoaffective disorder [20], suggesting that the differing response of the latter group is partly due to impaired attentional mechanisms. C. Contrast–Contrast Asynchrony All accounts of the contrast–contrast illusion so far have concentrated on the appearance of the center. But, just like Fig. 1(a), Fig. 2(a) contains two types of information: (1) the contrast of the disks, and (2) the difference between the contrast of the disks and contrast of the surround. The second source of information corresponds in principle to the difference between two textures. An important question for vision science is whether the visual system calculates this difference directly or whether there is a “later-stage” (and perhaps attentional) comparison between separate contrast calculations. We address this question with a visual stimulus that we have named the contrast–contrast asynchrony.
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In the basic version of the contrast–contrast asynchrony [Fig. 2(b)], the contrast of both disks modulates sinusoidally in time; one disk is placed against a low-contrast surround, and one disk is placed against a high-contrast surround. We will use the following terminology to make the discussion easier: C1center refers to the contrast of the center disks. C1center modulates in time and is always identical for the left and right center disks. C1surround refers to the contrast of the surround field. C1surround is always static. The left surround and right surround have the same mean luminance and typically have different contrast values. C2 refers to the contrast between C1center and C1surround . C2 modulates in time; C2 on the left typically modulates in antiphase relative to C2 on the right. Figure 2(c) plots the temporal changes of C1center and C2 as a function of time. As with the standard contrast asynchrony, we can discuss whether C2 is a different perceptual entity from C1center and C1surround . We attempt to determine the conditions under which observers see the disks as modulating asynchronously (corresponding to the C2 modulation) or synchronously (corresponding to C1center modulation). We investigate these differences with regard to modulation amplitude, temporal frequency, and texture size. The results show some similarities and some differences between the contrast–contrast asynchrony and the contrast asynchrony. The two most striking differences are that the asynchronous response is slower in the contrast–contrast asynchrony, and our observers exhibit substantial individual differences in their perceptual interpretation of the contrast–contrast asynchrony.
to each other. The dark pixels varied in time from 0.25 Amplitude sinF t and the bright pixels varied from 0.75 Amplitude sinF t 180°, where t is time and F is the frequency of modulation. The values of C2 were calculated as the absolute difference between the center and surround contrast, i.e., absC1center − C1surround .
2. GENERAL METHODS
Do contrast–contrast asynchronies exist, and if they do, how are they affected by changes in the amplitude and modulation frequency of C1center ? The standard contrast asynchrony is most apparent when the disks modulate between 3 and 5 Hz, and become stronger at higher modulation amplitudes. The fast temporal response for the asynchrony is consistent with a model in which contrast is computed earlier than a response corresponding to the brightness of the disks. If the contrast–contrast asynchrony is computed through similar channels, then we would expect the appearance of the contrast–contrast asynchrony to occur at similar rates as the contrast asynchrony.
A. Apparatus and Calibration The experiments were presented on a CRT monitor (Sony Trinitron Multiscan G520) using a computer running Windows 7. The luminance levels of the monitor were measured using a Spectrascan 650, and gamma was corrected using the driver software packaged with the computer graphics card (Catalyst Control Center on ATI Radeon HD 5970). Linearity and temporal response were checked with a photocell and oscilloscope. The mean luminance was 50 cd∕m2 , and the refresh rate of the monitor was 85 Hz. The stimuli were developed in Adobe Flash CS6. The data from the experiment were automatically saved in files on the computer. Observers viewed the monitor from a physical distance of 27 cm. B. Specification of Stimulus Contrast For both the center and surround fields, the average luminance of all the pixels was 50 cd∕m2 . In each field, 50% of the pixels were darker than the mean and 50% of the pixels were brighter than the mean, except when contrast for the field equaled 0, in which case all the pixels had the same value. The contrast values of C1center and C1surround were calculated as the Michelson contrast LumMax − LumMin ∕ LumMax LumMin . The pixels in the surround field were always static, i.e., they did not change from frame to frame. In the experiments presented here, the contrast of one surround field was always zero and the contrast of the other surround field was 1.0. The C1center values were always identical
C. Experimental Task Contrast asynchronies (and contrast–contrast asynchronies) produce two equally reasonable interpretations: observers can see the stimulus as “in phase” or as “out of phase.” Experiments with contrast asynchronies are therefore similar to Gestalt-like studies in which observers state whether they perceive one of two plausible responses, or respond “yes” or “no” as to whether they perceive the stimulus in a particular way. Such techniques are not the ideal psychophysical tasks since they give considerable control to observers. Nonetheless, previous studies with the standard contrast asynchrony have shown that observers’ responses follow rules that are systematic and repeatable. In all the experiments presented in this work, observers were asked to press keys on the keyboard to indicate whether the modulation appeared primarily in phase or out of phase (the “z” key on the left side of the keyboard indicated perceived in-phase modulation, and the “/” key on the right side of the keyboard indicated out-of-phase modulation). Observers had unlimited time to view the presentations. A beep indicated that the response had been logged by the computer and that a new trial had started.
3. EXPERIMENT 1: AMPLITUDE AND TEMPORAL FREQUENCY
A. Observers There were four observers who were between the ages of 20 and 25, two female and two male, all with normal or corrected vision. One of the observers was one of the authors (ARH); the other three were naíve to the aims of the experiment. B. Procedure There were two independent variables, contrast amplitude and temporal frequency. Each pixel was 0.1° of visual angle; each center was. 5° in diameter; each surround was a 1° × 1° square. There were 10 levels of contrast amplitudes (0.55 to 1) and three levels of temporal frequency (1, 3, and 6 Hz), and each condition was presented 10 times. There were 300 trials per observer; the trials were presented as a single block and in random order.
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Fig. 3. Results of Experiment 1: Documentation of the contrast– contrast asynchrony; proportion of trials reported as in phase as a function of modulation amplitude. The filled circles indicate 1 Hz modulation; the squares indicate 3 Hz modulation; the diamonds indicate 6 Hz modulation.
C. Results and Conclusion The experiment documents the existence of the contrast– contrast asynchrony, which indicates a perceptual response corresponding to the difference between the textured fields. In Fig. 3, the proportion of trials reported as out of phase are plotted as a function of contrast amplitude. Just like the standard contrast asynchrony, the proportion of trials reported as out of phase increases monotonically as a function of contrast amplitude. However, unlike the standard contrast asynchrony, the contrast–contrast asynchrony appears to be slower: the out-of-phase perception is strongest at 1 Hz. The difference is particularly evident at 6 Hz, a frequency that produces a strong standard contrast asynchrony, but for the contrast–contrast stimuli, observers report seeing out-ofphase modulation on less than half the trials, even at maximum modulation amplitude.
4. EXPERIMENT 2: SIZE OF TEXTURE PIXELS Here, we examine whether the probability of seeing asynchronous modulation depends on the pixel size of the textured field. In the contrast–contrast asynchrony, C2 modulates in antiphase, and C1center modulates in phase. One might assume that C1center would be more salient when the pixels are more identifiable as objects, and C2 would be more visible when the elements are identifiable as textures. One would expect, therefore, that the asynchrony would disappear when the pixel sizes become large. A. Observers There were three observers, two female, one male, all between the ages of 20 and 25 and with normal visual acuity. All observers were naïve to the aims of the experiment. B. Procedure The procedure was similar to Experiment 1. The contrast was fixed at 0.5, and the modulation frequency was fixed at 1 Hz. There were five pixel sizes; the pixel size of the center and surround were always equal to each other. Each condition
Fig. 4. Results of Experiment 2: Proportion of trials reported as in phase as a function of pixel size for the texture of the center fields. The contrast–contrast asynchrony appears in phase more frequently when the pixels are small.
was presented 20 times. There were 100 trials per observer; the trials were presented as a single block and in random order. C. Results and Conclusion The results are plotted as the proportion of trials perceived as asynchronous versus pixel size (Fig. 4). When the pixel sizes were small (0.1° and 0.2°), observers consistently saw the disks as modulating in antiphase; when the pixel sizes became larger, the disks appeared to modulate in phase. These results are therefore consistent with the hypothesis that larger pixel sizes make the C1 contrast more salient relative to C2 contrast.
5. EXPERIMENT 3: MONOCULAR VERSUS BINOCULAR CONTRAST Are the contrast responses in the contrast–contrast asynchrony available to the visual system under dichoptic viewing conditions? That is, can the asynchrony be perceived when the modulating disks are viewed with one eye while the contrast backgrounds are viewed with the other eye? If the asynchrony can be observed, then the C2 response may originate after the combination of signals from the two eyes (presumably in the visual cortex); if the asynchrony cannot be observed, then perception is dominated by monocular contrast, which presumably originates in the retina or in monocular portions of the cortex. A. Equipment The stimuli were presented on an Oculus Rift (OculusVR, Boston, Massachusetts), a head-mounted virtual reality system. The Oculus Rift system can be used as a 1200 × 800 dichoptic presentation device (640 pixels per eye; we did not use the parts of the screen that overlapped for the stimulus). The device was gamma-corrected with the procedure described in the General Methods section. The mean luminance of the display was 60 cd∕m2 , and the refresh rate was 60 Hz. The modulation at 1 Hz was approximately linear. The display subtended about 48° for each eye. The head-mounted display was preferable to our stereoscope/chinrest system because it
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eliminated any misalignments that can occur with use of a chinrest because of small head movements. The observers responded by pressing buttons on a computer keyboard. B. Observers There were two observers, both female, between the ages of 20 and 25 with normal visual acuity. Both observers were naïve to the aims of the experiment. C. Procedure The stimulus configurations can be seen in Fig. 5. The configurations approximated the stimuli from the other experiments. The center circles were 3.75° and the surrounds are 7.5° boxes. Each pixel was 0.075°. The stimuli were presented in either the monocular contrast condition or binocular contrast condition. In the binocular contrast condition, the center disks were presented to one eye against a gray background, and the surround fields were presented to the other eye. In the monocular condition, the modulating disks and contrast background were presented to one eye, and a gray field was presented to the other. In each condition, half the trials had modulating fields presented to the left and half to the right. The spatial configuration of the stimulus was similar to Experiment 1 (i.e., in Experiment 1 each pixel was 0.1° of visual angle, each center was 0.5 ° in diameter, and each surround was a 1° × 1° square). The modulation was always at 1 Hz. The task and response procedures were the same as in Experiment 1. D. Results and Discussion The results are plotted as a proportion of trials versus modulation amplitude in Fig. 6. The monocular contrast data follow the same pattern seen in Experiment 1; that is, observers perceived the asynchrony more frequently at higher modulation amplitudes. The binocular contrast data, on the other hand, remain flat and near zero. There were no significant differences between left and right eye presentations. The results therefore indicate that observers can see the contrast– contrast asynchrony with monocular contrast but not with
Fig. 6. Results of Experiment 3: Proportion of trials reported as in phase as a function of monocular contrast (circles) or binocular contrast (squares). The contrast–contrast asynchrony appears in phase when the modulating field is presented to one eye while the surrounding fields are presented to the other eye.
binocular contrast. The results add to findings that show that neither standard contrast asynchrony [15] nor achromatic contrast–contrast induction [5] can be seen in dichoptic presentation. Singer and D’Zmura [21], however, found weak interocular transfer for achromatic contrast–contrast induction and strong interocular transfer for chromatic (S and L–M) contrast–contrast induction. We have not investigated chromatic contrast–contrast asynchronies, so we cannot comment about chromatic contrast transfer at this stage.
6. EXPERIMENT 4: INDIVIDUAL DIFFERENCES As mentioned in the introduction, individual differences have been reported for the static version of the contrast–contrast illusion. We often present demonstrations in classroom-like settings, and we note that for the contrast–contrast asynchrony, unlike for the standard version of the contrast asynchrony, several people report only seeing the contrast– contrast disks as modulating synchronously. Here we repeat Experiment 1, but with three observers who reported not seeing the out-of-phase modulation. A. Observers There were three observers, two female, one male, all between the ages of 20 and 25 and with normal visual acuity. All observers were naïve to the aims of the experiment. All three observers were identified when they ran a different experiment in the lab and reported not seeing antiphase modulation when presented with the contrast–contrast asynchrony. B. Procedure The experiment was identical to Experiment 1.
Fig. 5. Spatial configuration for Experiment 3: In the binocular contrast condition, the contrast-modulated fields were presented to one eye and the surrounding contrast fields were presented to the other eye. In the monocular contrast condition, the contrast-modulated fields and the surrounding contrast fields were presented to the same eye.
C. Results and Conclusion The results are plotted in Fig. 7 (proportion of trials perceived as modulating asynchronously versus contrast amplitude). Consistent with the observers’ reports in earlier experiments, the observers saw the disks as modulating in phase at highcontrast modulations. Two of the observers saw the lowcontrast modulation as primarily out of phase, whereas one
A. Rose-Henig and A. G. Shapiro
Fig. 7. Results of Experiment 4. The same as Experiment 1, but conducted with three observers who were selected for the study because they reported not seeing the contrast asynchrony. The proportion of trials reported as in phase as a function of monocular or binocular contrast (the symbols are the same as in Fig. 3). These observers do not see the asynchronous modulation at high-contrast levels.
of the observers always saw in-phase modulation. There was no clear difference between the responses to different temporal frequency modulations. The results from these three observers are, therefore, the opposite of the results of the initial four observers used in Experiment 1.
7. DISCUSSION Here we have introduced the contrast–contrast asynchrony and shown some of the parameters that control the appearance of this effect. In the contrast–contrast asynchrony, the contrast levels of the two centers, C1center , modulate synchronously, but the contrast between each center relative to the contrast of the surround, C2, modulates in antiphase. All of the initial observers responded primarily to C2 instead of C1; that is, observers reported that they perceived out-ofphase modulation between the two disks when the contrast modulation was large, the pixel size was small, and the modulation frequency was slow, even though the contrast levels of the disks were always identical to each other. We have also shown that the asynchrony cannot be perceived binocularly (i.e., when the modulating centers are presented to one eye and the modulating surrounds are presented to the other) and that, like the static contrast–contrast illusion, there seem to be notable individual differences. The general finding is theoretically important because it documents that the processes that respond to differences in contrast levels are perceptually salient, and therefore responsible for more than just a contrast gain control. Previous studies with contrast–contrast illusions have shown the presence of lateral inhibition processes. For instance, when the contrast of the surrounding field (i.e., C1surround ) is modulated in time, the appearance of the static center also changes; i.e., when the C1surround has high contrast, the C1center appears to have low contrast and when C1surround has low contrast, the C1center appears to have high contrast [5,16]. In such experiments, the observer’s task is to adjust C1center so as to “null” the appearance of the contrast modulation in the center. Similar “nulling” techniques have been used to study surround effects on the center [9–11]. The
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assumption in such experiments is that a neural connection between the responses to the surround affects the appearance of the center. The functional purpose of the connection is to act as a neural gain control: when the surround field has high contrast, it is advantageous for the visual system to turn down the contrast response. The effect found in the contrast–contrast asynchrony, on the other hand, is inherently ambiguous. Whether observers perceive asynchronous or synchronous modulation depends upon whether they attend to the information provided by C1 or C2. Perceiving asynchronous modulation suggests that observers are attending to visual processes that encode C2. At the most general level, there are two ways that the visual system could construct a response to C2. One way, which we will refer to as a “salience encoding,” would be that the observer attends to the textured contrast in the center and to the textured contrast in the surround; a comparator at a later stage encodes whether these two textures are similar or dissimilar. Such an approach would presumably be slower and independent of binocular or monocular viewing. Another way, which we will refer to as a “direct encoding,” would be a process that responds directly to the contrast–contrast edge; that is, a perceptual process that responds directly to a difference in contrast and does not depend first on the coding of hue, brightness, or saturation of the lights in the display. The results here give some evidence for both the salience and direct encoding of C2 and therefore suggest that neither of the simple accounts can be completely correct. First, the asynchrony is seen monocularly but not binocularly, suggesting that the contrast–contrast signal is either retinal in origin or, at least, dominated by monocular cells within the visual cortex. If the asynchronous response depended only on the salience of the textures, then one would expect the binocular contrast to produce an asynchronous appearance since their perception of the binocular/monocular did not differ much. The result therefore seems to favor a direct model in which the visual system generates responses to C1 and C2. When the stimulus is presented monocularly, the response to C2 is perceptually stronger; but presented binocularly, the C2 response is absent or much weaker; consequently, the centers are perceived to modulate in phase. On the one hand, the best temporal frequency for seeing the contrast–contrast asynchrony is 1 Hz, whereas, for the standard asynchrony, the best temporal frequency is between 3 and 6 Hz. The fast response in the standard asynchrony has led to a model that contains a separate independent pathway for color contrast [6]; the slower temporal response for contrast–contrast asynchrony suggests that there isn’t an independent C2 response, and that contrast–contrast asynchronies require additional processing compared to standard asynchrony conditions. The additional processing could be based on some form of higher-level perceptual comparison (like that suggested by the salience model), almost as if the texture must be calculated prior to making a perceptual judgment. Also, the finding that the asynchrony is more visible for fine pixel sizes suggests that the C2 response is stronger for fine textures and the C1 response is stronger for coarser textures. For the standard asynchrony, the strongest evidence for a direct model approach is the speed of response, the strength of response at different color angles, and motion effects that
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arise at edges versus those that arise at greater spatial distances. Analogous experiments for the contrast–contrast asynchrony have been started but are not yet completed. A working hypothesis for these experiments is that under some (specifiable) conditions, the visual system acts as if it computes contrast directly, yet at other times the visual system acts as if it is making decisions based on attending to different features. The hypothesis is in line with motion studies that divide the motion into separate processes: a low-level motion sensor based on luminance or contrast (first- and secondorder motion), and other motion processes that are based on attending to objects or features of an object (third-order motion). Finally, we address the biggest difference between the standard asynchrony and the contrast–contrast asynchrony: individual differences. The standard contrast asynchrony is robust. The effect has been demonstrated hundreds of times, and nearly all observers see the asynchrony and are surprised to find out that the luminance of the centers is modulating in phase. For the contrast–contrast asynchrony, on the other hand, there appears to be a substantial number of observers who simply do not see the centers as modulating out of phase. This is most evident in public demonstrations, where there is an evident split between those who see in-phase modulations and those who see out-of-phase modulations. At this time we will not speculate as to the cause of this difference, but we note that investigations of the asynchrony may prove interesting since (as noted in the introduction) there have been attempts to correlate the static version of the contrast– contrast illusion to schizophrenia and other conditions, and there is at least one documented observer who reverses the direction of the static version of the contrast–contrast illusion. Investigations with the contrast–contrast asynchrony may therefore be more revealing than static versions of the contrast asynchrony for identifying these observers.
REFERENCES 1. 2. 3.
H. H. Helmholtz, Treatise on Physiological Optics, Vol. 2 (Dover, 1866). E. Hering, Outline of a Theory of Light Sense (Harvard University, 1905). M. E. Chevreul, The Principles of Harmony and Contrast of Colors and Their Applications to the Arts (Reinhold, 1839).
A. Rose-Henig and A. G. Shapiro 4. E. G. Heinemann, “Simultaneous brightness induction as a function of inducing and test-field luminance,” J. Exp. Psychol. 50, 89–96 (1955). 5. C. Chubb, G. Sperling, and J. A. Solomon, “Texture interactions determine perceived contrast,” Proc. Natl. Acad. Sci. USA 86, 9631–9635 (1989). 6. A. G. Shapiro, “Separating color from color contrast,” J. Vis. 8 (1):8, 1–18 (2008). 7. A. G. Shapiro, A. D. D’Antona, J. P. Charles, L. A. Belano, J. B. Smith, and M. Shear-Heyman, “Induced contrast asynchronies,” J. Vis. 4(6):5, 459–468 (2004). 8. A. G. Shapiro, J. P. Charles, and M. Shear-Heyman, “Visual illusions based on single-field contrast asynchronies,” J. Vis. 5 (10):2, 764–782– (2005). 9. J. Krauskopf and Q. Zaidi, “Induced desensitization,” Vis. Res. 26, 759–762 (1986). 10. Q. Zaidi, B. Yoshimi, N. Flanigan, and A. Canova, “Lateral interactions within color mechanisms in simultaneous induced contrast,” Vis. Res. 32, 1695–1707 (1992). 11. R. L. De Valois, M. A. Webster, K. K. De Valois, and B. Lingelbach, “Temporal properties of brightness and color induction,” Vis. Res. 26, 887–897 (1986). 12. A. G. Shapiro, A. D’Antona, J. B. Smith, L. A. Belano, and J. P. Charles, “Induced contrast asynchronies may be useful for luminance photometry,” Vis. Neurosci. 21, 243–247 (2004). 13. A. Shapiro and E. Knight, “Spatial and temporal influences on the contrast gauge,” Vis. Res. 48, 2642–2648 (2008). 14. A. G. Shapiro and A. M. Leaver, “Edges can eliminate the appearance of the contrast asynchrony,” Ophthalmic Physiolog. Opt. 30, 534–544 (2010). 15. O. J. Flynn and A. G. Shapiro, “The separation of monocular and binocular contrast,” Vis. Res. 93, 19–28 (2013). 16. B. Singer and M. D’Zmura, “Contrast gain control: a bilinear model for chromatic selectivity,” J. Opt. Soc. Am. A 12, 667–685 (1995). 17. M. W. Cannon and S. C. Fullenkamp, “Spatial interactions in apparent contrast: individual differences in enhancement and suppression effects,” Vis. Res. 33, 1685–1695 (1993). 18. S. Dakin, P. Carlin, and D. Hemsley, “Weak suppression of visual context in chronic schizophrenia,” Curr. Biol. 15, R822–R824 (2005). 19. M. S. Tibber, E. J. Anderson, T. Bobin, E. Antonova, A. Seabright, B. Wright, P. Carlin, S. S. Shergill, and S. C. Dakin, “Visual surround suppression in schizophrenia,” Front. Psychol. 4, 88 (2013). 20. D. M. Barch, C. S. Carter, S. C. Dakin, J. Gold, S. J. Luck, A. Macdonald, 3rd, J. D. Ragland, S. Silverstein, and M. E. Strauss, “The clinical translation of a measure of gain control: the contrast–contrast effect task,” Schizophr. Bull. 38, 135–143 (2012). 21. B. Singer and M. D’Zmura, “Color contrast induction,” Vis. Res. 34, 3111–3126 (1994).
J. Opt. Soc. Am. A / Vol. 31, No. 4 / April 2014
A. Rose-Henig and A. G. Shapiro
Contrast–contrast asynchronies Alex Rose-Henig1,2 and Arthur G. Shapiro1,2,* 1
Department of Psychology, American University, 4400 Massachusetts Avenue, NW, Washington, DC 20016, USA 2 Center for Behavioral Neuroscience, American University, 4400 Massachusetts Avenue, NW, Washington, DC 20016, USA *Corresponding author: [email protected] Received September 30, 2013; revised January 7, 2014; accepted January 10, 2014; posted January 13, 2014 (Doc. ID 198552); published February 12, 2014 We introduce the “contrast–contrast asynchrony,” a dynamic stimulus configuration that combines elements of the Shapiro contrast asynchrony with elements of the Chubb contrast–contrast illusion. In the contrast–contrast asynchrony, static textured fields surround two textured fields; one surround has high-contrast texture, and the other has low-contrast texture. The contrasts of the center fields modulate in phase with each other at 1 Hz, and as a consequence, the difference between the contrast of the centers and the contrast of the respective surround modulates in antiphase. Most observers report an antiphase appearance for high-contrast, fine-grained centers. These observers therefore respond to the difference between the center contrast and surround contrast. We also document three observers who do not see the asynchrony for high-contrast modulations of the center, suggesting possibly interesting individual differences. © 2014 Optical Society of America OCIS codes: (330.1720) Color vision; (330.5020) Perception psychology; (330.1800) Vision - contrast sensitivity; (330.6100) Spatial discrimination. http://dx.doi.org/10.1364/JOSAA.31.00A232
1. INTRODUCTION Generations of scientists have studied the effect of context on perception with the idea that such displays will give some indication of how the brain transforms light into appearance [1–4]. For instance, in a standard simultaneous contrast display, test patches with the same midluminance pixel values are placed in different spatial contexts: one disk is on a dark background, while the other is on a bright background. The disk on the bright background appears darker than the disk placed on the bright background, so the perceptual appearance differs even though the disks are physically identical. Here, we investigate what happens when we combine two variations of spatial context on perception: the contrast– contrast illusion [5] and the contrast asynchrony [6–8]. A. Contrast Asynchrony Contrast asynchronies are not standard simultaneous contrast displays; rather, the contrast asynchrony is a configuration that creates two competing perceptions by juxtaposing the luminance of identical disks with the contrast between the disks and their surrounds. Consider Fig. 1(a), which shows a series of identical disks placed on a gradient background. Each disk has two sources of information: one source of information corresponds to the luminance of the disks (i.e., the disks could all be called “white,” albeit slightly different achromatic shades); another source of information corresponds to the contrast between the disks and the gradient background (i.e., the disks on the left appear in high contrast with the background, while the disks on the right appear in low contrast with the background). By temporally modulating the luminance of the disks, contrast asynchronies separate the visual response to luminance from the visual response to contrast. 1084-7529/14/04A232-07$15.00/0
To illustrate this point, consider Fig. 1(b), which shows four frames of the contrast asynchrony, in which the surrounding ring on the left is dark while the ring on the right is bright. The luminance levels of the center disks are always identical to each other and modulate sinusoidally in time, and the contrast of the disks relative to the rings modulates in antiphase [see the plot in Fig. 1(c)]. So, when the luminance levels of the disks are high, both disks appear bright, but the left disk/ring pair has high contrast while the right disk/ring pair has low contrast, and when the luminance levels of the disks are low, the disks appear dark and the contrast levels are reversed. At low temporal frequencies, observers have the paradoxical effect of seeing the disks modulate out of phase (in line with the contrast modulation); yet, with attention, observers can also perceive that the disks are becoming bright and dark at the same time (in line with the luminance modulation). It needs to be emphasized that contrast asynchronies are different from dynamic simultaneous contrast, in which the modulation of the surround changes the appearance of the center [9–11]. The experiments measure the effects on the center typically through a nulling technique. In contrast asynchronies, however, the center disks’ difference from each other is not the main concern; rather, the question is which type of perceptual information predominates, the luminance of the center or the contrast between the center and the surround. Contrast asynchronies are useful because they are capable of separating the perception of color from the perception of color contrast. The contrast asynchrony has been used to study color [6,12], the effects of short-range and long-range motion [8], interactions of spatial changes in contrast [13], paradoxical effects of edges and contrast [14], and contrast in monocular and binocular configurations [15]. The appearance of the asynchrony is strongest when modulation is © 2014 Optical Society of America
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Fig. 1. Contrast asynchrony [5–7]. (a) Physically identical disks on a gradient background. The disk on the left has higher contrast relative to the background than the disk on the right. (b) Four frames from the contrast asynchrony which presents two physically identical disks, one with a dark surround and the other with a light surround. In each frame, the luminance levels of the disks are always the same; the contrast between each disk and surround modulates in antiphase. (c) Plots of the changes in luminance and contrast levels over time. The luminance levels of the disks are in phase; the contrast levels are in antiphase.
between 3 and 5 Hz, and when lights and background are in similar color directions and have higher modulation amplitude. B. Contrast–Contrast Illusion The contrast–contrast illusion [5], also referred to as the Chubb illusion, is a type of simultaneous contrast but with a textured patch. The effect can be seen by examining the textured disks in Fig. 2(a). The textures that fill the center disks have the same physical contrasts, yet the disks on the left are perceived to have a higher contrast texture than the disks on the right. Contrast–contrast illusions are important partly because they suggest the presence of a gain control that operates on texture [5,16]. This gain control is strongest when the texture of the surround matches that of the center, suggesting the presence of tuning curves for neural encoding of texture. Contrast–contrast illusions have also generated much attention because they seem to show individual differences and differences between some clinical and nonclinical populations. For instance, there has been documentation of an observer for whom the direction of the contrast– contrast effect was dramatically reversed [17]; and patients with schizophrenia have been reported to have weak or absent responses [18,19]. However, some caution regarding
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Fig. 2. Contrast–contrast asynchrony. (a) Version of the static contrast–contrast illusion. The disks all have the same internal contrast and are placed on a contrast gradient. The disk on the left appears to have lower internal contrast than the disk on the right. The paper makes a distinction between the physical contrast inside each disk (C1center ) and the difference between the contrast of the disk and the contrast of the surround (C2). (b) Four frames from the contrast– contrast asynchrony. The C1centers of the left and right disks are always the same; the C2 on the left is not always the same as the C2 on the right. (c) Plots of the changes in luminance, contrast, and contrast–contrast over time in the contrast–contrast asynchrony; the luminance levels of disks are always constant. The C1center of the left and right disks modulates in phase; the contrast–contrast (C2) modulates in antiphase.
conclusions about individual difference is advisable because other studies did not find a significant difference between normal observers and people with schizophrenia or schizoaffective disorder [20], suggesting that the differing response of the latter group is partly due to impaired attentional mechanisms. C. Contrast–Contrast Asynchrony All accounts of the contrast–contrast illusion so far have concentrated on the appearance of the center. But, just like Fig. 1(a), Fig. 2(a) contains two types of information: (1) the contrast of the disks, and (2) the difference between the contrast of the disks and contrast of the surround. The second source of information corresponds in principle to the difference between two textures. An important question for vision science is whether the visual system calculates this difference directly or whether there is a “later-stage” (and perhaps attentional) comparison between separate contrast calculations. We address this question with a visual stimulus that we have named the contrast–contrast asynchrony.
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In the basic version of the contrast–contrast asynchrony [Fig. 2(b)], the contrast of both disks modulates sinusoidally in time; one disk is placed against a low-contrast surround, and one disk is placed against a high-contrast surround. We will use the following terminology to make the discussion easier: C1center refers to the contrast of the center disks. C1center modulates in time and is always identical for the left and right center disks. C1surround refers to the contrast of the surround field. C1surround is always static. The left surround and right surround have the same mean luminance and typically have different contrast values. C2 refers to the contrast between C1center and C1surround . C2 modulates in time; C2 on the left typically modulates in antiphase relative to C2 on the right. Figure 2(c) plots the temporal changes of C1center and C2 as a function of time. As with the standard contrast asynchrony, we can discuss whether C2 is a different perceptual entity from C1center and C1surround . We attempt to determine the conditions under which observers see the disks as modulating asynchronously (corresponding to the C2 modulation) or synchronously (corresponding to C1center modulation). We investigate these differences with regard to modulation amplitude, temporal frequency, and texture size. The results show some similarities and some differences between the contrast–contrast asynchrony and the contrast asynchrony. The two most striking differences are that the asynchronous response is slower in the contrast–contrast asynchrony, and our observers exhibit substantial individual differences in their perceptual interpretation of the contrast–contrast asynchrony.
to each other. The dark pixels varied in time from 0.25 Amplitude sinF t and the bright pixels varied from 0.75 Amplitude sinF t 180°, where t is time and F is the frequency of modulation. The values of C2 were calculated as the absolute difference between the center and surround contrast, i.e., absC1center − C1surround .
2. GENERAL METHODS
Do contrast–contrast asynchronies exist, and if they do, how are they affected by changes in the amplitude and modulation frequency of C1center ? The standard contrast asynchrony is most apparent when the disks modulate between 3 and 5 Hz, and become stronger at higher modulation amplitudes. The fast temporal response for the asynchrony is consistent with a model in which contrast is computed earlier than a response corresponding to the brightness of the disks. If the contrast–contrast asynchrony is computed through similar channels, then we would expect the appearance of the contrast–contrast asynchrony to occur at similar rates as the contrast asynchrony.
A. Apparatus and Calibration The experiments were presented on a CRT monitor (Sony Trinitron Multiscan G520) using a computer running Windows 7. The luminance levels of the monitor were measured using a Spectrascan 650, and gamma was corrected using the driver software packaged with the computer graphics card (Catalyst Control Center on ATI Radeon HD 5970). Linearity and temporal response were checked with a photocell and oscilloscope. The mean luminance was 50 cd∕m2 , and the refresh rate of the monitor was 85 Hz. The stimuli were developed in Adobe Flash CS6. The data from the experiment were automatically saved in files on the computer. Observers viewed the monitor from a physical distance of 27 cm. B. Specification of Stimulus Contrast For both the center and surround fields, the average luminance of all the pixels was 50 cd∕m2 . In each field, 50% of the pixels were darker than the mean and 50% of the pixels were brighter than the mean, except when contrast for the field equaled 0, in which case all the pixels had the same value. The contrast values of C1center and C1surround were calculated as the Michelson contrast LumMax − LumMin ∕ LumMax LumMin . The pixels in the surround field were always static, i.e., they did not change from frame to frame. In the experiments presented here, the contrast of one surround field was always zero and the contrast of the other surround field was 1.0. The C1center values were always identical
C. Experimental Task Contrast asynchronies (and contrast–contrast asynchronies) produce two equally reasonable interpretations: observers can see the stimulus as “in phase” or as “out of phase.” Experiments with contrast asynchronies are therefore similar to Gestalt-like studies in which observers state whether they perceive one of two plausible responses, or respond “yes” or “no” as to whether they perceive the stimulus in a particular way. Such techniques are not the ideal psychophysical tasks since they give considerable control to observers. Nonetheless, previous studies with the standard contrast asynchrony have shown that observers’ responses follow rules that are systematic and repeatable. In all the experiments presented in this work, observers were asked to press keys on the keyboard to indicate whether the modulation appeared primarily in phase or out of phase (the “z” key on the left side of the keyboard indicated perceived in-phase modulation, and the “/” key on the right side of the keyboard indicated out-of-phase modulation). Observers had unlimited time to view the presentations. A beep indicated that the response had been logged by the computer and that a new trial had started.
3. EXPERIMENT 1: AMPLITUDE AND TEMPORAL FREQUENCY
A. Observers There were four observers who were between the ages of 20 and 25, two female and two male, all with normal or corrected vision. One of the observers was one of the authors (ARH); the other three were naíve to the aims of the experiment. B. Procedure There were two independent variables, contrast amplitude and temporal frequency. Each pixel was 0.1° of visual angle; each center was. 5° in diameter; each surround was a 1° × 1° square. There were 10 levels of contrast amplitudes (0.55 to 1) and three levels of temporal frequency (1, 3, and 6 Hz), and each condition was presented 10 times. There were 300 trials per observer; the trials were presented as a single block and in random order.
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Fig. 3. Results of Experiment 1: Documentation of the contrast– contrast asynchrony; proportion of trials reported as in phase as a function of modulation amplitude. The filled circles indicate 1 Hz modulation; the squares indicate 3 Hz modulation; the diamonds indicate 6 Hz modulation.
C. Results and Conclusion The experiment documents the existence of the contrast– contrast asynchrony, which indicates a perceptual response corresponding to the difference between the textured fields. In Fig. 3, the proportion of trials reported as out of phase are plotted as a function of contrast amplitude. Just like the standard contrast asynchrony, the proportion of trials reported as out of phase increases monotonically as a function of contrast amplitude. However, unlike the standard contrast asynchrony, the contrast–contrast asynchrony appears to be slower: the out-of-phase perception is strongest at 1 Hz. The difference is particularly evident at 6 Hz, a frequency that produces a strong standard contrast asynchrony, but for the contrast–contrast stimuli, observers report seeing out-ofphase modulation on less than half the trials, even at maximum modulation amplitude.
4. EXPERIMENT 2: SIZE OF TEXTURE PIXELS Here, we examine whether the probability of seeing asynchronous modulation depends on the pixel size of the textured field. In the contrast–contrast asynchrony, C2 modulates in antiphase, and C1center modulates in phase. One might assume that C1center would be more salient when the pixels are more identifiable as objects, and C2 would be more visible when the elements are identifiable as textures. One would expect, therefore, that the asynchrony would disappear when the pixel sizes become large. A. Observers There were three observers, two female, one male, all between the ages of 20 and 25 and with normal visual acuity. All observers were naïve to the aims of the experiment. B. Procedure The procedure was similar to Experiment 1. The contrast was fixed at 0.5, and the modulation frequency was fixed at 1 Hz. There were five pixel sizes; the pixel size of the center and surround were always equal to each other. Each condition
Fig. 4. Results of Experiment 2: Proportion of trials reported as in phase as a function of pixel size for the texture of the center fields. The contrast–contrast asynchrony appears in phase more frequently when the pixels are small.
was presented 20 times. There were 100 trials per observer; the trials were presented as a single block and in random order. C. Results and Conclusion The results are plotted as the proportion of trials perceived as asynchronous versus pixel size (Fig. 4). When the pixel sizes were small (0.1° and 0.2°), observers consistently saw the disks as modulating in antiphase; when the pixel sizes became larger, the disks appeared to modulate in phase. These results are therefore consistent with the hypothesis that larger pixel sizes make the C1 contrast more salient relative to C2 contrast.
5. EXPERIMENT 3: MONOCULAR VERSUS BINOCULAR CONTRAST Are the contrast responses in the contrast–contrast asynchrony available to the visual system under dichoptic viewing conditions? That is, can the asynchrony be perceived when the modulating disks are viewed with one eye while the contrast backgrounds are viewed with the other eye? If the asynchrony can be observed, then the C2 response may originate after the combination of signals from the two eyes (presumably in the visual cortex); if the asynchrony cannot be observed, then perception is dominated by monocular contrast, which presumably originates in the retina or in monocular portions of the cortex. A. Equipment The stimuli were presented on an Oculus Rift (OculusVR, Boston, Massachusetts), a head-mounted virtual reality system. The Oculus Rift system can be used as a 1200 × 800 dichoptic presentation device (640 pixels per eye; we did not use the parts of the screen that overlapped for the stimulus). The device was gamma-corrected with the procedure described in the General Methods section. The mean luminance of the display was 60 cd∕m2 , and the refresh rate was 60 Hz. The modulation at 1 Hz was approximately linear. The display subtended about 48° for each eye. The head-mounted display was preferable to our stereoscope/chinrest system because it
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eliminated any misalignments that can occur with use of a chinrest because of small head movements. The observers responded by pressing buttons on a computer keyboard. B. Observers There were two observers, both female, between the ages of 20 and 25 with normal visual acuity. Both observers were naïve to the aims of the experiment. C. Procedure The stimulus configurations can be seen in Fig. 5. The configurations approximated the stimuli from the other experiments. The center circles were 3.75° and the surrounds are 7.5° boxes. Each pixel was 0.075°. The stimuli were presented in either the monocular contrast condition or binocular contrast condition. In the binocular contrast condition, the center disks were presented to one eye against a gray background, and the surround fields were presented to the other eye. In the monocular condition, the modulating disks and contrast background were presented to one eye, and a gray field was presented to the other. In each condition, half the trials had modulating fields presented to the left and half to the right. The spatial configuration of the stimulus was similar to Experiment 1 (i.e., in Experiment 1 each pixel was 0.1° of visual angle, each center was 0.5 ° in diameter, and each surround was a 1° × 1° square). The modulation was always at 1 Hz. The task and response procedures were the same as in Experiment 1. D. Results and Discussion The results are plotted as a proportion of trials versus modulation amplitude in Fig. 6. The monocular contrast data follow the same pattern seen in Experiment 1; that is, observers perceived the asynchrony more frequently at higher modulation amplitudes. The binocular contrast data, on the other hand, remain flat and near zero. There were no significant differences between left and right eye presentations. The results therefore indicate that observers can see the contrast– contrast asynchrony with monocular contrast but not with
Fig. 6. Results of Experiment 3: Proportion of trials reported as in phase as a function of monocular contrast (circles) or binocular contrast (squares). The contrast–contrast asynchrony appears in phase when the modulating field is presented to one eye while the surrounding fields are presented to the other eye.
binocular contrast. The results add to findings that show that neither standard contrast asynchrony [15] nor achromatic contrast–contrast induction [5] can be seen in dichoptic presentation. Singer and D’Zmura [21], however, found weak interocular transfer for achromatic contrast–contrast induction and strong interocular transfer for chromatic (S and L–M) contrast–contrast induction. We have not investigated chromatic contrast–contrast asynchronies, so we cannot comment about chromatic contrast transfer at this stage.
6. EXPERIMENT 4: INDIVIDUAL DIFFERENCES As mentioned in the introduction, individual differences have been reported for the static version of the contrast–contrast illusion. We often present demonstrations in classroom-like settings, and we note that for the contrast–contrast asynchrony, unlike for the standard version of the contrast asynchrony, several people report only seeing the contrast– contrast disks as modulating synchronously. Here we repeat Experiment 1, but with three observers who reported not seeing the out-of-phase modulation. A. Observers There were three observers, two female, one male, all between the ages of 20 and 25 and with normal visual acuity. All observers were naïve to the aims of the experiment. All three observers were identified when they ran a different experiment in the lab and reported not seeing antiphase modulation when presented with the contrast–contrast asynchrony. B. Procedure The experiment was identical to Experiment 1.
Fig. 5. Spatial configuration for Experiment 3: In the binocular contrast condition, the contrast-modulated fields were presented to one eye and the surrounding contrast fields were presented to the other eye. In the monocular contrast condition, the contrast-modulated fields and the surrounding contrast fields were presented to the same eye.
C. Results and Conclusion The results are plotted in Fig. 7 (proportion of trials perceived as modulating asynchronously versus contrast amplitude). Consistent with the observers’ reports in earlier experiments, the observers saw the disks as modulating in phase at highcontrast modulations. Two of the observers saw the lowcontrast modulation as primarily out of phase, whereas one
A. Rose-Henig and A. G. Shapiro
Fig. 7. Results of Experiment 4. The same as Experiment 1, but conducted with three observers who were selected for the study because they reported not seeing the contrast asynchrony. The proportion of trials reported as in phase as a function of monocular or binocular contrast (the symbols are the same as in Fig. 3). These observers do not see the asynchronous modulation at high-contrast levels.
of the observers always saw in-phase modulation. There was no clear difference between the responses to different temporal frequency modulations. The results from these three observers are, therefore, the opposite of the results of the initial four observers used in Experiment 1.
7. DISCUSSION Here we have introduced the contrast–contrast asynchrony and shown some of the parameters that control the appearance of this effect. In the contrast–contrast asynchrony, the contrast levels of the two centers, C1center , modulate synchronously, but the contrast between each center relative to the contrast of the surround, C2, modulates in antiphase. All of the initial observers responded primarily to C2 instead of C1; that is, observers reported that they perceived out-ofphase modulation between the two disks when the contrast modulation was large, the pixel size was small, and the modulation frequency was slow, even though the contrast levels of the disks were always identical to each other. We have also shown that the asynchrony cannot be perceived binocularly (i.e., when the modulating centers are presented to one eye and the modulating surrounds are presented to the other) and that, like the static contrast–contrast illusion, there seem to be notable individual differences. The general finding is theoretically important because it documents that the processes that respond to differences in contrast levels are perceptually salient, and therefore responsible for more than just a contrast gain control. Previous studies with contrast–contrast illusions have shown the presence of lateral inhibition processes. For instance, when the contrast of the surrounding field (i.e., C1surround ) is modulated in time, the appearance of the static center also changes; i.e., when the C1surround has high contrast, the C1center appears to have low contrast and when C1surround has low contrast, the C1center appears to have high contrast [5,16]. In such experiments, the observer’s task is to adjust C1center so as to “null” the appearance of the contrast modulation in the center. Similar “nulling” techniques have been used to study surround effects on the center [9–11]. The
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assumption in such experiments is that a neural connection between the responses to the surround affects the appearance of the center. The functional purpose of the connection is to act as a neural gain control: when the surround field has high contrast, it is advantageous for the visual system to turn down the contrast response. The effect found in the contrast–contrast asynchrony, on the other hand, is inherently ambiguous. Whether observers perceive asynchronous or synchronous modulation depends upon whether they attend to the information provided by C1 or C2. Perceiving asynchronous modulation suggests that observers are attending to visual processes that encode C2. At the most general level, there are two ways that the visual system could construct a response to C2. One way, which we will refer to as a “salience encoding,” would be that the observer attends to the textured contrast in the center and to the textured contrast in the surround; a comparator at a later stage encodes whether these two textures are similar or dissimilar. Such an approach would presumably be slower and independent of binocular or monocular viewing. Another way, which we will refer to as a “direct encoding,” would be a process that responds directly to the contrast–contrast edge; that is, a perceptual process that responds directly to a difference in contrast and does not depend first on the coding of hue, brightness, or saturation of the lights in the display. The results here give some evidence for both the salience and direct encoding of C2 and therefore suggest that neither of the simple accounts can be completely correct. First, the asynchrony is seen monocularly but not binocularly, suggesting that the contrast–contrast signal is either retinal in origin or, at least, dominated by monocular cells within the visual cortex. If the asynchronous response depended only on the salience of the textures, then one would expect the binocular contrast to produce an asynchronous appearance since their perception of the binocular/monocular did not differ much. The result therefore seems to favor a direct model in which the visual system generates responses to C1 and C2. When the stimulus is presented monocularly, the response to C2 is perceptually stronger; but presented binocularly, the C2 response is absent or much weaker; consequently, the centers are perceived to modulate in phase. On the one hand, the best temporal frequency for seeing the contrast–contrast asynchrony is 1 Hz, whereas, for the standard asynchrony, the best temporal frequency is between 3 and 6 Hz. The fast response in the standard asynchrony has led to a model that contains a separate independent pathway for color contrast [6]; the slower temporal response for contrast–contrast asynchrony suggests that there isn’t an independent C2 response, and that contrast–contrast asynchronies require additional processing compared to standard asynchrony conditions. The additional processing could be based on some form of higher-level perceptual comparison (like that suggested by the salience model), almost as if the texture must be calculated prior to making a perceptual judgment. Also, the finding that the asynchrony is more visible for fine pixel sizes suggests that the C2 response is stronger for fine textures and the C1 response is stronger for coarser textures. For the standard asynchrony, the strongest evidence for a direct model approach is the speed of response, the strength of response at different color angles, and motion effects that
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arise at edges versus those that arise at greater spatial distances. Analogous experiments for the contrast–contrast asynchrony have been started but are not yet completed. A working hypothesis for these experiments is that under some (specifiable) conditions, the visual system acts as if it computes contrast directly, yet at other times the visual system acts as if it is making decisions based on attending to different features. The hypothesis is in line with motion studies that divide the motion into separate processes: a low-level motion sensor based on luminance or contrast (first- and secondorder motion), and other motion processes that are based on attending to objects or features of an object (third-order motion). Finally, we address the biggest difference between the standard asynchrony and the contrast–contrast asynchrony: individual differences. The standard contrast asynchrony is robust. The effect has been demonstrated hundreds of times, and nearly all observers see the asynchrony and are surprised to find out that the luminance of the centers is modulating in phase. For the contrast–contrast asynchrony, on the other hand, there appears to be a substantial number of observers who simply do not see the centers as modulating out of phase. This is most evident in public demonstrations, where there is an evident split between those who see in-phase modulations and those who see out-of-phase modulations. At this time we will not speculate as to the cause of this difference, but we note that investigations of the asynchrony may prove interesting since (as noted in the introduction) there have been attempts to correlate the static version of the contrast– contrast illusion to schizophrenia and other conditions, and there is at least one documented observer who reverses the direction of the static version of the contrast–contrast illusion. Investigations with the contrast–contrast asynchrony may therefore be more revealing than static versions of the contrast asynchrony for identifying these observers.
REFERENCES 1. 2. 3.
H. H. Helmholtz, Treatise on Physiological Optics, Vol. 2 (Dover, 1866). E. Hering, Outline of a Theory of Light Sense (Harvard University, 1905). M. E. Chevreul, The Principles of Harmony and Contrast of Colors and Their Applications to the Arts (Reinhold, 1839).
A. Rose-Henig and A. G. Shapiro 4. E. G. Heinemann, “Simultaneous brightness induction as a function of inducing and test-field luminance,” J. Exp. Psychol. 50, 89–96 (1955). 5. C. Chubb, G. Sperling, and J. A. Solomon, “Texture interactions determine perceived contrast,” Proc. Natl. Acad. Sci. USA 86, 9631–9635 (1989). 6. A. G. Shapiro, “Separating color from color contrast,” J. Vis. 8 (1):8, 1–18 (2008). 7. A. G. Shapiro, A. D. D’Antona, J. P. Charles, L. A. Belano, J. B. Smith, and M. Shear-Heyman, “Induced contrast asynchronies,” J. Vis. 4(6):5, 459–468 (2004). 8. A. G. Shapiro, J. P. Charles, and M. Shear-Heyman, “Visual illusions based on single-field contrast asynchronies,” J. Vis. 5 (10):2, 764–782– (2005). 9. J. Krauskopf and Q. Zaidi, “Induced desensitization,” Vis. Res. 26, 759–762 (1986). 10. Q. Zaidi, B. Yoshimi, N. Flanigan, and A. Canova, “Lateral interactions within color mechanisms in simultaneous induced contrast,” Vis. Res. 32, 1695–1707 (1992). 11. R. L. De Valois, M. A. Webster, K. K. De Valois, and B. Lingelbach, “Temporal properties of brightness and color induction,” Vis. Res. 26, 887–897 (1986). 12. A. G. Shapiro, A. D’Antona, J. B. Smith, L. A. Belano, and J. P. Charles, “Induced contrast asynchronies may be useful for luminance photometry,” Vis. Neurosci. 21, 243–247 (2004). 13. A. Shapiro and E. Knight, “Spatial and temporal influences on the contrast gauge,” Vis. Res. 48, 2642–2648 (2008). 14. A. G. Shapiro and A. M. Leaver, “Edges can eliminate the appearance of the contrast asynchrony,” Ophthalmic Physiolog. Opt. 30, 534–544 (2010). 15. O. J. Flynn and A. G. Shapiro, “The separation of monocular and binocular contrast,” Vis. Res. 93, 19–28 (2013). 16. B. Singer and M. D’Zmura, “Contrast gain control: a bilinear model for chromatic selectivity,” J. Opt. Soc. Am. A 12, 667–685 (1995). 17. M. W. Cannon and S. C. Fullenkamp, “Spatial interactions in apparent contrast: individual differences in enhancement and suppression effects,” Vis. Res. 33, 1685–1695 (1993). 18. S. Dakin, P. Carlin, and D. Hemsley, “Weak suppression of visual context in chronic schizophrenia,” Curr. Biol. 15, R822–R824 (2005). 19. M. S. Tibber, E. J. Anderson, T. Bobin, E. Antonova, A. Seabright, B. Wright, P. Carlin, S. S. Shergill, and S. C. Dakin, “Visual surround suppression in schizophrenia,” Front. Psychol. 4, 88 (2013). 20. D. M. Barch, C. S. Carter, S. C. Dakin, J. Gold, S. J. Luck, A. Macdonald, 3rd, J. D. Ragland, S. Silverstein, and M. E. Strauss, “The clinical translation of a measure of gain control: the contrast–contrast effect task,” Schizophr. Bull. 38, 135–143 (2012). 21. B. Singer and M. D’Zmura, “Color contrast induction,” Vis. Res. 34, 3111–3126 (1994).
