VisionRes.Vol. 30,No. II, pp. 17%1?61,1990 Printed

in Great

Britain

MOTION AT ISOLUMINANCE: DISCRIE?[INATION/ DETECTION RATIOS FOR MOVING ISOLUMINANT GRATINGS DELWINT.

m&s

of ‘~~~010~

LINDSEY** and DAMDA Y. TEL&~

and z~~o~~/Biopby~, WA 98195, U.S.A.

University of Won,

(Remived 10 August 1989; in rcvisedjbm

Seattle,

I April 1990)

Abstract-Subjects viewed a 2.3 x 2.3 dcg patch of a moving 1.34&g, 3.75 Hz sinusoidal grating, centered 1.8 dcg from fixation. Two-altcmative forced-choice contrast tbm3holds were measured along the luminance axis and 10 chromatic axes at isoluminance for tbrec tasks: detection (O), form disrimination (F), and discrimination of upward from downward motion (M). F/D threshold ntios averaged approx. 1: 1 on all axes. M/D ratios were approx. 1:1on the luminance axis, but varied from 3: 1 to indetumkately large with chromatic axis at isoluminance. We conclude that under the prwcnt conditions there are large, highly spcciftc losses of direction-of-motion information at isolumkance. Themsultsimplytbcuisterrt of chromatic channels that are labeled for form but not for direction of motion at thrubold. Tbc pattern and significance of variations in MID ratios witbin the isoluminant plane is also dkwsed. Color vision

Isoluminancz

Motion

L.abclcd detectors

INTRODUCTION

form and motion (e.g. Livingstone & Hubel, 1987). There is evidence to suggest that a wide variety The literature on motion perception is heteroof visual functions are compromised when the geneous. For example, Cavanagh et al. (1984) stimulus being processed is isoltinant; i.e. made quantitative measurements of the perwhen the spatiotemporal pattern of visual ceived velocity of moving high contrast isostimulation is made up only of variations in last red&een and y~ow~l~ sinusoidal chromati~ty, without a~orn~n~ng variations gratings in near-foveaI viewing. A 10 x 10 deg in luminance. These visual functions include stimulus was used, with a 2 deg central, horixonborder perception (Boynton, 1978; Tansley tal strip obscured. The subject’s task was to & Boynton, 1978), spatial (Mullen, 1985) and vary the velocity of a high contrast ltinancetemporal (de Lange, 1958) contrast sensitivity modulated grating, presented in the upper halfat high frequencies, stereopsis (Lu & Fender, field, to match the perceived velocity of a 1972; de Weert & Sadxa, 1983), vernier acuity chromatically-modulated grating of the same (Morgan & Aiba, 1985; Fare11 & Krauskopf, spatial frequency moving in the opposite direc1988, 1989), accommodation (Wolfe & Owens, tion in the lower half-field. Cavanagh et al. 1981), phase discrimination (Troscianko, 1987; found that the greatest relative loss of perceived Troscianko & Harris, 1988), and, particularly, velocity was found at low spatial and temporal the perception of motion (R~ac~andran & frequencies (e.g. 0.8 cjdeg and 0.3 deg/sec). Gregory, 1978; Cavanagh, Tyler 6t Favreau, Under such conditions, the chromatic gratings 1984; Troscianko, 1987; Troscianko 65;Fahle, “appeared appreciably slowed and sometimes 1988). These phenomena have attracted wide stopped, even though the bars of the grating interest because they are suggestive of the theorcould be easily resolved”. Cavanagh et al. also etical notion that different visual functions are found that small amounts of chromatic moduprocessed in parallel, and that visual reprelation, added to a luminance-modulated stimusentations initiated by isoluminant chromatic lus, reduced rather than increased its perceived stimuli do not access the neural machinery vebcity. required for the processing of higher levels of More recently, Cavanagh and Anstis (1991) have used motion nulling techniques to evahaate *To whom reprint quests should be address&. the deg of loss of motion at ~l~n~~. They 1751

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DELWIN T. LINDSSY

found that luminance mod~ations of 515% were required to null the perceived motion of isoluminant stimuli, and concluded that isoluminant stimuli do sustain the perception of motion to some degree. Similarly, Gorea and Papathomas (1989), using an apparent-motion paradigm, showed that variations of chromaticity, even at isoluminance, can act as effective tokens in sustaining the perception of motion. In fact, in one variant of their paradigm, chromati~ty and luminance motion cues were pitted against one another and the results suggest that chromaticity, rather than luminance, is the more salient token. Thus, the impairment of motion perception at isoluminante is not all-or-none, but varies considerably with the specific motion perception task and with variation of stimulus parameters. The same may well be true for other visual functions. Several features of experimental design arc important if one wishes to quantify the degree of loss at isoluminancc and compare it across visual functions and stimulus conditions. First, a metric is needed that allows such ~rnpa~~ns to be made. One appealing possibility is to use the individual detection threshold for each stimulus as the unit of comparison and to scale threshold contrasts on various discrimination (or identification) tasks by the corresponding detection threshold contrasts. Discrimination/ detection ratios (see Thomas, 1985) can then be compared for chromatic vs luminance-modulated stimuli, as well as across chromatic axes at isoluminancc and across visual functions. Di~~mination~det~tion ratios have been directly compared for l~nan~-modulated and isoluminant, chromatically-modulated targets in several recent studies. The visual functions studied include the contrast dependency and spatial selectivity of simultaneous masking (Switkes, Bradley & De Valois, 1988), orientation discrimination and spatial frequency discrimination (Webster, De Valois & Switkes, 1990), vernier acuity (Mulligan & Krauskopf, 1983; Fare11 & Krauskopf, 1988, 1989), and dir~tion-of-motion disc~mination th~sholds (Cavanagh & Anstis, 1991; Mullen & Boulton, 1989). In all of these studies, discrimination/detection ratios for chromatically modulated stimuli are reported to be remarkably similar to those for luminance-modulated stimuli, and never exceed them by more than a factor of about two. Thus, when discrimination thresholds are normalized to detection thresholds for the same stimuli, minimal losses

and DAMDA Y.TELLER are found at i~l~nan~. The question then arises as to whether, evaluated in this way, all of the apparent losses of sensitivity at isoluminance will be much reduced or nonexistent, or whether there may be some visual functions that, even evaluated in this way, are still poorly sustained at isoluminance. Have the capacities of chromatic channels been unjustly maligned, or do some deficiencies remain even when discrimination/detection ratios are used as the unit of comparison? A common di~~mination/ detection metric is useful in allowing such comparisons to be made. A second desirable feature of experimental design is the use of systematic variations of chromatic axis. To date, most studies of motion perception at isoltinance have been carried out with only one or a few chromatically distinct isoltinant stimuli. However, it seems likely that the choice of chromatic stimulus will influence the degree of loss found at isoltinance for at least some tasks. In particular, t&an stimu~isol~i~t stimuli designed for the study of signals i~tiat~ by the shortwavelength-sensitive (SWS) cones in isolationare known to be particularly deficient in supporting the perception of borders at isoluminance (Tansley 8~ Boynton, 1976). To the degree that mechanisms necessary for border perception are also required for certain other visual functions, one would expect relatively large losses along the tritan axis for such functions. Similarly, the intrusion of other visual mechanisms and/or higher levels of chomatic processing, as well as the presence of wavelen~h-de~ndent experimental and/or biological luminance artifacts (see below) can all influence the axes on which minima and maxima in discrimination/detection ratios are found. These potential asymmetries of chromatic vision at isolminance can be evaluated only if visual function is examined systematically as a function of the chromatic axis of stimulation. A third desirable feature is the joint study of two or more disc~mination tasks by highly similar techniques, so that the data can be compared across tasks in detail (e.g. Webster, De Valois & Switkes, 1988). If, for example, motion perception were found to be significantly impaired at isoluminance, it might be argued that the impairment is due to failure of the visual system to construct a representation of the spatial structure of the moving stimulus adequate for motion perception. The comparison of scaled threshold contrasts for motion

Motion at i!3olumlMnce

discrimination with those for form discrimination tasks, such as the discriinination of vertical from horizontal orientation, permits one to sort out this question and perhaps to link impairment of motion perception to visual mechanisms which specifically code for motion. Finally, particular attention must be paid to the analysis and minimization of luminance artifacts. Even if a particular visual function (such as motion) is apparently sustained at isohuninance, there are a number of potential artifacts that may limit the interpretation of the data. These potential artifacts include such factors as chromatic aberration, quantization error associated with the production of computer-generated isoluminant stimuli, temporal phase shifts (Lindsey, Pokomy & Smith, 1986; Swanson, Pokomy & Smith, 1988) or frequency-doubling nonlinearities (Lee, Martin & Valberg, 1989) within the pathways which classical isohuninant stimuli are designed to silence, the intrusion of rod-initiated signals, variation of the isoltinant point locally or with retinal eccentricity, and/or individual differences in any of the above factors. The presence of such artifacts allows the possibility that nominally isoluminant stimuli introduce salient spatiotemporal modulations in luminance, or in the channels that nominally code luminance information, somewhere in the visual pathways, and that these signals rather than signals in chromatic pathways sustain the observed performance at isoluminance. If so, one will overestimate the capabilities of the chromatic channels. The goal of the present research was to re-investigate the detection and discrimination of moving stimuli at isohuninance under the following constraints. First, we have used stimuli and experimental conditions designed to minimize potential experimental and biological artifacts. Second, we have employed luminance modulated stimuli as well as isoluminant chromatically modulated stimuli, and have varied systematically the chromatic axis of the chromatic stimuli. And third, we have used virtually identical moving stimuli to measure contrast thresholds for three different psychophysical tasks: grating detection, discrimination of vertical from horizontal gratings, and discrimination of the direction of motion of the gratings. These stimulus variations allow the use of discrimination/detection ratios as the unit of comparison, and quantitative comparison of performance between luminance- and chromati-

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cally-modulated stimuli, across chromatic axes and across visual tasks. Under the conditions tested, our results show highly task-specific losses of motion sensitivity at isoltinancc on all chromatic axes. METHODS Overview

Using forced-choice procedures, contrast thresholds for a series of three different tasks were measured in two observers using nearly identical moving patches of sinewave grating across task. The tasks included-(l) grating detection (D): the detection of the presence of moving gratings without regard for the direction of motion, (2) form discrimination (F): the discrimination of horizontally- from verticallyoriented moving gratings without regard for the direction of motion, and (3) motion discrimination (M): the discrimination of upward- from downward-moving gratings. Observers

One of the authors (DL) and a laboratory technician (CA) served as observers. The two observers had normal color vision as assessed by Nagel Anomaloscope and FM 100 Hue tests and normal acuity with corrective contact lenses. Both were highly practiced by the time final data were taken. Viewing was monocular with the right eye. Apparatus The principal components of the apparatus were a computerized color graphics system for stimulus generation and display, a Powell fiveelement achromatizing lens (observer DL) or 2 mm artificial pupil (observer CA), and a bite bar mounted on an xyz-positioner. Neutral density filters were used with the achromatizing lens, in order to equate retinal illuminance for the two observers. Motion discrimination thresholds at isoltinancc were measured with the apparatus in this configuration. For the determination of grating detection and form discrimination thresholds at isohnninance, as well as all determinations of threshold for luminance-modulated stimuli, a uniform field of white light produced by an auxiliary CRT display was combined optically by beamsplitter with the image produced by the primary color graphics display. The purpose of the auxiliary field was to reduce the effective contrast of the stimulus display, in order to avoid quantization

khL.WlN

I754

T. Lmn~

error while allowing accurate control of the very low contrasts required for these threshold measurements. Occasional control data taken without the auxiliary field gave similar results. The stimuli were generated at a 60 Hz (noninterlaced) frame rate by a color graphics system (Adage 3006) and displayed on a highresolution RGB monitor (Conrac 7235). The chromaticity coordinates of the red, green and blue phosphors of the CRT are, respectively (0.62, 0.35), (0.31, 0.59) and (0.15, 0.08). The color CRT was located 137cm from the observer. Retinal illuminances were approx. 125 td under all conditions. A comprehensive treatment of the image generation and display hardware as well as calibration procedures has been presented previously (Lindsey & Teller, 1989).

As depicted in Fig. lA, stimuli consisted of moving 1.3 c/deg, 3.75 Hz sinewave gratings. The grating patch subtended 2.3 deg on a side and contained 3 cycles of the grating. These spatial and temporal frequency values were chosen because they fall well within the spatiotemporal range over which sensitivity to isoMOVEMENT

and Davm~ Y. Tm

luminant chromatic modulation is high (de Lange, 1958; Noorlander t Koenderink, 1983; Mullen, 1985), and also well within the range over which direction of motion discrimination is possible at detection threshold for luminance modulated stimuli (Watson, Thompson, Murphy & Nachmias, 1980; Graham, 1989). In addition, the use of low spatial frequency sinusoids and small fields mitigates the influence of chromatic aberration and retinal inhomogencity. The gratings were horizontally oriented, except in the form discrimination task, in which both horizontal and vertical gratings were used. The remainder of the display was dark except for a small fixation square located 0.75 deg from the left-hand edge of the stimulus field. Eccentric placement of the gratings reduced the tendency to track the moving gratings (which would have rendered the temporal characteristics of the retinal image indeterminate) and the fixation target helped to maintain accommodation during a trial. Pilot studies showed that the 0.75 deg displacement of the stimulus away from fixation did not appreciably alter grating detection thresholds. The temporal envelope of contrast change during a trial is schematized in Fig. 1B. Contrast onset and offset were smoothed in time, following approximately a cumulative normal function. The maximum contrast for a given trial was held constant for 267 msec. This duration was sufficient to allow each cycle of the grating to be displaced spatially by 1 cycle within the exposure duration. The duration of halfmaximal or greater contrast was 830msec. Stimulus space: &~nition of axes

TIME

Fig. I. Spatial and temporal characte~stics of the stimuli. (A) Spatial characteristics: stimuti were 2.3 x 2.3 deg patches of moving, 1.3 c/deg, 3.75 Hz sinusnidal gratings, centered at 1.8 deg (in nasal retina) from fixation. For detection and motion discrimination, the gratings were horizontal and moved either upward or downward. For form discrimination, horizontal or vertical gratings moved either upward-downward or left-right. (B) Temporal waveform: the stimuli were ramped on and off with an approximately Gaussian waveform.

The gratings consisted of sinusoidal modulations in luminance, chromaticity, or both as specified for a stimulus space derived from the work of MacLeod and Boynton (1978) and Derrington, Krauskopf and Lennie (1984). The origin of our stimulus space was an 18 cd/mm2 white. For the motion discrimination task at isoluminance, the white had x,y-chromaticity coordinates of 0.31, 0.33 (the nominal white point of the monitor). For me~urements of threshold involving the auxiliary display, the chromaticity coordinates of the white point differed slightly from these values. All stimuli had a space-time average luminance and chromaticity equivalent to the white point. The stimulus space isaefined by three orthogonal axes through the white point: two chromatic axes which span a provisional isoluminant

Motion at isoltinaw

plane calculated from Judd’s revised l-‘,f~tion (Wysxecki & Stiles, 1982),and a third axis which represents variations in ltinance contrast of the white. One of the chromatic axes represents stimuli which produce complementary variations in LWS (long-wavelength-sensitive) and MWS (middle-wavelength-sensitive) cone excitation at constant luminance without concomitant changes in SWS (short-wavelengthsensitive) cones. The other chromatic axis represents all lights which are tritan metamers with the white point; at isol~nan~, modulations along this axis produce variations in SWS cone excitation only. We will call these two chromatic axes the R/G and tritan axes respectively. In this space, stimuli are characterized by three parameters: azimuth, elevation and amplitude of sinusoidal modulation, as shown in Fig. 2. No consistent convention for the choice of metrics on the three axes has yet evolved. We chose to scale the axes of our space following the procedure used by Derrington et al. (1984). Along the luminance axis, mutation is measured in units of Michelson contrast. Modulation along the two chromatic axes is de6ned in terms of the maximum available modulation of our CRT’s blue phosphor around the white point at isoluminance. In order to document the correctness of our chromatic axes, several observers were tested on two psychophysical calibration procedures which employed the method of adjustment. In LWINANCE 0 : azimuth

Fig, 2. The stimulus space, adopted from Derrington et al. (1984). The reddish end of the R/G axis is at Ode& and the yellow-greenish end of the t&an axis is at 9Odeg. Stimuli were modulated along the luminance axis, and at 10 azimuth8 within the isoluminant plane: 0 (r/g axis), 20, 40, 60, 80, 90 (Wan axis), 100, 120, 140, MOdcg. For each condition. contrast thresholds were measured for three tasks: detection, form discrimination (horizontal vs vertical), and direction of motion.

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both cases a 2.3 x 2.3 deg test field was used. To validate the tritan axis, the field was made bipartite by spatially juxtaposing two isohuninant fields: a rectangular white field and a field of variable chromaticity. Observers adjusted the chromaticity of the variable field to produce “melting” minimally distinct borders (Boynton, 1978). In a second psychophysical procedure, the white half-field was ignored and observers determined the chromaticity required for the perception of unique yellow, employing mixtures of the red and green phosphors. The results were consistent with predicted values to within experimental error. The determination of isoluminance was ap proached as follows. For each azimuth, “true” isoluminance for an individual observer can be defined as the stimulus elevation that yields that observer’s wont discrimination performance. In general, isohuninance defined in this way will deviate slightly from the provisional V, isohuninant plane, due to a combination of individual differences, calibration errors, and apparatus instability. On the other hand, since detection, motion, and form discrimination are all threshold tasks involving virtually identical stimuli (cf. Cavanagh, MacLeod 8c Anstis, 1987), variation across tasks in the elevation required for worst performance was jud8ed to be unlikely. Thus, a single determination of isoltiuance was made within each individual run, using the motion task. The elevation of worst performance for motion was used for the second threshold estimate (detection or form) made within the same experimental run (see below). Thus comparisons of performance across task were made with identical stimuli. Thresholds for grating detection and discrimination of direction of motion were obtained at the following azimuths: 0,20,40,60,80,90,100, 120, 140 and 16Odeg. Form discrimination thresholds were obtained for a subset of these azimuths. Thresholds for all three tasks were also measured for modulations along the luminance axis.

Each observer sat in a darkened room with his or her head restrained by a bite bar attached to an xyz-positioner. Each experimental session began with the alignment of the observer with either the achromatizing lens (DL) or the artificial pupil (CA). The observer then adapted in a darkened room for 5 min before any data were collected.

DEL-

1756

T. -

After the a~p~tion Period, an initial estimate of the individual observer’s elevation of isoitinance at the azimuth appropriate for the given session was made by the method of adjustment. Contrast was set to a value of 0.9 and viewing was continuous. The observer adjusted a knob controlling the elevation of the stimulus until the percept of motion was minimally distinct. The settings were not difficult to make, since at the appropriate elevation the grating slowed and often appeared stationary; at other times the grating would either fade to a uniform white field or appear to flicker and/or jump randomly in an upward or downward direction. The initial estimate of the elevation required for isoluminance was based on the average of 10 settings. During the remainder of the experimental session, thresholds were determined using the two-alternative forced-choice method of constant stimuli, with 20-40 trials per stimuhts. First, motion discrimination thresholds were determined for at least 10 elevations spanning the elevation determined by the method of adjustment in steps of 0.1 or 0.2 deg of elevation. The elevation of maximum threshold for motion .discrimination was usually sharply defined, with large changes in threshold occurring over variations of elevation of 1 deg or less. At the end of the session, the elevation of maximum motion discrimination threshold was determined by inspection of the psychometric functions, and the detection threshold was measured for stimuli modulated at that elevation. In a separate session, the elevation of maximum threshold for motion divination

and DAVIDAY. TIUER

LUMINANCE

was redetermined, followed by the rn~~~nt of threshold for form caption at that elevation. Motion, detection and form discrWnation thresholds were also determined for luminance modulations. Because of the need to collect motion discrimination data at several elevations, only one azimuth could be tested per session, and data collection took several weeks for each subject. Thus, the elevations of maximal motion infolds, across azimuths, are a between sessions variable, and are not optimal for further analysis of individual differences in isoltinant planes. This question is under further study. RESULTS

Psychometric functions for detection, form and motion thresholds for luminance modulation for observation DL are shown in Fig. 3. The functions are regular and, plotted on a log abscissa, do not vary systematically with task: there is little variation in either the slope or the estimated threshold for the functions. Psychometric functions for isoluminant stimuli at four selected azimuths for observer CA are shown in Fig. 4. The psychometric functions obtained for the detection and form discrimination tasks are regular and the thresholds are similar for these two tasks, although a small difference (less than 0.3 log units) was obtained for the tritan stimulus AZIMUTH 90

AZIMUTH 50 lWL

so 60

MODULATION

40

AtlMUTH

;$i

140

I

4pool

.Ol

.l

CONTRAST

Fig. 3. Psychometric functions for luminance-modulated stimuli. Observer DL. Percent correct is plotted as a function of log,, contrast. Triangles, circles and squares represent data obtained for detection, motion and form discrimination, respectively.

Fig. 4. Psychometric functions for isoluminant, chromatically-modulated stimuli for four azimuths. Observer CA. Detaztion and form discrimination thresholds remain similar, while motion discrimination thresholds are much higher. At azimuth = 140 deg. the motion threshold was not measurable.

Motion at isoltinana

(azimuth Wdeg) for this observer. However, for motion discrimination, thresholds at all azimuths depicted were clearly elevated above detection and form the corresponding thresholds. In fact, for the aximuth 140 condition, percent correct for motion discrimination was never signikantly above chance for this observer. The thresholds for detection, and form and motion emanations for all azimuths are shown for both observers in Fig. 5. In each graph, the symbols plotted for azimuths between 0 and 16Odeg represent the actual threshold contrasts obtained by experiment. The additions points were added by retleetion of each data point through the origin, in order to enhance the visual interpretation of the results. The open circles at aximutbs of 120 and 140 in observer CA’s graph plot the maximum (A)

TRITM 90

Oba: CA

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available contrast of our color monitor, since motion discrimination threshsolds were indeterminate at these azimuths. For both observers, the aximuths of minimum and maximum threshold contrast, while roughly 90deg apart, do not fall on the cardinal axes, but instead at azimuths of 40-60 deg and 140 deg respectively. Finally, M/D and F/D ratios as a function of aximuth, as well as those values obtained for l~nan~-rn~~~ gratings, are shown on a log,, axis in Fig. 6. As shown in the right-hand side of the panel for each observer, these ratios are at or near 1: 1 for luminance-modulated gratings. Similarly, in the case of form dkcrimination at i~l~~~, F/D is always reasonably close to 1: 1 and is always less than 2: 1, regardless of the aximuth tested. However, in the case of motion dkimination, M/D for isoltinant gratings is always greater than 3 : 1 and varies systematically with azimuth. (Al Oba: CA

0 N/G

180

1 .

Chromatic

deteotim

0

(B)

TRlTAN 80

Ohs: DL

Obs: DL

0 RIG

180

270

0 form

Fig. 5. Thmhokb

for detection, and form and motion ~~~ p4ottedin the ~S~M~t piane for both obwrvers. The radial coordinate is contrast. With the enarptkm of CA’s results for tritan stimuli, dctoctioa and form diecrimination thresholds are approximately equal. Motion thresholds am much luger. The largest contrast thresholdx for motion discrimination arc found at about 14Odcg~for this aximuth, the motioa/~on ratio is approx. 16:l for DL and i&err&ate for CA.

0

80

180

AzwJlll Fig. 6. Log M/D (squares) and F/D (circks) for aU chromatic axa (l&k)8nd for l~~rn~~~ stimuli (tight) for both observws.

DELWXN T. hnxw

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For observer DL, the variation in M/D with azimuth parallels the variation in motion discrimination alone with azimuth, whiie for observer CA these two functions differ somewhat more. Although DL’s M/1) ratios are consistently smaller than CA’s, the pattern of variation with azimuth is similar for both observers, with the minimum and maximum values of M/D occurring, respectively, at azimuths of 20-60 and 120-140 deg. DISCUSSION

and DAVILM Y. TILLER

ratios of 3 : 1 or 4: 1, consistently less than those found at 120 and 140 deg of azimuth. Their use of relatively large stimulus fields (8 x 8 deg) may also contribute to the quantitative di&ences between the two studies. The two most novel aspects of our results, then, are the finding of large motion/detection ratios at isohuninance, and the pattern of those ratios with azimuth variations; i.e. the minimum and maximum thresholds for motion discrimination do not coincide even approximately with the r/g and tritan axes.

Dkxrimination /detection ratios

Orientation of the motion discrimination ellipse

Many of the results of the present study are consistent with earlier reports in the vision literature. Our results for form perception (V vs II) are consistent with prior studies of orientation difference thresholds. For example, for l~~~rn~~at~ stimuli, Thomas and Gille (1979) found o~en~~on/det~tion ratios of approx. 1: 1 for orientation differences at or above 20-30 deg. Webster et al. (1990) found orientation/detection ratios at or near 1: 1 for 90deg orientation differences, for both ltinante- and i~l~Mnt chro~ti~~y-rn~u~t~ stimuli. In the case of motion, discrimination/ detection ratios between 1: 1 and 2 : 1 have been reported for luminance-modulated stimuli of low spatial and high temporal frequencies (e.g. Watson et al., 1980). Thus the literature, in conjunction with our results, implies that the impairment in motion discrimination at isohnninance cannot be attributed to any general insensitivity of the visual system to motion, nor to any gross failure to reconstruct a representation of the spatial structure of moving stimuli, nor to any generalized failure to process isoluminant stimuli. One apparent inconsistency between our results and those of others is that for chromatically modulated stimuli, Cavanagh and Anstis ( 1991) have reported maximal motion~de~on ratios slightly greater than 2: 1 for red/green gratings and near 1: 1 for blue/yellow gratings (see also Mullen & Boulton, 1989).In our study, motion/detection ratios are always larger and sometimes much larger than those reported by Cavanagh and Anstis. However, the inconsistencies between the two studies are smaller than they initially appear. In our color space, Cavanagh and Anstis’s red/green and blue/ yellow stimuli would be represented at azimuths of 0 and roughly 45 deg respectively. In our study, these azimuths yielded motion~det~on

The choice of the R/G and tritan axes as the cardinal chromatic axes of a color space (MacLeod & Boynton, 1978) has a variety of empirical justifications. The semi-axes of discrimination ellipses often coincide fairly closely with the R/G and tritan axes (e.g. Noorlander & Koenderink, 1983) and adaptational in%ences are minimal between these axes (Krauskopf, Williams & Healey, 1982). Moreover, the azimuths of optimal response of most lgn cells conform closely to these axes (Derrington et al., 1984). Thus, it seems likely that these axes represent a good description of chromatic coding at early post-receptoral levels of visual processing. In this context, the finding of maximum motion ratios and motion/detection thresholds in the vicinity of 14Odeg azimuth requires explanation. This result was unexpected and emphasizes the importance of testing visual function at isoluminance along a number of different azimuths. Although the issue cannot be resolved at present, there are two general classes of possible reasons for this finding. The first possibility is that the shift of chromatic axes is real, in the sense that it reflects the motion-discrimination limitations of a pair of recoded chromatic channels, having spectral properties which differ from those embodied in the R/G and tritan axes, It is interesting to note that the azimuth of minimal thresholds (40-60 deg) corresponds at least roughly with the “yellow/blue” axis proposed by Hurvich and Jameson (1957). Krauskopf, Williams, Mandler and Brown (1986) have recently reasserted the need for recoding of the axes of color space to form higher-level chromatic channels. The second possibility is that some or all of the motion discrimination thresholds we have measured are controlled by one or a ~mbination of experimental and/or biological

Motion

at isolumi~~~

artifacts which produce an unwanted luminance @ml from stimuli intended to be isoltinant. One such biological artifact would be a m&ibution from rods. Gur stimuli, centered at 2 deg nasal, fall outside the human rod-free xone of the fovea (see Curcio, Sloan, Kalina & Hendrickson, 1990, for recent estimates of this zone). This interpretation is supported by the facts that the mean retinal illuminance of our stimuli is at least a factor of 10 below that classically required for rod saturation (Aguilar & Stiles, 1954), and the directions of maximum and minimum motion/discrimination contrast ratio correspond at least roughly to those predicted by rod intrusion (D’Zmura 8c Lennie, 1986). However, we also note that the rod density at 2 deg nasal is quite low and may not be sufficient to support the analysis of motion, so this possibility cannot be endorsed without further evidence and theoretical analysis. With regard to other potential artifacts, we have ruled out a sole contribution by chromatic aberration by using a carefully-aligned achromatizing lens or artificial pupil, and testing with low spatial frequency sinusoidal gratings. We have also analyzed for the possible effects of calibration and quantixation error on motion discrimination and find that artifactuallyproduced luminance contrast, while present, is generally about a factor of 2.0 below that required for motion discrimination. Thus, such errors cannot solely account for the variation in motion discrimination with azimuth at isoluminance. We have also analyzed for the possible contribution to motion discrimination by an eccentricity-dependent gradient in the optical density of macular pigment across the stimulus field. Although we did not analyze for the magnitude of such a contribution, we did solve for the azimuths at which such effects should be minimal. Unlike our motion discrimination data, the resulting axis lies close to the R/G cardinal axis of our color space, thus ruling out the possibility that macular pigment variations alone can account for our results. We have also considered the possibility that one or more of the following potential biological artifacts might account for our results: temporal phase differences (Lindsey et al., 1986; Swanson et al., 1988) and frequency-doubling nonlinearities (Lee et al., 1989)within or among mechanisms which form the luminance channel, and local and/or eccentricity-dependent variations in the relative radiances of the three CRT primaries required to achieve isoluminance. It is

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unlikely that any of these factors can solely account for our results, since each hypothetical factor predicts that the maximum (or indeterminate) motion discrimination threshold will fall on the tritan axis. In sum, the reasons for the unexpected finding of maximum motion thresholds and motion/ detection ratios in the vicinity of 140deg aximuth have not yet been determined and are under further investigation. We here emphasize the important logical point that the possibility of artifact only enhances the likelihood that a major loss of motion perception occurs at isohuninance. If the motion thresholds we measured are determined by experimental and/or biological artifacts, one must posit that the chromatic channels are even less sensitive to motion than would be indicated by the present data. Losses specific to motion discrimination Finally, we return to the original question. As reviewed in the introduction and above, when discrimination/detection ratios are used as the metric of comparison, isohuninant stimuli apparently hold their own with respect to luminance-modulated stimuli in supporting a variety of psychophysical tasks, including masking, orientation discrimination, vernier acuity, and gross form discrimination. Why then are deficits found for discrimination of the direction of motion at isohuninance? In the psychophysical literature, discrimination/detection ratios at or near 1: 1 are usually taken to imply detection of the two to-bediscriminated stimuli by two different, independent, labeled channels or mechanisms (Thomas, 1985). In the case of our motion discrimination task, these would be detectors tuned to the detection of upward vs downward motion; and in the case of form discrimination, to vertical vs horizontal orientation. For hnninancemodulated stimuli, the finding of 1: 1 discrimination/detection ratios for motion in opposite directions (e.g. Watson et al., 1980) and for appropriately large variations of orientation (Thomas & Gille, 1979) and spatial frequency (Watson 8t Robson, 1981) provide major bases of support for models positing multiple, spatial frequency-, orientation- and motion-tuned channels (Graham, 1989). In the case of isohuninant chromatic modulation, we have found that form/detection ratios, which in our paradigm require discrimination of orientation, remain near 1: 1, while

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DELWINT. Lerrwey and DA~XDA Y. m

mo~on~det~tion ratios are always 3 : 1 or more. These results, in conjunction with the previous Aguilar, M. & Stiles, W. S. (1954). Saturation of the rod literature on visual function at isoluminance, mechanism of the retina at high levels of stimulation suggest that the channels which detect isoOptic0 Acta, I, 59. luminant stimuli, while spatial frequency and Boynton, R. M. (1978). Ten years of reseat& with the minimally distinct border. In Armington, J. C.. Krauskopf, J. orientation tuned, are not tuned or not labeled & Wootea, B. R. @Is.). Visual psychophysics andphysifor the direction of motion. Additional contrast, dogy (pp. 193-207). New York Academic Press. above that required for detection, is necessary Cavana8h, P. & Anstis, S. (1991).The contribution of color to encode and extract direction-of-motion to motion in normal and color-deficient observers. V&on information from isoltinant stimuli. For iso*sear&, in pmss. luminant stimuli, motion di~nation may Cavana& P., Tyler, C. W. & Favreau, 0. E. (1984). Perceived velocity of moving chromatic gratings. Journal not be directly mediated by those m~h~sms of tb Optical Society of America A, I, 893-899. which are responsible for detection and gross Cavana& P., MacLeod, D. I. A. & Anstis, S. M. (1987). form discrimination. Equihmtinance: Spatial and temporal factors and the Theories of local motion processing require contribution of blue sensitive cones, Journal of the Optical Society of America A, 4, 1428-1438. the existence of processing units which extract Cdo, C. A., Sloan, K. R., Ralina, R. E. & Hendrickson, the spatial derivative of location with respect to A. E. (1990) Human photoreceptor topography. Joumul time (see the June, 1985 issue of Journal of the of CompnratioeNeurology, in press. Optical Society of America for a comprehensive Derrington, A. M., Rrauskopf, J. & Len&, P. (1984).Chrotreatment of this issue). Within the context of matic mechamsms in lateral geniculate nucleus of macaque. &urn& of Physiology, London, 357,241-265. such theories, factors which reduce the accuracy of encoding either spatial or temporal infor- DZmura, M. t Len&e, P. (1986). Shared pathways for rod and cone vision. Vision Research, 26, 1273-1280. mation would be expected to degrade the Fare& B. & Rrauskopf, J. (1988). Intluence of chromatic processing of motion i~o~tion. Troscianko content on vernier ~01~. ~~st~at~ Ophthal(1987) has argued that the fundamental deficit mology and VisuaiScience (Suppl.), 29, 371. at isoluminance is the loss of precise spatial Famll, B. & Krauskopf, J. (1989).Comparison of stereo and vernier offset thresholds for stimuli modulated chromatilocalization, and that losses of motion discally and in luminance. Ineestigative Ophthalmologyad crimination occur as a consequence of such VW Scietwe (Suppl.), * 129. spatial uncertainty. It is not clear how the Gorea, A. & Papathomas, T. V. (1989). Motion processing magnitude of the losses of spatial information by chromatic and achromatic visual pathways. Journal of the Optical Society of America A, 6, 590602. documented to date can account for the large losses of motion discrimination seen in the Graham, N. (1989). V&t& pattern analyzers. New York: Gxford university Press. present study. Hurvich, L. M. B Jameson, D. (1957).An opponent-process In conclusion, under the present con~tions, theory of color vision. Ps~holog~~ Reuiews, 64, we find large, highly task-specific losses in 384404. motion discrimination at isohnninance. The Krauskopf, J., Williams, D. R. & Hreley, D. W. (1982). Cardmat directions in color space. Vision Research, 2.2 possibility that the residual capacity for motion 1123-1131. discrimination is mediated by artifactual in- Rrauskopf, J., Williams, D. R., Mandler, M. 8. & Brown, trusion of the luminance channel cannot be A. M. (1986). Higher order color mechanisms. Vision ruled out; but, if luminance artifacts do provide Reseurch, 26, 23-32. the cue for residual motion discrimination, then de Lange, H. (1958). Research into the dynamic nature of the human fovea-cortex systems with intermittent and motion discrimination could only be even worse modulated light. II. Phase shift in brightness and delay if complete isolation of chromatic channels in color perception. Jotmal of the Optical Society of could be achieved. In any case, the finding America, 48, 784-789. of fo~~det~tion ratios of about 1:l and Lee, B. B., Martin, P. R. & Valberg. A. (1989). Nonlinear s~tion of M- and L-cone inputs to phasic retinal motion~det~on ratios much larger than 1: 1 ganglion cells of the macaque. Joumal of Neuroscience,9, suggests the existence of chromatic channels 1433-1442. that are labeled for orientation but not labeled Lindsey, D. T. &Teller, D. Y. (1989).Influence of variations for the direction of motion. in edge blur on minimally distinct border jud8ement.s: A Acknowledgements-This research was supported by NIH grant EY 07078 to DL. We thank Corinne Mar for writing the software that controlled and analyzeddata collection for our experiments and Cynthia Ames for her role as an observer.

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