VLrlon

Rcs. Vol.

3U. NO. 3, pp. 439-448,

aMz-6989/90

I99U

Printedin Great Britain.All rights reserved

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Copyright Q 1990 Pergamon Press pk

METACONTRAST AND

MASKING BETWEEN CYCLOPEAN LUMINANCE STIMULI

ROBERTPATTIX.WN’ and ROBERTFox* Department of Psychology, Montana State University, Bozeman, MT 59717 and 2Department of psychology, Vanderbilt University, Nashville, TN 37240, U.S.A. (Received

19 June 1985; in revisedform 25 July 1989)

study investigated the functional equivalence between cyclopean (global stereoscopic) and luminance (local stereoscopic) stimuli. To do so, a metacontrast masking paradigm was employed to determine the level of perceptual interaction between the two stimulus domains. Four target and mask combinations were used: cyclopean target-cyclopean mask, luminance target-luminance mask, cyclopean target-luminance mask, and luminance target*yclopean mask. Substantial intradomain masking occurred, with the form of masking being similar for both domains. Moreover, signScant interdomain masking also occurred, in qua1 measure for the cyclopean and luminance stimuli, although the magnitude of masking was one-half that of intradomain masking. These results imply that there is a functional quivaknce at some stage of the visual system between the mechanisms representing cyclopean and luminance stimuli. Abr%ract-This

Cyclopean perception element stereograms

Global stereopsis

Local stereopsis

INT’RODUCTION

Of the several contributions to an understanding of binocular vision promoted by the development of the random element stereogram (Julesz, 1960, 1971), perhaps the most signif% cant is the discovery of hypercyclopean perception. That term refers to the replication in the stereoscopic domain, by means of a random element stereogram, perceptual phenomena from the physical or luminance domain (Tyler, 1983; Julesz & Schumer, 1981). In this line of inquiry the emphasis is not so much on the process that gives rise to global stereoscopic forms but, rather, on the subsequent spatial interaction of those forms. Do they mimic, in the hypercyclopean domain, the actions of their physical counterparts? A positive answer has been obtained for a variety of phenomena including geometric visual illusions (e.g. Hochberg, 1963; Julesx, 1971; Patterson 8c Fox, 1983), classical figural aftereffects (e.g. Walker 8c Kruger, 1972), metacontrast masking (Lehmkuhle & Fox, 1980)and the spatial modulation transfer function (Tyler, 1974; Schumer % Ganz, 1979). In all of these studies, the anticipated perceptual effects have been found and follow relationships functionally quivalent to those produced by physical stimuli. Physical is used here to denote stimuli

perceptual equivalence

Random

that can be specified at the level of the receptor in terms of gradients of luminance. Global stereoscopic stimuli, on the other hand, cannot be so specified because they arise in the central visual system only after the information in the random element matrices has been merged and analyzed. Because of their rather diverse origins, the functional similarity between global stereoscopic forms and physical ones is of particular interest. Emphasis must be placed, however, on the functional character of the similarity because hypercyclopean phenomena differ from their luminance domain counterparts quantitatively along a number of dimensions, a notable one being scale. An excellent sample is provided by the stereoscopic analogue of the contrast sensitivity function (Tyler, 1974) which specifies the relationship between the spatial frequency of a sinusoidally modulated global. stereoscopic surface and the minimum disparity required to resolve it. The stereoscopic function has the same shape as the contrast sensitivity function with both low and high frequency declines in sensitivity, defined in terms of minimum disparity. But overall sensitivity is reduced by a factor of IO-the stereoscopic cut off frequency is on the order of 3 c/deg as compared to the luminance-contrast cut off frequency of 3Oc/deg.

439

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ROBERTPATTERSON and ROBERTFox

The functional equivalence between phenomena from the hypercyclopean and luminance domains naturally inspires questions about the degree to which both are products of similar mechanisms. One way to explore this issue is to investigate the extent to which interdomain interactions can occur. To that end we used metacontrast masking to examine interactions between the cyclopean and luminance domains. Masking was selected because it yields large, yet controllable, changes in target detectability and many relevant variables have been identified (see Breitmeyer, 1984, for a review). Moreover, it is known that masking among cyclopean stimuli (intradomain masking) is quite similar in character to masking among stimuli from the luminance domain (Lehmkuhle & Fox, 1980). Our experiments were designed to address the question of whether masking could occur between cyclopean and luminance stimuli, i.e. a cyclopean mask vs a luminance target and vice versa. A positive answer to the question of interdomain interaction is suggested by Tyler (1975) who found that a luminance domain adaptation stimulus could induce a tilt aftereffect in a cyclopean test stimulus. At the neurophysiological level, several investigations of disparity sensitive cortical neurons in alert behaving monkeys by Poggio and coworkers (Poggio, Motter, Squatrito & Trottter, 1985; Poggio & Poggio, 1984) have found patterns of responses that could provide a neural substrate for interdomain interaction. Two classes of cells were observed, one of which (complex cells) responded equally well to luminance contours with retinal disparity and to contours formed from random element stereograms. The other class (simple cells), however, responded only to disparate luminance contours. If it is assumed that the magnitude of masking is a function of the total number of neurons activated, then this differential responsiveness suggests that while interdomain masking would occur it would be of lesser magnitude than intradomain masking. This is because in intradomain masking the same populations of cells would be activated whereas in interdomain masking partially nonoverlapping populations would be stimulated. One additional consideration in establishing the optimal conditions for interdomain masking is the relative depth position of the mask and target stimuli. Maximal interaction occurs only when both occupy the same depth position (Lehmkuhle & Fox, 1980). To meet this require-

ment, stimuli from the luminance domain must be placed in stereoscopic depth. From that perspective, this study can be thought of as an investigation of the interaction between local and global stereopsis. That emphasis, however, is only incidental to the more fundamental objective of this study, which is to learn more about the mechanisms underlying hypercyclopean perception. GENERAL

DESIGN

AND METHOD

To measure the equivaknce of cyclopean and luminance stimuli, four target and mask combinations were employed in four separate masking experiments; cyclopean target and cyclopean mask, luminance target and luminance mask (these two cases will be referred to as intradomain masking), luminance target and cyclopean mask, and cyclopean target and luminance mask (these two cases will be referred to as interdomain masking). The methods used for generating these stimuli are described later. The indicator task used for measuring masking was four-alternative (left, right, up or down) forced-choice recognition of gap position of a target configured as a Landolt C. The mask was an annulus that surrounded the C. In all experiments, recognition performance of three observers under a no-mask baseline condition was compared to performance obtained when target and mask were presented together at various stimulus onset asynchronies (SOAs). Preceding formal data collection, two methodological requirements had to be satisfied. First, before masking could be extended across stimulus domains, it had to be established within each domain. Second, the parameters chosen for intradomain masking had to be compatible with interdomain masking. For instance, the luminance level of the luminance forms could not impair the visibility of the random elements comprising the cyclopean forms. To determine a compatible set of stimulus parameters, several preliminary experiments varied stimulus size, luminance and SOA. The values obtained in these experiments were used in the formal experiments. In the next section, the apparatus, stimuli, observers and general procedures common to all experiments are described. Apparatus The main apparatus consisted of two systems, the cyclopean (dynamic random element)

Mctacontrast

stereogram generation system and the luminance stereogram generation system. These systems are described below. The random element stereogram system has been described previously (Shetty, Brodersen & Fox, 1979; Lehmkuhle & Fox, 1980; Fox & Patterson, 1981). It consists of three main components: the display, the stereogram generator and the optical programmer. The display was a large lenticular projection screen (1.42 x 1.75 m) upon which matrices of ted and green dots were projected via a projection color television receiver (Advent, Model ImA). Stereoscopic viewing was achieved by the anaglyph method, in which appropriately matched red (Wratten No. 29) and green (Wratten No. 58) filters were placed before the eyes of the observers. The stereogram generator was a hard-wired electronic unit that performed several functions: (1) operating in the raster-scan mode, it generated random matrices each composed of over 5000 red or green dots that were dynamically replaced at the field rate (60 Hz) of the video receiver; the replacement produced apparent motion of the dots which permitted the cyclopcan forms to be presented briefly without introducing monocular cues (this motion did not impair the visibility of the forms); (2) it specified the X-Y coordinates of the cyclopean form that was displayed; (3) it produced the retinal disparity between subsets of dots by introducing a slight delay in the output of one of the electron guns; (4) it generated random elements, without disparity, that precisely filled the gap produced by the delay; (5) it controlled the duration of exposure of the cyclopean form (in multiples of 16.7 msec). The optical programmer was a black and white video camera, modified to operate as an image digitizer. Two-dimensional, high-contrast forms scanned by the programmer provided the code for the generation of stereoscopic forms of the same configuration. This was achieved by synchronizing the scan rate of the camera with that of the receiver, and using the digitized signal from the camera to specify the X-Y coordinates where disparity was inserted. With respect to the production of stereograms in the luminance domain, a system composed of two pairs of yoked 35 mm projectors was constructed. Each projector was securely mounted on a positioning carriage that permitted the projector to be positioned precisely in the X-, Y-, and Z-axes. As with the viewing of the

masking

441

random element system, stereoscopic viewing of the luminance forms was achieved by the anaglyph method: one projector of each pair had a red filter in front of its lens and the other a green filter, which matched the filters worn by the observers. To vary retinal disparity, the angle of rotation between each pair of projectors (indicated by dial indicators) was manipulated by a Vernier drive. To manipulate exposure duration, an electronic shutter in front of each projector lens was opened for durations controlled by an electronic timer. The images cast by the projectors were projected onto the lenticular screen used by the projection color television receiver. This made it possible to combine optically the cyclopean and luminance forms. The background elements, which defined a flat plane of random dots at zero disparity, remained on all times. Now consider the operation of both systems. The luminance stereogram system was located above and behind the observer’s station, which was situated 347 cm from the display screen. In front of the station the video projector was located. When both classes of form were exposed, they were perceived clearly in the visual space between observer and display screen. Stimuli The dimensions of the target and mask were the same for both stimulus domains. The outer diameter of the target was 2.6 deg. with a gap of 1.2 deg. The inner diameter of the annular mask was 4.0 deg, and its outer diameter was 7.4 deg. When the luminance stimuli were projected onto the background elements of the screen, they yielded a luminance of 12.2cd/m’ (combined right- and left-half images). The luminance of the random elements comprising the cyclopean stimuli was 10.8 cd/m’. The difference between the luminance stimuli and the random elements comprising the cyclopean stimuli was 0.06 log units; a preliminary experiment revealed that this small differena was necessary in order for both classes of form to be seen clearly when presented simultaneously. The red and green filters worn by the observers attenuated luminance by 0.6 and 0.9 log units, respectively. Both target and mask were presented at a crossed disparity of 0.99 deg. which corresponds to a half-image separation of 6 cm. The forms appeared at half the distance to the screen in accordance with the depth interval predicted by the geometry of stereopsis. This disparity value, which was used in all

ROBERTPATTEIWN and ROBERTFox

442

experiments, falls within the fusional range given the size of the stimuli employed.

Three persons, JK, LTA and BF (age range: 23-28 yr), all possessing normal or corrected-tonormal acuity, good binocular vision and considerable experience in perceiving cyclopean and luminance stereoscopic forms, served as paid observers in one or more experiments. The observers were unaware of the hypotheses under consideration. General procedure

The paradigm used for measuring masking has been employed previously (e.g. Eriksen & Lappin, 1964, Eriksen, Becker, & Hoffman, 1970). It involved establishing a baseline for each observer individually so that performance was set at 80-90% correct recognition when the target was presented alone. This was accomplished by manipulating the exposure duration of the target so that performance remained at that level during several preliminary sessions in which the observer practiced the recognition task without the presence of the mask. When asymptotic performance was achieved, the exposure duration of the target was fixed at the most recent value. All observers attained asymptotic performance prior to formal data collection. After baseline performance stabilized, the mask was introduced at each of six SOA values relative to the onset of the target: - 100, -50, 0, +50, + 150 and +250 msec. The exposure duration of the mask was equal to that of the target, which was within the range of 133-167 msec for the cyclopean forms and within the range of 80-120msec for the luminance forms. The order of presentation of the six SOA conditions (100 trials collected under each) was randomly determined for each observer. INTRALlOhlAIN MASlUNG

1

Before masking between cyclopean and luminance forms could be investigated, we had to establish that substantial masking could occur within each stimulus domain, using the parameters of the stimuli based on the results of the preliminary experiments. In the present experiment, masking between a cyclopean target and mask was investigated. This inquiry was

MASK;

CYCLOPEAN

TARGET

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Observers

Experiment

CYCLOPEAN JK-. LTA- 0

BF- A

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I

,

1

330 f

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100

200

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CE6CElVED SOL

Fig. 1. Visual masking between cyclopean target and mask. Percent corr~ recognition and equivalent d’ scores are shown for the no-mask baseline (control) condition, where the target was presented alone, and the six perceived WA conditions, where the target and mask were troth presented at various onset asynchronies. Positive values on the abscissa indicate the target was presented before the mask (backward masking), and negative values indicate the target was presented after the mask (forward masking).

facilitated by information gleaned from a similar study by Lehmkuhle and Fox (1980). Results. Figure 1 shows percent correct recognition and the corresponding d’ values ubtained under the no-mask (control) and the six SOA conditions for each of the three observers. Inspection of Fig. 1 shows that for all observers the mask produced significant decrements in recognition performance relative to the no-mask condition, under both forward and backward directions. The maximum decrement in d’ amounted to a factor of about 3 or 4 for all observers. For JK and LTA, the maximum decrement occurred at SOA = 0, while for BF it occurred at SOA = -50. The d’ scores were analyzed individually for each observer by a one-way analysis of variance (ANOVA) for randomized block designs. This analysis revcaM significant effects of SOA on recognition perfurmance for all observers: for JK, F(6,24) = 29.6, P ~0.001; for LTA, F(6,24)= 16.0, P ~0.001; and for BF, F(6,24) = 44.8, P < 0.001. These results are similar to those found by Lehmkuhk and Fox (1980). In that study, the

maximum decrement in performance occurred at SOA = 0, with symmetric in&eases in performance (i.e. decreases in masking) in both forward and backward directions when SOA was increased. This pattern of masking is consistent with what has been called Type A masking (Breitmeyer, 1984). The purpose of this experiment was to ensure that masking between a luminance target and mask could occur under the luminance level specified by the results of the preliminary experiments. Results. Figure 2 shows percent correct recognition and the corresponding d’ values obtained under the no-mask (control) and the six SOA conditions for each of the three observers. Inspection of Fig. 2 shows that for all observers the mask produced significant decrements in recognition performance relative to the no-mask condition, in both forward and backward directions. The maximum decrement in d’ amounted to a factor of about 3 for all observers. For JK and LTA, the maximum decrement occurred at SOA = 0, while for BF it occurred at SOA = + 50. The d’ scores were analyzed individually for each observer by a one-way ANOVA for randomized block designs. This analysis revealed significant effects of SOA on recognition performance of all observers: for JK F&24) = 32.5, P < 0.001; for LTA, F(6,24) = 14.7, P < 0.001; and for BF, F(6,24) = 12.4, P < 0.001. These results are consistent with those from previous studies investigating visual masking in which forced-choice recognition served as the dependent measure. Those studies, as did this one, yielded Type A masking. The results of Experiments 1 and 2 show that the parameters of the stimuli derived from the results of the preliminary experiments produce considerable masking within each stimulus domain, cyclopean and luminance. Further, the temporal pattern of masking is similar for the two stimulus classes. MASKING

Experiment 3 Now that the parameters of the stimuli for the cyclopean and luminance domains have been determined, the next step was to investigate masking across domains. This was, of course, the principle purpose of this study. To do so,

MlWS II -0 LTA-0

3.

DF -A

l A 3.1

Experiment 2

INTERDOMAIN

LUMIWAME MASK;LUWJAIICE TARGET

0

-I

d’

1. I

ti I

= 1 S.E.

300 CERCEIVLD

SOL

Fig. 2. Visual masking between luminance target and mask. Percent conrct recognition and equivalent d’ scores are shown for the no-mask baseline (control) condition, where the target was preacatcd alone, and the six perceived SOA conditions, where the target and mask were both prcsented at various onset asynchronies. Positive values on the abscissa indicate the target was presented before the mask (backward masking), and negative values indicate the target was presented after the mask (forward masking).

however, required special consideration of the timing parameters of the two stimulus domains, because cyclopean forms are perceived less rapidly than luminance forms (e.g. Julesz, 1971; Tyler, 1983). This means that the same physical time of onset will produce a difference in perceived onset. To eliminate this difference, estimates of perceived onset simultaneity for cyclopean and luminance forms were obtained in this experiment. These estimates were used to convert physical SOA to perceived SOA in Experiments 4 and 5, the interdomain masking experiments (in Experiments 1 and 2, perceived SOA should have been equivalent to physical SOA). The methods employed in this experiment were different from those used in the masking experiments. These methods are described below. Stimuli. One cyclopean and one luminance target were presented side by side (edge to edge lateral separation was 4.0 deg) at a crossed disparity of 0.99 deg. Observers. Obwvers JK and BF served. In addition, one other person, DM, who possessed

444

ROBERTPATTERSON and ROBERTFox

corrected to normal acuity and good binocular vision, served as paid observer. DM had considerable experience in perceiving cyclopean and luminance stimuli, but was naive with respect to the hypothesis under test. Procedure. By varying the time of onset of the stimuli, estimates of perceived onset simultaneity could be obtained. This was done by using the method of constant stimuli, in which the physical onset of the luminance form was varied in accordance with seven different SOA values relative to the onset of the cyclopean form. The SOA values were 0, + 30, + 60, + 90, + 120 and + 150 msec. The observer fixated the center of the display screen before he or she initiated each trial. On each trial, the two forms were presented together for an exposure duration of 300 msec and the observer judged whether the perceived onset of the luminance form was early or late relative to that of the cyclopean form. The order of presentation of the seven SOA conditions was randomly determined for each observer. Results. The data from each observer were scored as the proportion of trials that the onset of the luminance form was judged “late” relative to the onset of the cyclopean form for each of the seven SOA values. The threshold at 50% (point of subjective equality) as determined by probit analysis served as the measure of perceived onset simultaneity for each observer. By this measure, the time delay between cyclopean and luminance forms necessary for simultaneity was determined. Figure 3 shows the data for all observers. For JK, the 50% threshold for the luminance form was 52.9 msec, for BF it was 73.9 msec and for DM it was 75.2 msec. Probit analysis revealed that these values were significantly different from physical simultaneity (i.e. SOA = 0 msec) at the 0.01 level of statistical significance. Subsidiary experiments were performed to investigate whether the luminance difference between the cyclopean and luminance forms, light onset, or an auditory cue associated with the operation of the shutters may have influenced the judgments of simultaneity. The results showed that they did not. The values of 52.9, 73.9 and 75.2 represent the amount of time in msec that the cyclopean form would have to precede the luminance form for the two to appear simultaneous in onset. These values are consistent with a study by Staller, Lappin and Fox (19&O),which measured the reaction time required to classify groups

CYCLOPEAN

ST.; LUMINANCE

VAR.

subj*ctr

x 5 I

A0 -

H L

**O- / . 0

It. .pi: I

.

,

.

,

300090120lw

SOA (msec)

Fig. 3. Determination of perceived onset simultaneity of cyclopean and luminance stimuli by the method of constant stimuli. The luminance stimulw was presented at seven different SOA valuea relative to the onset of the cyclopean stimulus. The threshold at 50%. as determined by probit analysis, served as the measure of perceived simultaneity for each observer.

of cyclopean and luminance forms, and found that the class&ation of cyclopean forms required about 6Omsec longer duration than the classification of luminance forms. Experiment 4 Now that the perceived onset of the cyclopean and luminance forms could be equati by the estimates of simultaneity derived from Experiment 3, masking across stimulus domains was investigated. This involved combining and using interchangeably as target and mask the two classes of form in two masking experiments. The first of these experiments, Experiment 4, investigated interdomain masking by employing a luminance target and a cyclopean mask. To equate perceived onset of the two classes of form, the mask was presented earlier by an amount equal to each individual’s simultaneity threshold, determined in Experiment 3, under each SOA condition. For LTA, for whom no estimate was available, the mask was presented 67 msec earlier, which is the average of the thresholds of the other observers. Stimulus onset asynchrony values that have been equated for perceived onset of the cyclopean and luminance forms will be referred to as perceived SOA. Remh. Figure 4 shows percent correct recognition and the corresponding d’ values obtained under the no-mask (control) and the six

Metacontrast masking perceived SOA conditions for each observer. Inspection of Fig. 4 shows thatfor all observers the mask produced significant decrements in recognition performance relative to the no-mask condition, in both forward and backward directions. The maximum decrement in d’ amounted to a factor of about 1.5 for all observers. For LTA and BF, the maximum decrement occurred at perceived SOA = 0, while for JK no clearly defined maximum was obtained. The d’ scores were analyzed individually for each observer by a one-way ANOVA for randomized block designs. This analysis revealed significant effects of perceived SOA on recognition performance for JK and BF: for JK, F(6,24) = 6.5, P < 0.001; for BF, F(6,24) = 2.5, P = 0.05. For LTA, the effect of perceived SOA on recognition performance only approached significance F(6,24) = 1.7, P > 0.05. A subsidiary experiment was performed to investigate whether the difference in luminance between the cyclopean and luminance forms may have influenced masking. The results showed that it did not. Experiment 5 The purpose of this experiment was to investigate masking between a cyclopean target and a luminance mask. In so doing, special consideration was required because of the large luminance annulus used as the mask. It seemed possible that the brief presentation of the annulus might reduce the visibility of the elements comprising the cyclopean target due to the presence of veiling glare. That possibility did not arise previously because the luminance mask-cyclopean target pair had not been employed. To check on that possibility, recognition performance had to be measured in the presence of the annulus under conditions that did not produce masking. This was accomplished by separating in depth the positions of the target and mask, i.e. presenting the annulus at a disparity value of 0, because it is known that masking is absent whenever target and mask are separated by that amount (Lehmkuhle & Fox, 1980). In terms of the perceived depth interval, the separation in depth was on the order of 170cm. The results of this experiment were used for correcting the data obtained in the main experiment. As in Experiment 4, the simultaneity thresholds of the observers were used for equating perceived onset of the two classes of form.

445 CYCLOPEAN MAslc;LUMIWANCE TARGET

SIllI IIt -.

3.0

Cd&

-100

0

100

PERCEIVED

SD1

200

300

Fig. 4.

Visual masking between luminance target and cyclopam mask. Percent correct recognition and equivalent d’ scores are shown for the no-mask baseline (control) condition. where the target was presented alone, and the six perceived SOA conditions (derived from Experiment 3). where the target and mask ware both presented at various onset asynchronies. Positive values on the abscissa indicate the target was presented before the mask (backward masking), and negative values indicate the target was presented after the mask (forward masking).

Results. With respect to the control experiment on glare, Fig. 5 shows percent correct recognition and the corresponding d’ values obtained under the no-mask (control) and the six perceived SOA conditions for each observer. Inspection of Fig. 5 shows that for LTA and BF the annulus produced significant reductions in recognition performance relative to the no mask condition only in the forward direction. The maximum decrement in d’ amounted to a factor of about 1.7 for both observers. For both, the maximum decrement occurred at perceived SOA = - 100. The d’ scores were analyzed individually for each observer by a one-way ANOVA for randomized block designs. The analysis revealed no significant effect of the annulus for JK, while significant effects were present for LTA and BF (P < 0.001). These effects, however, were due entirely to interference in the forward direction (as revealed by a Newman-Keuls test for a posteriori comparisons). The absence of a significant decrement in performance in the backward direction

ROBERT PATTERSON and ROBERT Fox

446

demonstrates that masking did not occur when the positions of the target and annulus were separated in depth. These results indicate that the interference between target and annulus in the forward direction arose from sources other than metacontrast masking. The data from the experiment on glare were used for adjusting the data of the main experiment. This involved revising upwards the d’ scores for forward masking only by an amount (in d’ units) equal to the interference effect for each observer, an approach that assumes that the no-masking and the masking effects are additive. With respect to the main experiment, Fig. 6 shows the percent correct recognition and the corresponding d’ values obtained under the no-mask (control) and the six perceived SOA conditions for each observer. Inspection of Fig. 6 shows that for all observers the mask produced significant decrements in recognition performance relative to the no-mask condition

in both forward and backward directions. The maximum decrement in d’ ranged from a factor LUMJNANCE

MASK; CYCLOPEAN

TARGET

3.0

2.a

1’

1.a

I _+--

C.rn,,.l

-100

3

0

100

?LRCLlvcD

SOA

1. 6.E.

300

Fig. 5. Glare or non-specific interference effects between cyclopean target and luminance mask (the mask was presented at disparity ~0). Percent correct recugnition and equivaknt d’ scores are shown for the no-mask baseline (control) condition, where the target was presented alone, and the six perceived SGA conditions (derived from Fixperiment 3), where the target and mask were both presented at various onset asynchronies. Positive v&es on the abscissa indicatz the target was presented before the mask (brckwud masking), and negative values indicate the target was presented after the mask (forward masking).

LUMINANCE

MASK; CYCLOPEAN

TARGET

Srbj*ctr II LTA-0

3,

8 2.

d’

1.

I= 1

-100

0 ?ERCLlVED

100

S.E.

200

300

JO1

Fig. 6. Visual masking between cyclopean target and luminance mask (corrected for glare or non-specific interference). Percent correct ruxqnition and equivalent d’ scores are shown for the no-mask &line (cotztrol) condition, where the target was presented alone, and the six penrived SGA conditions (derived from Experiment 3). where the target and mask were both presented at various onset asynchronies. Positive values on the abscissa indicate thetaqetwaspreaentedbefonthemask@ackwardmasking), and negative v&us indicate the target was presMed after the mask (forward masking).

of about 1.6 to a factor of about 2.6 across observers. For JK and BF, the maximum decrement occurred at perceived SOA = 0 (though not well defined), while for LTA it occurred at perceived SOA = - 50 msec. The d’ scores were analyzed individually for each observer by a one-way ANOVA fq randomized block designs. The analysis revealed significant effects of peroeived SOA on recognition performance for all observers: for JK, F(6,24) = 10.0, P < 0.001; for LTA, F(6,24) = 13.7, P 0.05, nor was the difference between the two interdomain experiments, t( 14) = 2.0, P > 0.05. But the greater decrement in performance produced by intradomain masking relative to interdomain masking was significant, t(14) = 6.5, P < 0.001. DIXUS!SION The results of this study can be summarized in the following way. First, int~domain masking produces a greater impairment in performance than does interdomain masking. Second, interdomain masking is equivalent for the two classes of form, cyclopean and luminance. Considering first the difference in masking ma~itude between the interdomain and intradomain conditions, the reduction of masking for the interdomain case fits well with the differential responsiveness of cortical neurons to disparate luminance contours and cyclopean contours as described in the “Introduction”. Recall that stereoscopic stimuli defined by luminance differences excite both simple and complex cells responsive to retinal disparity. Cyclopean stimuli, however, excite only complex cells. This means that in intradomain

ROBERTPA~RSON and ROBERTFox

448

masking both mask and target stimuli will engage identical cell populations. But, in interdomain masking, there will be an incomplete overlap between the cells excited by the mask and target stimuli. A cyclopean mask will not influence that portion of a luminance target that is represented by simple cortical cells. Similarly, a luminance mask will influence a cyclopean target only to the extent to which a portion of the mask stimulates complex cells. Although interdomain masking is less than intradomain masking, it is significant that the magnitude of masking between the interdomain conditions is the same. This implies that there is a functional equivalence at some stage of the visual system between the mechanisms representing cyclopean stimuli and those that represent stimuli defined by luminance differences.

tion & Psychophysics, 8, 245-250.

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Patterson & Fox (1983). Depth separation and the Ponzo illusion. Perception & Psychophysics, 34. 25-28. Poggio, G. F., Motter, B. C., Squatrito, S. & Trotter, Y. (1985). Responses of neurons in visual cortex (VI and V2) of the alert macaque to dynamic random-dot stereograms. Vision Research 25, 397-406. Poggio, G. F. & Poggio, T. (1984). The analysis of stereop sis. Annual Review of Neuroscience 7. 379. Schumer. R. & Ganz, L. (1979). Independent stereoscopic channels for different extents of spatial pooling. Vision Research,

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Breitmeyer, B. G. (1984). Visual masking: An integrative approach. New York: Oxford University Press. Eriksen, C. W., Becker, B. A. & Hoffman, J. E. (1970). Safari to masking land: A hunt for the elusive U. Percep-

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