INHIBITORY INTERACTION BETWEEN VISUAL PATHWAYS TUNED TO DIFFERENT ORIENTATIONS JAMESP. THOMASand KEIKO K. SHIMAMURA Department of Psychology, University of California. Los Angeles, CA 90024. U.S.A. (Received 14 October

1974; in rruiwd

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16 February 1975)

Abstract-When subjects judge the orientation of a single line presented at threshold luminance, their performance conforms to predictions derived from psychophysical estimates of receptive field properties. When subjects detect stimuli which simultaneously activate channels tuned to two different orientations, performance falls below that expected from receptive field properties. The discrepancy is attributed to inhibition between channels tuned to different orientations. Such inhibition appears lo be strongest when the orientations differ by 15-Z”.

The visual pathways which mediate detection of single lines are selectively sensitive with respect to orientation. Part of the evidence for this conclusion comes from masking studies. The threshold for a single line is elevated by a masking stimulus consisting of a grating (Sekuler, 1965) or a single line (Parlee, 1%9). However, the magnitude of the masking effect is reduced if the test and masking stimuli differ in orientation. Other evidence comes from studies of subthreshold summation. Kulikowski, Abadi and King-Smith (1973) showed that the sensitivity of a line-detecting mechanism to a superimposed grating fell sharply as the orientation of the grating deviated from that of the line detector. Finally, as will be shown in this paper, subjects accurately perceive the orientation of threshold lines which indicates that the pathways which respond to threshold lines also provide information about orientation. This paper concerns the mechanisms which produce such selectivity or tuning with respect to orientation. One probably source of tuning is the shape of the receptive fields of the pathways which mediate the detection of lines. If the receptive field is elongated in one direction. the pathway will be most sensitive to lines which parallel the long axis of the field. The simple cortical cell illustrates this mechanism (Hubel and Wiesel. 1962). However, a number of investigators have suggested that this selectivity is augmented by inhibitory interaction between channels tuned to different orientations. Most of the evidence for this suggestion is psychophysical (Andrews 1965; Blakemore, Carpenter and Georgeson, 1970; Atkinson, 1972). However. Blakemore and Tobin (1972) have provided physiological evidence for such inhibition, and Benevento. Creutzfeldt and Kuhn (1972) have argued on physiological grounds that interchannel inhibition is the major source of orientation tuning. This paper provides additional psychophysical evidence for the occurrence of inhibition between pathways tuned to different orientations. The orientation tuning attributable to receptive field properties is derived from published data on the properties of linedetecting mechanisms. It is shown that this derived I373

tuning adequately accounts for perceptual performance in a situation which minimizes the effects of interchannel inhibition, but fails to account for performance in a situation which maximizes the effects of such inhibition. The conceptual schema which underlies the analysis is illustrated in Fig. 1. The long, upward-pointing arrows represent parallel visual pathways which serve the same part of the visual field. Each is driven by a receptive field of the type shown. The only difference between the receptive field of one channel and that of another is in the orientation of the long axis of the field. This orientation varies systematically from channel to channel. The centers of the fields coincide although, for clarity, the fields are laterally displaced in the figure. Because of the shape of its receptive field, each channel is selectively sensitive with respect to orientation, responding best to lines which parallel the long axis of the field. The question addressed in this study is whether this selectivity is augmented by inhibitory interactions between channels tuned to different orientations. These possible interactions are represented in the figure by the horizontal arrows. The outputs of the channels, whether Response

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or not modified by interchannel interactions. are presumed to provide all of the information used by the subject m detecting, line stimuli and in judging the orientations of the hnes. Exactly how the subject uses the information from the different channels depends upon the stimuli used and the type of judgment made by the subject. This task~ont~ngent processing is represented in the diagram by the box labeiled Decision process. The essential feature of this processing is that it is task contingent. It is not presumed that the subject has any conscious awareness of this processing. lmpiemen~tion of the analysis requires quantitative data about the receptive fields of the line-detecting channels. The effects of inducing lines. bars, edges. and gratings of various sizes and positions upon the visibility of a test line have been measured by several investigators (Fiorentini and Mazzantini. 1966; Fiorentini and Zoli. 1966, 1967; Thomas. Rourke and Wilder, 1968; Ku~ikowsk~ and King-Smith. 1973). When appropriately analyzed, such data yield a sensitivity map of the receptive field of the mechanism which mediates detection of lines: i.e. the sensitivity of the mechanism to each point in the visual field as a function of the distance of the point from the major axis or centerline of the receptive field of the m~hanism. Figure 2 presents two such sensitivity functions. The solid line is the sensitivity function derived by Kulikowski and King-Smith (1973) from the changes produced in the visibility of a test line when the line was superimposed on subthreshold gratings of various spatial frequencies and phases. The function shown here is the mean of the two functions which they present for different subjects. They obtained a virtually identical function from the effects upon the visibility of the test line of two subthreshold lines placed at different distances from the test line. Kulikowski and King-Smith state that the mechanism which is described mediates detection of lines up to 6’ wide. The open circles indicate sensitivities derived from the data of Thomas, Rourke and Wilder (1968).

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They measured the changes produced in the \tsibiht! of a test line. ?’ wide. when it \vas flanked h> mduiing fields of various widths. The luminance of the inducing fields was always half that of the test line. (The sensitivity function presented by Thomas cr ~1. does not have as many points as are given here: the additional points were obtained by drawing smooth curves through the raw data before deriving the sensitivities.) The cross-hatching will be discussed later. The sensitivity functions from these hvo different studies agree reasonably well. although the Kuhkowski and King-Smith function clearly has a deeper inhibitory trough. The Kulikowski and King-Smith function is used in the analyses which follow because it provides the more rigorous test for interchannel inhibition. The sensitivity functions in Fig. 2 provide onlv a onedimensional description of the receptive field..i.e. how sensitivity varies along a direction perpendicular to the major axis of the field. For the purposes of this study. it is also necessary to know how sensitivity varies along the direction parallel to the major axis. Kulikowski et al. (1973) estimate that, along the major axis, sensitivity is greater than half the maximum value for a distance of about 100’. For the purposes of our analysis, we assume that sensitivity along the major axis and along any line parallei to the major axis is uniform for a distance of at least SO’, the length of our stimuli. This assumption not only simplifies the calculations, but also provides the most rigorous test of interchannel inhibition. Using the above assumption and the Kulikowski and King-Smith sensitivity function shown in Fig. 2, the sensitivity of each channel to any stimu’tus can be computed by procedures described elsewhere (Thomas, 1970; Kulikowski and King-Smith, 1973). The question is whether these sensitivities, which reflect only receptive field characteristics, are consistent with the subject’s actual performance, or whether interchannel inhibitjon must also be invoked in order to amount for performance. GENERAL

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METHODS

AND PROCEDURE

The test stimuli were superimposed on a IOOO-tdbackground field which was IO” wide by 15” high. At the center of the background were four tiny black marks, located at equal intervals on the circumference of an imaginary circle, 2” dia. The test stimuli were presented one at a time within this area, each so placed that its center coincided with the center of the imaginary circle. The subject fixated the center of the area. The background was illuminated continuously. The stimuli were presented in Maxwellian view by a computer~ontrolied apparatus which has been previously described (Thomas and Shimamura, 1974). Three subjects were used. LN was an undergraduate student, well trained as an observer, but unaware of the empirical and theoretical background of the experiments. KS and JT are the authors. All three are myopic. LH and KS wore cornea1 contact lenses. A corrective spectacle lens was placed at the artificial pupil for JT. Because of the length of time required by the full study, only KS participated in all experiments. Each session was divided into a series of trials, i set long, which were separated by 2-see intervals. A tone sounded during each trial period. There were 480 or 560 trials in a session. with a rest every 100 trials. Detection performance was measured by a signal detection procedure.

inhibitory interaction between visual pathways A test stimulus was presented on approximately half the trials. the rest were blanks. The subject’s task was simply to discriminate between the two types of trials. He rated each trial on a 6-point scale according to his estimate of the probability that a test stimulus had been presented. Rating-ROC curves were constructed from the data and the arca beneath each curve used as the measure of detection performance (Green and Swets, 1966). This measure varies between 05 (chance) and I.0 (perfect) and is comparable to the proportion correct in a two-alternative forcedchoice task. During each daily session, each stimulus was presented at one luminance on 40 signal trials. These signal trials were randomly intermixed with noise trials and signal trials on which other stimuli were presented. Thus the subject did not know which of the possible stimuli. if any. would appear on a given trial. A different procedure was followed for one set of data in Experiment II: the session was divided into blocks of 100 trials (50 signal, 50 noise); a single stimulus, which was shown to the subject before the first trial was used throughout each block. Detection threshold was defined as the luminance yielding an area under the ROC of @8. This luminance was estimated for each stimulus by varying the luminance of the stimulus from one daily session to another in steps of Qi Iog units so as to continuously bracket the @8 level. Data were gathered during six daily sessions and linear regression was used to estimate the threshold luminance and its standard error (Thomas and Kerr, 1971). The standard error seldom exceeded @03 log units. EXPERIMENT

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In this experiment, subjects identified the orientation of a just-detectable line. This task minimixes the effects of interchanncl inhibition because activity is restricted to those channels tuned to a singIe orientation. Thus, regardless of whether or not there is inhibition between channels tuned to ditkrent orientations, performance on this task should depend largely on receptive field characteristics. The stimulus was a bar of light, 5’ wide by SO’ iong. Detection thresholds were obtained for the stimulus when it was presented at each of eight orientations: vertical and rotated from vertical 18, 36, 54, 72 and 90” counterclockwise and 18 and 36” clockwise. Two sets of thresholds were obtained for each of two subjects. one set before and one after the orientation judgments described below. The two sets were simiIar. except for a slight general decrease from the first to the second, and were averaged for presentation in Fig. 3. As would be expected, thresholds are higher for oblique orientations than for vertical or horizontal. There is also a trend which is confirmed by thresholds taken in Experiment II. for horizontal thresholds to be I.2

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Fig. 3. Detection threshold for a single line as a function of its orientation. CircIes: subject KS. Squares: subject LH. Open symbols: clockwise deviation. Closed symbols: counterclockwise deviation.

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higher than vertical ones, This trend is probably an instrumental artifact: because the test channel has a narrow. vertically oriented filament, depth of field is greater for vertical contours than for horizontal contours. The broken curve in Fig. 3 is used in the theoretical analysis of the results of Experiment II. For the orientation judgments. the stimulus was presented in six orientations: vertical and rotated 18. 36, 54. 72 and 90; from vertical. Two luminanccs were used for each orien~tion. They were @I log units apart and bracketed the detection threshold for that orientation. Orientation and luminance were varied randomly from trial to trial, On each trial, the subject indicated in which of the six possible orientations the stimulus had been presented. That is. he rated each stimulus on a &point scale. giving the lowest rating when he believed the stimulus was horizontal. the highest rating when he thought the stimulus was vertical. and interm~~te ratings for ~rres~nding intermediate positions. He gave a response on each trial, even if he felt he could only guess. Each daily session contained 40 trials at each luminance. at each orientation (total = 480). There were six daily sessions. Figure 4 shows distributions obtained by tallying the orientation judgments over all 6 days. Each graph shows the distribution of responses on those trials on which the stimulus was presented at one particular orien~tio~ shown in the corner of the graph. At each orientation. responses were tallied separately for the two luminances and are shown by different curves. The response distributions are unimodal and exhibit sharp. narrow peaks which systematically change with the orientation of the stimulus. With one exception, the higher of the two luminances at each orientation yields the more sharply peaked distribution. However, the effect of the @I log unit luminance difference is small. Clearly, the subjects perceive the differences in orientation with relatively high accuracy. The channels which respond to the just-detectable bar must carry information about orientation. To obtain a quantitative evaluation of the subject’s performance. a measure was computed which is analogous to the measure used in assessing detection performaw. This measure. which will be called a discrimination area, evaluates the extent to which the subject discriminates between two stimuli in the Sense that he gives different responses to them. The measure is computed by treating those trials on which one stimulus is presented as noise trials; treating those trials on which the second stimulus is presented as signal trials; and constructing a rating-ROC from the orien~tion judgments, which are treated as ratings on a 6-point scale. The area under the ROC is the discrimination area. Its value varies from 0.5. when the subject shows no discrimination and gives the same distribution of responses to both stimuli.‘to 1.0, when the subject shows perfect discrimination by giving non-overlapping distributions of responses to the two stimuli. Discrimi~tion areas were computed for each possible pair of orientations and for both luminance levels. Areas were computed for each daily session and then averaged over days. The modal standard error of these averages was 002 and 9096 of the standard errors were 004 or less. Figure 5 shows discrimination area as a function of the difference in orientation between the two stimuli for which the area is computed. The values for a difference of 18 were obtained by averaging over the five possible pairs of orientations differing by 18”; those for 36’ by averaging over the four possible pairs, etc. Values for the two luminance levels were also averaged together. The mean difference between the discrimination areas for the two luminance levels was 0.033. The broken curves are theoretical predictions which will be discussed below. The major interest in Fig. 5 is in the comparison of the data points with the theoretical curves. However. two points should be noted before that comparison is taken

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As indicated earlier. the present task minimizes the effects of interchannel inhibition. Thus. performance should be determined by receptive field characteristics. The purpose of the analysis is to validate the description of the receptive field presented earlier h!, showing that the description is consistent with the performance obtained on the task. On each presentation of the stimulus at a given orientation. j. there is a response or level of activity in each of the tuned channels. This response will be taken as normally distributed over trials with unit variance and mean = lj.Si,, where I, is the effective intensity; of the stimulus when presented at orientatton j. and S,i, is the sensitivity of channel I; to the stimulus when the latter is presented at orientation j. Reliable discrimination among the orientations depends upon differences among the distributions of responses. i.e. upon the variables Ij and Sj,. In addition, the subject’s performance depends upon how he makes use of these differences, i.e. upon the decision process. These three determinants of performance will now be considered.

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Fig. 4. Distributions of orientation judgments. Each graph shows the distribution of judgments obtained when stimulus was presented at the orientation indicated. A Lower luminance. o Higher luminance. up. First, although the stimulus was presented at luminances which afford only imperfect detection performance (detection areas clustered about 08), the subjects approached perfect performance in discriminating the larger differences in orientation. The second point concerns the homogeneity of the data averaged for each difference in orientation. Andrews (1965) suggested that channels tuned to vertical and horizontal orientations have narrower sensitivity bands than those tuned to oblique orientations. Were such the case, the discrimination areas for a particular difference in orientation, e.g. 18”. should

SIt This function was derived from the Kulikowski and King-Smith sensitivity function presented earlier. The stimulus was projected upon the receptive field with the major axis of the stimulus coincident with that of the receptive field and with the axis of the stimulus rotated various amounts from the axis of the field. At each orientation, sensitivity to the stimulus was calculated by integrating the sensitivity of the receptive field over the stimulated region. The normalized results are shown in Fig. 6 and represent the sensitivity of the channel driven by the receptive field to the stimulus as a-function of the orientation of the latter. Of course, the function represents sensitivity as determined by receptive field properties only and does not reflect the effects of any interchannel inhibition, As discussed above, the data do not indicate that the width of the tuning function varies systematically with orientation. Therefore, the function shown in Fig. 6 is taken as applying to all channels. The only difference between the sensitivity functions for different channels is in the orientation to which the channel is most sensitive. 10

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Ij This is a measure of signal strength which is some unspecified function of luminance. Its unit is the standard deviation of the response distributions. The value of 11 corresponding to the luminances used in the orientation judgments was estimated from the detection data according to the following model. It was assumed that there was a channel tuned to each of the eight orientations used in the detection task. The subject monitors all eight channels and responds positively if the activity in one or more channels exceeds a critical value. For the rating task which was used, the subject establishes five critical values. He gives the highest rating if the highest critical value is exceeded in one or more channels; if no channel exceeds that value, but one or more channels exceed the next highest critical value, he gives the next highest rating; and so on. On blank trials, lj is taken as zero. Thus, on blank trials the distribution of responses in each channel has a mean of zero and a standard deviation of 1. On signal trials, the distribution of responses in each channel has, as indicated above, a mean IpIk and a standard deviation of 1. In the terms of signal detection theory, IjSj, is the value of d’ for the particular channel and orientation. Given the foregoing, the value of Ij which yields an area of Q8 under the detection ROC for each orientation is 2.135. This value was used for each orientation because the two luminances for each orientation yielded detection areas bracketing 08. Decision

process

The possible strategies are too numerous to examine individually. Rather. a range of possible performance levels was estimated by applying an optimal decision process and a much less efficient process. Both strategies assume that there is a channel which is optimally tuned to each of the six orientations used for the judgment task. The optimal performance values were calculated on the premise that. on each trial, the subject chooses from only two alternative orientations. Thus, these values represent an upper limit which can be approached. but not achieved. since the subject was actually confronted with a choice among six alternatives. According to this stra-

I?77

between visual pathways

tegy. the subjedt monitors the two channels tuned to the possible orientations, subtracts the response to the stimulus of one channel from the response of the other channel, and assigns his rating on the basis of this difference. This difference is normally distributed over trials with sigma = L 2 and mean = I,(S,, S12) on trials on which the stimulus is presented at orientation 1 and 12(S2, - Szt) on trials on which the stimulus is presented at orientation 2. The values of I and S are determined as indicated above and the rating-ROC is constructed by plotting the integral of one distribution against the integral of the other. The resulting areas. or performance levels, are shown as the upper broken curve in Fig. 5. According to the less efficient strategy, the subject monitors all of the six channels which are optimally tuned to the six possible orientations. On each trial. the subject determines which channel is most active and gives the judgment which corresponds to that channel. The distribution of judgments expected for each orientation of the stimulus was determined by calculating for each channel the probability that the response in that channel would be greater than the response of any other channel. Expected performance levels were then calculated from the expected distributions of judgments in the same way that observed levels were calculated from the observed distributions. The expected performance values are shown by the lower broken curve in Fig. 5. Because of the approximation procedures used in the calculations for this strategy, performance is somewhat underestimated for the largest differences in orientation. For the smaller differences in orientation. the observed values lie between the two curves, i.e. within the expected range. For the two largest differences, the observed values straddle the upper limit. At first glance, it might appear that in the latter case subjects were attaining impossible levels of performance. However, in this region of large differences in orientation and high performance, the consequences of various strategies converge and some nonoptimal strategies yield results which are experimentally indistinguishable from the obtained results. Thus. the data can be said to lie within the expected range. In summary, orientation judgments were gathered in a situation which minimizes the effects of interchannel inhibition. The Kulikowski and King-Smith sensitivity function was used to estimate the accuracy which could be expected simply on the basis of receptive field properties. The data fall within the range of estimates so generated. This agreement is taken as supporting the soundness of the analytical model and the Kulikowski and King-Smith sensitivity function. EXPERIMENT

II

This experiment examines a task which maximizes the effects of interchannel inhibition. The stimuli are shown in Fig. 7. Except for the single vertical and horizontal bars, each stimulus has two differently oriented components. Thus. each simultaneously activates channels tuned to two different orientations, providing an opportunity for the effects of interchannel inhibition to be observed. The subject’s task was to detect the stimuli when presented one at a time. A detection task was used rather than identification or discrimination of orientation because the latter might involve decision processes. such as subtracting one

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JAMES P.THOMS

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&+=+Fig. 7. Stimuli used in Experiment II. The non-vertical components are rotated 5, 15. 25. 45, 65 and 90” from vertical. channel’s output from another, which would mimic the effects of interchannel inhibition. The components of the stimuli were 5’ wide by 50’ long. The luminance of’ each stimulus was uniform, i.e. there was no increase in luminance where the two components overlapped. The orientations of the non-vertical components were 5, 15. 25, 45, 65 and 90” from vertical. Figure 8 shows log detection sensitivity for each of the stimuli. where sensitivity is the reciprocal of the detection threshold. Data were gathered for both subjects by the fully randomized procedure: the stimulus presented on each signal trial was randomly selected from the pool of eight and the subject did not know from one trial to the next which stimulus. if any, would be presented. In addition, data was gathered for one subject by the blocked procedure: only one test stimulus, previously identified to the subject, was used in each block of 100 intermixed signal and noise trials. As might be expected, the subject who was tested under both conditions performed somewhat better in the blocked case, when she knew which stimulus to look for. However, the difference is small and, in log coordinates, her two curves have essentially the same shape. As was noted in connection with Experiment I, sensitivity is higher to the single vertical line than to the single horizontal line, a difference which may be instrumental in origin. For the theoretical analysis. each set of data was normalized with respect to sensitivity to the single vertical bar. Figure 9 presents the normalized sensitivities in linear coordinates. Sensitivity to the single horizontal stimulus is omitted.

KEIKO

K.

SHIMAMCUA

As was done in the analysis of Experlmenr 1. ;: range of expected performances is defined b! examming the least efficient decision strategy. on the one hand. and the optimal strateg. on the other hand. In the least efficient strateg. the subject monitors only the channel which is tuned to the vertical component of the stimulus. The activity in this channel is taken as normally distributed with standard devlation = c and mean = LJ,,, where L, is the proportional to the luminance of stimulus 111and S,, is the sensitivity of vertically tuned channel (i, = 1) IO stimulus ITI.The proportional relationship bemeen L,

and the luminance of the stimulus implies that the channel is linear to this point. Actually. this assumption is not new. having been used in the derivation of the sensitivity functions for the receptive fields (Thomas, 1970; Kulikowski and King-Smith, 1977). The ability of the subject to detect the stimulus. i.e. to distinguish between noise trials and trials on which stimulus m is presented. depends upon the separation of the distributions produced in the channel on the two types of trials. This separation depends upon the ratio (LJm,Jia which is taken as constant at the detection threshold. The value of S,,. is computed as indicated above. a is taken as constant. and the relative threshold luminance is determined for each stimulus. The detection sensitivities so obtained are shown by the lower solid line in Fig. 9. The most efficient strategy is developed for the case, represented by one set of data, in which only one test stimulus is used at a time. According to this strategy, the subject monitors the channels tuned to the orientations of both components of the stimulus and responds on the basis of the sum of the outputs. In this case, the overlap of the noise and signal distributions, and thus detection performance, is determined by the ratio L,G?W + CGl,) a, ‘2 where L, S,,,,, and a have the same values as in the first strategy; Smt is the sensitivity of the channel 1 0

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THEORETICAL. ANALYSIS For the purposes of this analysis, it is assumed that

there is a channel which is optimally tuned to each of the orientations of the individual components of the stimuli. The analysis consists of two steps. First, the sensitivity of each channel to each of the test stimuli is calculated by projecting the stimulus on the receptive field of the channel and integrating sensitivity over the stimulated am. Second, the responses of the different channels are combined according to different decision rules in order to derive the relative visibilities of the stimuli. If there is no inhibition between channels, the derived visibilities should agree with the observed visibilities. However, if there is inhibition, it will reduce the outputs of the channels and observed visibility will be lower than that calculated from receptive field properties alone.

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Inhibitory interaction between visual pathways

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tuned to the orientation, k, of the second component of the stimulus; and bk is a weighting factor which reflects the relative sensitivity of the visual system to dinerent orientations and which varies as the reciprocal of the broken line in Fig. 3. Noise in the two channels is taken as uncorrelated. When the stimulus is the single vertical bar, this strategy reduces to the strategy described above (the least eflicient one) and performance is determined by the ratio given in that connection. In both cases, it is assumed that the relevant ratio is equal to the same constant at the detection threshold. The relative detection sensitivities which follow from this strategy are shown by the upper solid line in Fig. 9. For most of the stimuli. the observed detection sensitivities fall within the range defined by the two theoretical lines. In the case of the stimuli with components separated by 15 and 25”, however, performance falls below the prediction of even the least efIicient strategy. For these stimuli, observed performance is worse than that expected on the basis of receptive field properties, which is precisely the expected result if the outputs of the channels are reduced by interchannel inhibition.

DISCUSSION

Detectability was measured for stimuli designed to activate simultaneously channels tuned to two different orientations. In most cases, the detectabilities were consistent with the independently established characteristics of the receptive fields of mechanisms which detect single lines. When the orientations of the components of the stimuli differed by 15 and 25”, however, detection performance was less than that expected on the basis of receptive field characteristics. It is proposed that this shortfall in performance occurs because the outputs of the individual channels are reduced below the expected level by inhibitory interaction between the channels. However. alternative explanations must be examined and rejected before this conclusion can be accepted. The alternatives can be grouped according to whether they deal with the decision processes by which the outputs of the various channels are combined or with the sensi-

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tity fun&ons used to calculate the outputs of the individual channels. Could the outputs of the channels have been combined in an even less efficient manner than that used in the analysis? For the purposes of the analysis, the data were normalized with respect to the single vertical bar. In this context, an inefficient procedure is one which maximizes sensitivity to the single, vertical stimulus and/or minimizes sensitivity to the other stimuli. The strategy analysed does both of these things. Sensitivity to the vertical stimulus is maximized not only because the channel monitored is the one best tuned to that stimulus. but also because the amount of noise processed is reduced by virtue of the fact that only a single channel is monitored. On the other hand, the detectabilities of the other stimuli are minimized because information from channels tuned to their non-vertical components is completely ignored. In theory, the ratio between the sensitivities to the vertical and nonvertical stimulus components would be greater if the single channel monitored were tuned to an orientation counterclockwise from vertical. However. since none of the stimuli contained a component which was rotated counterclockwise from vertical, such a monitoring procedure is not a realistic possibility. Thus, the processing rule examined (monitoring only the vertically tuned channel) is the least efficient realistic procedure. Can the discrepancies between observed and predicted sensitivities be attributed to errors in the sensitivity function adopted to describe the receptive field? To assess this possibility, the data in Fig. 9 were averaged and analyzed in reverse in or&r to determine the sensitivity function which would predict those data in the absence of interchannel inhibition. The result is represented by the cross-hatched band in Fig. 2. The upper boundary was obtained using the least efficient decision processing and the lower boundary using the most efficient processing. The deviation between the independently established sensitivity curves and the function derived from the present data on the presumption of no inhibition is too great to attribute to error of measurement or differences in procedure. Another possibility is astigmatism. If the optics of the apparatus or of the subjects’ eye were such as to blur vertical contours more than non-vertical components, then the effective sensitivity function of the channel tuned to the vertical component would be altered in the direction of the cross-hatched function in Fig. 2. However, as has been noted, the bias of the apparatus is in the opposite direction. Of the two subjects involved in Fig. 9. one wore contact lenses which provided a total correction for astigmatism. The other used a spectacle lens which did not correct for his slight astigmatism (@50D), but that astigmatism favors vertical components. Finally. the detection sensitivities to single lines in Fig. 8 show both subjects to be more sensitive to vertical than to horizontal lines. Thus, astigmatism is not the explanation. Finally. it must be noted that the Kulikowski and King-Smith sensitivity function yielded accurate prediction of performance when applied to the data of Experiment I, data gathered in a situation which minimized the possibility of interchannel inhibition.

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JAMS

P. THOMAS and KEIKO li. SHIMAMLKA

In summary. when the selective sensitivity of the visual system to differences in orientation was examined in a situation which minimized possible inhibition between channels. the performance observed was consistent with the selectivity expected from receptive field properties alone. When detection performance was examined in a situation which facilitated inhibition between channels, performance was lower than predicted. as would be the case if the outputs of the individual channels were reduced by inhibitory interaction between the channels. Various alternatives were examined and rejected. Thus. the most reasonable conclusion is that the reduced visibility results from inhibition between channels tuned to different orientation. The deviations between predicted and observed values in Fig. 9 occur when the difference in orientation of the two components of the stimulus is 15 or 25’, which suggests that the greatest inhibition occurs between channels which are tuned to orientations which differ by 15-25”. This conclusion is in substantial agreement with that of Blakemore et al. (1970) who found inhibition greatest between channels which differed by 15”. The fact that the data for other stimuli fall within the predicted zone does not, of course, mean that no inhibition is occurring between the channels involved. Rather, inhibition may occur but be too small to be revealed by the present procedures. Throughout the paper, the conceptualization of the problem and of the results has been framed in terms of receptive fields and the selectivity with respect to orientation which can be derived from receptive field properties. However, the problem and analysis can also be formulated in strictly stimulus terms, with no reference to the possible physiological mechanisms involved. The sensitivity functions obtained by Thomas et al. (1968) and Kulikowski and King-Smith (1973) show only what might be called the lateral component of the spatial interactions which affect the perception of single line stimuli. That is, in obtaining the function. both groups varied only the distribution of light along an axis perpendicular to the stimulus. Neither group varied either the height or the orientations of the stimuli. In this strictly operational sense, only the lateral components were measured and not the longitudinal or orientational components. The question raised in this study is whether these lateral components and a conservative assumption about the longitudinal components, are sufficient to predict performance when the stimuli vary in orientation. The answer is no: the spatial interactions include orientation components which are not derivable from the lateral components.

Ackrto,~/[,dgentar~~-This research was supported m part b! United States Public Health Service Research Grant EYOO.760from the National Eye Institute.

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