Reaction Time As a Measure of Binocular Interaction in Human Vision

Reaction time as a measure of binocular interaction in human vision

Randolph Blake, William Martens, and Anthony Di Gianfilippo

In a series of psychophysical experiments monocular and binocular reaction times (RTs) were measured in response to the presentation of sinusoidal grating patterns. Over a wide range of contrast values, binocular RT was consistently faster than monocular RT, even at high-contrast levels where RT had reached asymptotic levels. For observers with good stereopsis this binocular summation effect was greater than that expected on the basis of probability summation alone, whereas observers with deficient stereopsis performed at the level of probability summation. For normal observers broadband random noise presented to one eye produced an elevation in RT to gratings presented to the other eye; no such dichoptic masking effect was found, in a stereoblind observer. These results validate the use ofRT as an efficient, reliable measure of binocular interaction in human vision.

Key words: binocular summation, dichoptic masking, reaction time, stereoblindness

Human observers with good stereopsis are binocular mechanisms in human vision. For more sensitive on visual threshold tasks when instance, it has been possible to measure the 2 3 4 5 using both eyes than when using either eye spatial ' and temporal * characteristics of alone. This general finding, referred to as human binocular vision by determining just "binocular summation," holds for a variety of how different the views of the two eyes may threshold tasks such as increment detection, be before binocular summation breaks down. form recognition, and contrast sensitivity (see The bulk of the research on binocular review by Blake and Fox1). As a rule, this summation has dealt with threshold mea- superiority of binocular viewing is greater sures of sensitivity, wherein visual targets are than that expected on the basis of probability rendered difficult to see by virtue of their low summation alone, which means that binocu- contrast or intensity or because they were lar summation must arise from some form of only very briefly presented. Although this excitatory neural interaction between the two approach provides important information eyes. Consequently, binocular summation about binocular visual performance near provides a potentially useful tool for studying threshold, it does not readily allow us to ex- trapolate to suprathreshold conditions of stimulation such as those typically involved From the Cresap Neuroscience Laboratory, Department in our everyday visual environment. To over- of Psychology, Northwestern University, Evanston, come this limitation, we have turned to a dif- 111. This research was supported by grant EY01596 from the ferent approach to the study of binocular National Institutes of Health and by Career Develop- summation, one involving the measurement ment award EY00106 to R. B. of simple reaction time (RT). During mon- Submitted for publication July 13, 1979. ocular or binocular viewing, an observer Reprint requests: Randolph Blake, Cresap Neuroscience makes a manual response immediately upon Laboratory, Department of Psychology, Northwestern University, Evanston, 111. 60201. presentation of a clearly visible stimulus.

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Previous work6 8 has documented that bin- Table I. Left eye (LE) and right eye (RE) ocular RT is consistently faster than monocu- contrast thresholds for detection of a vertical lar RT, but those studies made no attempt to grating pattern at two spatial frequencies* deal with the contribution of probability 1 cy/deg 4 cy/deg summation. In our work we chose to study binocular LE RE LE RE RT summation using grating patterns, the R. B. 0.0047 0.0049 0.0031 0.0032 contrast or spatial frequency of which can be A. G. 0.0058 0.0055 0.0037 0.0035 varied without altering the overall level of W. M. 0.0055 0.0055 0.0030 0.0030 C. B.t 0.0049 0.0073 0.0035 0.0073 light adaptation. Visual displays of this type P. G.t 0.0049 0.0045 0.0038 0.0032 have been used rather extensively2' 5l 9' 10 *These values were obtained by having the observer adjust the within the context of threshold measures of contrast of the grating to the point where the pattern was just binocular summation, which means we have barely visible. a convenient basis for comparison of binocu- tStereoblind. lar summation at threshold vs. suprathreshold levels. In this paper we demonstrate the plays. Great care was taken to adjust the mirrors of magnitude of binocular RT summation and the stereoscope so that fusion of the two CRT dis- seek to establish empirical tests for assessing plays was easily maintained. the contribution of probability summation to Procedures. On each trial both CRT screens this superiority of binocular RT. Also, we initially appeared uncontoured (homogeneous ras- ter). Using his or her left hand, the observer de- have combined the simple RT task with a si- pressed a lever switch which initiated a variable multaneous masking paradigm to develop a interval foreperiod, at the end of which a grating suprathreshold index of binocular inhibitory abruptly appeared on one or both CRTs. The vari- interactions in human vision. able foreperiod consisted of a fixed 1 sec interval followed by a series of 100 msec intervals during Methods each of which the probability of grating onset was Visual displays. The visual stimuli used to mea- 0.10. The observer was instructed to release the sure RTs were one-dimensional, vertical grating spring-loaded lever just as soon as the grating ap- patterns of sinusoidal luminance profile. These peared; the switch release served to extinguish the gratings were generated electronically" on two grating and to stop an electronic counter which matched cathode-ray tubes (CRTs) with P31 phos- started the instant the grating was presented. In phors. The details of our particular setup are given this manner latencies could be measured within an elsewhere,12 and so only the major features are accuracy of 1 msec. Observers were allowed to set provided here. their own trial-to-trial pace. The various viewing The two matched CRTs were viewed separately conditions (left eye, right eye, both eyes) were by the two eyes through a mirror stereoscope, tested in 30-trial blocks, with the first five RTs in with a viewing distance of 114 cm. The screens each block treated as practice. Observers were un- subtended 7 deg by 5 deg, and the space average informed about the particular viewing condition luminance of both was 7 cd/m2 regardless of the under test, and the order of blocks was counterbal- spatial frequency or contrast of the gratings. Grat- anced across observers. ing contrast could be varied independently on the Prior to formal data collection, each observer two CRTs in 0.05 log unit steps with the use of was provided with several sessions of practice on precision decade attenuators. In addition, one- the task in order to minimize learning effects dur- dimensional random noise could be displayed on ing the course of the experiments. Observers were either CRT by driving the beam current amplifier encouraged to respond as quickly as possible with- with the output from a broadband digital noise out habitually making "false alarms," which we generator. arbitrarily defined as RTs less than 100 msec and Observers viewed these displays through natu- excluded from data analysis. For no observer did ral pupils while their heads were firmlypositione d the false alarm rate exceed 8%; typically the stan- on a dental impression board. The only source of dard deviation of the RTs were about 10% to 15% light within the darkened test chamber was the of the arithmetic mean. ambient illumination from the pair of CRT dis- Observers. A total of six individuals participated

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Of the six observers only the authors (R. B., _ 300 T. D., W. M.) were aware of the purposes of the S 280 o-monocular •-binocular experiments. E 26 Z ° Results ~ 240 Binocular summation of simple RT. In our c 2 220 first experiment monocular and binocular o RTs were measured at 1 and at 4 cy/deg. At | 200 both spatial frequencies RTs were measured 0.3 0.6 0.9 1.2 over a range of contrast values, starting at a Grating contrast level 6 dB (0.3 log units) above threshold and (log-units above threshold) sampling in 3 dB steps. For each observer Fig. 1. Monocular and binocular RT in response to contrast visibility thresholds were measured presentation of a 1 cy/deg grating, the contrast of monocularly with a method of adjustment; which is plotted along the abscissa. Each point the resulting values are given in Table I. For represents the arithmetic mean of RTs over 25 none of the normal observers did the thres- trials. holds for the two eyes differ by more than 1 dB. For stereoblind Observer P. G. the two 375 WM eyes differed by 2 dB at 1 cy/deg and by 4 dB o-monocular at 4 cy/deg, with the nonamblyopic eye being -binocular the more sensitive. For C. B., who is also E 325 stereoblind, the differences in sensitivity were more pronounced (5 and 8 dB at 1 and 4 cy/deg, respectively). For both stereoblind 275 observers, maintaining these contrast differences between the two eyes at suprathres- o

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1.10 £ 1.05 1 1.00 I1 RB AG WM PG CB

Normal stereopsis Stereoblind Fig. 3. These histograms plot the ratio of monocular to binocular RTs averaged across all contrast values tested; the vertical bars denote ±1 S.E.M. Open histograms give the results for 1 cy/deg, the filled histograms for 4 cy/deg. ner, even though we went to some lengths to tween monocular and binocular RTs is con- match the two eyes in terms of apparent consistently smaller than at 1 cy/deg. trast. Consequently, for data analysis in her The results for the two stereoblind ob- case we always compared binocular per- servers are summarized by the two pairs of formance with that of the nonamblyopic eye. histograms in the right-hand portion of Fig. 3. For all normal observers the binocular RTs For both of these observers, binocular RTs were consistently faster than the monocular were shorter than monocular RTs but by an RTs. This was true without exception at both amount less than that found for those ob- spatial frequencies and at all contrast levels. servers with normal stereopsis. For the ste- The typical pattern of results is illustrated in reoblind observers, in six out of 24 conditions Figs. 1 and 2, which plot average monocular the monocular RTs were equal to or less than and binocular RTs as the function of grating the corresponding binocular RTs; this was contrast. Following the lead of Harwerth and never the case for observers with normal ste- Levi,13 we used template curves to fit smooth reopsis. An analysis of variance performed on lines to the data points. There was no clear the data from stereoblind observers revealed trend toward the biphasic relationship be- only the main effect of contrast (F(7,7) = tween RT and contrast reported by Harwerth 4.45, p < 0.01) to be statistically significant; and Levi,13 but this may simply stem from the main effect of viewing condition was only the fact that our measurements, unlike theirs, marginally significant (F(l,l) = 13.04, p < were confined to contrast values within 1.5 0.15). The average difference between mon- log units of threshold. An analysis of variance ocular and binocular RTs was 4.25%. The revealed statistically significant main effects finding of reduced summation in stereoblind of contrast (F(7,14) = 33.45, p < 0.01) and observers is a point we shall return to in the viewing condition (F(l,2) = 21.97, p < 0.01) next section. and a marginally significant interaction be- Probability summation. Having ascer- tween spatial frequency and viewing condi- tained the superiority of binocular RT, we tion (F(l,2) = 14.58, p < 0.10). There was, now must consider whether probability sum- on the average, a 10% difference between mation alone can account for this superiority. monocular and binocular RTs. This differ- By probability summation we mean an im- ence can be seen in the case of normal ob- provement in binocular performance due servers in the three pairs of histograms in the strictly to the statistical fact that viewing with left-hand portion of Fig. 3. Note that at 4 two eyes provides an observer with two op- cy/deg the magnitude of the difference be- portunities to produce a response, not just

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300 obvious that probability contributes to per- Left eye formance, since the stimulus itself is clearly detectable. Still, it is possible to concep- 150 tualize the decision process leading to a response in probabilistic terms. Indeed, there is a class of well-developed models that in- a- corporate probabilistic mechanisms,14"16 and 0} f 300 Right eye all these latency models would predict shorter RTs with binocular viewing as a result of 150 probability summation between two independent channels (i.e., the two eyes). These various models differ primarily in terms of the nature of the putative underlying sto- 0*- chastic process and the particulars of the de- 0.75 1.00 1.25 1.50 cision rule leading to a response. Normalized reaction time In addition to a priori predictions derived Fig. 4. Frequency distributions of RTs measured from models, one can employ various ex- with the left eye only or the right eye only. Both perimental strategies to derive empirically an distributions contain data pooled across observers, estimate of probability summation. These contrast levels, and spatial frequency. Prior to various strategies have in common the aim of pooling, each RT value was normalized by divid- ing the individual values by the average RT for structuring the conditions of binocular stimu- that condition. Inspection of these two distri- lation so that neural summation is precluded, butions, as well as comparisons of the mean and leaving only probability summation as a variability for the individual conditions, supports mechanism for improved performance. In the assumption of equivalence of the two distri- past work such strategies have included butions. To model the level of binocular per- separating the two monocular inputs in time4 formance expected on the basis of probability or in space17 or using stimuli that are summation, we have assumed a decision process sufficiently dissimilar along some dimension which samples once from each distribution in a (e.g., orientation) to preclude neural sum- random fashion and takes the faster of the two mation. As an example of this latter strategy, sampled values as the binocular RT. The mean of 18 the binocular RTs generated by this strategy will Westendorf and Fox found that simulta- be lower than the obtained mean of either mon- neous presentation of a light increment to ocular distribution by the factor 0.57 times the one eye and a light decrement to the other S.D. of the monocular RTs. eye yielded forced-choice detection performance that was better than that measured with monocular viewing but was worse than one as in the case of monocular viewing; the that produced by identical binocular stimu- logic of the probability summation argument lation (e.g., increments to both eyes). In fact, has been detailed elsewhere.1 the detection performance with the dissimi- In the case of detection threshold mea- lar pair of dichoptic stimuli was equivalent to sures, where binocular performance is ex- that predicted by several models of probabil- pressed as percent correct, it is possible to ity summation. Westendorf and Fox3 ob- derive a priori predictions of performance tained essentially the same result in the case expected on the basis of probability summa- of single line stimuli that were orthogonally tion, with one of several available models of oriented for the two eyes. probability summation. In the derivation of Given these two complementary ap- such predictions in this analytical fashion, the proaches to the problem of predicting binocu- major concern is selecting a model that is ap- lar RT performance expected on the basis of propriate for the structure of the threshold probability summation, we have chosen to task. In the case of an RT task, it may be less utilize both strategies, analytical and empiri-

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•o 360 z 360

* • X* 240 'I* **** - 240 lar 3 O O " * • V. c */ • •• / m * *1 i i i i / i i i i 1 1 1 2.00 240 280 320 240 280 320 360 Binocular reaction time (predicted) Fig. 5. Two scatterplots showing binocular RTs predicted on the basis of probability summation plotted against the actual value of binocular RTs. The predicted values which assume independence of the two eyes were obtained by multiplying the S.D. of a set of monocular RTs by 0.57 and subtracting this value from the mean of those monocular RTs. Points consistently below the diagonal indicate binocular summation in excess of that expected on the basis of probability.

cal, in the hopes of converging to a common dom samples from the monocular RT distri- estimate of that level of performance. bution, with the predicted binocular RT cor- Analytical prediction. To derive an a priori responding to the smaller of these two val- estimate of binocular RT assuming probabil- ues. On the basis of sampling theory,21 it can ity summation, we have adopted a version of be shown that the new distribution of binocu- the statistical decision theory of simple RT,19 lar RTs created by repeating these paired which in its formal structure resembles the samplings would have a mean (/xB) given by familiar signal detection model.20 According to this view, sensory events lead to the ac- /XB = XM-O-M(0.57) (1) cumulation of inputs at some decision center, whereX M and CTMrefe r to the mean and stan- and when some criterion number of inputs is dard deviation of the monocular RT distribu- exceeded, a response is triggered; for our tion. This formulation makes no assumption purposes it makes no difference whether this about the form of the monocular distribution decision center examines these inputs on the of RTs other than the equivalence of left-eye basis of counting14 or timing.16 Just as in sig- and right-eye distributions. It is necessary to nal detection theory, the observer is as- assume that the bias level remains constant sumed to select a criterion, or bias, level with monocular and binocular testing and which is influenced by the particulars of the that the decision center samples from the task (e.g., temporal uncertainty about stimu- two monocular channels in an unbiased lus onset). In the case of binocular viewing, fashion. the decision center samples inputs from two When we applied this model to the data for sensory channels which, for our purposes, the three observers with normal stereopsis, are assumed to be independent. It is in 43 out of 54 comparisons the obtained im- further assumed that the frequency distri- provement in binocular RT exceeded that butions of RT are identical for the two eyes, predicted on the basis of probability summa- an assumption which is consistent with our tion; a scatterplot of these predicted and ob- findings (Fig. 4). tained values appears in Fig. 5, a. This con- Given these assumptions, the binocular sistent underestimation of binocular RT im- case can be considered as involving two ran- plies that the improvement in binocular

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28Or two different conditions of dichoptic stimulation designed to preclude neural summation. AG RB One condition involved the presentation of 260 orthogonally oriented contours to the two eyes, and the other involved binocular stimulation of noncorresponding retinal areas. J 240 Both of these conditions have been used previously to estimate probability summation on threshold tasks.3'17 Results from these two 220 tests will be described in turn. For the experiment with orthogonal pat- 200 terns, the left eye received a vertical grating, \ and the right eye received horizontal, a con- V/H V/V VV V/H V/V dition we shall denote by 'V/H'. Both ortho- -/v H/H H/- H/H H/- gonally oriented gratings were 4 cy/deg and 1 Viewing condition log unit above threshold in contrast. For purposes of comparison, RTs were also mea- Fig. 6. Average RT (n = 25) for various conditions of monocular and binocular stimulation. Vertical sured under conditions where just the left (V) or horizontal (H) gratings were presented to eye received vertical (V/ —), just the right eye the left eye and/or the right eye, as explained in received horizontal (—/H), and both eyes re- the text. Vertical bars denote ±1 S.E. ceived horizontal (H/H) or vertical (V/V). The order of conditions was randomly in- Table II. Binocular RTs (msec) predicted termixed. We tested normal observers R. B. on the basis of probability summation and and A. G. those measured empirically under dichoptic The resulting RTs for these various condi- conditions designed to preclude binocular tions are shown in Fig. 6. For both observers neural summation the fastest RTs were associated with the binocular conditions involving identical stimula- Orthogonal contours Disparate tion to both eyes (H/H and V/V). The binocu- Observer Predicted Obtained Predicted Obtained lar dissimilar condition (V/H) yielded latencies that were longer than the binocular R. B. 218 216 264 269 A. G. 242 248 299 296 identical conditions but were shorter than either monocular condition by 4.6%. Using the latency model described in the performance over monocular involves more previous section and the monocular RT data than simply probability summation. For in Fig. 6, we derived estimates of binocular stereoblind observers, however, probability RT based on probability summation, and summation appears to offer a more reason- those predictions together with the obtained able account of binocular RT, as shown in results are shown in Table II. For both ob- Fig. 5, b. Here the predicted and obtained servers, the probability predictions were values clustered more closely on either side very similar to performance on the V/H con- of the diagonal. Of course, these conclusions dition, thus supporting the validity of this rest on the assumptions of the independence condition as an index of performance due to model. The next section, by approaching the probability summation. It is also noteworthy problem of probability summation empiri- that this condition for the normal observers cally, provides a test of the validity of this yielded the same degree of improvement model. in binocular RT as that measured for ste- Empirical estimates of probability summa- reoblind observers with identical binocular tion. To estimate probability summation em- stimulation. pirically, we measured binocular RT under One might object that the 'H/V stimulus

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condition, in addition to precluding neural 340 summation, also generates inhibition in the o 32 form of binocular rivalry suppression. But in 1 AG RB fact, binocular rivalry takes some time to de- I ° velop, and for at least the first few hundred | 300 milliseconds, potentially rivalrous displays are seen in their entirety with no hint of sup- | 280 T pression. Others22' 23 have shown that bin- o i. ocular suppression produces an increase in o RT well beyond the monocular level mea- r 260 sured during dominance. We see no way in which such an elevated RT could contribute 2 p 240 5 o »' to the improvement in binocular RT found for I I condition 'H/V, which leads us to reject ar- I ! guments which invoke binocular inhibition. Q • The second condition for estimating prob- Fig. 7. Average RT (n = 25) for monocular view- ability summation involved presenting a 4 ing and for binocular viewing with stimulation of cy/deg vertical grating on noncorresponding corresponding retinal areas (fused) or with stimu- retinal areas. To achieve this, both CRT dis- lation of noncorresponding retinal areas (dispa- plays were partitioned into two circular re- rate). Vertical bars denote ±1 S.E. gions, each 2 deg in diameter, which appeared 1 deg on either side of a central fixa- with that actually measured under the dispa- tion point. With this arrangement, binocular rate condition (Table II). RTs were measured with the grating to the In summary, several converging lines of left eye presented just to the left-hand por- evidence indicated that in normal observers tion of the display and the grating to the right the enhancement in binocular performance eye presented just to the right-hand position, on the RT task exceeded that expected on the a condition we shall call "disparate." For sake basis of probability summation when the two of comparison, RTs also were collected under eyes were stimulated with identical patterns conditions of corresponding binocular stimu- on corresponding retinal areas; when either lation (i.e., left- and right-eye gratings to the of these conditions was violated, binocular same retinal areas, either to the left or right performance was equivalent to that expected of fixation) and under monocular stimulation merely on the basis of two independent (i.e., a single grating to just one portion of chances to perceive. Even under optimal one eye). To eliminate uncertainty as to conditions, however, stereoblind observers target location, the observer was told in performed at a level comparable to that pre- which region(s) of the display the grating dicted by probability, a finding which has would appear but was not told whether also been reported for threshold tasks.24 stimulation would be monocular or binocu- Interocular masking of RT. The previous lar. The observer was instructed always to experiments dealt with a form of binocular maintain central fixation. interaction which in all likelihood was ex- Results for this experiment are summa- citatory in nature. It is easy to envision the rized in Fig. 7. Binocular stimulation of cor- enhancement in binocular RT as the product responding retinal areas yielded RTs that of neural facilitation between the two eyes. were on the average 10.6% faster than mon- There are, of course, other stimulus situ- ocular RTs; with binocular stimulation of non- ations wherein the two eyes seem to interact corresponding areas, RTs were 5.3% faster in an inhibitory, not an excitatory, fashion. than with monocular viewing. The RT pre- For instance, detection thresholds for a vari- dicted on the basis of probability summation ety of types of visual targets can be elevated with the latency model compared favorably by the presentation of certain other types of

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TD 400 o- binocular g 350 •- monocular o- binocular to •- monocular • - interocular o- interocular 350 I 300

300 250 250 0 5 10 15 0 5 10 15

Average noise contrast (%) Average noise contrast (%) Fig. 8. Reaction time to a 4 cy/deg grating pre- Fig. 9. Same type of plots as in Fig. 8. Observer sented to one or both eyes in the presence of P. G. exhibits no evidence of stereopsis. one-dimensional random noise, the contrast of which is plotted along the ordinate. T. D. is an observer with good stereopsis. Alternatively, it was possible to route the grating to one CRT while displaying noise on just the other CRT (interocular condition). targets to the contralateral eye.25 This gen- For each of these various conditions we mea- eral outcome, known as dichoptic masking, is sured RTs over a range of noise contrast typically conceptualized in terms of inhibi- levels calibrated in terms of the mis voltage tory interactions between the two eyes.26 We of the signal. were interested in developing a measure of The results for Observer T. D., one of our dichoptic masking within the context of a RT normal subjects, are shown in Fig. 8; we also paradigm like that used in our binocular tested W. M. on this task, and his data were summation work. This interest arose from the qualitatively the same as those shown in this need for a simple task with which to assess figure. For graphic presentation we have binocular inhibition in individuals with defi- pooled the data for the two monocular condi- ciencies in stereopsis and visual resolution. tions (left eye only and right eye only) and the This section presents results from our initial data for the two interocular conditions (noise: efforts at devising such a task. left eye/grating: right eye, and vice versa), With the same displays and stereoscope, since the particulars of these conditions made we generated one-dimensional random noise no difference. For each condition RT in- on one or both CRTs. The noise spectrum creased with noise contrast, indicating that was flat out to a spatial frequency of 25 cy/ one-dimensional random noise does interfere deg, and its energy distribution over time with the detection of the sinusoidal grating. was uniform (i.e., rectangular), not Gaussian. At the highest noise level tested, this inter- This noise was continuously present and ference was on the order of a 50% elevation served as the background in which the RT in RT. Note that despite the large masking stimulus appeared, itself a sinusoidal grating effect there still was evidence for a binocular that was 4 cy/deg and 20% in contrast. As in summation effect, since RT under the mon- the previous experiments, this grating was ocular condition was consistently longer than abruptly presented after some variable inter- under the binocular condition. Of signifi- val following the observer's depression of the cance for our purposes is the clear evidence RT switch. With a linear summing amplifier for interference under the interocular condi- it was possible to mix the grating with the tion where noise and test target are pre- noise electronically and thereby present the sented to separate eyes. This particular result combined target to one eye (monocular con- demonstrates the feasibility of RT as a mea- dition) or to both eyes (binocular condition). sure of dichoptic masking. Note, too, that

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masking under this interocular condition was equivalent RTs of, say, 250 msec, the mon- reduced in magnitude relative to the monocular contrast must be 0.20 to 0.30 log units ocular condition. This partial interocular ef- higher than the binocular contrast. For any fect is characteristic of threshold measures of particular observer, this contrast difference is dichoptic masking,27 and it also has an not strongly dependent on the particular cri- analogue in the case of interocular transfer of terion RT selected because the monocular visual aftereffects.28 and binocular functions are approximately Next look at Fig. 9 which plots comparable parallel, differing mainly in their positions data from stereoblind Observer P. G. Al- along the abscissa. This 0.2 to 0.3 log unit though exhibiting clear evidence for masking summation effect actually represents a larger under the monocular and binocular condi- enhancement in binocular performance than tions, this observer displayed no increase in that typically measured at threshold levels of monocular RT when the other eye received contrast, where 0.15 log units is more or less random noise; this result was true regardless the rule of thumb.10 It is interesting to note of whether noise was presented to her that there is a trend for binocular RT sum- amblyopic eye or to her nonamblyopic eye. mation to be greater for the lower of the two For this observer the binocular RT was only spatial frequencies tested; this tendency also marginally faster than monocular RT at three has been observed in contrast threshold mea- of the four noise levels. sures of binocular summation.5' 9 The present results also show that the ab- Discussion sence of binocular neural summation in ste- The present results serve to validate the reoblind observers on threshold tasks24'2!) ex- use of simple RT as a psychophysical gauge of tends to conditions of suprathreshold stimu- binocular interaction in human vision. With lation: the two eyes continue to perform at this measure of visual performance it is pos- the level expected on the basis of probability sible to assess both binocular summation and summation alone. Very recently Levi et al.30 dichoptic masking, phenomena which tradi- reported that human amblyopes, when tested tionally have been studied with threshold on a task much like that used in this paper, measures of performance. Moreover, one can performed no better with two eyes than with derive empirical estimates of probability just the nonamblyopic eye; in some instances summation with RT and thereby determine there was evidence that binocular perfor- the contribution of neural summation to the mance was actually poorer than monocular. superiority of binocular RT. They made no attempt to compensate for the In previous work involving contrast thres- poorer contrast sensitivity of the amblyopic hold measures,2'5> 9l 10 it has been found that eye, however, which means that with binocu- binocular performance can surpass monocular testing, the two eyes differed in apparent lar by as much as 40%. The present results, contrast. Our results show that such a con- on the other hand, show improvement in pensation can serve to elevate binocular per- binocular RT to be on the order of 10% when formance to the level of probability summa- expressed in the time domain. This modest tion. Both our results and those of Levi et difference between monocular and binocular al.30 are consistent with the hypothesis that in RTs actually represents a rather robust sum- stereoblind persons there is a paucity of vi- mation effect when considered in terms of sual neurons receiving excitatory input from the contrast necessary to produce equivalent both eyes. RTs. From plots like those in Figs. 1 and 2, it In some of the earlier work on RT,6 it was is possible to determine the contrast values found that the dominant eye yielded consis- necessary to produce some criterion RT tently faster RTs than did the nondominant under monocular and binocular viewing. eye, where eye dominance was defined by Depending on the observer, to produce sighting tests. Our results for normal ob-

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servers show no effect of eye dominance, in clusions. We are grateful to them for sharing that performance for the two eyes was not their findings with us. significantly different. Of course, in our experiments grating contrast was set relative to REFERENCES the separate threshold of each eye, a maneu- 1. Blake R and Fox R: The psychophysical inquiry into ver that could serve to minimize any effect of binocular summation. Percept Psychophys 14:161, eye dominance. Still, though, for these ob- 1973. 2. Blake R and Levinson E: Spatial properties of bin- servers the two eyes differed little, if any, in ocular neurons in the human visual system. Exp terms of contrast threshold (Table I). For one Brain Res 27:221, 1977. of the two stereoblind observers, C. B., there 3. Westendorf D and Fox R: Binocular detection of was a consistent superiority of the dominant vertical and horizontal line segments. Vision Res (i.e., nonamblyopic) eye. Despite the match 15:471, 1975. 4. Matin L: Binocular summation at the absolute in apparent contrast between left- and right- threshold for peripheral vision. J Opt Soc Am eye gratings, the RTs of her right eye were 52:1276, 1962. slower by about 10% to 15% than those of her 5. Blake R and Rush C: Temporal properties of binocu- left eye. Had we not matched the monocular lar mechanisms in the human visual system. Exp gratings in apparent contrast, this difference Brain Res 38:333, 1980. 6. Poffenberger A: Reaction time to retinal stimulation would have been exaggerated, especially at 4 with special reference to the time lost in conduction cy/deg where her two eyes differed by 0.4 log through nerve centers. Arch Psychol 23:1, 1912. units in sensitivity. 7. Minucci P and Connors, M.: Reaction time under Finally, our results show that dichoptic three viewing conditions: binocular, dominant eye, masking with broadband noise can produce a and nondominant eye. J Exp Psychol 67:268, 1967. 8. Gilliland K and Haines R: Binocular summation and substantial increase in RT, approaching 20% peripheral vision response time. Am J Optom at higher levels of noise energy. Somewhat Physiol Opt 52:834, 1975. surprising is the absence of interocular mask- 9. Rose D: Monocular versus binocular contrast thres- ing in stereoblind Observer P. G. Other psy- holds for movement and pattern. Perception 7:195, chophysical evidence31 points .to the exis- 1978. 10. Campbell FW and Green D: Monocular versus bin- tence of potent inhibitory connections be- ocular visual acuity. Nature 208:191, 1965. tween the two eyes of stereoblind people, 11. Enroth-Cugell C and Robson J: The contrast sen- and there is physiological evidence for bin- sitivity of retinal ganglion cells of the cat. J Physiol ocular inhibitory effects in the visual cortex of 187:517, 1966. monocularly deprived cats which lack binocu- 12. Blake R and Cormack R: On utrocular discrimina- 32 33 tion. Percept Psychophys 26:53, 1979. lar excitatory connections. ' It could be 13. Harwerth RS and Levi DM: Reaction time as a mea- that, in our task, Observer P. G. was able to sure of suprathreshold grating detection. Vision Res adopt a viewing strategy that selectively iso- 18:1579, 1978. lated the eye receiving the test grating; recall 14. McGill W: Stochastic latency mechanism. In Hand- that trials were run in blocks in which the book of Mathematical Psychology, Luce RD, Bush RR, and Galanter E, editors. New York, 1963, John same eye was predictably tested. Other Wiley & Sons, Inc., vol. 1. 34 data show that stereoblind persons, unlike 15. Pike R: Response latency models for signal detec- normal ones, can easily distinguish left-eye tion. Psychol Rev 80:53, 1973. from right-eye stimulation and can attend at 16. Luce RD and Green DM: A neural timing theory for will to just one eye. It would be interesting to response times and the psychophysics of intensity. Psychol Rev 79:14, 1972. assess interocular masking under conditions 17. Westendorf D and Fox R: Binocular detection of where the stereoblind observer was unable to disparate light flashes. Vision Res 17:697, 1977. anticipate the eye to be tested. 18. Westendorf D and Fox R: Binocular detection of Following submission of an earlier version positive and negative flashes. Percept Psychophys of this paper, we learned that Harwerth et 15:61, 1974. 35 19. John, I. D.: A statistical decision theory of simple al. were performing similar experiments reaction time. Aust J Psychol 19:27, 1967. which lead to substantially to the same con- 20. Green DM and Swets J: Signal Detection Theory

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and Psychophysics. New York, 1966, John Wiley & 29. Lema S and Blake R: Binocular summation in nor- Sons, Inc. mal and stereoblind humans. Vision Res 17:691, 21. David J: Order Statistics. New York, 1970, John 1977. Wiley & Sons, Inc. 30. Levi DM, Harvverth RS, and Manny RE: Supra- 22. Fox R and Check R: Detection of motion during threshold spatial frequency detection and binocular binocular rivalry suppression, J Exp Psychol 78:388, interaction in strabismic and anisometropic ambly- 1968. opia. INVEST OPHTHALMOL VIS SCI 18:714, 1979. 23. Blake R and Fox R: Binocular rivalry suppression: 31. Levi D, Harwerth R, and Smith E: Human subjects insensitive to spatial frequency and orientation with amblyopia have binocular interactions tuned to change. Vision Res 14:687, 1974. size and orientation. INVEST OPHTHALMOL VIS SCI 24. Westendorf D, Langs ton A, Chambers D, and Al- 18(ABVO Suppl.):150, 1979. legretti C: Binocular detection by normal and ste- 32. Duffy FH, Snodgrass SR, Burchfield JL, and Con- reoblind observers. Percept Psychophys 24:209, way J: Bicuculline reversal of deprivation amblyopia 1978. in the cat. Nature 260:256, 1976. 25. Henning GB and Hertz BG: Binocular masking level 33. Kratz K, Spear P, and Smith D: Postcritical period differences in sinusoidal grating detection. Vision reversal of effects of monocular deprivation on Res 13:2455, 1973. striate cortex cells in the cat. J Neurophysiol 39:501, 26. Abadi R: Induction masking—a study of some in- 1976. hibitory interactions during dichoptic viewing, Vi- 34. Blake R and Cormack R: Psychophysical evidence sion Res 16:269, 1976. for a monocular visual cortex in stereoblind humans. 27. Turvey M: On peripheral and central processes in Science 203:274, 1979. vision: inferences from an information-processing 35. Harwerth RS, Smith E, and Levi DM: Supra- analysis of masking with patterned stimuli. Psychol threshold binocular interactions for grating patterns. Rev 80:1, 1973. Percept Psychophys 27:43, 1980. 28. Blake R, Overtoil R, and Lema S: interocular transfer of visual aftereffects. J Exp Psychol (in press).

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