What can the visual evoked potential (VEP) tell us about the tuning characteristics of binocular versus monocular visual mechanisms? In particular, are binocular mechanisms tuned to the same range of spatial frequencies as monocular mechanisms? To start, let us review some relevant information from other sources. First there are data from animals. In the monkey, which is the most appropriate animal to compare with man, the geniculate afferents terminate mostly in layer IV. Most cells in the visual cortex are binocularly driven, but layer IV cells are monocular, and in addition they have the smallest receptive fields of all cortical cells.’ These findings might suggest that monocular neurons are tuned to higher spatial frequencies than binocular neurons. Whether the receptive field sizes translate into a difference in spatial frequency tuning functions is not clearly established, however. Binocular cells with large receptive fields (complex cells, for example) may be driven by subunits which themselves have small receptive fields.* As a consequence, the spatial frequency tuning function may not differ between cells having different receptive field sizes. Thus, the results of single unit research are as yet inconclusive. I n terms of human psychophysics, it is possible to compare the contrast sensitivity functions for monocular and binocular viewing. They both have the same shape, with the sensitivity of the monocular curve being about 1.4 times less than that for the binocular curve.3 This might suggest that the spatial frequency tuning for binocular and monocular mechanisms was identical, but this conclusion would be unwarranted. I f the binocular system had higher thresholds, for example, both sets of thresholds may be determined by monocular mechanisms alone, revealing nothing about binocular mechanisms. In fact, the ratio of binocular to monocular sensitivity is close to 6, a ratio that would be predicted if the threshold detection in the binocular situation were due to probability summation of independent monocular mechanisms. Thus, these threshold results do not argue for or against a different spatial frequency tuning for each type of mechanism. Moreover, they suggest that the use of contrast threshold psychophysics may be inappropriate to examine the difference between monocular and binocular mechanisms. More apt would be to examine the question with stimuli that were suprathreshold and would be sure to stimulate binocular mechanisms. This can be done either using different types of psychophysical technique or using visual evoked potential techniques. Psychophysical experiments using such supratheshold stimuli favor the view that binocular mechanisms are tuned to lower spatial frequencies. First, there is an increase in perceived spatial frequency of random noise on closing one eye. For example, random noise viewed monocularly appears to be composed of smaller texture “grains” than the same noise viewed binoc~lar ly . ‘~~ Secondly, stereoscopic tilt from a spatial frequency between the two eyes6 does not
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