Studies on Normal Adult Cats In the mid-nineteenth century, Wheatstone (1838), using a stereoscope, showed that not only do two bidimensional images viewed with each eye independently appear as a fused single image, but that the image is also seen in depth. This three-dimensional stereoscopic binocular vision appears to depend upon binocular retinal disparities along the horizontal axis. The neural mechanisms underlying stereo-perception seem to be based on the convergence of information from the two eyes onto single cortical cells. This binocular convergence and subsequent binocular fusion are mediated by two pathways: the crossed reti-nothalamocortical pathway (via the optic chiasm) and the transcallosal route. Over the past few years we have carried out a series of behavioral and electrophysiolog-ical experiments to determine the contribution of each of these pathways to stereoperception (reviewed by Ptito et al., 1991). These studies were first done on normal cats to show that this species indeed possesses the neural substrates to mediate stereoperception and on cats with sections of the optic chiasm or/and the corpus cal-losum. A number of studies carried out on cats and monkeys (see reviews by Ptito et al. (1986, 1991) and Wang and Dreher (1996) for the cats and Poggio (1991) for the monkeys) have indicated the presence of cells in the visual cortex that are sensitive to binocular disparity. For example, using single-cell recording techniques, we have found four response profiles in area 17 of the cat that characterize cells that are differentially sensitive to stimulus disparities and that constitute 70% of all sampled neurons. The tuned excitatory disparity detectors (TEDD) and the tuned inhibitory disparity detectors (TIDD) represent cells that respond with excitation or inhibition to stimuli situated at or very close to the fixation plane (Figures 6.7a and 6.7c). The near or crossed disparity detectors and the far or uncrossed disparity detectors (UDD) are sensitive to disparities that would place the image farther in front of or behind the fixation plane (Figures 6.7b and 6.7d). It seems, moreover, that

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