Physiological experiments indicate that the middle temporal visual area (MT) of primates plays a prominent role in the cortical analysis of visual motion. We investigated the role of MT in visual perception by examining the effect of chemical lesions of MT on psychophysical thresholds. We trained rhesus monkeys on psychophysical tasks that enabled us to assess their sensitivity to motion and to contrast. For motion psychophysics, we employed a dynamic random dot display that permitted us to vary the intensity of a motion signal in the midst of masking motion noise. We measured the threshold intensity for which the monkey could successfully complete a direction discrimination. In the contrast task, we measured the threshold contrast for which the monkeys could successfully discriminate the orientation of stationary gratings. Injections of ibotenic acid into MT caused striking elevations in motion thresholds, but had little or no effect on contrast thresholds. The results indicate that neural activity in MT contributes selectively to the perception of motion. Extrastriate visual cortex in primates comprises a mosaic of visual areas that can be distinguished on the basis of visual topography, anatomical connections, cortical architecture, and physiological response properties. A growing corpus of data indicates that several of these areas are specialized for the analysis of visual motion information. These areas appear to constitute a motion pathway that originates in striate cortex and terminates in higher cortical areas of the parietal lobe (reviewed by Maunsell and Newsome, 1987) . The salient physiological feature of this pathway is an elevated percentage of directionally selective neurons at each level. The pathway begins in layer 4B of striate cortex, which is enriched in such cells relative to other striate laminae (Dow, 1974; Blasdel and Fitzpatrick, 1984; Livingstone and Hubel, 1984; Michael, 1985) . Layer 4B projects, in turn, to the middle temporal visual area (MT), in which over 80% of the neurons are directionally selective (e.g., Dubner and Zeki, 197 1; Maunsell and Van Essen, 1983a; Albright, 1984) . MT receives additional inputs from striate cortex via indirect routes through V2 and V3
Newsome REF found the selective impairment of motion perception following lesions of MT.
A selective impairment of motion perception following lesions of the middle temporal visual area (MT
Journal of Neuroscience
Medicine Biology