Psychophysical studies in humans and in cats have identified two classes of contrast detecting mechanisms, directionally selective and non-directional, each operating at different but overlapping spatial and temporal frequencies. This proposal describes studies in cats, that could not be done in humans, designed to elucidate the neural basis of these two mechanisms. In all proposed studies contrast sensitivity for detection of a moving grating will be compared to the sensitivity for discriminating the grating's direction. Fixation locus will be controlled behaviorally and monitored with eye coils. In the first project the major stimulus parameters that determine visibility of the direction of stimulus motion will be identified in normal cats by measuring detection and direction discrimination over a range of spatial and temporal frequencies, at several retinal eccentricities. The separate contribution of the two major cortical structures, areas 17 and 18 to the preception of motion will be examined. Focal lesions will be placed in physiologically identified portions of either area 17 or 18 and sensitivity measured for detection and direction discrimination of moving gratings in corresponding portions of the visual field. Localized lesions will also be made in the lateral suprasylvian area (LS) to examine its role in later stages of motion processing. Cats with LS lesions will be tested with simple gratings as well as with more complex motion patterns (dynamic random dots). The final project will determine the locus of directionally selective neurons, involved in the ability to discriminate opposite directions of motion. Previous behavioral studies of cats with a selective loss of directional selectivity in visual cortex (cats reared in 8 hz stroboscopic illumination), found greatly reduced contrast sensitivity for stimulus direction but nearly normal direction discrimination performance at high stimulus contrasts. Localized lesions will be made in area 17, 18 or LS in these cats to determine which of the few remaining directionally selective neurons contribute to their residual sensitivity for motion direction.
