CORTICAL SUBSTRATES OF VISUAL MOTION PERCEPTION

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The visual environment of an active observer is constantly changing, even though its individual features may remain constant. To preserve continuity of visual stimulation, the observer has to not only encode visual information, but also preserve it over brief periods of time. This research is aimed at examining properties of this temporary storage mechanism. We will also explore the role of selected extrastriate cortical areas in storing one of the fundamental features of the visual stimulus, its direction of motion. We chose this feature because its neural coding is well understood and some cortical areas involved in its processing have been identified. These studies will provide important new information concerning the possible participation of visual cortical areas in circuitry subserving visual short-term memory. Proposed experiments will use psychophysical measures, microstimulation and lesions of physiologically identified regions in selected extrastriate cortical areas to investigate the temporary storage (i.e. short-term memory) of directional information. I. Psychophysical properties of mechanisms mediating short-term memory for stimulus motion - The first group of experiments will explore the nature of the process underlying the storage of motion information, in particular its spatio-temporal and directional tuning, retinotopy, and the efficacy of different types of stimulus noise in degrading storage. The monkeys will be tested in a visual task in which two visual stimuli, the sample and the test, separated in time will be judged as the same or different. These studies will provide a better understanding of the mechanisms mediating the retention of motion information, help determine the level of cortical processing at which visual information is stored and set the stage for subsequent studies of neural substrates underlying visual memory. II. The role of areas MT, V3 and V4 in discrimination and storage of stimulus direction and orientation - The goal of this portion of the project is to determine the role of visual areas identified with motion analysis, areas V3 and MT, in the processing and storage of information about stimulus direction. We will use intracortical microstimulation of physiologically identified sites and the lesion approach. To determine the specificity of the effects produced by these manipulations, we will also use non-motion tasks requiring discrimination and retention of stimulus orientation, and apply the same manipulations to localized portions of area V4, which has been identified with form processing. This will help clarify how hierarchical and parallel cortical streams mediate both perception and recall of stimulus features.