CORTICAL SUBSTRATES OF VISUAL SHORT TERM MEMORY

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This research is aimed at examining the properties and the neural substrate of the temporary storage mechanism necessary to ensure continuity of visual stimulation of an active observer. We use psychophysical measures and lesions of physiologically identified regions in selected extrastriate cortical areas in macaque monkeys to explore the role of these cortical areas in storing one of the fundamental features of the visual stimulus, its direction of motion. During the past year we performed a number of experiments that not only provide new insights into the mechanisms of short-term sensory storage but also suggest and point to new psychophysical tools for the study of short-term memory of active observers. (1) We found that the decline in retention of low- and high-level motion stimuli with time is similar for the two types of motion and is accelerated in the presence of motion noise. This result suggests that the process underlying temporary storage of direction information may be common to the two types of motion. It also points to the existence of inhibitory interactions within the networks encoding and storing sensory information. (2) The use of motion stimuli masked by noise also allowed us to uncover selective contribution of areas MT and MST to motion processing. Lesions of MT/MST produced a large, permanent deficit in the discrimination of direction only with motion stimuli masked by noise. (3) We recently found that lesions of areas MT/MST severely limit the retention of information about motion direction. If confirmed, this result for the first time demonstrates the involvement of intermediate visual cortical areas in temporary storage of visual information. These studies provide an important link between the experiments involving saccade-contingent display updating (Hayhoe, Pelz and Ballard) and psychophysical studies of patients with brain lesions (Merigan). In particular, we plan to use noise masking, which allowed us to uncover many of the properties of the temporary storage mechanism, in the study of short-term memory of active human observers performing visual memory tasks. Moreover, the finding of the deficit in motion erception in the presence of directional noise and in short-term memory after MT/MST lesions suggests a number of new tasks for the study of the effects of cortical lesions in humans.