MODELING THE TEMPORAL DYNAMICS OF IT NEURONS IN VISUAL-SEARCH - A MECHANISM FOR TOP-DOWN SELECTIVE ATTENTION

We propose a neural model for object-oriented attention in which vario us visual stimuli (shapes, colors, letters, etc.) are represented by c ompeting, mutually inhibitory, cell assemblies. The model's response t o a sequence of cue and target stimuli mimics the neural responses in infero temporal (IT) visual cortex of monkeys performing a visual sear ch task: enhanced response during the display of the stimulus, which d ecays but remains above a spontaneous rate after the cue disappears. W hen, subsequently, a display consisting of the target and several dist racters is presented, the activity of all stimulus-driven cells is ini tially enhanced. After a short period of time, however, the activity o f the cell assembly representing the cue stimulus is enhanced while th e activity of the distracters decays because of mutual competition and a small top-down ''expectational'' input. The model fits the measured delayed activity in IT-cortex, recently reported by Chelazzi, Miller, Duncan, and Desimone (1993a), and we suggest that such a process, whi ch is largely independent of the number of distractors, may be used by the visual system for selecting an expected target (appearing at an u ncertain location) among distractors.