Microstimulation of area V 4 has little effect on spatial attention 1 and on the perception of phosphenes evoked in area V 1

27 Previous transcranial magnetic stimulation (TMS) studies suggested that feedback from 28 higher to lower areas of the visual cortex is important for the access of visual information to 29 awareness. However, the influence of cortico-cortical feedback on awareness and the nature 30 of the feedback effects are not yet completely understood. In the present study, we used 31 electrical microstimulation in the visual cortex of monkeys to test the hypothesis that 32 cortico-cortical feedback plays a role in visual awareness. We investigated the interactions 33 between the primary visual cortex (V1) and area V4 by applying microstimulation in both 34 cortical areas, at various delays. We report that the monkeys detected the phosphenes 35 produced by V1 microstimulation but that subthreshold V4 microstimulation did not 36 influence V1 phosphene detection thresholds. A second experiment examined the influence 37 of V4 microstimulation on the monkeys’ ability to detect the dimming of one of three 38 peripheral visual stimuli. Again, microstimulation of a group of V4 neurons failed to 39 modulate the monkeys’ perception of a stimulus in their receptive field. We conclude that 40 conditions exist where microstimulation of area V4 has only a limited influence on visual 41 perception. 42