Two controversial issues were investigated. One concerned the question of whether cen· trifugal fibers project to the human retina, and the other, the question of whether precortical fil· ters which differentially transmit relevant and irrelevant information during selective attention exist. The issues were addressed jointly by studying the effects of selective attention on evoked responses obtained from the retina and over the occipital cortex. Selective attention was manipulated by requiring subjects to respond to stimuli presented at various locations in the visual field while attempting to ignore stimuli presented at other locations. Retinal responses to stimuli presented at attended to locations were found to be larger than those at unattended to locations. A similar result was obtained for responses recorded over the occipital cortex. The results are interpreted as follows: (1) They support the hypothesis that sustained voluntary at· tention to specific locations in space is mediated, at least partly, by centrally controlled cen· trifugal pathways which differentially modify synaptic transmission at a precortical level; (2) such differential alteration can occur as far peripherally as the retina; and (3) such alteration at the retinal level necessarily requires the existence of centrifugal optic nerve fibers. The results are discussed within the context of existing evidence bearing on the issues.

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