UNDERSTANDING HUMAN VISUAL-CORTEX - THE ROLE OF FUNCTIONAL IMAGING

Visual cortex in the nonhuman primate (NHP) consists of a hierarchy of interconnected representations of the visual field arranged in parall el pathways. Knowledge of human visual cortex has been inferred from l esion and electrophysiological studies of the NHP and accidental patho logy in human subjects. Recently, the functional imaging techniques of positron emission tomography (PET) and single-photon emission compute d tomography (SPECT) have for the first time provided a direct demonst ration of functional specialization within human visual cortex. This a rticle reviews NHP research and lesion studies in humans, which have s uggested a segregation of visual information in the cortex for percept ion (areas V1, V2, V3) and the interpretation of form (areas V3, V4), color (area V4), and motion (area V5). The applications of functional imaging to support these data and, in some cases, verify the respectiv e deficits associated with damage to these visual areas, such as visua l field defects, akinetopsia (failure to appreciate motion), achromato psia (failure to appreciate colors), and prosopagnosia (inability to r ecognize familiar faces), are detailed. In particular, several clinica l settings in which PET and SPECT may be employed to evaluate visual c ortex are discussed.