Involvement of striate and extrastriate visual cortical areas in spatial attention

We investigated the cortical mechanisms of visual-spatial attention while s ubjects discriminated patterned targets within distracter arrays. Functiona l magnetic resonance imaging (fMRI) was used to map the boundaries of retin otopic visual areas and to localize attention-related changes in neural act ivity within several of those areas, including primary visual (striate) cor tex. Event-related potentials (ERPs) and modeling of their neural sources, however, indicated that the initial sensory input to striate cortex at 50-5 5 milliseconds after the stimulus was not modulated by attention. The earli est facilitation of attended signals was observed in extrastriate visual ar eas, at 70-75 milliseconds. We hypothesize that the striate cortex modulati on found with fMRI may represent a delayed, re-entrant feedback from higher visual areas or a sustained biasing of striate cortical neurons during att ention. ERP recordings provide critical temporal information for analyzing the functional neuroanatomy of visual attention.