Visual activation of frontal cortex: segregation from occipital activity.

Studies in primates have found visually responsive neurons that are distributed beyond cortical areas typically described as directly involved in vision. Among these areas are premotor cortex, supplementary motor area, dorsolateral prefrontal cortex and frontal eye fields. Given these findings, visual stimulation would be expected to result in activation of human frontal cortex. However, few human studies have described sensory activations in frontal regions in response to simple visual stimulation. Such studies have classically described event-related potential (ERP) components over occipital regions. The present study sought to further characterize the spatiotemporal dynamics of visually-evoked electrocortical responses elicited by simple visual stimuli using scalp current density measures derived from high-density ERP recordings, with particular emphasis on the distribution of stimulus-related activity over frontal cortex. Hemiretinal stimuli were viewed passively and during a simple ipsi- or contramanual (RT) task. The motor requirement was included to investigate the effects of response preparation on premovement frontal activations. The results indicate early frontocentral activation, particularly over the right hemisphere (peak magnitude 124-148 ms) that is independent of input visual field or motor response requirement, and that is clearly separate in timecourse from the posterior responses elicited by visual input. These findings are in accord with the multiplicity of visual inputs to frontal cortex and are discussed in terms of frontal lobe functions as may be required in these tasks.