Effects of attention and arousal on early responses in striate cortex.

Humans employ attention to facilitate perception of relevant stimuli. Visual attention can bias the selection of a location in the visual field, a whole visual object or any visual feature of an object. Attention draws on both current behavioral goals and/or the saliency of physical attributes of a stimulus, and it influences activity of different brain regions at different latencies. Attentional effect in the striate and extrastriate cortices has been the subject of intense research interest in many recent studies. The consensus emerging from them places the first attentional effects in extrastriate areas, which in turn modulate activity of V1 at later latencies. In this view attention influences activity in striate cortex some 150 ms after stimulus onset. Here we use magnetoencephalography to compare brain responses to foveally presented identical stimuli under the conditions of passive viewing, when the stimuli are irrelevant to the subject and under an active GO/NOGO task, when the stimuli are cues instructing the subject to make or inhibit movement of his/her left or right index finger. The earliest striate activity was identified 40-45 ms after stimulus onset, and it was identical in passive and active conditions. Later striate response starting at about 70 ms and reaching a peak at about 100 ms showed a strong attentional modulation. Even before the striate cortex, activity of the right inferior parietal lobule was modulated by attention, suggesting this region as a candidate for mediating attentional signals to the striate cortex.