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Attentional effects in the striate and extrastriate cortex have been the subject of intense research interest in many recent studies using single unit recordings in animals and neuroimaging methods in humans. The consensus emerging from these studies places attentional effects first in extrastriate areas. Attentional influence in this view is reaching V1 rather late, some 150 ms after stimulus onset, well after the main N1/P1 complex which peaks about 50 ms earlier. We show here that an earlier wave of attentional effects is present in V1 beginning soon after 50 ms and reaching a peak between 90 and 100 ms after stimulus onset. This effect is identified in a comparison of passive viewing of stimuli and an active GO/NOGO task using the same stimuli. The effect was best seen in the tomographic analysis of single trial magnetoencephalographic (MEG) data, designed to cope with simultaneous activations throughout the brain at each timeslice. This early attentional modulation might still be interpreted as a modulation of the striate cortex from extrastriate areas because attention related changes are present in the prestimulus period in both dorsal and ventral extrastriate areas.

Old ideas about attention and consciousness seemed to be irrelevant today. We re-evaluated them here in the light of recent neuroimaging data, data from pathological conditions where attentional mechanisms are impaired and the knowledge that activity following a stimulus spreads rapidly very widely. We present the case that the multiple systems both cortical and subcortical involved in attentional processes can best be studied using tomographic, millisecond by millisecond single trial analysis of magnetoencephalographic data.