Role of the calcarine cortex (V1) in perception of visual cues for saccades.

Objective: To determine the initial level at which the pathways for cue perception, saccades and antisaccades diverge.

Methods: Two procedures: single pulse transcranial magnetic stimulation (sTMS) over posterior occiput and backward masking were used. A visual cue directed saccades to the left or right, either a pro-saccade (to the side of the cue but beyond it) or an antisaccade, i.e., contraversive saccade. No visual target was presented.

Results: Latencies of the two types of saccades did not differ. Focal sTMS applied unilaterally over V1 suppressed both perception of a cue flashed 80-90ms earlier contralaterally (but not ipsilaterally) and the appropriate saccade. Masking at a delay of 100ms abolished the appropriate saccade and cue perception.

Conclusions: V1 is essential for the perception of a flashed cue and for executing appropriate pro- and contraversive saccades. Masking may occur beyond V1, where the pathways for perception and for saccades at least to the next visual processing level start separating.

Significance: VI is needed for rapid, accurate perceptual and motor responses to the crudest (left versus right) cues. It is unlikely that the "where" system can have a major direct input bypassing V1.