Peripheral attention versus central fixation: modulation of the visual activity of prelunate cortical cells of the rhesus monkey.

Single unit activity was monitored in the prelunate gyrus of monkeys trained to execute or suppress goal-directed saccades to a peripheral target in the presence or absence of a central fixation spot. Throughout the experiments in the dimming paradigm was used. We observed the previously reported spatially selective enhancement of the visual on-response and the extra activation before saccades to a continuously visible target. However, we also observed a sudden increase of activity when the central fixation spot was extinguished in the presence of a peripheral target which the monkey decided not to look at, although he could detect its dimming correctly. This activity is also spatially selective: it occurs only if the peripheral target is within the receptive field of the cell. The presence or absence of just any foveal spot, which is behaviourally not relevant, does not make any difference. The activity occurs about 220 ms after fixation point offset. Thus it can be interpreted as a sign of the animal having directed attention to the peripheral target and/or having stopped active fixation being ready for the next goal-directed saccade without necessarily executing it.