Components of visual attentiveness in suppression of the occipital alpha rhythm.

This study examined whether either visual input or "attention" exerts an influence on the alpha rhythm beyond that produced by oculomotor maneuvers such as movement and fixation. Alpha-suppressive effects of an attentional "set" were demonstrated in subjects anticipating a response to an unknown visual stimulus in the dark. In a second condition, subjects moved their eyes laterally between two visual targets. When visual input was increased by interspersing additional lights between the targets, a significant increase in alpha suppression was observed beyond that produced by eye movement alone. In the third condition, subjects fixated on a point of light (to stabilize oculomotor influences across tasks) while adjacent points were displayed at random intervals. Attentional "demand" was varied by requiring subjects alternately to respond to these flashing lights or passively view them. Both tasks produced alpha block relative to baseline periods, but relative suppression did not differ between tasks.