Average Evoked Potentials and Intramodality Selective Attention

An attempt was made to determine the effects of attention on average evoked potentials when no general change in the alertness of the subject and when no peripheral gating of sensory inputs can be assumed to operate. Human subjects viewed a 50 msec flash of light superimposed on a fluctuating background (e.g., a circle alternating aperiodically with a square). In one of the two experimental conditions, the subject was instructed to ignore the background alternations and to respond to the flash; in the other condition, the subject was required to ignore the flash and to respond to the fluctuations in the background. It was found that the stimulus to which the subject had to respond elicited an AEP with a considerably enhanced late positive component (latency to peak 250–300 msec)