Selective attention to spatial frequency gratings affects visual processing as early as 60 msec. poststimulus.

Event-related potentials (ERPs) were recorded from occipital sites to investigate early selection mechanisms and to determine the time at which attention modifies the processing activity of the visual cortex in humans. 19 right-handed participants served as paid volunteers. The task consisted in paying selective attention to a combination of spatial frequency and location and then responding to target stimuli while ignoring other combinations of features. Sensory-evoked components were analyzed by measuring mean amplitude values within the latency ranges of 60-80, 80-100, 100-120, and 120-140 msec. poststimulus. Stimuli relevant in frequency and/or location elicited larger evoked C1 responses than unattended stimuli as early as 60-80 msec. poststimulus, a range that likely corresponds to sensory activity in the striate cortex, although due to the small number of recording sites, the activity could not be precisely localized.