Effects of display and memory load on event-related potentials during a visual search task

We recorded event-related potentials (ERPs) in nine normal adult subjects to investigate the effects of display load (number of positions to be processed) and memory load (memory set size) on ERPs in visual search tasks. The stimulus consisted of a horizontal array of five different alphabets. In search task, subjects were required to respond only to stimuli containing a target letter. In a simple reaction task, they were required to respond to all the stimuli. The results showed that display load affected N200 and NA deflections recorded at occipital and posterior temporal electrodes, although memory load did not affect them. We also found the different effects of display load and memory load on search-related negativities. That is, in latency, search-related negativities with increasing display load appeared before those with memory load. The difference in topography between display and memory load effects on search-related negativities was not confirmed statistically. The validity of ERPs as indices for the visual and memory search processes was discussed.