Electrophysiological evidence of dynamic ignoring

We used electroencephalographic (EEG) recordings to examine the processes that enable subjects to intentionally ignore transient, task-irrelevant visual stimuli, keeping such stimuli from interfering with short-term memory for similar, behaviorally relevant visual stimuli (Yotsumoto & Sekuler, in press). Subjects recognition memory was tested for brief, sequentially-presented compound sinusoidal gratings that varied in spatial frequency; in one condition, one of two study items was consistently task irrelevant. Event related potentials (ERPs) were derived from EEG signals synched either to the onset of the to-be-remembered item or to the onset of the to-be-ignored item. When these two ERPs were compared at varying times following stimulus onset and on varying locations on the scalp, we found that early on, the occipital cortex was indifferent to whether the stimulus was to be ignored or remembered. Differences between ignoring and remembering were found in the left frontal area at about 200 msec after stimulus onset, followed by the parietal area at 250 msec, and the occipital area at 300 msec. The early differences in frontal areas suggest top-down inhibitory control; the relatively late onset of occipital modulation suggests that intentional ignoring becomes effective well-after the stimulus becomes visible. Because the world affords far more visual information than can be processed effectively, visual processing must be selective, modulating incoming information, privileging what is behaviorally relevant over what is behaviorally irrelevant. To gain insight into 10 mechanisms that support one critical, but little studied form of visual selectivity we assessed subjects’ visual memory while they attempted to gate or exclude particular briefly-presented items (Tsotsos, 1997; Kastner & Ungerleider, 2000). Ordinarily, as subjects view and try to remember study items in a series, the presence of irrelevant, distractor items can interfere with memory. However, if subjects could completely exclude irrelevant items, visual mem15 ory performance would be unimpaired by the presentation of those distractors. Yotsumoto and Sekuler (in press) recently demonstrated that subjects can successfully ignore one of a Supported by NIH grant 5R01MH068404, NSERC Discovery Grants 42133 and 105494, the Canada Research Chair program, and the Canadian Institute for Health Research fellowship program.