Effects of rare non-target stimuli on brain electrophysiological activity and performance.

In order to assess the effects of non-target stimuli on task performance and electrophysiological activity, 16 subjects performed reaction time (RT) experiments under 3 conditions. In all conditions, subjects had to press a button upon detection of rare (15%) target stimuli (1600 Hz) presented among frequent (85%) non-target stimuli. The 3 conditions differed based on their non-target stimuli. In one condition, the non-targets consisted of 'standard' stimuli (900 Hz). In the two other conditions, rare and deviant non-target stimuli were randomly added to the standard stimuli. These deviant non-target stimuli consisted of either constant (700-Hz tones) or novel (buzzes, filtered noises and other unusual sounds) stimuli. Both the rare target and non-target stimuli elicited P300, responses. Behavioral (RT) and electrophysiological (event-related potential) data showed that stimuli that followed standard stimuli were processed differently compared to stimuli that followed deviant non-target stimuli. In the conditions containing deviant non-target stimuli, the P3b to the target stimuli was smaller and later, and the mean RT longer than in the condition with no deviant stimuli. These behavioral and electrophysiological changes induced by the deviant non-target stimuli were discussed with reference to two factors, distraction and increased level of task difficulty. It was suggested that each of these factors were differentially sensitive to the novelty of the rare deviant stimuli.