Neural mechanisms of involuntary attention to acoustic novelty and change

Behavioral and event-related brain potential (ERP) measures were used to el ucidate the neural mechanisms of involuntary engagement of attention by nov elty and change in the acoustic environment. The behavioral measures consis ted of the reaction time (RT) and performance accuracy (hit rate) in a forc ed-choice visual RT task where subjects were to discriminate between odd an d even numbers. Each visual stimulus was preceded by an irrelevant auditory stimulus, which was randomly either a "standard" tone (80%), a slightly, h igher "deviant" tone (10%), or a natural, "novel" sound (10%). Novel sounds prolonged the RT to successive visual stimuli by 17 msec as compared with the RT to visual stimuli that followed standard tones. Deviant tones, in tu rn, decreased the hit rate but did not significantly affect the RT. In the ERPs to deviant tones, the mismatch negativity (MMN), peaking at 150 msec, and a second negativity, peaking at 400 msec, could be observed. Novel soun ds elicited an enhanced N1, with a probable overlap by the MMN, and a large positive P3a response with two different subcomponents: an early centrally dominant P3a, peaking at 230 msec, and a late P3a, peaking at 315 msec wit h a right-frontal scalp maximum. The present results suggest the involvemen t of two different neural mechanisms in triggering involuntary attention to acoustic novelty and change: a transient-detector mechanism activated by n ovel sounds and reflected in the N1 and a stimulus-change detector mechanis m activated by deviant tones and novel sounds and reflected in the MMN. The observed differential distracting effects by slightly deviant tones and wi dely deviant novel sounds support the notion of two separate mechanisms of involuntary attention.