Neural mechanisms of involuntary attention to acoustic novelty and change

n Behavioral and event-related brain potential (ERP) measures were used to elucidate the neural mechanisms of involuntary engagement of attention by novelty and change in the acoustic environment. The behavioral measures consisted of the reac-tion time (RT) and performance accuracy (hit rate) in a forced-choice visual RT task where subjects were to discriminate between odd and even numbers. Each visual stimulus was preceded by an irrelevant auditory stimulus, which was ran-domly either a “standard ” tone (80%), a slightly, higher “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 turn, decreased the hit rate but did not signicantly 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 sounds 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 with a right-fron-tal scalp maximum. The present results suggest the involve-ment of two different neural mechanisms in triggering involuntary attention to acoustic novelty and change: a tran-sient-detector mechanism activated by novel sounds and reected in the N1 and a stimulus-change detector mechanism activated by deviant tones and novel sounds and reected in the MMN. The observed differential distracting effects by slightly deviant tones and widely deviant novel sounds support the notion of two separate mechanisms of involuntary atten-tion.