Abstractn 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 reaction time (RT) and performance accuracy (hit rate) in a forcedchoice 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 randomly 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 signi cantly 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-frontal scalp maximum. The present results suggest the involvement of two different neural mechanisms in triggering involuntary attention to acoustic novelty and change: a transient-detector mechanism activated by novel sounds and re ected in the N1 and a stimulus-change detector mechanism activated by deviant tones and novel sounds and re ected 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 attention. n

The sequence of neurophysiological processes elicited in the auditory system by a sound is analyzed in search of the stage at which the processes carrying sensory information cross the borderline beyond which they directly underlie sound perception. Neurophysiological data suggest that this transition occurs when the sensory input is mapped onto the physiological basis of sensory memory in the auditory cortex. At this point, the sensory information carried by the stimulus-elicited process corresponds, for the first time, to that contained by the actual sound percept. Before this stage, the sensory stimulus code is fragmentary, lacks the time dimension, cannot enter conscious perception, and is not accessible to top-down processes (voluntary mental operations). On these grounds, 2 distinct stages of auditory sensory processing, prerepresentational and representational, can be distinguished.

This article reviews recent event-related brain potential (ERP) studies of involuntary attention and distractibility in response to novelty and change in the acoustic environment. These studies show that the mismatch negativity, N₁ and P_3a ERP components elicited by deviant or novel sounds in an unattended sequence of repetitive stimuli index different processes along the course to involuntary attention switch to distracting stimuli. These studies used new auditory-auditory and auditory-visual distraction paradigms, which enable one to assess objectively abnormal distractibility in several clinical patient groups, such as those suffering from closed-head injuries or chronic alcoholism.

The widely accepted “memory-mismatch” interpretation of the mismatch negativity (MMN) event-related brain potential (ERP) suggests that an MMN is elicited when an acoustic event deviates from a memory record describing the immediate history of the sound sequence. The first variant of the memory-mismatch theory suggested that the memory underlying MMN generation was a strong auditory sensory memory trace, which encoded the repetitive standard sound. This “trace-mismatch” explanation of MMN has been primarily based on results obtained in the auditory oddball paradigm. However, in recent years, MMN has been observed in stimulus paradigms containing no frequently repeating sound. We now suggest a different variant of the memory-mismatch interpretation of MMN in order to provide a unified explanation of all MMN phenomena. The regularity-violation explanation of MMN assumes that the memory records retaining the history of auditory stimulation are regularity representations. These representations encode rules extracted from the regular intersound relationships, which are mapped to the concrete sound sequence by finely detailed auditory sensory information. Auditory events are compared with temporally aligned predictions drawn from the regularity representations (predictive models) and the observable MMN response reflects a process updating the representations of those detected regularities whose prediction was mismatched by the acoustic input. It is further suggested that the auditory deviance detection system serves to organize sound in the brain: The predictive models maintained by the MMN-generating process provide the basis of temporal grouping, a crucial step in the formation of auditory objects.