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The current study assessed the normal development of cortical auditory evoked potentials (CAEPs) in humans presented with pure tone stimuli at relatively fast stimulus rates. Traditionally, maturation of sound processing indexed by CAEPs has been studied in paradigms using inter-stimulus intervals (ISIs) generally slower than 1 Hz. While long ISIs may enhance the amplitude of CAEP components, speech information generally occurs at more rapid rates. These slower rates of sound presentation may not accurately assess auditory cortical functions in more realistic sound environments. We examined the effect of temporal rate on the elicitation of the P1-N1-P2-N2 components to unattended sounds at four levels of stimulus onset asynchrony (SOA, onset to onset, 200, 400, 600, and 800 ms) in children grouped separately by year (ages 8, 9, 10, 11 years), in adolescents (age 16 years) and in one group of young adults (ages 22-40 years). We found that both age and stimulus rate produced profound changes in CAEP morphology. Between the ages of 8-11 years, the P1 and N2 components dominated the ERP waveform at all stimulus rates. N1, the dominant CAEP component in adults, appeared as a bifurcation in a broad positive peak at earlier ages, and did not emerge as a separate component until adolescence. While the P1-N1-P2 components are more "adult-like" than "child-like" in the adolescent subjects, the N2 component, a hallmark of the child obligatory response, was still present. Faster rates resulted in the suppression of discrete components such that by 200 ms, only P1 in the adults and adolescents, and both P1 and N2 in the youngest children were discernable. We conclude that both age and ISI are important variables in the assessment of auditory cortex function and maturation. The presence of N2 in adolescents indicates that auditory cortical maturation persists into teen years.

We tested the effects of predictability on involuntary attention switching to task-irrelevant sound changes (distraction). Behavioral and neurophysiological evidence are provided, showing that the predictability of task-irrelevantsound changes eliminates effects of distraction even though the automatic auditory change detection system remains responsive. Two indices of distraction, slower task performance and cortical brain responses associatedwith attention switching, were seen only in the unpredictable condition, in which the irrelevant acoustic changes were unexpected. Attention was not involuntarily drawn away from the primary task when the subjects had foreknowledge of when the irrelevant changes would occur. These results demonstrate attentional control over orienting to sound changes and suggest that involuntary attention switching occurs mainly when an irrelevant stimulus change is unexpected. The present data allowed observation of the temporal dynamics of attention switching in the human brain.

There is uncertainty concerning the extent to which the auditory streaming effect is a function of attentive or preattentive mechanisms. The mismatch negativity~MMN!, which indexes preattentive acoustic processing, was used to probe whether the segregation associated with the streaming effect occurs preattentively. In Experiment 1, alternating high and low tones were presented at fast and slow paces while subjects ignored the stimuli. At the slow pace, tones were heard as alternating high and low pitches, and no MMN was elicited. At the fast pace a streaming effect was induced and an MMN was observed for the low stream, indicating a preattentive locus for the streaming effect. The high deviant did not elicit an MMN. MMNs were obtained to both the high and low deviants when the interval between the across-stream deviance was lengthened to more than 250 ms in Experiment 2, indicating that the MMN system is susceptible to processing constraints.Descriptors: Auditory stream segregation, Mismatch negativity, Event-related potentials, Streaming effect, Auditory sensory memory Multiple sources of acoustic energy can impinge on the ear constantly in our everyday experience. It is not uncommon for the sounds of voices and office equipment or the ringing of telephones to occur simultaneously. The task of the brain is to tease apart the cacophony of these sounds, forming meaningful representations of the incoming acoustic information. This process requires a mechanism for segregating the inputs into their original sources. A now classic example, the cocktail party phenomenon, illustrates the ease with which the brain is able to perform this task. Amid the steady din of party sounds~e.g., tinkling of glasses, multiple conversations, music!, the brain keeps the sources distinct. Auditory cues such as the location of the sound or the pitch of a speaker's voice help this process of segregating the total stream of sound, which has been called auditory stream segregation~Bregman, 1990!.When tones of a sufficient frequency separation are alternated continuously at a fast enough rate for a period of time, a streaming effect occurs~Bregman, 1978, 1990Bregman & Campbell, 1971!. The streaming effect, an aspect of stream segregation, is a perceptual phenomenon governed by both the rate of stimulation and the frequency relationship of a tonal sequence. The perception is that the sets of high and low tones split into separate streams of sound, one formed of the high tones and one formed of the low tones. It sounds somewhat like counterpoint in music, as though the two streams are occurring independently and simultaneously. The purpose of this perceptual segregation is presumably to sort the tones in terms of sound sources, thereby improving the ability to perceive patterns within them. Natural differences of acoustic properties emanating from different sound sources are often reflected in pitch~e.g., the voice of a man vs. the voice of a woman!. Therefore, within a mixture of sounds striking the ear, it is likely that sounds in a high fr...

The perceptual world of neonates is usually regarded as not yet being fully organized in terms of objects in the same way as it is for adults. Using a recently developed method based on electric brain responses, we found that, similarly to adults, newborn infants segregate concurrent streams of sound, allowing them to organize the auditory input according to the existing sound source. The segregation of concurrent sound streams is a crucial step in the path leading to the identification of objects in the environment. Its presence in newborn infants shows that the basic abilities required for the development of conceptual objects are available already at the time of birth.

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There is controversy over whether stream segregation is an attention-dependent process. Part of the argument is related to the initial formation of auditory streams. It has been suggested that attention is needed only to form the streams, but not to maintain them once they have been segregated. The question of whether covert attention at the beginning of a to-be-ignored set of sounds will be enough to initiate the segregation process remains open. Here, we investigate this question by (1) using a methodology that does not require the participant to make an overt response to assess how the unattended sounds are organized and (2) structuring the test sound sequence to account for the covert attention explanation. The results of four experiments provide evidence to support the view that attention is not always required for the formation of auditory streams.

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