Neural correlates of auditory stream segregation in primary auditory cortex of the awake monkey

An important feature of auditory scene analysis is the perceptual organizat ion of sequential sound components, or 'auditory stream segregation'. Audit ory stream segregation can be demonstrated by presenting a sequence of high and low frequency tones in an alternating pattern, ABAB. When the tone pre sentation rate (PR) is slow or the frequency separation (DeltaF) between th e tents is small(< 10%), a connected alternating sequence ABAB is perceived . When the PR is fast or the <Delta>F is large, however, the alternating se quence perceptually splits into two parallel auditory streams, one composed of interrupted 'A' tones, and the other of interrupted 'B' tones. The neur ophysiological basis of this perceptual phenomenon is unknown. Neural corre lates of auditory stream segregation were examined in Al of the awake monke y using neuronal ensemble techniques (multiunit activity and current source density). Responses evoked by alternating frequency sequences of tones, AB AB, were studied as a function of PR (5, 10, 20 and 40 Hz). 'A' tones corre sponded to the best frequency (BF) of the cortical site, while 'B' tones we re situated away from the BF by an amount DeltaF. At slow PRs, 'A' and 'B' tones evoked responses that generated an overall pattern of activity at the stimulus PR. In contrast, at fast PRs,'B' tone responses were differential ly suppressed, resulting in a pattern of activity consisting predominantly of 'A' tone responses at half the PR. The magnitude of 'B' tone response su ppression increased with DeltaF. Differential suppression of BF and non-BF tone responses at high PRs can be explained by physiological principles of forward masking. The effect of DeltaF is explained by the hypothesis that r esponses to tones distant from the BF are more susceptible to suppression b y BF tones than responses to tones near the BF. These results parallel huma n psychoacoustics of auditory stream segregation and suggest a cortical bas is for the perceptual phenomenon. (C) 2001 Elsevier Science B.V. All rights reserved.