Complex tone processing in primary auditory cortex of the awake monkey. II. Pitch versus critical band representation

Noninvasive neurophysiological studies in humans support the existence of a n orthogonal spatial representation of pure tone frequency and complex tone pitch in auditory cortex [Langner et al., J. Comp. Physiol. A 181, 665-676 (1997)]. However, since this topographic organization is based on neuromag netic responses evoked by wideband harmonic complexes (HCs) of variable fun damental frequency (f(0)), and thus interharmonic frequency separation (Del ta F), critical band filtering effects due to differential resolvability of harmonics may have contributed to shaping these responses. To test this hy pothesis, the present study examined responses evoked by three-component HC s of variable f(0) in primary auditory cortex (Al) of the awake monkey. The center frequency of the HCs was fixed at the best frequency (BF) of the co rtical site. Auditory evoked potential (AEP), multiunit activity, and curre nt source density techniques were used to evaluate Al responses as a functi on of f(0) (=Delta F). Generally, amplitudes of nearly all response compone nts increased with f(0), such that maximal responses were evoked by HCs com prised of low-order resolved harmonics. Statistically significant increases in response amplitude typically occurred at Delta Fs between 10% and 20% o f center frequency, suggestive of critical bandlike behavior. Complex tone response amplitudes also reflected nonlinear summation in that they could n ot be predicted by the pure tone frequency sensitivity curves of the cortic al sites. A mechanism accounting for the observed results is proposed which involves mutual lateral inhibitory interactions between responses evoked b y stimulus components lying within the same critical band. As intracortical AEP components likely to be propagated to the scalp were also strongly mod ulated by Delta F, these findings indicate that noninvasive recordings of r esponses to complex sounds may require a consideration of critical band eff ects in their interpretation. (C) 2000 Acoustical Society of America. [S000 1-4966(00)05407-2].