Transient changes in neural discharge patterns may enhance the separation of concurrent vowels with different fundamental frequencies

Abstract

Two vowels presented concurrently are better identified by human listeners when their fundamentals (F0’s) differ. Neurophysiological correlates of this phenomenon were investigated using single fiber recordings from the auditory nerve of anesthetized cats. Results show that aggregate interspike interval distributions (the summed autocorrelation histograms of many fibers with different CF’s) contain sufficient information to characterize single vowels. When two concurrent vowels have the same F0, the resulting interval distributions resemble that of one ‘‘dominant’’ vowel constituent. When the two vowels have different F0’s, running interval distributions additionally show short time segments when they resemble the other, ‘‘nondominant’’ vowel. These short time segments and accompanying changes in interval distributions are associated with shifting relative onset times of vowel periods (i.e., changes in vowel period alignments) created by the different F0’s. Particular period alignments can enhance or diminish the similarity of the interval distribution with that of either vowel constituent. These results along with recent psychophysical findings [P. Assmann and Q. Summerfield, J. Acoust. Soc. Am. 95, 471–484 (1994)] suggest that differences in F0 may allow the auditory system to ‘‘hear out’’ each vowel in moments when period alignments are favorable. [Work supported by NIH DC00119.]