Cortical processing of the periodicity of speech sounds

Abstract

The periodicity of speech sounds which is produced by the vibration of the vocal folds, plays a significant role in speech communication. In the auditory system, sound periodicity is extracted along the neural pathway and is, according to several studies of the human brain, represented in the cortical level by a periodicity-specific neural population. Such a population could encode the periodicity of speech sounds. The evidence for cortical periodicity-sensitivity, however, rests mostly on measures of brain activity elicited by non-speech stimuli that differ from speech sounds with respect to their acoustic features and perceptual qualities. Thus, the generalizability of these results to natural speech communication may be limited. The work presented in this thesis investigated cortical processing of the periodicity of speech sounds by using controlled manipulations in the periodicity of vowel stimuli and by measuring brain activity elicited by these stimuli with magnetoencephalography. The results indicate larger amplitudes and more anterior source locations for the responses elicited by periodic as opposed to aperiodic vowel stimuli. While such an effect of periodicity was observed for a range of fundamental frequencies (F0), degrees of periodicity, and durations of the periodic vowel stimuli, the cortical periodicity-specific activity was also modulated by these parameters. Furthermore, evidence for aperiodicity-sensitive activity was found through stimulus-specific release from adaptation when aperiodic vowel stimuli were alternated with periodic rather than with aperiodic adaptors. The results of the thesis, thus, indicate that the degree of speech sound periodicity, determined by the vocal fold vibration, is represented in the auditory cortex. Such sensitivity to periodicity might reflect the activity of distinct neural populations that are selective to sound periodicity and aperiodicity. Importantly, this view of distinct feature-selective populations can, based on the current results, be generalized to describe the neural mechanisms of speech perception. The dependency of the observed periodicity-sensitivity on the acoustic features of the vowel stimuli, further, appears to reflect cortical encoding of auditory-perceptual aspects of voice quality