The Role of the Right Hemisphere in Processing Phonetic Variability Between Talkers

Abstract

Neurobiological models of speech perception posit that both left and right posterior temporal brain regions are involved in the early auditory analysis of speech sounds. However, frank deficits in speech perception are not readily observed in individuals with right hemisphere damage. Instead, damage to the right hemisphere is often associated with impairments in vocal identity processing. Herein lies an apparent paradox: The mapping between acoustics and speech sound categories can vary substantially across talkers, so why might right hemisphere damage selectively impair vocal identity processing without obvious effects on speech perception? In this review, I attempt to clarify the role of the right hemisphere in speech perception through a careful consideration of its role in processing vocal identity. I review evidence showing that right posterior superior temporal, right anterior superior temporal, and right inferior / middle frontal regions all play distinct roles in vocal identity processing. In considering the implications of these findings for neurobiological accounts of speech perception, I argue that the recruitment of right posterior superior temporal cortex during speech perception may specifically reflect the process of conditioning phonetic identity on talker information. I suggest that the relative lack of involvement of other right hemisphere regions in speech perception may be because speech perception does not necessarily place a high burden on talker processing systems, and I argue that the extant literature hints at potential subclinical impairments in the speech perception abilities of individuals with right hemisphere damage.

Keywords: functional neuroimaging; phonetic variability; right hemisphere; speech perception; talker identity; vocal identity.

Figure 1.

Vocal identity processing is supported by a right-lateralized system involving the posterior superior temporal cortex (green), the anterior superior temporal cortex (blue), and the inferior/middle frontal cortex (pink). The right superior temporal cortex has been implicated in mapping vocal acoustic information to a person’s identity, with posterior regions underlying the early sensory analysis of voices and more anterior regions supporting vocal identity recognition. Left temporal regions (not shown) may contribute to vocal identity processing, with their involvement potentially depending on the familiarity of the voice being processed. Right inferior and middle frontal regions play a role during the categorization of vocal stimuli into task-relevant categories, as well as when listeners must compare a target voice to a vocal sample in working memory, respectively.

Figure 2.

Speech perception involves interactions between left posterior temporal regions implicated in phonetic processing and right posterior temporal regions associated with the perceptual analysis of vocal information. These interactions may specifically reflect the process of conditioning phonetic identity on talker information. In this figure, posterior temporal regions are depicted by green circles with the label “PT,” and the established functional connection between them is indicated via a solid black line. However, the literature on speech perception does not suggest a strong role for other regions involved in vocal identity processing—namely, right anterior temporal cortex (blue circle labeled “AT”) and right inferior/middle frontal cortex (pink circle labeled “F”). It may be the case that these other right hemisphere regions only interact with left posterior temporal cortex (dashed gray lines) when demands on talker processing are high.