Automatic phonetic category processing in premotor cortex is functionally coupled with the dorsal auditory stream in human
Understanding how the human brain is able to extract phonetic information from acoustics of speech, given that it is highly variable due to a number of factors including phonetic context (e.g., in English “p” in “port” and “sport” are acoustically quite different) and voice characteristics of different speakers, has inspired both empirical and theoretical work. In the so-called auditory models, speech perception is accomplished solely based on analysis of the auditory information. In contrast, speech motor theory postulates that auditory speech inputs are mapped to motor schemas that form the basis for speech perception. While neuroimaging studies have provided support for both models, the precise role played by the speech motor system / dorsal processing stream (that involves auditory cortical structures posterior to the primary auditory cortex, inferior parietal and frontal cortical areas, as well the (pre)motor cortex) in speech perception has remained an open question.
In their recent study, Dr. Mark Chevillet et al. (2013) examined, in healthy volunteers using a specific functional magnetic resonance imaging adaptation paradigm, whether there is phonetic category selectivity in human premotor cortex and, further, using structural equation modeling, the authors sought answer to the question of whether the premotor cortex is functionally more coupled with anterior or posterior auditory cortical areas. Using pairs of morphed stimuli that varied equally in terms of acoustic distance but either belonged the same vs. different phonetic category, the authors indeed managed to see phonetic categorization taking place in premotor cortex that correlated with behavioral categorization performance. Further, as revealed by structural equation modeling, the premotor cortex was functionally coupled with posterior rather than anterior auditory cortical areas.
These findings are highly important in shedding light on the role of human premotor cortex in processing of phonetic information and, further, add to the growing pool of evidence linking the dorsal auditory processing stream with motor-schemata based speech perception. The authors elegantly utilize an adaptation functional neuroimaging paradigm combined with carefully designed continuums of auditory phonetic stimuli that retain constant acoustic distance with vs. without crossing of the phonetic category boundary. The structural equation modeling further provides important information about functional connectivity between the premotor cortex and auditory cortical areas. Since the subjects were actively engaged in a task other than phonetic categorization suggests that phonetic categorization that takes place in premotor cortical areas is highly automatic in nature, which further augments the importance of the findings.
Reference: Chevillet MA, Jiang X, Rauschecker JP, Riesenhuber M. Automatic phoneme category selectivity in the dorsal auditory stream. Journal of Neuroscience (2013) 33: 5208–5215. http://dx.doi.org/10.1523/JNEUROSCI.1870-12.2013