Speech-specific brain regions?

Article cited: Dick, F., Lee, H.L., Nusbaum, H., & Price, C.J. (2010). Auditory-Motor Expertise Alters “Speech Selectivity” in Professional Musicians and Actors. Cerebral Cortex.

There is a long-standing debate in the literature regarding how ‘specialised’ the brain is for speech processing. This is of particular relevance in my field where over the past few years I have seen growing comparisons made between the processing of speech and music in the brain. The most notable work for me being Ani Patel’s  Shared Syntactic Integration Resource Hypothesis (SSIRH) .

This week I heard about the publication of a new related paper that looks directly at brain activation (fMRI) during language and music listening tasks, and interestingly looks at the differences between professional violinists and actors.  The aim of the study was to investigate the role of expertise in tuning previously identified “speech-selective regions” of the brain.

This work had the potential to disambiguate whether speech-selective regions in the brain are driven by general ‘acoustic expertise’ or whether they are indeed specific to speech content in any way. The following hypotheses are possible:

1)      If there are general acoustic specialisation regions then presumably they should be active in both groups of participants for speech but show more activation for violinists while listening to violin music.

2)      If, however, there are speech – specific regions then they should show more activation when listening to speech compared to violin music in both groups.

3)      A third intriguing possibility is the idea of locating what the authors’ term the effects of “performing arts training” – where violinists show more activation to hearing music compared to speech and actors show more activation to hearing speech compared to music.

They tested 15 violinists and 15 actors who were all currently active professionals. Participants listened to short (10s) excerpts from the violin literature and from dramatic monologues for female characters.  They manipulated the familiarity of the music for both groups of stimuli. There were also scrambled versions of the violin and speech segments that were used as control stimuli.

The authors studied closely the fMRI data and concluded that they had evidence to parcellate so called “speech-selective regions” of the brain into those related to auditory expertise and those related to acoustic/information content. They also differentiated activation that was selective to the sounds that the performers had been trained to produce. So there is a little bit of a result for everyone! None of the effects appeared to be driven by familiarity with the music in question.

Region 1: Speech selective regions also activated by music sounds in violinists. These regions are associated with expertise in fine grained auditory analysis, speech categorisation, audiomotor integration and auditory sequence processing. (Right superior temporal cortex mostly (posterior STS), but also 3 smaller regions in the Left STC).

Region 2: Selective for speech above music in all participants. (Middle and anterior superior temporal sulcus, superior temporal gyrus and middle temporal gyrus). The authors are keen to point out here that this result still doesn’t imply the existence of a “speech-specific region”, as they don’t know whether or not it could be activated by other types of auditory stimuli.

Region 3: Selective to sounds in which the performers had expertise. (Primarily in motor control regions such as dorsal premotor regions and right cerebellum)

In conclusion it seems that this study has gone some way towards breaking down some of the rigidity that comes with assigning brain regions specific processing capacities that are associated with the stimuli we encounter in the modern world (like speech).  A friend of mine used to refer to this practice as “the dark side of postcode neuroscience!”

The authors themselves state that ‘…such results should make us wary of ascribing innate response preferences to certain brain regions that show consistent “category-specific” response preferences in most individuals, in that these may simply be the result of experience and task demands’. Well put. 🙂