Individual differences in premotor and motor recruitment during speech perception.

Although activity in premotor and motor cortices is commonly observed in neuroimaging studies of spoken language processing, the degree to which this activity is an obligatory part of everyday speech comprehension remains unclear. We hypothesised that rather than being a unitary phenomenon, the neural response to speech perception in motor regions would differ across listeners as a function of individual cognitive ability. To examine this possibility, we used functional magnetic resonance imaging (fMRI) to investigate the neural processes supporting speech perception by comparing active listening to pseudowords with matched tasks that involved reading aloud or repetition, all compared to acoustically matched control stimuli and matched baseline tasks. At a whole-brain level there was no evidence for recruitment of regions in premotor or motor cortex during speech perception. A focused region of interest analysis similarly failed to identify significant effects, although a subset of regions approached significance, with notable variability across participants. We then used performance on a battery of behavioural tests that assessed meta-phonological and verbal short-term memory abilities to investigate the reasons for this variability, and found that individual differences in particular in low phonotactic probability pseudoword repetition predicted participants' neural activation within regions in premotor and motor cortices during speech perception. We conclude that normal listeners vary in the degree to which they recruit premotor and motor cortex as a function of short-term memory ability. This is consistent with a resource-allocation approach in which recruitment of the dorsal speech processing pathway depends on both individual abilities and specific task demands.