Abstract
In the neuroscience of language, phonemes are frequently described as multimodal units whose neuronal representations are distributed across perisylvian cortical regions, including auditory and sensorimotor areas. A different position views phonemes primarily as acoustic entities with posterior temporal localization, which are functionally independent from frontoparietal articulatory programs. To address this current controversy, we here discuss experimental results from functional magnetic resonance imaging (fMRI) as well as transcranial magnetic stimulation (TMS) studies. On first glance, a mixed picture emerges, with earlier research documenting neurofunctional distinctions between phonemes in both temporal and frontoparietal sensorimotor systems, but some recent work seemingly failing to replicate the latter. Detailed analysis of methodological differences between studies reveals that the way experiments are set up explains whether sensorimotor cortex maps phonological information during speech perception or not. In particular, acoustic noise during the experiment and ‘motor noise’ caused by button press tasks work against the frontoparietal manifestation of phonemes. We highlight recent studies using sparse imaging and passive speech perception tasks along with multivariate pattern analysis (MVPA) and especially representational similarity analysis (RSA), which succeeded in separating acoustic-phonological from general-acoustic processes and in mapping specific phonological information on temporal and frontoparietal regions. The question about a causal role of sensorimotor cortex on speech perception and understanding is addressed by reviewing recent TMS studies. We conclude that frontoparietal cortices, including ventral motor and somatosensory areas, reflect phonological information during speech perception and exert a causal influence on language understanding.
Highlights
Establishing links between the human ability to speak and understand language and the underlying neuronal machinery of the human brain is a key to modern cognitive neuroscience
Contrasting with the crossmodal links suggested by biological and motor theories, a classic position in the neuroscience of language had been that speech motor and speech perception networks are relatively independent from each other (Wernicke, 1874; Lichtheim, 1885), a position inherited by more recent approaches
Even in the absence of any task, Möttönen et al (2013) found that an attention-independent neurophysiological index of speech sound processing known as the mismatch negativity or MMN (Näätänen et al, 1997), was reduced following transcranial magnetic stimulation (TMS) to lip motor cortex
Summary
Establishing links between the human ability to speak and understand language and the underlying neuronal machinery of the human brain is a key to modern cognitive neuroscience. Auditory areas are strongly connected by way of adjacent inferior frontal and superior temporal areas, so that (c) well-established Hebbian learning implies that auditory-motor neurons activated together during phoneme production will be bound together into one distributed neuronal ensemble (Pulvermüller and Fadiga, 2010) In this action-perception integration perspective, speech sounds with different places of articulation have their cortical correlates in different activation topographies across superiortemporal and fronto-parietal areas, including the articulatory sensorimotor cortex. Comparing studies against each other shows that the crucial methodological factors which predict acoustically induced phonological activation of, and information in, fronto-parietal areas are: (i) the use of ‘‘silent gap’’, or ‘‘sparse’’ imaging (Hall et al, 1999; Peelle et al, 2010) and (ii) the absence of a requirement to perform button presses during the experiments Both of these features are amongst those that distinguished Arsenault and Buchsbaum (2016) from Pulvermüller et al (2006). IFG (decoding of right inferior postcentral gyrus, place/manner) precentral gyrus
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