Abstract
According to a traditional view, speech perception and production are processed largely separately in sensory and motor brain areas. Recent psycholinguistic and neuroimaging studies provide novel evidence that the sensory and motor systems dynamically interact in speech processing, by demonstrating that speech perception and imitation share regional brain activations. However, the exact nature and mechanisms of these sensorimotor interactions are not completely understood yet. Transcranial magnetic stimulation (TMS) has often been used in the cognitive neurosciences, including speech research, as a complementary technique to behavioral and neuroimaging studies. Here we provide an up-to-date review focusing on TMS studies that explored speech perception and imitation. Single-pulse TMS of the primary motor cortex (M1) demonstrated a speech specific and somatotopically specific increase of excitability of the M1 lip area during speech perception (listening to speech or lip reading). A paired-coil TMS approach showed increases in effective connectivity from brain regions that are involved in speech processing to the M1 lip area when listening to speech. TMS in virtual lesion mode applied to speech processing areas modulated performance of phonological recognition and imitation of perceived speech. In summary, TMS is an innovative tool to investigate processing of speech perception and imitation. TMS studies have provided strong evidence that the sensory system is critically involved in mapping sensory input onto motor output and that the motor system plays an important role in speech perception.
Highlights
Previous lesion studies provided evidence that brain areas of language production and perception are separate and independent; speech production links to the motor system of the inferior frontal area while speech perception is processed in the sensory system of the posterior temporal area in the left hemisphere (Hayward et al, 1977; Mazzocchi and Vignolo, 1979; Cappa and Vignolo, 1983; Kertesz, 1993)
Recent speech models have claimed that the sensory system is involved in speech production and that the motor system is activated in speech perception, suggesting that the sensory and motor systems dynamically interact in speech processing (Hickok and Poeppel, 2004, 2007; Rauschecker and Scott, 2009; Pulvermuller and Fadiga, 2010; Hickok et al, 2011)
We investigated to what extent changes in motor evoked potential (MEP) amplitudes are graded with speech perception difficulty by providing subjects, while they were viewing speech-related lip movements, with variable speeds of lip movements, or while they were listening to speech, with different levels of contamination by background noise
Summary
Previous lesion studies provided evidence that brain areas of language production and perception are separate and independent; speech production links to the motor system of the inferior frontal area while speech perception is processed in the sensory system of the posterior temporal area in the left hemisphere (Hayward et al, 1977; Mazzocchi and Vignolo, 1979; Cappa and Vignolo, 1983; Kertesz, 1993). Watkins et al (2003) reported that the MEP amplitude from OO increased while there was no notable change in the MEP amplitude from a hand muscle, the first dorsal interosseous (FDI) muscle, when listening to speech and viewing speech related lip movements, supporting the notion that observation-execution matching processes link speech-related visual and auditory input in a somatotopically specific fashion to effector representation in M1 (Watkins et al, 2003).
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