Infants' brain responses to speech suggest analysis by synthesis

Abstract

Historic theories of speech perception (Motor Theory and Analysis by Synthesis) invoked listeners' knowledge of speech production to explain speech perception. Neuroimaging data show that adult listeners activate motor brain areas during speech perception. In two experiments using magnetoencephalography (MEG), we investigated motor brain activation, as well as auditory brain activation, during discrimination of native and nonnative syllables in infants at two ages that straddle the developmental transition from language-universal to language-specific speech perception. Adults are also tested in Exp. 1. MEG data revealed that 7-mo-old infants activate auditory (superior temporal) as well as motor brain areas (Broca's area, cerebellum) in response to speech, and equivalently for native and nonnative syllables. However, in 11- and 12-mo-old infants, native speech activates auditory brain areas to a greater degree than nonnative, whereas nonnative speech activates motor brain areas to a greater degree than native speech. This double dissociation in 11- to 12-mo-old infants matches the pattern of results obtained in adult listeners. Our infant data are consistent with Analysis by Synthesis: auditory analysis of speech is coupled with synthesis of the motor plans necessary to produce the speech signal. The findings have implications for: (i) perception-action theories of speech perception, (ii) the impact of "motherese" on early language learning, and (iii) the "social-gating" hypothesis and humans' development of social understanding.

Keywords: brain imaging; phonetic perception.

Fig. 1.

Mean MEG magnetic waveforms from the left and right ST (green dot) and IF (red dot) regions for 7-mo-old infants (A), 11-mo-old infants (B), and adults (C), for native English (Middle) and nonnative Spanish (Bottom) deviants.

Fig. 2.

The maximum peak magnitude of the MMR MNE-current normalized by the baseline noise magnitude. (A) Main effects of age and brain region: mean maximum peak magnitude recorded from 7-mo-old infants, 11-mo-old infants, and adults; mean maximum peak magnitude recorded from ST and IF regions. (B) Interaction effects: mean maximum peak magnitude for native English and nonnative Spanish recorded from ST (light gray) and IF (dark gray) regions from 7-mo-old infants, 11-mo-old infants, and adults. E, native English; S, nonnative Spanish.

Fig. 3.

Mean MMRs for native (red) and nonnative (blue) contrasts relative to prestimulus baseline, and for the native minus nonnative (green) for ST (A), Broca’s area (B), and the cerebellum (C). Waveforms for 7-mo-old and 12-mo-old infants are shown on the left and right, respectively. Significant temporal clusters are shown below waveforms, FWER-corrected, P < 0.05. Positive t values (red) indicate native > nonnative; negative t values (blue) the reverse. Red circles highlight the timing of significant differential activation for native and nonnative contrasts in each brain area.