Optical Brain Imaging Reveals General Auditory and Language-Specific Processing in Early Infant Development

Abstract

This study uses near-infrared spectroscopy in young infants in order to elucidate the nature of functional cerebral processing for speech. Previous imaging studies of infants' speech perception revealed left-lateralized responses to native language. However, it is unclear if these activations were due to language per se rather than to some low-level acoustic correlate of spoken language. Here we compare native (L1) and non-native (L2) languages with 3 different nonspeech conditions including emotional voices, monkey calls, and phase scrambled sounds that provide more stringent controls. Hemodynamic responses to these stimuli were measured in the temporal areas of Japanese 4 month-olds. The results show clear left-lateralized responses to speech, prominently to L1, as opposed to various activation patterns in the nonspeech conditions. Furthermore, implementing a new analysis method designed for infants, we discovered a slower hemodynamic time course in awake infants. Our results are largely explained by signal-driven auditory processing. However, stronger activations to L1 than to L2 indicate a language-specific neural factor that modulates these responses. This study is the first to discover a significantly higher sensitivity to L1 in 4 month-olds and reveals a neural precursor of the functional specialization for the higher cognitive network.

Espectroscopia de luz próxima ao infravermelho e processamento sensorial auditivo em lactentes

“…Although the auditory cortex decodes fast acoustic modulations, relevant for the phonemic decoding within the speech flow in a bilateral way, the phonemic contrasts preferentially activate the left hemisphere of the temporal region in babies. The prosodic contrasts predominantly activated the right temporal region 10,15,21,22 . Reinforcing the role of this region for the sensory processing of emotional speech signals.…”

“…Reinforcing the role of this region for the sensory processing of emotional speech signals. This mechanism is also essential as the prosodic organization of speech facilitates the acquisition of language by children 10,15,19 .…”

“…The studies that used vocal sounds and other stimuli have investigated brain hemodynamic responses to evaluate the perceptual processing during the exposure to audiovisual stimuli 18 ; brain lateralization for speech sounds and non-vocal sounds (native and non-native speech, human onomatopoeia and monkey sounds) 10 ; processing of vocal and prosodic specificity 19 ; ability of the newborn to memorize words 6 ; processing for linguistic and non-linguistic sounds in bilingual and monolingual children 20 . These studies showed important results, including the usefulness of the NIRS technology as a tool to monitor the hemodynamic activity in children, an activity that was found in greater proportion in the left temporal region in response to audiovisual stimuli compared only with visual stimulus 18 ; greater lateralization in the left temporal region for the speech processing in comparison to non-vocal sounds 10 .…”

“…These studies showed important results, including the usefulness of the NIRS technology as a tool to monitor the hemodynamic activity in children, an activity that was found in greater proportion in the left temporal region in response to audiovisual stimuli compared only with visual stimulus 18 ; greater lateralization in the left temporal region for the speech processing in comparison to non-vocal sounds 10 . They also show increased hemodynamic responses to human voice in the right and left temporal cortex when compared to non-vocal sounds in infants aged 07 months, and greater activation in the right temporal region in response to vocal stimuli modulated by emotion 19 .…”

“…It has a good spatial resolution and better temporal resolution compared with Functional Magnetic Resonance Imaging. It has been successfully employed in the presentation of auditory stimuli for investigation of the auditory skills in newborns and young infants 2,[7][8][9][10] . Normally the coupling of the optodes in the head, reduction of artifacts in the signal and increase of penetration of light in the cortex is better in the child population than in the adult population 5 due to their thinner hair, skin and skull.…”