Influence of early experience of speech and language on their processing and neural representation: A study of hearing infants with deaf mothers

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This project aims to determine the impact of early speech and language experience on language processing and representation in the brain. I will study hearing infants of deaf mothers who use a sign language as their dominant language (HoD infants) in comparison to hearing infants of hearing mothers (HoH infants). Despite normal hearing, HoD infants have a different early experience of speech and language to that of HoH infants. First, since deaf signing mothers are less likely to use auditory speech than hearing mothers, their infants are likely to have reduced exposure to auditory spoken language. Second, the experience of HoD infants includes both a language in the visual modality, e.g. British Sign Language (BSL), and one in the auditory modality, e.g. English (used by the mother, as well as by hearing relatives and the rest of the hearing community). Since HoD infants are exposed to two languages, they will be compared to HoH individuals who are exposed to two spoken languages. I hypothesise that the experience of HoD infants will influence their neural representation of language, their patterns of eye gaze during spoken and signed language perception and their audio-visual speech integration. Three groups of hearing infants will be recruited: 30 infants from a monolingual English speaking environment, 30 infants from a bilingual environment in which two spoken languages are frequently used by the mother, 30 infants with a deaf mother for whom BSL is the dominant language. Experiment 1- Neural representation of spoken language: I will use near infrared spectroscopy (NIRS) to study brain activation during spoken language perception. In infancy, spoken language activates similar areas of the left temporal hemisphere as in adulthood. It has been hypothesised that this pattern develops prenatally while receiving the input of the mother's spoken language. I will test this hypothesis by studying HoD infants with reduced early experience of auditory spoken language. Auditory sentences will be presented in English and an unfamiliar language. I predict that reduced early experience of auditory spoken language leads to a reduction in language activation in the temporal lobe.Experiment 2- Neural representation of signed language: I will use NIRS to study brain activation while infants are presented with sentences in BSL and an unfamiliar signed language. Adult neuroimaging studies demonstrate that signed language is processed in a similar brain network as spoken language, but this network has never been studied in infancy. I hypothesise that HoD infants will show increased brain activity in the temporal cortex for BSL compared to unfamiliar gestures. I will compare any preferential activation for BSL with that for a familiar spoken language in Experiment 1.Experiment 3- Signed and spoken language perception: I will use eye tracking to study infants' eye gaze while watching sentences presented either in audio-visual English or BSL. Unlike spoken languages, signed languages require visual attention to the speaker, and adult native signers focus on the signer's face and not their hands while perceiving signed language. I hypothesise that HoD infants will spend more time than HoH infants looking at the speaker's face during signed and spoken language perception. Experiment 4- Audio-visual speech integration: I will use eye tracking and an established audio-visual speech perception paradigm to compare how HoD and HoH infants use visual cues of articulation to discriminate speech sounds. Infants as young as two months can integrate the audio-visual aspects of speech. HoD adults outperform HoH adults in lip-reading, suggesting that they extract more information from visual cues of articulation. I hypothesise that HoD infants will rely more on the visual cues of articulation in speech perception.Together, the results of this study will shed light on the role of early experience in shaping brain development and language learning.