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    • The auditory social versus non

    2018-10-25

    The auditory social versus non-social contrast, which was used at all five age points from 0 to 24 months of age, revealed evidence of a developmental change in specialisation for auditory social stimuli across infancy. Whilst the 0–2 (Cohort 1) and 4–8 (Cohort 2) month olds evidenced significant HbO2 auditory non-social selectivity (non-social>social) in a region of trpv1 localized over the pSTS-TPJ, Cohort 2 at 9–13 and 12–16 months of age and the 18–24 month olds (Cohort 3) evidenced significant HbO2 auditory social selectivity (social>non-social) in a region of channels localized over the anterior temporal cortex. Furthermore, the group averaged haemodynamic responses of the infants from 9 to 24 months of age to the social and non-social auditory stimuli are remarkably similar, despite the contributed data being derived from different combinations of infants at each age point (within the longitudinal cohort it was dependent on those available for follow up, and who then contributed valid data, and Cohort 3 compromised a cross-sectional sample). In contrast the averaged haemodynamic responses of the 4–8 month olds (Cohort 2) suggest that the lack of social selectivity and evident non-social selectivity may be due to a delayed response to the social stimuli. The non-social selectivity is only apparent during the earlier time epoch and the response to both stimuli becomes equivalent at a later time point during stimulus presentation. Although speech perception in newborn infants is well described (Dehaene-Lambertz and Spelke, 2015; Gervain et al., 2008; Pena et al., 2003; Vannasing et al., 2016), responses to human vocalisations are less well known. To allow us to compare these responses with a UK cohort, in collaboration with colleagues in Cambridge, we were able to use the auditory paradigm with newborn infants at the local maternity hospital. The findings from the UK cohort of 1–4day old newborns were consistent with the current findings (Chuen Wai Lee, Topun Austin et al. unpublished results). Widespread non-social responses (non-social>silence) were found, whilst an isolated region of the anterior temporal lobe responded to social sounds (social>silence). Furthermore a significant non-social>social selective response was localized within the pSTS-TPJ region of the cortex in these newborns. These findings are also in line with other recent research (Cristia et al., 2014) suggesting auditory responses to such cues are less specialized at this age. This comparison with previous research at 0–2 months is also important, as whilst in these other studies all infants were asleep, in the current study some of our participants in Cohort 1 were in a quiet alert state. Therefore it is important to see that the responses were largely replicated across the Gambian and UK infants despite these differences in their state of alertness at time of test. Previous research with 4–7 month olds would suggest that auditory social>non-social selectivity emerges from the first months of life becoming more robust in the second half of the first year of life (Grossmann et al., 2010; Lloyd-Fox et al., 2012). Grossmann and colleagues only found social selectivity once the infants reached 7 months of age, and Lloyd-Fox and colleagues reported the strength of the response across 4–7 months of age correlated with age of infant. In the current work, we did find evidence of a social>non-social response at this age point but only within the HHb response. Interestingly, this was also the case for one of our previous studies in UK infants (Lloyd-Fox et al., 2012), whilst in other infants we have found a significant HbO2 selective response (Lloyd-Fox et al., 2013). Collectively, this research supports the view that this age range marks a shift in specialisation to social over non-social sounds and future research should focus on both HbO2 and HHb responses in this age group. In contrast, by 9 months of age onwards the social>non-social selectivity (and absence of non-social>social selectivity) becomes a robust response localized within a specific anterior temporal region of the cortex with all three of the older aged cohorts showing responses in this area. Furthermore, in support of these findings, the location and selectivity of this response was replicated in our UK 12–16-month-old cohort (see Box 1).