Neural processing of repetition and non-repetition grammars in 7- and 9-month-old infants

Abstract

An essential aspect of infant language development involves the extraction of meaningful information from a continuous stream of auditory input. Studies have identified early abilities to differentiate auditory input along various dimensions, including the presence or absence of structural regularities. In newborn infants, frontal and temporal regions were found to respond differentially to these regularities (Gervain et al., 2008), and in order to examine the development of this abstract rule learning we presented 7- and 9-month-old infants with syllables containing an ABB pattern (e.g., "balolo") or an ABC pattern (e.g., "baloti") and measured activity in left and right lateral brain regions using near-infrared spectroscopy (NIRS). While prior newborn work found increases in oxyhemoglobin (oxyHb) activity in response to ABB blocks as compared to ABC blocks in anterior regions, 7- and 9-month-olds showed no differentiation between grammars in oxyHb. However, changes in deoxyhemoglobin (deoxyHb) pointed to a developmental shift, whereby 7-month-olds showed deoxyHb responding significantly different from zero for ABB blocks, but not ABC blocks, and 9-month-olds showed the opposite pattern, with deoxyHb responding significantly different from zero for the ABC blocks but not the ABB blocks. DeoxyHb responses were more pronounced over anterior regions. A grammar by time interaction also illustrated that during the early blocks, deoxyHb was significantly greater to ABC than in later blocks, but there was no change in ABB activation over time. The shift from stronger activation to ABB in newborns (Gervain et al., 2008) and 7-month-olds in the present study to stronger activation to ABC by 9-month-olds here is discussed in terms of changes in stimulus salience and novelty preference over the first year of life. The present discussion also highlights the importance of future work exploring the coupling between oxyHb and deoxyHb activation in infant NIRS studies.

Keywords: NIRS; auditory processing; infancy; language; optical imaging.

Figure 1

(A) Images of the neoprene headband used to hold the NIRS probes in place in the present experiment. (B) Schematic of probe placement over the left and right lateral regions of the infant head. Due to individual variation, analyses were based on groups of channels to create standardized regions of interest across infants. The left anterior region included channels 1–5; the left posterior region included channels 6–12, the right anterior region included channels 20–24, and the right posterior region included channels 13–19.

Figure 2

Time course of oxyHb and deoxyHb. Illustration of the average oxyHb and deoxyHb concentration at each time point across the 16-s time window for the ABB and ABC grammar in the group of 7-month-old infants (left) and the group of 9-month-old infants (right). Concentrations are averaged across all 24 channels.

Figure 3

Average deoxyHb concentration. Analysis of mean deoxyHb concentration revealed a marginal age by condition interaction (p = 0.08). Post hoc analyses revealed that 7-month-olds show a negative response significantly different from zero for the ABB grammar, while the 9-month-olds show a negative response significantly different from zero for the ABC grammar. Error bars represent ±SE.

Figure 4

Average deoxyHb concentration over time. Analysis of deoxyHb concentration during the first four blocks and the last four blocks revealed a significant interaction between condition and time (p = 0.025), with a significantly larger deoxyHb response for the ABC grammar during the early blocks than the later blocks (p < 0.02) and no change in ABB grammar responding across time (p = 0.45). Additionally, a marginal interaction was found between condition and age (p = 0.056). One-sample t-tests revealed that the 7-month-old ABB response during Blocks 1–4 and the 9-month-old ABC response during Blocks 1–4 are significantly different from zero. Error bars represent ±SE.

Figure A1

Channel-by-channel t-tests for 7-month-old infants in the present study.

Figure A2

Channel-by-channel t-tests for 9-month-old infants in the present study.