The neonate brain detects speech structure

Abstract

What are the origins of the efficient language learning abilities that allow humans to acquire their mother tongue in just a few years very early in life? Although previous studies have identified different mechanisms underlying the acquisition of auditory and speech patterns in older infants and adults, the earliest sensitivities remain unexplored. To address this issue, we investigated the ability of newborns to learn simple repetition-based structures in two optical brain-imaging experiments. In the first experiment, 22 neonates listened to syllable sequences containing immediate repetitions (ABB; e.g., "mubaba," "penana"), intermixed with random control sequences (ABC; e.g., "mubage," "penaku"). We found increased responses to the repetition sequences in the temporal and left frontal areas, indicating that the newborn brain differentiated the two patterns. The repetition sequences evoked greater activation than the random sequences during the first few trials, suggesting the presence of an automatic perceptual mechanism to detect repetitions. In addition, over the subsequent trials, activation increased further in response to the repetition sequences but not in response to the random sequences, indicating that recognition of the ABB pattern was enhanced by repeated exposure. In the second experiment, in which nonadjacent repetitions (ABA; e.g., "bamuba," "napena") were contrasted with the same random controls, no discrimination was observed. These findings suggest that newborns are sensitive to certain input configurations in the auditory domain, a perceptual ability that might facilitate later language development.

Fig. 1.

Details of the procedure used in experiments 1 and 2. (A) The experiments' design. The upper boxcar shows how the consecutive stimulation blocks unfold. The lower boxcar indicates the sequence of sentence types within a block. (B) The placement of the probes overlaid on a schematic neonate brain. Although individual variation cannot be excluded, this placement ensured recording from perisylvian and anterior brain regions. The dashed white lines separate anterior and posterior ROIs. The red ellipses indicate the channels included in the frontal area of interest (LH: channels 2 and 5; RH: channels 13 and 15). The blue ellipses indicate channels included in the temporal area of interest (LH: channels 3 and 6; RH: channels 17 and 19).

Fig. 2.

The grand average results of experiment 1. Channels are plotted following the same placement as in Fig. 1B. The x-axis represents time in seconds; the y-axis shows concentration in mmol·mm. The rectangle along the x-axis indicates time of stimulation. The continuous red and blue lines in the graphs represent oxyHb and deoxyHb concentrations, respectively, in response to the ABC grammar. The dashed magenta and cyan lines represent oxyHb and deoxyHb concentrations, respectively, in response to the ABB grammar.

Fig. 3.

Statistical maps (t-maps) comparing the responses to the repetition and control grammars in experiment 1 (A) and experiment 2 (B). Channels are plotted following the same placement as in Fig. 1B. The t-values for each channel are color-coded as indicated on the color bar. Significance levels are indicated for each channel (*, P < 0.05; **, P < 0.01; ***, P < 0.001).

Fig. 4.

The time course of the responses in experiment 1. (A) The linear regression lines of the oxyHb concentrations fitted on the data points provided by the 14 consecutive blocks for the two grammars. The light-gray line represents ABC; the dark-gray line, ABB. The r² values are r² = 0.00002 for ABC and r² = 0.3427 for ABB. (B) The bars indicate the average oxyHb concentration in the left frontal area (channels 2 and 5) for the first and the last four blocks for the two grammars. The y axis shows the average totalHb concentration in mmol·mm. The light-gray bars represent ABC; the dark-gray bars, ABB.

Fig. 5.

The grand average results of experiment 2. All graphical conventions are the same as those in Fig. 2.