Fig. 2. Low-frequency (Δ-band) cortical encoding of categorical phonetic features in the first year of life.
A Schematic diagram of the analysis paradigm. Forward TRF models were fit to describe the relationship between speech features (including nuisance regressors) and low-frequency EEG signals. Speech features included the acoustic spectrogram (S), half-way rectified envelope derivative (D), visual motion (V), and phonetic features (F). B (Left) Hypotheses: The cortical encoding of phonetic feature categories was expected to progressively increase across the first year of life. Hp1-3: phonetic encoding emerging from 11, 7, and 4 months of age respectively. (Right) Phonetic feature encoding measured as the EEG prediction correlation gain when including phonetic features in the TRF (violin plots with mean value across participants). Only the frequency bands showing significant effects of age for TRFF were studied. Stars indicate significant effects of age in infants (one-way repeated measures ANOVA; *p ≤ 0.05; **p ≤ 0.01, ***p ≤ 0.001). Statistically significant effects were measured in the Δ-band (p = 0.003) and in the Θ-bands (p = 0.009). C TRF weights corresponding to phonetic features for the TRF in Δ- and Θ-bands (1–8 Hz). The figure was built using MATLAB software.