Fig. 2 |. Distributed encoding and clustering of phonemes along phonetic categories.

a. Average mTRF kernel for phoneme feature. The shaded area indicates the 95% confidence region of the surrogate (chance-level) distribution. The phoneme kernel was significant at two time periods, centered around 200 and 400 ms. b. mTRF kernels identified on each single unit for one example phoneme /k/. The units are sorted increasingly by their mean firing rate. Brighter colors indicate higher values of the kernel. These kernels are then projected into a two-dimensional space using principal component analysis (PCA), resulting in one phoneme trajectory. Numbers along the trajectory indicate the time at which the trajectory reached the corresponding location. c. PCA variance explained. Four PCs explain 50% of variance (dotted line) and 15 explain 90% (dashed line). d. Principal component (PC) coefficients of isolated single units. Several units are represented in the first PCs, indicating distributed encoding of phonemes. e. The schematics show how the clustering index is computed as intra-cluster distance (full lines) subtracted from between-cluster distances (bold lines). f. Clustering index for vowels grouped by the first formant (high and low tongue position). The shaded area represents the 95% confidence region of the surrogate (chance-level) distribution. This clustering index is significant during a time interval centered around 200 ms. g. Distribution of vowels in the two-dimensional PC space at 0 and 200 ms. Vowels are color-coded based on their first formant (high and low tongue position). Mirroring the increase in the clustering index, the separation between those two phonetic features in the PC space is absent at time 0, becoming evident at 200 ms. h. Clustering index for vowels grouped by the second formant. i. Clustering index for consonants grouped by the manner of articulation revealed two significant periods, centered around 200 and 400 ms. j. Clustering index for consonants grouped by the place of articulation.