(A) The T5-phonemes dataset consists of the participant speaking 420 unique words which together sampled 41 American English phonemes. We constructed firing rate vectors for each phoneme using a 150 ms window centered on the phoneme start (one element per electrode), averaged across every instance of that phoneme. This dissimilarity matrix shows the difference between each pair of phonemes’ firing rate vectors, calculated using the same neural distance method as in Figure 1E. The matrix is symmetrical across the diagonal. Diagonal elements (i.e. within-phoneme distances) were constructed by comparing split halves of each phoneme’s instances. The phonemes are ordered by place of articulation grouping (each group is outlined with a box of different color). (B) Violin plots showing all neural distances from panel A divided based on whether the two compared phonemes are in the same place of articulation group (‘Within group’, red) or whether the two phonemes are from different place of articulation groups (‘Between groups’, black). Center circles show each distribution’s median, vertical bars show 25th to 75th percentiles, and horizontal bars shows distribution means. The mean neural distance across all Within group pairs was 30.6 Hz, while the mean across all Between group pairs was 42.8 Hz (difference = 12.2 Hz). (C) The difference in between-group versus within-group neural distances from panel B, marked with the blue line, far exceeds the distribution of shuffled distances (brown) in which the same summary statistic was computed 10,000 times after randomly permuting the neural distance matrix rows and columns. These shuffles provide a null control in which the relationship between phoneme pairs’ neural activity differences and these phonemes’ place of articulation groupings are scrambled. (D) A hierarchical clustering dendrogram based on phonemes’ neural population distances from panel A. At the bottom level, each phoneme is placed next to the (other) phoneme with the most similar neural population activity. Successive levels combine nearest phoneme clusters. By grouping phonemes based solely on their neural similarities (rather than one specific trait like place of articulation, indicated here with the same colors as in the panel A groupings), this dendrogram provides a complementary view that highlights that many neural nearest neighbors are phonetically similar (e.g. /d/ and /g/ stop-plosives, /θ/ and /v/ fricatives, /ŋ/ and /n/ nasals) and that related phonemes form larger clusters, such as the left-most major branch of mostly vowels or the sibilant cluster /s/, /ʃ/, and /dʒ/. At the same time, there are some phonemes that appear out of place, such as plosive consonant /b/ appearing between vowels /ɑ/ and /ɔ/ (we speculate this could reflect neural correlates of co-articulation from the vowels that frequently followed the brief /b/ sound).