Temporal structure of the language hierarchy within small cortical patches

TL;DR

This study reveals that small cortical patches in the brain encode multiple levels of speech features simultaneously and dynamically, facilitating complex language production without clear macroscopic organization.

Contribution

It demonstrates that cortical patches multiplex phonetic, syllabic, and lexical information dynamically, challenging previous assumptions of organized hierarchical encoding.

Findings

Cortical patches encode multiple speech features simultaneously.

Encoding schemes dynamically change over time during speech production.

Small cortical patches organize speech hierarchy without clear macroscopic segregation.

Abstract

Speech production requires the rapid coordination of a complex hierarchy of linguistic units, transforming a semantic representation into a precise sequence of articulatory movements. To unravel the neural mechanisms underlying this feat, we leverage recordings from eight 3.2 x 3.2 mm 64-microelectrode arrays implanted in the motor cortex and inferior frontal gyrus of two patients tasked to produce twenty thousand sentences. We show that a hierarchy of linguistic features are robustly encoded in most of these small cortical patches. Contrary to our expectations, instead of a clear macroscopic organization between patches, we observe a multiplexing of phonetic, syllabic and lexical representations within each cortical patch. Critically, this coding scheme dynamically changes over time to allow successive phonemes, syllables and words to be simultaneously represented without interference. Overall, these results, reminiscent of position encoding in transformers, show how small cortical patches organize the unfolding of the speech hierarchy during language production.