Shared neural geometries for bilingual semantic representations in human hippocampal neurons

The human brain has the remarkable ability to comprehend and express similar concepts in multiple languages. To understand how it does so, we examined responses of hippocampal neurons during passive listening, directed speaking, and spontaneous conversation in both English and Spanish in a small group of balanced bilinguals. We found a small number of putative "cross-language neurons," whose responses to equivalent words (e.g., "tierra" and "earth") are correlated. However, neurons' semantic tunings differed substantially by language, suggesting language-specific neural implementations. Instead, the crucial driver of translation was a preserved geometric organization of neural responses between the two languages, one that did not depend on neuron-level functional overlap. Indeed, that geometry was implemented by a common set of neurons along distinct readout axes; this difference in readout may help prevent cross-language interference. Together, these results suggest that the hippocampus encodes a language-independent internal model for meaning.