Pronouns reactivate conceptual representations in human hippocampal neurons

During discourse comprehension, every new word adds to an evolving representation of meaning that accumulates over consecutive sentences and constrains the next words. To minimize repetition and utterance length, languages use pronouns, like the word ‘she’, to refer to nouns and phrases that were previously introduced. It has been suggested that language comprehension requires that pronouns activate the same neuronal represen­tations as the nouns themselves. We recorded individual neurons in the human hippocampus during a reading task. Brain-imaging studies have gained insight into the brain regions that activate during sentence and discourse comprehension. However, the resolution of these methods does not suffice to track the neuronal assemblies that encode individual concepts in the human brain during reading. It has become possible to directly record the activity of single neurons in patients who are implanted with electrodes to locate the source of their epilepsy. These studies demonstrated the existence of ‘concept cells’ in the medial temporal lobe. Concept cells have an invariant and multimodal selective response to a concept. They contribute to the representation of meaning because they not only activate when the participant sees a picture of a specific individual for example, but also when the participant hears or reads the name of this person, or recalls this individual from memory. We hypothesized that monitoring the activity of concept cells during reading could provide insight into the dynamics of semantic representations during language comprehension. We found that cells that were selective to a particular noun were later reactivated by pronouns that refer to the cells’ preferred noun. These results imply that concept cells contribute to a rapid and dynamic semantic memory network that is recruited during language comprehension.