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.