Ensembles of human MTL neurons "jump back in time" in response to a repeated stimulus

Marc W. Howard, Indre V. Viskontas, Karthik H. Shankar, Itzhak Fried

Research output: Contribution to journalArticlepeer-review

Abstract

Episodic memory, which depends critically on the integrity of the medial temporal lobe (MTL), has been described as "mental time travel" in which the rememberer "jumps back in time." The neural mechanism underlying this ability remains elusive. Mathematical and computational models of performance in episodic memory tasks provide a specific hypothesis regarding the computation that supports such a jump back in time. The models suggest that a representation of temporal context, a representation that changes gradually over macroscopic periods of time, is the cue for episodic recall. According to these models, a jump back in time corresponds to a stimulus recovering a prior state of temporal context. In vivo single-neuron recordings were taken from the human MTL while epilepsy patients distinguished novel from repeated images in a continuous recognition memory task. The firing pattern of the ensemble of MTL neurons showed robust temporal autocorrelation over macroscopic periods of time during performance of the memory task. The gradually-changing part of the ensemble state was causally affected by the visual stimulus being presented. Critically, repetition of a stimulus caused the ensemble to elicit a pattern of activity that resembled the pattern of activity present before the initial presentation of the stimulus. These findings confirm a direct prediction of this class of temporal context models and may be a signature of the mechanism that underlies the experience of episodic memory as mental time travel.

Original languageEnglish
Pages (from-to)1833-1847
Number of pages15
JournalHippocampus
Volume22
Issue number9
DOIs
StatePublished - Sep 2012

Keywords

  • Contiguity effect
  • Episodic memory
  • Temporal context

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