No Pattern Separation in the Human Hippocampus

• Published in: Trends in cognitive sciences Vol. 24; no. 12; pp. 994 - 1007
• Main Author: Quian Quiroga, Rodrigo
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.12.2020
• Subjects: Concept Cells; conjunctive coding; engram; episodic memory; Hippocampus - physiology; human intelligence; Humans; Memory, Episodic; neural coding; Neurons - physiology; episodic memory; human intelligence; engram; conjunctive coding; Concept Cells; neural coding
• ISSN: 1364-6613; 1879-307X
• DOI: 10.1016/j.tics.2020.09.012

Pattern separation is a basic principle of neuronal coding that precludes memory interference in the hippocampus. Its existence is supported by numerous theoretical, computational, and experimental findings in different species. However, I argue that recent evidence from single-neuron recordings suggests that pattern separation may not be present in the human hippocampus and that memories are instead coded by the coactivation of invariant and context-independent engrams. This alternative model prompts a reassessment of the definition of episodic memory and its distinction from semantic memory. Furthermore, I propose that a lack of pattern separation in memory coding may have profound implications that could explain cognitive abilities that are uniquely developed in humans, such as our power of generalization and of creative and abstract thinking. In contrast to claims based on theoretical and modelling studies, findings with single-neuron recordings in rats and monkeys, and indirect evidence from fMRI activations, I argue that pattern separation may not be present in the human hippocampus.Relatively recent evidence from human single-neuron recordings shows that, contrary to pattern separation, episodic memories are coded by context-independent and invariant engrams in the human hippocampus.Associations constitute the skeleton of episodic memories and are coded with partially overlapping assemblies, which prompts us to reconsider the view of episodic memory as mental time travel, and the distinction between episodic and semantic memory.A lack of pattern separation may explain human cognitive abilities, such as our unique powers of generalization and of creative and abstract thinking.