Concept Cells and the Neural Bases of Human Memory

Single-neuron recordings from the medial temporal lobe of patients undergoing epilepsy surgery have revealed "concept cells" that respond selectively and invariantly to meaningful stimuli such as specific people, places, or objects. These responses offer a unique window into how individual neurons encode high-level, multimodal representations-the building blocks of episodic memory-that differ from the more distributed, often hierarchical representations supporting semantic memory in the neocortex. Episodic and semantic memory, the systems for storing past experiences and conceptual knowledge, have traditionally been regarded as distinct. However, converging evidence from neuroimaging, lesion studies, and electrophysiological recordings challenges this strict dichotomy. This review synthesizes findings from human single-neuron recordings to re-examine the traditional distinction between episodic and semantic memory. We propose that the primary difference between the two systems lies in the structure of the associations they support: sparse, arbitrary links supporting episodic memory in the hippocampus versus ordered, hierarchical representations supporting semantic memory in the neocortex.