Memory Circuits

There are two main types of explicit memory: episodic and semantic memory. Episodic memory is likened to autobiographic memory, as an episode of one’s life is recalled (remembering a certain vacation to the beach). Semantic memory refers to memory about facts, and general knowledge that is unrelated to a specific experience (for instance, I know that a zebra has stripes). Not surprisingly, CA1 and the subiculum undergo selective neuronal degeneration in AD. The loss of connections between the hippocampal formation and entorhinal cortical neurons, which project to the hippocampus via the perforant pathway, account for the clinical presentation of explicit memory problems in AD patients. Implicit memory, on the other hand, is “unconscious,” can sometimes be linked to an emotion, and can be procedural (for instance, remembering how to drive a car).

The study of one particular patient, H.M., provided significant insights into the formation of memory and the role of the mesial temporal structures in memory. H. M. underwent a bilateral resection of the medial temporal regions as part of an experimental treatment for medically refractory seizures. Subsequently, he developed profound loss of personal memories but had preserved language, attention, procedural memory, and general intellectual ability.

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