Long-term memory is a term that encapsulates the brain’s ability to store information. It is subdivided into two main types: explicit memory (also known as declarative memory) and implicit memory (also known as nondeclarative memory). Explicit memory refers to the acquisition of information about objects, stimuli, and information that is consciously noted and recallable. The mesial temporal lobe, which includes the hippocampal formation (CA1, CA3, and dentate gyrus) and ento- rhinal cortex, is the region responsible for this process. While the hippocampal formation stores memories, the entorhinal cortex mediates learning and memory via its interaction with the hippocampus and neocortex. For instance, neocortical information from a visual stimulus is translated via the entorhinal cortex to higher-order complex memory representations such that an emotion can trigger a visual memory. Layer II of the entorhinal cortex is the first region affected in Alzheimer disease (AD). The memory circuit that integrates the mesial temporal lobe and hippocampal formation includes several pathways: the perforant pathway (input to the hippocampus from the entorhinal cortex), Mossy fiber pathway (dentate gyrus to CA3 region), Schaffer collateral/associational commissural pathway (from CA3 to CA1 region), and CA1-subiculum-entorhinal cortex pathway (the principal output of the hippocampus).
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.