Understanding memory dysfunction

Abstract

Background: Although traditionally memory has been viewed as a simple concept, converging and complementary evidence from patient studies and more recent neuroimaging research suggest that memory is a collection of mental abilities that use different neuroanatomical systems within the brain. Neurologic injury may cause damage to one or more of these memory systems.

Review summary: In this review a number of different memory systems are discussed, including their function, neuroanatomy, and the different disorders that disrupt them. Episodic memory, the most clinically relevant memory system, depends upon the hippocampus and other medial temporal lobe structures, the limbic system, and the frontal lobes. Several other kinds of memory are contrasted with episodic memory, including semantic memory, simple classic conditioning, procedural memory, priming, and working memory.

Conclusion: Improved understanding of these different types of memory will aid the clinician in the diagnosis and treatment of the memory disorders of their patients. As more specific therapeutic strategies are developed for the treatment of diseases which cause memory dysfunction, this knowledge will become increasingly important.

FIGURE 1.

Episodic memory. The medial temporal lobes, including the hippocampus and parahippocampus, form the core of the episodic memory system. Other brain regions are also necessary for episodic memory to function correctly. In addition to being involved in episodic memory, the amygdala is also important for the autonomic conditioning. (Adapted with permission from Budson and Price, New England Journal of Medicine, 2005).

FIGURE 2.

Ribot’s law.

FIGURE 3.

Detailed anatomy of the medial temporal lobe. PHG, indicates parahippocampal gyrus; Pr, presubiculum; v, ventricle; S, subiculum. CA1, CA2, and CA3 are subregions of the hippocampus, and ML, GL, and PL are different regions of the dentate gyrus of the hippocampus. (Adapted with permission from Martin JH. Neuroanatomy: Text and Atlas. New York, NY.: Elsevier, 1989, p. 391; permission granted by Elsevier).

FIGURE 4.

Areas of the cerebral cortex, including sensory areas, are connected bidirectionally to the parahippocampal region, which is in turn bidirectionally connected to the hippocampus. (Adapted with permission from Eichenbaum, 1997;permission granted by Science Magazine).

FIGURE 5.

Schematic representation of encoding in the medial temporal lobe.

FIGURE 6.

Schematic representation of retrieval in the medial temporal lobe.

FIGURE 7.

A neural network model. See text for details.

FIGURE 8.

Semantic, procedural, and working memory. The anterior and inferolateral temporal lobes are important in the naming and categorization tasks by which semantic memory is typically assessed. However, in the broadest sense, semantic memory may reside in multiple and diverse cortical areas that are related to various types of knowledge. The basal ganglia, cerebellum, and supplementary motor area are critical for procedural memory. The prefrontal cortex is active in virtually all working memory tasks; other cortical and subcortical brain regions will also be active, depending on the type and complexity of the working memory task. In addition to being involved in procedural memory, the cerebellum is also important for the motoric conditioning. (Adapted with permission from Budson and Price, New England Journal of Medicine, 2005).