Anatomic Localization Of Memory Systems

A considerable body of evidence indicates that the various memory systems described earlier are supported by distinct anatomic structures. These conclusions have been derived from a number of strategies. Among these is

Memory

Declarative (Explicit)

Facts -

Skills and habits

Nondeclarative (implicit)

Priming

Simple classical conditioning

Nonassociative learning

Emotional responses

Skeketal musculature

Medial temporal lobe Striatum Neocortex Amygdala Cerebellum Reflex diencephalon pathways

FIGURE 1 Multiple memory systems and their associated brain structures. Memory systems in the brain can be categorized as declarative and nondeclarative. Each system is supported by distinct anatomical regions of the brain. (Modified from Squire, LR, Knowlton, BJ, Memory, hippocampus, and brain systems. In: Gazzanige, MS, ed. The coginitive neurosciences. Cambridge: MIT Press, 1995:825-837.)

the use of modern imaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), which show regions of the brain engaged by specific memory tasks (Fig. 2). Studies of learning and memory deficits in experimental animals with lesions have also helped identify areas of the brain that are involved in learning and memory processes. Finally, brain regions involved in learning and memory have been identified through detailed behavioral assessments of patients with learning and memory deficits produced either by injury (e.g., trauma, tumors, vascular incidents such as stroke) or by surgical removal of regions of the brain which became necessary to treat disorders such as epilepsy.

A particularly revealing early case that has had a major impact on the development of modern views on the distributed representation of memory systems was the study by Brenda Milner and her colleagues of a patient named H.M., who underwent surgery in 1953 to treat severe epileptic seizures. The surgery included removal of the medial temporal region of the brain, which included the amygdala, anterior two-thirds of the hippocampus, and hippocampal gyrus. Following the surgery, H.M. appeared normal in many respects except for a severe anterograde amnesia. Specifically, he could not form any new long-term declarative memories. For example, on the day following an interview, he had no recollection that the interview on the previous day had occurred nor any memory of any events associated with the interview. H.M. also could not add new words to his vocabulary. Thus, he appeared to be unable to form any new memories for events, facts, or concepts. Although he could not form any new long-term episodic or semantic memories, his early childhood memories were intact. Presumably, these memories are stored in a region of the brain outside the medial temporal region, which was removed during surgery. Of particular interest was the finding that H.M. retained the ability to acquire new skills at a level comparable with normal individuals. For example, H.M. was able to learn a difficult mirror-drawing task. He also learned to play the Tower of Hanoi puzzle. Although he could acquire these new skills (i.e., nondeclarative memory), he had no conscious recollection of ever acquiring them (i.e., no declarative memory).

Studies of the type described earlier indicate that the medial temporal lobe and diencephalon are critical for declarative memories (see Fig. 1). Anatomic loci for nondeclarative memories are more diverse and seem to depend on the particular brain structure or structures engaged by the task. Thus, the learning of certain skills and habits involves the striatum, whereas the learning of certain movements can involve the cerebellum or spinal cord. Conditioning of emotional responses depends on the amygdala (see Fig. 1).

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