Thomson

Annu. Rev. Psychol. 2005.56:1-23. Downloaded from arjournals.annualreviews.org by Universidad Nacional Autonoma de Mexico on 09/29/06. For personal use only.

Annu. Rev. Psychol. 2005. 56:1–23 doi: 10.1146/annurev.psych.56.091103.070239 Copyright c 2005 by Annual Reviews. All rights reserved First published online as a Review in Advance on June 10, 2004

IN SEARCH OF MEMORY TRACES
RichardF. Thompson
Neuroscience Program, University of Southern California, Los Angeles, California 90089-2520; email: thompson@neuro.usc.edu
Annu. Rev. Psychol. 2005.56:1-23. Downloaded from arjournals.annualreviews.org by Universidad Nacional Autonoma de Mexico on 09/29/06. For personal use only.

Key Words learning, hippocampus, cerebellum, inactivation, localization ■ Abstract The key issue inanalyzing brain substrates of memory is the nature of memory traces, how memories are formed, stored, and retrieved in the brain. In order to analyze mechanisms of memory formation it is first necessary to find the loci of memory storage, the classic problem of localization. Various approaches to this issue are reviewed. A particular strategy is proposed that involves a number of different techniques(electrophysiological recording, lesions, electrical stimulation, pathway tracing) to identify the essential memory trace circuit for a given form of learning and memory. The methods of reversible inactivation can be used to localize the memory traces within this circuit. Using classical conditioning of eye blink and other discrete responses as a model system, the essential memory trace circuit isidentified, the basic memory trace is localized (to the cerebellum), and putative higher-order memory traces are characterized in the hippocampus.

CONTENTS
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Simplified Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Strategies to Study Memory Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 OUR MODEL SYSTEM: EYEBLINK CONDITIONING . . . . . . . . . . . . . . . . . . . . . 6 Motor Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 The Hippocampus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 8 The Essential Circuitry: The Cerebellum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

INTRODUCTION
A major achievement of recent research on brain substrates of learning and memory, to which our work hascontributed, is the recognition that there are several different forms or aspects of memory involving different brain systems (Figure 1). Squire (1992) has argued eloquently for the distinction between declarative and nondeclarative forms of memory, as has Schacter (1987). The distinction between episodic and semantic memory—“What did you have for breakfast?” versus “Where is the Eiffel Tower?”—has beenstressed by Tulving (1985).
0066-4308/05/0203-0001$14.00

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Annu. Rev. Psychol. 2005.56:1-23. Downloaded from arjournals.annualreviews.org by Universidad Nacional Autonoma de Mexico on 09/29/06. For personal use only.

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Figure 1 A current view of the various forms of learning and memory and their putative brain substrates.

LOCALIZATION OF MEMORY TRACES

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Implicit ornondeclarative memory is very much a grab bag. In general, explicit memory involves awareness of the memory whereas implicit memory does not necessarily involve being aware of the memory (aware meaning verbal report). The schema of Figure 1 is of course oversimplified. When an organism learns something, a number of brain systems can become engaged. However, in most cases there is one critical...