In Search of Memory Traces

Abstract

■ Abstract The key issue in analyzing 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 is identified, 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Strategies to Study Memory Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 OUR MODEL SYSTEM: EYEBLINK CONDITIONING . . . . . . . . . . . . . . . . . . . . . 6 Motor Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . INTRODUCTIONA major achievement of recent research on brain substrates of learning and memory, to which our work has contributed, 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 been stressed by Tulving (1985). LOCALIZATION OF MEMORY TRACES 3Implicit or nondeclarative 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 brain system, which when damaged causes permanent impairment in the particular form of learning and memory.Many readers will recall Lashley's (1950) pessimistic conclusion that his series of experiments "has yielded a good bit of information about what and where the memory trace is not" in his famous article, "In Search of the Engram." In all fairness to Lashley, the existence of several different forms of memory with differing neuronal substrates was not recognized at that time, nor were modern analytic techniques ava...

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