Explicit and Implicit Memory in the Elderly: Evidence for Double Dissociation Involving Medial Temporal- and Frontal-Lobe Functions

Rotman Research Institute of Baycrest Centre for Geriatric Care Groups of normal old people living in institutions or in the community and young adults were administered tests of implicit (IM) and explicit (EM) memory with word-stem (WSC) and word-fragment (WFC) completion paradigms. Neuropsychological tests sensitive to frontal and medial temporal lobe function were also administered. Age differences were observed on both tests of EM and on all neuropsychological tests. Priming effects on WSC were smaller in the institutionalized group than the other 2 groups. Comparisons of EM and IM test results with neuropsychological test scores revealed several effects, including significant correlations (a) between EM scores on both tests and performance on standard memory tests in both aged groups and (b) between IM scores of both aged groups on WSC and frontal-lobe test performance. The results provide evidence of a double dissociation with respect to involvement of brain regions in EM and IM. They also indicate that repetition priming in WSC and WFC involve different mechanisms and that frontal-lobe dysfunction is a factor in reduced priming on the WSC test. Memory loss is a frequent complaint of the elderly, but extensive research over several decades has shown that not all aspects of memory function decline at the same rate. Age differences favoring young adults are most consistently re- ported on explicit (or direct) tests of memory that use recall or recognition techniques to assess conscious recollection of contextually based information. This type of memory loss is often attributed to progressive deterioration of the hippocam- pus, a medial temporal lobe brain region that is critical to memory function and known to be particularly sensitive to the aging process (Albert & Stafford, 1986; Milner, 1966; Tomlin- son, 1972). Implicit (or indirect) tests of memory do not refer to specific events. In these tests, memory is assessed by the effects of experience on some measure of performance that does not involve conscious awareness of any part of the prior experi- ence. Tests of general knowledge, skill learning, and repetition priming in which exposure to a word biases subsequent Gordon Winocur, Rotman Research Institute of Baycrest Centre for Geriatric Care, Toronto, Ontario, Canada, Department of Psychol- ogy, Trent University, Peterborough, Ontario, Canada, and Depart- ments of Psychology and Psychiatry, University of Toronto, Toronto, Ontario, Canada; Morris Moscovitch, Department of Psychology, Erindale College, University of Toronto and Rotman Research Insti- tute of Baycrest Centre for Geriatric Care; Donald T. Stuss, Rotman Research Institute of Baycrest Centre for Geriatric Care and Depart- ments of Psychology and Medicine (Neurology), University of Toronto, Toronto, Ontario, Canada. This research was supported by grants from the Medical Research Council of Canada and the Ontario Mental Health Foundation. We thank Lars Nyberg and Marko Jelicic for comments on an earlier version of this article. Portions of this research were presented at the Annual Meeting of the Memory Disorders Research Society, Boston, October 1994. Correspondence concerning this article should be addressed to Gordon Winocur, Rotman Research Institute of Baycrest Centre for Geriatric Care, 3560 Bathurst Street, North York, Ontario, Canada, M6A 2El. Electronic mail may be sent to winocur@psych.utoronto. ca. identification in a word completion test are common examples of implicit tests of memory. In general, cognitive processes that underlie normal performance on such tests are preserved in old age (see Howard, 1991; Light & LaVoie, 1993, for reviews), but there are notable exceptions (Chiarello & Hoyer, 1988; Davis et al., 1990; Howard, Shaw, & Heisey, 1986; Hultsch, Masson, & Small, 1991; Rose, Yesavage, Hill, & Bower, 1986). A primary purpose of the present study was to investigate one type of implicit memory (word-stem completion) on which age differences have been reported and to examine these differences from a neuropsychological perspective. Word completion as a measure of priming is usually studied in one of two ways. In word-fragment completion, the individual is presented initially with the complete word and then at test is given a sample of the word's letters (e.g.,