204 LARRY R.SQUIRE ald,199).It is pos sible.too,that the amygdala has a mor cit that can be are selected role in Mish 197 SK 78 d on amyedal y ofskill learn ing that t ir th Cohen pa /980 ation a thetinmcofretricralIincicitingrc call in anke in c patients cam emerge from ed hippocamp ons can be increa by additiona as the su lum or the alveus (Jar ard,98;Morris.Schenk ions fus ts (Gra uced of da 8er1978 on)As de ribed the s me is also true in hu unders d as an example of ord prim (eg patient R.B.vs.H.M) and a large body of work has accumulate both norma Multiple Memory Systems kind of memory than the kind that is mura in ider ctures and cor ections that nak up the oral I m ce m of dife 07 ates in m in thisachi ing,1985 Weisk 1987 ght that ind of ocamp 9)w d greatly b htmensy5notasneeeatmyrconsisisoritpilepe see Hintzman 99 pal ing v n m yis no can be found ine and human of dev lopm tablished In 1962 the se (Bereson 1911:me and of day-t o-day im y(Ryle, mory for the practice and proce ning and to fo the uni ary nature of the res men tory Amne on should be made for m amnesia In addition the idea that the ipp angs of u on the ha ag Spe esions 9974H h1974 OKeefe&Nad Picture s and experimental animals.in 968 1970,19 others that foll ved m nd related cture mes good o at least better than m ,1980)in thes ing to mind or dec ected, 1976.Tbe some aksare simply casier than oth asks c r D rkwithhuman sub has be al T ed that ce ce are one' in normal subjects and amnesi nts alike. ever arative men es men 204 LARRY R. SQUIRE 1987; Gallagher, Graham, & Holland, 1990; Kesner, in press; LeDoux, 1987; Nachman & Ashe, 1974; Sutherland & McDon￾ald, 1990). It is possible, too, that the amygdala has a more general role in forming associations between stimuli, for exam￾ple, in making associations across modalities (Murray & Mish￾kin, 1985). However, when these ideas are based on amygdala lesions prepared by a direct surgical approach, the contribution of underlying cortical regions included in the amygdala lesions needs to be evaluated. The second important and anatomically relevant finding to emerge from work with rats is that the deficit associated with restricted hippocampal lesions can be increased by additional damage to anatomically related structures and fiber tracts such as the subiculum or the alveus (Jarrard, 1986; Morris, Schenk, Tweedie, & Jarrard, 1990). These findings are in agreement with the findings from monkeys that different levels of impair￾ment can be produced depending on the extent of damage within the medial temporal lobe (e.g., the H+ lesion vs. the H++ lesion). As described earlier, the same is also true in humans (e.g., patient R. B. vs. H. M.). Multiple Memory Systems Progress in identifying the structures and connections that make up the medial temporal lobe memory system has been paralleled by gains in understanding how this system partici￾pates in memory functions. An important step in this achieve￾ment was the insight that the hippocampal formation is impor￾tant for only a particular kind of memory. The implication was that memory is not a single entity but consists of multiple pro￾cesses or systems. Converging evidence about the selective role of the hippocampal formation in memory is now available from rats, monkeys, and humans. It took time for the idea of multiple memory systems to be￾come firmly established. In 1962, the severely impaired amne￾sic patient H. M. was reported to be capable of day-to-day im￾provement in a hand-eye coordination skill, despite having no memory for the practice sessions (Milner, 1962). Nevertheless, subsequent discussions of memory in general and amnesia in particular tended to set aside motor skill learning and to focus on the unitary nature of the rest of memory. Amnesia was con￾sidered to impair memory globally, with the recognition that an exception should be made for motor skills. Findings of unexpectedly good learning by amnesic patients on tasks not requiring motor skills were also reported many years ago. Specifically, patients performed well when the reten￾tion test provided partial information (e.g., fragments) about previously presented pictures or words (Milner, Corkin, & Teuber, 1968; Warrington & Weiskrantz, 1968, 1970, 1974, 1978). However, there were two reasons why these reports, and others that followed, did not lead to the idea of multiple mem￾ory systems. First, although the performance of amnesic pa￾tients was sometimes good, or at least better than might have been expected, it was often below normal levels. Accordingly, the data were open to a proportionality interpretation, namely, that some tasks are simply easier than other tasks or provide more sensitive measures of memory. It could therefore be ar￾gued that certain task conditions simpJy improve performance in normal subjects and amnesic patients alike. Second, even when amnesic patients appeared to perform normally, the data could be interpreted as evidence that amnesia is a retrieval defi￾cit that can be reversed when the appropriate tasks are selected (Warrington & Weiskrantz, 1970; Weiskrantz, 1978). Subsequently, it was discovered that motor skills are just one example of a broader category of skill learning that is intact in amnesic patients (Cohen, 1984; Cohen & Squire, 1980; Squire, 1982). At the same time, the success of partial information at the time of retrieval in eliciting recall in amnesic patients came to be better understood (e.g., word stems like inc or mot— as cues for recently studied words). It turned out that only one kind of instruction yields normal performance (complete the stem to form the first word that comes to mind; Graf, Squire, & Mandler, 1984). With conventional memory instructions (use the stem as a cue to recall a recently presented word), normal subjects maintain their advantage over amnesic patients (Graf et al., 1984; Squire, Wetzel, & Slater, 1978). Intact performance by amnesic patients on such tasks, when indirect instructions are used, is now understood as an example of word priming, and a large body of work has accumulated with both normal subjects and amnesic patients in support of the idea that prim￾ing reflects a different kind of memory than the kind that is tapped in conventional memory experiments (Shimamura, 1986; Tulving & Schacter, 1990). The emergence of the idea that memory consists of different systems (Cohen, 1984; Moscovitch, 1982; Schacter, 1987; Squire, 1982; Tulving, 1985; Weiskrantz, 1987; Wickelgren, 1979) was influenced greatly by work with amnesic patients. In addition, experimental work with normal subjects was influen￾tial (for reviews, see Hintzman, 1990; Polster, Nadel, & Schacter, 1991; Richardson-Klavehn, & Bjork, 1988). Distinc￾tions between kinds of memory can be found in earlier writings that reflect the traditions of developmental psychology (Bruner, 1969; memory with record and memory without rec￾ord), psychology (Bergson, 1911; memory and habit), philo￾sophy (Ryle, 1949; knowing how and knowing that), and artifi￾cial intelligence (Winograd, 1975; Winston, 1977; declarative and procedural). The tradition of work with amnesic patients explains why the idea of multiple memory systems led naturally to a consider￾ation of what kind of memory depends on the integrity of the brain structures, including hippocampus, that are damaged in amnesia. In addition, the idea that the hippocampus might be involved in only one kind of memory appeared independently in the animal literature, on the basis of the selective effects of limbic lesions (Gaftan, 1974; Hirsch, 1974; O'Keefe & Nadel, 1978; Olton et al., 1979). The sections that follow suggest that the findings from humans and experimental animals, includ￾ing rats and monkeys, are now in substantial agreement about the kind of memory that depends specifically on the hippo￾campus and related structures. This kind of memory has been termed declarative (Cohen & Squire, 1980) in the sense that one can bring to mind or declare the content of this kind of memory (for its earlier use in psychol￾ogy, see Anderson, 1976). The term declarative was derived from work with human subjects and has been difficult to apply usefully to experimental animals. The problem is not that de￾clarative memory seems to imply an ability to declare one's knowledge verbally. Indeed, declarative memory includes mem-