Distinguishing False from True in Human Memory

Abstract

Studies with mesial temporal amnesic patients have shown that although they are impaired on explicit tests of memory such as recall and recognition requiring conscious recollection of a prior study episode, they are often unimpaired on implicit memory measures in which retention is assessed without conscious awareness. Since this class of findings suggests that medial temporal structures are critical for explicit, but not implicit memory (e.g.Squire 1992xSquire, L.R. Psychol. Rev. 1992; 99: 195–231CrossRef | PubMedSee all ReferencesSquire 1992), one might expect to observe activation there during explicit, but not implicit memory tasks. The PET findings have not consistently supported this prediction, however. First, there are several examples of PET studies of explicit memory for verbal materials which have failed to show left medial temporal effects (e.g.12xShallice, T, Fletcher, P, Frith, C.D, Grasby, P, Frackowiak, R.S.J, and Dolan, R.J. Nature. 1994; 368: 633–635CrossRef | PubMed | Scopus (627)See all References, 16xTulving, E, Kapur, S, Markowitsch, H.J, Craik, F.I.M, Habib, R, and Houle, S. Proc. Natl. Acad. Sci. USA. 1994; 91: 2012–2015CrossRef | PubMedSee all References). In addition, studies comparing implicit and explicit memory directly have produced various outcomes ranging from failure to observe reliable medial temporal activation for either type of test (e.g.,Buckner et al. 1995xBuckner, R.L, Petersen, S.E, Ojemann, J.G, Miezin, F.M, Squire, L.R, and Raichle, M.E. J. Neurosci. 1995; 15: 12–29PubMedSee all ReferencesBuckner et al. 1995, word stem completion), finding left medial temporal activation for explicit but not implicit memory (Schacter et al. 1996axSchacter, D.L, Alpert, N.M, Savage, C.R, Rauch, S.L, and Alpert, M.S. Proc. Natl. Acad. Sci. USA. 1996; 93: 321–325CrossRef | PubMed | Scopus (483)See all ReferencesSchacter et al. 1996a, word stem completion), and even left hippocampal formation activation for both explicit and implicit tasks (Blaxton et al. 1996xBlaxton, T.A, Bookheimer, S.Y, Zeffiro, T.A, Figlozzi, C.M, Gaillard, W.D, and Theodore, W.H. Can. J. Exp. Psychol. 1996; 50: 42–56CrossRef | PubMedSee all ReferencesBlaxton et al. 1996, word fragment completion).In summary, the regions of activation reported for true and false target memory in the Schacter et al. study are ones that have been identified previously across a variety of PET paradigms in which memory for true targets was studied. These new findings make important contributions to our knowledge of the brain structures subserving memory function and constitute an important addition to this literature. As may be seen from Table 1, Table 2 , however, there are still many interesting riddles to be worked out concerning the role of left temporal structures in human memory, particularly as evidenced by functional neuroimaging techniques.*Present address: 1831 Abbotsford Drive, Vienna, Virginia 22182.