Implicit Memory: How It Works and Why We Need It David W. L. Wu1 1University of British Columbia Correspondence to: [email protected] ABSTRACT Since the discovery that amnesiacs retained certain forms of unconscious learning and memory, implicit memory research has grown immensely over the past several decades. This review discusses two of the most intriguing questions in implicit memory research: how we think it works and why it is important to human behaviour. Using priming as an example, this paper surveys how historic behavioural studies have revealed how implicit memory differs from explicit memory. More recent neuroimaging studies are also discussed. These studies explore the neural correlate of priming and have led to the formation of the sharpening, fatigue, and facilitation neural models of priming. The latter part of this review discusses why humans have evolved with highly flexible explicit memory systems while still retaining inflexible implicit memory systems. Traditionally, researchers have regarded the competitive interaction between implicit and explicit memory systems to be fundamental for essential behaviours like habit learning. However, recently documented collaborative interactions suggest that both implicit and explicit processes are potentially necessary for optimal memory and learning performance. The literature reviewed in this paper reveals how implicit memory research has and continues to challenge our understanding of learning, memory, and the brain systems that underlie them. Keywords: implicit memory, priming, neural priming, habit learning INTRODUCTION several days despite having no recollection of the learning trials. For a time it was Not until Scoville and Milner believed that the acquisition of motor studied the amnesic patient H.M. in 1957 skills was the only type of learning H.M. did investigations into the biological and was capable of (Corkin, 1968). It would neurological basis of memory begin in take another decade until it was earnest. Among the many contributions to discovered that other non-motor skills science that H.M. provided, one of the could be learned as well. What was known most important and fascinating was the about learning skills retained during first convincing evidence of multiple amnesia was again broadened when it was memory systems. Milner (1962) showed demonstrated that amnesiacs had the that in ten trials, H.M. was able to learn to ability to learn perceptual skills like trace a star only by using its mirror image. mirror-reading (Cohen & Squire, 1980). Impressively, he retained the skill across These early investigations of H.M. and 1 JYI | July 2011 | Vol. 22 Issue 1 2011 The Journal of Young Investigators other amnesiacs formed the foundation habit learning becoming addiction—but for research involving the relationship also in enhancing beneficial behaviours. between memory and awareness. This For example, understanding motor paper will look into the implicit memory memory may lead to the development of research that has been conducted since optimal strategies in skill acquisition. It then, with a focus on two key questions: may also lead to better teaching strategies how it works, and why we need it. for people who do not learn effectively with conventional teaching methods, like First, an examination of the amnesiacs or Alzheimer’s patients, or even behavioural and neuroimaging evidence of an ill-behaved household pet. priming will lead to a discussion on neural models which attempt to explain how MECHANISMS OF IMPLICIT MEMORY implicit memory may work. Historic behavioural priming studies of both The term implicit memory is a amnesic and non-amnesic subjects general label which refers to the provide convincing evidence that implicit observation that past experiences can memory and explicit memory systems are modify subsequent task performance, in fact distinct. More recent neuroimaging despite the learner lacking conscious studies have discovered the neural recollection of the learning episode correlate of priming, which has led to (Schacter, Chiu & Ochsner, 1993). Many sharpening, fatigue, and facilitation neural skills are acquired and maintained models of priming. Secondly, interactions implicitly, for example typing. Most between implicit and explicit memory people do not remember when or how systems will be used as evidence in the they learned to type, nor do they need to discussion of the need for an implicit consciously monitor their typing. Implicit memory system for human behaviour. memory is thus expressed through Behaviours like habit learning require a performance as opposed to explicit competitive interaction between implicit memory, which is expressed through and explicit memory systems, but more recollection (Squire, 2009a). While the recently theorized collaborative previous example illustrated procedural interactions may be necessary in memory, the most extensively studied producing optimal learning and memory. implicit memory paradigm is priming. (Schacter et al., 1993). Implicit memory encompasses a wide range of phenomena, including Early Priming Studies procedural and motor memory, habit learning, classical conditioning, and Priming refers to the phenomenon emotional memory (Squire, 2009b). where a recent encounter with a stimulus Because of this, implicit memory research creates an improvement in the ability to is important not only because it increases identify or process a related stimulus our basic understanding of the human (Tulving & Schacter, 1990). One of the brain, but also because of it role behind a main differences between priming and wide range of behaviours. Such an other types of implicit skill learning is that understanding is not only be necessary in priming takes only takes a single or a developing solutions where such small number of learning trials, whereas behaviours become maladaptive—like skill learning requires numerous learning 2 JYI | July 2011 | Vol. 22 Issue 1 2011 The Journal of Young Investigators trials (Schacter et al. 1993). In the 1980s, samples began to paint a more detailed visual word priming was the most widely picture of how these systems differed. studied form of priming. Some of the most popular experimental paradigms One such difference revealed by included stem or fragment completion, early priming studies was the endurance where subjects are given the first three of priming effects. Fragment completion letters or fragments of the word and tasks have been shown to persist from as choose between multiple possible long as one week (Tulving, Schacter, & completions (i.e., for___ or f_re_t Stark, 1982) to over a year (Sloman, (forest)); and word identification, where Hayman, Ohta, Law, & Tulving, 1988). subjects are shown a word for a very brief While initial studies of amnesic patients duration (i.e., 35ms) and asked to identify failed to find priming effects at long delays it (Schacter et al. 1993). (Squire, Shimamura & Graf, 1987), it was later shown that the amnesiac K.C. The key principle that emerged retained priming effects over the course of from early studies of visual word priming a year (Tulving, Hayman & MacDonald, was the fact that processes involved in 1991). These long term effects are thought priming are distinct from ones pertaining to be related to the fact that manipulations to explicit memory. Strong support for of interference that normally impair this principle came from studies of explicit memory exhibited no effect on amnesic patients who had comparable performance on fragment and stem levels of priming as non-amnesic subjects completion tasks (Graf & Schacter 1987; in stem completion tasks (Graf, Squire & Sloman et al., 1988). The endurance of Mandler, 1984), provided that the priming effects added to the growing instructions did not suggest an explicit picture of how implicit memory was strategy. It was also shown that normal different from explicit memory. subjects had similar explicit/implicit dissociations. Manipulations to depth of Lastly, visual word priming was encoding (i.e., semantic study tasks which established to be relatively specific. focus on the meaning of a word, compared Priming has little cross-modality to non-semantic tasks which focus on the capabilities, a fact evident when priming physical properties of a word) produced was diminished with auditory—as opposed equal levels of priming despite large to visual—target stimuli in stem differences in explicit memory (Graf, completion (Graf, Shimamura, & Squire, Mandler & Hayden, 1982; Graf & Mandler, 1985), and word identification 1984). Post-experiment questionnaires (Hashtroudi et al., 1988). When subjects revealed that subjects who were not aware studied mixed list of words and pictures in of the relationship between the completion a word or a picture-fragment completion task and the learning task still showed task, priming was nearly abolished in equivalent priming effects as subjects who picture completion tasks despite free recall were aware (Bowers & Schacter, 1990). of the pictures being better than words Amnesiacs provided the first piece of the (Weldon & Roediger, 1987). Bilingual puzzle that the implicit memory system subjects also showed little priming when was distinct from the explicit memory they studied words in one language but system, while studies on non-patient were tested in another (Durgunoglu & Roediger, 1987). Even changing the 3 JYI | July 2011 | Vol. 22 Issue 1 2011 The Journal of Young Investigators typecase or font of the words can adversely studies (Buckner et al.,