The Death of Implicit Memory

The thesis of this article is that implicit memory does not exist. Implicit memory phenomena are distinct from explicit memory phenomena at a neural and information processing level, but there is such variety among the implicit memory phenomena that nothing holds them together in a common category. Other researchers have distinguished among different types of implicit memory, but have retained the superordinate category. Extant data is evaluated in light of how classification systems should be developed, and it is concluded that there is currently not a reason to retain the construct "implicit memory. " 1. What Implicit Memory Might Be 1.1 As defined by Graf & Schacter (1985) implicit memory refers not to a memory system, but to a set of memory tasks. Their distinction is based on characteristics of the experimental situation. Implicit memory tasks make no reference to the initial encoding episode and are not necessarily associated with awareness of engaging in recall. Explicit memory tasks make reference to the initial encoding episode, and are associated with subjective awareness of engaging in recall. The terms implicit/explicit have in some sense been hijacked by memory researchers, and now are often used to refer to hypothetical memory systems; this practice has become widespread enough that Schacter has felt compelled to comment on the topic (Schacter & Tulving, 1994). With apologies to Schacter, this article uses the terms implicit/explicit in this common (but strictly speaking, incorrect) sense. The arguments made here apply to many bicameral memory classification schemes, including procedural/declarative (Cohen & Eichenbaum, 1993; Cohen & Squire, 1980), nondeclarative/declarative (Squire, 1992) and so on. 1.2 All of these classification schemes rely on two sets of data for support: neuroanatomic and cognitive. The neuroanatomic data demonstrate dissociations between implicit and explicit memory based on brain lesion or brain imaging data. Thus, the two types of memory are separate because they rely on distinct anatomic structures for normal operation. For example, patients with global amnesia perform poorly on tasks that rely primarily on explicit memory such as recognition and recall tasks (Scoville & Milner, 1957) but these same subjects perform normally on tasks that rely primarily on implicit memory, such as motor skills (e.g., Corkin, 1968) and repetition priming (e.g., Warrington & Weiskrantz, 1970) 1.3 The second data set lending support to the implicit/explicit distinction is purely cognitive. Implicit and explicit memory are thought to use different processes and/or representations in their normal operation. For example, the level of performance in explicit memory tasks is strongly influenced by the depth of processing at encoding, but is not influenced by the match of the perceptual characteristics of the stimuli (e.g. type font of written words) at encoding and retrieval. Measures of implicit memory, on the other hand, are often insensitive to level of encoding, at least in some paradigms, but are sensitive to physical characteristics of the stimuli such as type font (e.g., Graf, Mandler, & Haden, 1982; Jacoby & Hayman, 1987). 1.4 The answer, then, to "What is implicit memory? " can take two forms. First, the answer might be that "implicit memory is a memory system supported by particular brain structures. " The second answer is that "implicit memory is a memory system using particular processes and representations. " Naturally a third answer is that both of the previous answers are true because both distinctions apply. We will argue that neither distinction applies. Implicit memories have nothing in common; implicit memory does not exist as a coherent system. 1.5 The point is this. A number of different tasks purportedly offer relatively pure measures of implicit memory (although it is accepted that tasks are not process-pure, Jacoby, 1991): motor skill learning (Nissen & Bullemer, 1987), perceptual learning (Cohen & Squire, 1980), perceptual after-effects (Savoy & Gabrieli, 1991) and classical conditioning (Weiskrantz & Warrington, 1979). These tasks may be differentiable on a number of dimensions, but if they are all going to be called implicit, they must share some characteristic. What is the need for a superordinate category called "implicit memory" that ties these tasks together? To examine that question more closely, it is worth considering how classification systems in general are created. 2. Creating Classification Systems of Memory 2.1 Creating a classification system of anything (including memories) entails creating categories of objects based on their attributes. Objects have more attributes than will be used to create categories. How then can one select which attributes are the important ones? There are, in fact, many, many possible classification systems for any group of objects, and these different classification systems might serve different functions. For example, someone concerned with safety will develop a different classification system for automobiles than someone concerned with economy and these two classification systems will use different attributes of automobiles. The function of classification systems of memory is different; the goal is to create a classification system that "carves nature at its joints"--a system that captures important divisions in the nature of memory itself. 2.2 That desire leads in turn to the question "What important attribute must be captured? " The answer to this question might be called the conceptual basis of a classification system (Ridley, 1986). For example, one might develop a system based on neuroscientific criteria. The attributes used to differentiate memories will, in that case, be their neuroanatomic or neuropharmacologic basis, for example. Or one might select information processing as a conceptual basis; memories might be identified as different (or the same) depending on the processes and representations they share. These conceptual bases may lead to different classification systems. For example, using an neuroscientific representation, one might differentiate between two types of memories with nonoverlapping neuroanatomic bases, but if there was good reason to think that the cognitive processes and representations supporting the memories were identical despite the diverse anatomic substrates, a classification system using an information processing conceptual basis would not differentiate the memories. 2.3 Once a conceptual basis has been selected, one must select attributes that will differentiate memories. For example, a memory classification system using a neuroscientific conceptual basis might use the attribute "effect of basal forebrain acetylcholine (ACh) on the memory. " Naturally, the conceptual basis limits the possible attributes, but does not dictate them. 2.4 After the attributes have been selected, cut-points of the attributes that determine category membership must be developed. For example, given that "effect of basal forebrain ACh on the memory" is an important attribute, how is that attribute used? Should there be two levels, say "unaffected, " and "affected? " Perhaps there will be three levels of this attribute, "impaired, " "enhanced, " and "unaffected. " 2.5 The point of this section is to highlight that classification systems represent choices about what the researcher takes to be important, defining evidence for differences among memories. The classification systems that have been developed thus far use neuroscientific and information processing conceptual bases. The validity of the category "implicit memory" should be evaluated within the same framework used to develop classification systems. 3. Implicit Memory as a Neuroscientific Concept 3.1 Within neuroscientific conceptual bases, the one most often used is neuroanatomy. Although there has been some work on distinguishing implicit and explicit memories based on neuropharmacology (e.g., Nissen, Knopman, & Schacter, 1987), the bulk of the work with human subjects has used anatomy. The primary finding is that a set of interconnected structures in the medial temporal lobe, basal forebrain, and diencephalon support the formation of new explicit memories, but do not appear to contribute to the formation of new implicit memories (Squire & Zola-Morgan, 1991). If one or more of these sites is damaged, explicit memory is impaired, but implicit memory is not. This finding allows one to assign coherence to explicit memory because explicit memory is anatomically localized--remember, neuroanatomy is the conceptual basis. While there has been some debate about possible differences in the amnesic syndromes caused by damage to the medial temporal lobe vs. that caused by diencephalic damage, it now appears that there is not an appreciable difference between them (Squire, Knowlton, & Musen, 1993). This finding does not, however, allow one to assign coherence to what is spared in amnesia. What is spared in amnesia is simply "memory not in the medial temporal lobe, etc." For it to merit the term "implicit memory" there must be coherence in these type of memories within the conceptual basis of the system, in other words, anatomic consistency. Although there were some suggestions that implicit memory might reside in a single anatomic system (Mishkin, Malamut, & Bachevalier, 1984), that view has not been popular in recent years, for reasons described in the next section. 3.2 There does not appear to be anatomic consistency within implicit memory. Different implicit memory tasks are differentially affected by brain damage, i.e., they are dissociable. Tulving and Schacter (1990) and Keane, Gabrieli, Fennema, Growdon and Corkin (1991) have argued that repetition priming in fact represents two types of priming: a perceptual representation system in the occipital and posterior temporal cortices, and a semantic system in the anterior temporal cortices. Heindel, Salmon, Shults, Walicke,