Explicit-Implicit Memory: A Developing Theoretical Framework

The review o f experimental work investigating the explicit-implicit taxonomy o f memory provided in Chapter 1 highlighted the difficulty in providing an unequivocal localisation o f age effects within each form o f memory (cf. La Voie & Light, 1994; Light & La Voie, 1993b). Nonetheless, it does appear that age-related differences in memory are reduced when performance is mediated by automatic, unconscious processes (Dywan & Jacoby, 1990; Jacoby et al., 1993b), and when the environment support is high (Craik, 1986; Craik & M cDowd, 1987). Hay and Jacoby (1996) argued that instances in which performance was mediated by unconscious processes reflected strong stimulus-response links o f the form found in ‘hardw ired’ functions or habitual responses. In contrast, age-related differences are expected to be greatest when m em ory performance is mediated by self-initiated, conscious processes because o f the greater burden on cognitive resources.

Before articulating the theoretical issues that relate specifically to age-related differences in conscious and unconscious m em ory processes, a discussion o f the theories that have been proposed to explain the cognitive architecture o f explicit and implicit m em ory is necessary, since many o f the issues addressed in the experimental chapters are based on this theoretical work. The discussion will be limited to the domain o f theoretical work that attempts to explain the phenomenon o f verbal priming. In this regard, four principal categories o f theory can be identified, each o f which concentrates on how the distinction between explicit and implicit m em ory can be conceptualised: (1) multiple independent memory systems (Moscovitch et al., 1993; Schacter, 1994b; Squire, 1994); (2) transfer appropriate processing framework, represented by m ultiple dichotomous process distinctions (Blaxton, 1989; Roediger, 1990b); (3) com ponents-ofprocessing framework (Johnson, 1983; Tenpenny & Shoben, 1992; W itherspoon & Moscovitch, 1989); and (4), post-perceptual selection accounts (Masson & MacLeod, 1996; Ratcliff & M cKoon, 1997; Ratcliff, McKoon, & Verwoerd, 1989).

At a basal level, the multiple memory system framework is modelled in terms o f the ontology and emergent properties o f neurological structures that give rise to m emory (Schacter, 1992b), rather than as a primarily epistemological explanation o f the mechanisms that mediate memory based information processing (Bower, 1996). The multiple memory systems framework represents a formalisation o f the task dissociation approach for studying memory within a structural, cognitive neuroscientific perspective, where a dissociation between memory tasks is interpreted as evidence o f discrete ‘Fodor-type’ memory systems with dissociable neural correlates. Each empirically independent memory system, o f which at least four are widely recognised (Schacter & Tulving, 1994b), is characterised by distinct properties and attributes. In contrast, the transfer appropriate processing framework is derived from hypothesised mental procedures, such as levels o f processing and perceptual and conceptual processing (Blaxton, 1989; Craik & Lockhart, 1972; Roediger, 1990b). The component-of-processing framework identifies the processes that mediate task performance, which are determined by an analysis o f the nature o f the task and the strategies that are adopted (Johnson, 1983; Moscovitch, 1994b; Tenpenny & Shoben, 1992; W itherspoon & Moscovitch, 1989). The components-of-processing processing framework developed tangentially from the processing dichotomies based on the transfer appropriate processing framework. The most recent development has been the post-perceptual selection theories that locate repetition priming effects at the level o f bias effects that occur in response selection (Light & Kennison, 1996a; but see. Light & Kennison, 1996b; e.g., McKoon & Ratcliff, 1996; Ratcliff & M cKoon, 1997).

Three o f these four primary theoretical approaches are described in the following subsections. The process and multiple memory systems approaches are not mutually exclusive approaches to the explicit-implicit memory distinction (Hayman & Tulving, 1989a; Tulving, 1999), since a degree o f convergence is inevitable (Kelley & Lindsay, 1996). Further, the dialogue between proponents o f these two approaches should not be characterised as that between multiple memory systems and a unitary mnemonic system (Foster & Jelicic, 1999; M asson & Graf, 1993; cf. Tulving, 1999). Therefore, a levelling treatment is not required for the integration o f process and system approaches. Post-perceptual selection theories are not addressed because they do not have a direct bearing on the experimental work included in this thesis, and these theories are at present under specified, and are not yet able to account for a large domain o f the data in implicit memory. In addition, the forced-choice cueing procedure adopted to evaluate this class o f theory may be responsible for the bias effects reported by the advocates o f this theory (Masson & MacLeod, 1996; Ratcliff & McKoon, 1997), and also may contaminate indirect tests with conscious processes

3. Conscious and Unconscious M emory Processes and Ageing

( see also, Bowers, 1999; Light & Kennison, 1996b; but see, McKoon & Ratcliff, 1996). Thus, a detailed treatise o f the post-perceptual selection theories is beyond the scope o f the present chapter.

The multiple memory systems view will be discussed first, with emphasis given to the perceptual representation system (Tulving & Schacter, 1990), posited to mediate perceptual item-specific priming, and the recently proposed conceptual representation system (Gabrieli, 1999), posited to subserve conceptual item-specific priming. The process approaches represent a collection o f dichotic distinctions between types o f processing; each o f these will be examined individually, with particular emphasis given to accounts that are directly relevant to experimental work presented in this thesis. Finally, the com ponents-of processing approach will be discussed, since this approach represents an important rapprochement between the process and system approaches. However, it will eventually be necessary to specify an intermediate, more specific neural network based accounts in which the operation o f distinct, interlocked brain processes operate as a functional system.

3.3.1 Multiple Memory Systems: Mechanisms of Explicit and Implicit Memory

The multiple memory systems approach is designed to relate performance on a behavioural task to the underlying neural structures and processes, in order to provide a detailed morphology o f memory. However, a general criticism o f this approach is that the functional relation between task demands and prior encoding does not really represent a substantive unit o f analysis on which to base a theoretical framework that seeks to explain the neural substrate o f memory, nor is it sufficient to provide empirical falsification or confirmation o f discrete memory systems (Ratcliff & M cKoon, 1996; Toth & Hunt, 1999; Weldon, 1999). Specifically, these functional categories o f description are not isomorphic with the neural structures that are inferred from them (Luria, 1973). A further difficulty with using memory tasks as the unit o f analysis (as extensively discussed in Chapter 2) is that they can be performed in multiple ways, since each instance o f a particular response can reflect a quite different instantiation o f processes (Tulving, 1985c).

An additional failing o f this class o f approach is that it is sometimes unclear whether the postulated memory systems represent a taxonomic classification o f memory, a descriptive construct, or a formal theory (Eichenbaum, 1994; Nadel, 1994). This may, in part, reflect the fact that the mechanisms that mediate memory and other forms o f cognitive processing are often assumed to be

cognitive processes and other memory systems. Nonetheless, none o f these concerns are restricted to the multiple memory systems approach, since they are also equally relevant to the interpretation o f task performance within process accounts.