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Comparing Direct (Explicit) and Indirect (Implicit) Measures to Study Unconscious Memory

Philip M. Merikle and Eyal M. Reingold

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Comparisons of the relative sensitivity of direct and indirect tasks can provide definitive evidence for unconscious memory when the direct and indirect tasks are matched on all characteristics except instructions. To demonstrate unconscious memory in normal adults, subjects first viewed pairs of words and named one cued word in each pair. During the subsequent assessment of memory, new words and either previously cued words (Experiment 1) or previously uncued words (Experiments 2A and 2B) were presented against a background mask. The subjects judged whether a word was old or new (direct task) or whether the contrast between a word and the mask was high or low (indirect task). For cued words, the direct task was more sensitive than the indirect task. However, for uncued words, the indirect task was initially more sensitive than the direct task, even though the direct task exhibited hypermnesia so that it became more sensitive than the indirect task across trial blocks. The greater initial sensitivity of the indirect task implicates unconscious processes underlying memory for the uncued words. These results indicate that unconscious processes in normal adults can be revealed through comparisons of comparable direct and indirect measures.

Comparing Direct (Explicit) and Indirect (Implicit) Measures to Study Unconscious Memory

Questions regarding the relationship between consciousness and memory are attracting increasing attention (e.g., Jacoby, Woloshyn & Kelley, 1989; Kelley & Jacoby, 1990; Kihlstrom, 1987; Schacter, 1989). This increased interest begins to rectify what Tulving (1985) described as the "benign neglect of consciousness" (p. 1) in the study of human memory. However, in spite of the growing number of studies directed at demonstrating memory without awareness, definitive studies distinguishing conscious from unconscious memories in normal adults are rare. As noted by Schacter (1989), even though studies of amnesic patients provide compelling evidence for memory in the absence of conscious recollection, the evidence from studies with normal adults is not nearly as clear cut. A major reason it has been so difficult to distinguish conscious and unconscious memories in normal adults is that there is no general agreement as to what constitutes an adequate measure of conscious experience (Erdelyi, 1986; Reingold & Merikle, 1990). For this reason, we previously proposed an alternative approach to the study of memory and consciousness that is based on comparisons of the relative sensitivity of different types of tasks (Reingold & Merikle, 1988; 1990). In the present paper, we demonstrate that this alternative approach provides compelling evidence for memory without conscious awareness in normal adults.

A basic distinction among tasks used to study memory and consciousness concerns whether a given task provides a direct or an indirect measure of memory (Johnson & Hasher, 1987; Reingold & Merikle, 1990; Richardson-Klavehn & Bjork, 1988). Tasks in which subjects are explicitly instructed to discriminate old or previously presented stimuli from new stimuli that have not been presented within the experimental context are defined as direct measures of memory. In contrast, if task instructions do not make any reference to the old/new discrimination, then such tasks are defined as indirect measures of memory. Examples of direct memory tasks are stimulus recognition and recall. In these tasks, subjects are explicitly instructed either to make an old/new discrimination or to recall only stimuli that were presented previously within the experimental context. Examples of indirect tasks are word-stem completion (e.g., Graf & Mandler, 1984) and perceptual identification (e.g., Jacoby & Dallas, 1981). These tasks provide indirect measures of memory in the sense that no explicit reference is made in the task instructions to the distinction between old and new stimuli.

Any classification of memory tasks based on the distinction between direct and indirect measures is identical to a classification of tasks based on distinction between explicit and implicit measures of memory (e.g., Graf & Schacter, 1985; Schacter, 1987). We prefer the distinction between direct and indirect measures, however, because the explicit/implicit terminology is potentially very misleading (see Dunn & Kirsner, 1989; Reingold & Merikle, 1990; Richardson-Klavehn & Bjork, 1988). Specifically, the terms implicit and explicit have been used to classify both memory processes and memory tasks. This dual meaning of the terms has blurred the distinction between theoretical constructs and empirical measures. On the other hand, the direct/indirect distinction is based solely on task instructions and does not require any speculation as to either underlying memory processes or possible phenomenal experiences (e.g., awareness, intentionality) correlated with the different types of tasks. It is precisely for this reason that we and others ( Hintzman, 1990; Reingold & Merikle, 1990; Richardson-Klavehn & Bjork, 1988) have suggested that the terms direct and indirect rather than the terms explicit and implicit are better labels for these two classes of memory tasks.

To date, many dissociations between direct and indirect measures of memory have been reported (see Richardson-Klavehn & Bjork, 1988; Schacter, 1987; for reviews). However, there is no general consensus as to the best interpretation of many of these reported dissociations (cf. Dunn & Kirsner, 1989). This is the case because direct and indirect tasks are often quite dissimilar. Thus, the observed dissociations may simply reflect task differences. In general, dissociations between measures are usually more informative the more similar the tasks, just as associations between measures are usually more impressive and informative the more dissimilar the tasks. For this reason, a useful strategy when comparing direct and indirect tasks is to match the tasks on all characteristics except task instructions. This strategy constrains the number of possible interpretations of any observed dissociations.

Reingold and Merikle (1988) suggested a number of methodological guidelines for evaluating the comparability of direct and indirect tasks. For present purposes, we describe the three most common differences between the direct and indirect tasks used in studies of memory that have revealed dissociations.

1) Retrieval or test cues: Direct memory tasks such as recognition and recall, and indirect memory measures such as word-stem completion and perceptual identification often present subjects with very different test or retrieval cues (e.g., TRA__ in a stem completion task versus TRAVEL in a recognition task). Thus, in any study in which the retrieval or test cues are not physically identical across tasks, dissociations between measures may simply reflect the fact that different test cues were used with each task (see Schacter, Bowers & Booker, 1989 for a related discussion).

2) Memory sensitivity versus response bias: Performance levels on direct and indirect measures may also be differentially affected by response bias. For example, while a recognition task permits independent assessment of memory sensitivity and response bias, sensitivity and bias cannot be distinguished in indirect tasks such as perceptual identification and word-stem completion. Often, differences in bias across tasks are accompanied by differences in retrieval cues. However, even if identical retrieval cues are used, the influence of response bias may not be equivalent across direct and indirect measures. For example, Graf and Mandler (1984) reported a dissociation between a direct cued recall task and an indirect stem completion task under conditions in which identical test cues were used across tasks. However, while subjects were required to provide a response to every cue in the stem completion task, they were not required to do so in the cued recall task. Thus, the reported dissociation may have simply reflected a difference in response bias across the two tasks.

3) Response metric: Memory measures such as stimulus recognition, recall, word-stem completion, and perceptual identification represent very different response metrics. The functions relating performance levels across tasks are unknown. Thus, observed dissociations between direct and indirect memory tasks may simply reflect differences in measurement scales and not reflect differences in underlying processes.



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