This special issue of the Canadian Journal of Experimental Psychology is titled "Implicit Memory Research in 1996." Implicit memory now occupies centre stage in the study of memory, and as such, the editor of the journal saw the need for a special issue. As guest editor of the special issue on implicit memory, I solicited research articles from eleven prominent laboratories. As you will see in reading the issue, the eleven papers report new and innovative research on a range of empirical and theoretical issues dealing with implicit memory.
Because the study of implicit memory is still quite new, and the Canadian Journal of Experimental Psychology attracts readers with different research backgrounds, I am providing below a general introduction to implicit memory and to the eleven articles contained in the special issue.
The way we think about memory has evolved rather dramatically in recent years. Check your dictionary for the term memory and you will likely find the traditional definition: Memory is the faculty of retaining and recalling past experiences, an act or instance of remembrance or recollection (The American Heritage Dictionary, 1985). Ask a cognitive psychologist to define memory and you will likely find that the traditional definition is no longer complete because of a recent distinction between explicit memory and implicit memory (Graf & Schacter, 1985).
We now appreciate that memory for a past event or experience can be revealed in at least two fundamentally different ways. On the one hand, memory can be expressed through the conscious recollection or recalling of past events, which is now called explicit memory. On the other hand, memory for a past event or experience can be revealed through behaviour that does not entail conscious recollection of the event, which is referred to as implicit memory(f.1).
Implicit memory encompasses many kinds of phenomena. In a review of the history and current status of implicit memory, Schacter (1987) describes many instances of implicit remembering that occur in both everyday and experimental situations. These include simple conditioning, savings during relearning, priming, and social behaviour, to mention only a few. Researchers are beginning to investigate many of these and other expressions of implicit memory, although the contemporary study of implicit memory focuses extensively on priming.
Priming is the phenomenon of implicit memory whereby a previous encounter with a stimulus influences the speed or accuracy of performance on a task, a task that does not require conscious recollection of the prior encounter with the stimulus. The contemporary study of priming has focused on the phenomenon whereby a prior presentation of a stimulus, usually a word or picture, facilitates subsequent processing of that stimulus or a related stimulus. A prevalent illustration would be the identification of a perceptually - degraded or fragmented form of the stimulus. For instance, one popular priming task requires subjects to identify a stimulus flashed very briefly on a computer screen; this is referred to as perceptual identification or masked word identification. Another popular task requires subjects to complete a fragmented stimulus, as in word fragment completion ( - 1 - ph - t) or word stem completion (ele ____). In these priming tasks, a target item presented at study (elephant) is cued at test by its perceptually - cued form, so the beneficial effect of study on test performance has been labeled perceptual priming.
Priming on perceptually - cued tests plays a major role in the study of implicit memory. Initial interest in memory began with reports that dense amnesic patients with severely impaired explicit memory show normal priming on word fragment completion tasks. Many further studies have shown that a variety of subject and experimental variables have a dissociative effect on implicit memory assessed by priming, and on explicit memory assessed by recall or recognition. These kinds of findings imply to many researchers that priming on perceptually cued tests is governed by principles rather different from those that govern explicit retrieval of previously studied information. The relevant empirical facts and theoretical explanations have been reviewed recently for normal human subjects and brain - damaged patients with memory disorders (e.g., Graf & Masson, 1993; Moscovitch, Vriezen, & Goshen - Gottstein, 1993; Roediger & McDermott, 1993; Schacter, Chiu, & Ochsner, 1993).
In this special issue, several articles investigate priming on perceptually - cued tests and contribute to our understanding of the nature of implicit retrieval onperceptually - cued tests.
The article by Masson and MacLeod presents empirical support for a very provocative interpretation of priming in masked word identification. In a series of experiments, they examined priming on different versions of a word identification task. Priming occurred in the conventional free report task but not in forced choice or probe matching tasks that eliminated certain bias effects. The results imply that priming, at least in word identification, does not reflect enhanced sensory processing. Rather, priming reflects the fluency with which the studied target comes to mind.
The Curran, Schacter, and Bessenoff article investigates an important aspect of priming on perceptually - cued tests: When and how do changes in the physical form of the stimulus at study or at test influence priming? They examined the effects of changes in letter case on word stem completion, which is particularly valuable because previous findings are ambiguous. Their research shows that specificity effects of letter case can be obtained under certain conditions, thereby providing insights into the nature of specificity effects on a perceptual - cued test.
McCauley, Eskes, and Moscovitch examined the effects of an interesting combination of experimental and subject variables on word stem completion. Young and old adults read a word or imaged a word's referent, and then completed word stems under implicit or explicit test instructions. The results show a novel double dissociation with an important combination of variables. The findings have empirical and theoretical implications.
Priming tasks can take various forms. Aside from perceptually - cued tests, there is a growing interest in conceptually - cued tests. In these priming tasks, the cue at test is meaningfully related to the target item, in the absence of any perceptual similarity between them. Examples of such tasks include answering questions (What animal did Hannibal use to cross the Alps?) and generating exemplars from a category cue (animals____). Here, priming is the increased likelihood that subjects produce the target word if it was presented at study, which has been called conceptual priming.
The idea that we need to distinguish between priming on perceptually - cued and conceptually - cued tests is rather new, but it is gaining support. In a seminal study, Blaxton (1989) showed that priming onthese two kinds of test was affected quite differently by several experimental variables. More recent studies have shown that other experimental and subject variables dissociate priming on perceptually - cued and conceptually - cued tasks. For reviews of the research, see Moscovitch et al. (1993) and Roediger and McDermott (1993).
In this issue, an article by Blaxton and her colleagues provides important new evidence for the distinction between perceptual and conceptual tasks. In the experiment, subjects performed four memory tasks while regional cerebral blood flow was measured using positron emission tomography (PET). The results show that perceptual and conceptual tasks activate somewhat different areas of the brain, providing novel evidence that conceptual and perceptual memory (processes) are subserved, at least in part, by different neurological substrates in the human brain.
The separation of perceptually and conceptually - cued tests is consistent with contemporary theoretical views of priming and implicit memory. According to a processing approach (e.g., Roediger, 1990; Roediger, Weldon, & Challis, 1989; also see Graf & Ryan, 1990), memory tests benefit to the extent that the type of processing promoted at study overlaps with the type of processing required for completion of the test. Implicit conceptually - cued retrieval, as well as explicit retrieval on tests such as recall and recognition, depend on meaning - based or conceptually - driven processes for their completion. On the other hand, implicit perceptually - cued retrieval, such as word fragment completion, depends on data - driven processing.
Another prevalent view of implicit memory is in terms of multiple memory systems (e.g., Tulving & Schacter, 1990; Schacter, 1994). According to this view, perceptual priming is subserved by a perceptual representative system, or its particular subsystems, that represent information of a presemantic or perceptual nature. On the other hand, conceptual priming depends on a semantic memory system that represents information of a semantic or conceptual nature. Explicit retrieval depends on episodic memory, which also represents information of a conceptual nature.
The processing and memory systems views, as noted by their proponents, can be seen as complementary rather than antagonistic. Both views accommodate the idea that implicit retrieval may entail perceptual or conceptual information, with the relative contribution of the two types of information depending, at least partially, on the nature of the retrieval cue. Both views can accommodate the observed dissociations between perceptual and conceptual priming, and between perceptual priming and measures of explicit memory, as well as the similarities between conceptual priming and explicit recall and recognition. Of course, the compatibility of the processing and memory systems views means that the two views are faced with similar kinds of questions and challenges.
One such challenge is presented by the McDermott and Roediger article in this issue. They compare tests previously classified as conceptual (free recall, category instance generation, category cued recall). At study, a target item is repeated as a word or as word and picture, under different encoding conditions. Contrary to the processing account of dissociations among memory tests, the conceptual memory tests did not exhibit a similar pattern of results. They then consider how the processing theory might account for the dissociation among conceptual tests.
So far, my discussion has focused on priming tasks where a target item is cued at test by its perceptually - cued form or the cue is conceptually related to the target item, in the absence of any perceptual similarity between them. Of course, we need to ask about priming tasks that combine perceptual and conceptual information in the test cue.
This question is addressed by Weldon and Massaro with innovative experimental and modeling work. In their experiment, they manipulate the perceptual and conceptual makeup of the test cue by varying the number of letters in a word fragment and the number of semantically related words beside the fragment. They vary level of processing at study and retrieval instructions at test. The results show, in conjunction with the fuzzy logic model of perception, how the contribution of perceptual and conceptual information relates to the nature of the test cue and instructions.
The study of priming has been dominated by the use of verbal tasks, like the tasks used in the previous articles. However, priming can be revealed in many ways, and it is important that researchers explore the nature of priming in a variety of experimental situations so as to broaden our knowledge and understanding of implicit memory. Several articles in this special issue do this.
Uttl and Graf report a series of experiments on priming for colour photographs of objects. They presented photos with a cardinal orientation (e.g., helicopter) or photos of a non - cardinal object (e.g., pencil). The orientation of the photo was varied at study and test. On the priming test, subjects tried to identify the objects in a photo that was faded in slowly on a computer monitor. Priming occurred in all conditions, although the magnitude was affected by the display orientation and type of photo. The findings provide insights into the coding and contribution of object orientation information to performance on implicit and explicit memory tests.
Musen investigated priming for object location associations. Subjects were presented with a series of lower case letter or symbol targets in a 9 - field rectangle. Subjects named each target as it was presented, or they named the location in which the target was displayed. Implicit memory for the learning trials was revealed by slower performance when target locations changed. The results, which show a revealing pattern for naming and location identification, imply that implicit memory depends on the attention demands of the test situations.
In the study of implicit memory, a crucial question is whether a memory test provides a pure measure of implicit or explicit memory. The question really has two sides. First, we can ask whether performance on a priming task reflects, at least partially, conscious recollection of studied items. There is no objective way to determine the subject's strategy, but there is an abundance of evidence supporting the claim that subjects do employ different strategies under implicit and explicit test conditions. In this issue, for instance, many of the articles provide support for the assertion that the implicit test was not treated as a recall test, by showing that an experimental variable had a clear dissociation on the implicit and explicit test. This notwithstanding, the issue of explicit retrieval on measures of implicit memory is a controversial one, and interested readers will find an excellent discussion in Roediger and McDermott (1993).
The second side of the question, and the one addressed extensively in this special issue, is whether implicit retrieval contributes to performance on conventional measures of explicit memory. A number of researchers have proposed that recognition performance reflects two fundamentally different components or processes, what have been called recollection and familiarity (e.g., Mandler, 1980). These two processes can be equated to explicit and implicit memory, respectively. This idea, and the nature of the processes, are examined in innovative ways by several articles in this issue.
An article by Gardner, Java, and Richardson - Klavehn employed an experiential approach to investigate the contribution of recollection and familiarity to recognition memory. Subjects reported their state of awareness when selecting an item on a recognition test: Do you remember the item occurring in the study phase, do you know that the item was presented although you do not remember it, or are you guessing? Experiments examined the effect of study variables on subjects' responses on several types of recognition tests. The experimental results and a meta - analysis of previous work provide important insights into the subjective report procedure (e.g., "know" responses are not commensurate with guessing). As well, the authors provide a stimulating discussion on issues associated with the subjective report procedure.
Toth investigates the intriguing idea that the familiarity component of recognition memory is mediated, at least in part, by implicit conceptual memory processes. In his experiments, he manipulates modality of presentation and level of processing, and tests recognition memory using response - signal delays, an opposition procedure, and the process dissociation procedure. The results support his idea that implicit conceptual processes contribute to familiarity, which has considerable theoretical implications.
Smith, Meiran, and Besner employ a successive test procedure to compare the sensitivity of recognition against two priming tasks. Subjects performed a lexical decision task and then completed a recognition or implicit semantic categorization test for the words that appeared in the lexical decision task. Even though semantic priming did not show itself in lexical decision, the effect was preserved in a recognition test: Primed words were better remembered. In contrast, implicit semantic categorization did not reveal the effects of prior semantic priming. The results provide insights into the nature of recognition in relation to priming.
This concludes the overview of the eleven articles contained in this special issue. As you will see in reading them, the eleven articles cover a wide range of topics on implicit memory. Each articles stands on it own and makes a novel and important contribution. At the same time, the articles share similarities which also helped to determine the order of presentation of the articles in the issue. I encourage you to read all of them, for every article reports new and innovative work that clearly advances our knowledge and understanding of implicit memory.
Communications concerning this article should be addressed to Bradford H. Challis, Institute of Psychology, University of Tsukuba, Ibaraki 305, Japan
(email: challis@human.tsukuba.ac.jp)
My role in the preparation of the special issue was supported by Natural Sciences and Engineering Research Council of Canada Grant A8632. I am grateful to Colin MacLeod for asking me to edit the special issue, and for his constructive comments on my introductory remarks. I thank all of the contributors to the special issue for providing excellent articles in a timely fashion. I also thank Lucy Pickering for her technical help.
References
The American Heritage Dictionary. (1985). Boston: Houghton Mifflin.
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Roediger, H.L., Weldon, M.S., & Challis, B.H. (1989). Explaining dissociations between implicit and explicit measures of retention: A processing account. In H.L. Roediger & F.I.M. Craik (Eds.), Varieties of memory and consciousness: Essays in honour of Endel Tulving (pp. 3 - 14). Hillsdale, NJ: Erlbaum.
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Footnote:
(f.1) Those who study implicit memory know, but others may not, that other terms exist to describe the phenomena that are investigated in this special issue. The term implicit memory is a case in point; it has been equated with terms like unconscious memory, unintentional memory or unconscious influences of memory. In these introductory remarks, I use the most popular terms of the day, implicit memory being one. Some variation in terms does occur across the articles in this issue. I will not take the time to introduce the various terms and consider the relative merits of each term, other than to say that the variations probably share much the same meaning so that researchers can and do communicate. This will be evident in reading this articles in this special issue.
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