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Word Picture Word

Study modality

Figure 3 The picture superiority effect is not always seen on implicit memory tests. (Left) Overlap in perceptual features between the study and test phases influences priming on perceptual implicit memory tests. Robust priming occurs on a verbal perceptual implicit memory tests (e.g., word fragment completion) following visual presentation of words (e.g., seeing the word ''whistle'' primes solving the fragment ''w h _ _ _ l _''). However, seeing a picture corresponding to the concept of interest (e.g., a picture of a whistle) does not facilitate performance on word fragment completion. (Right) The converse set of results is seen for picture fragment identification. Seeing a picture primes the identification of its later fragmented form, whereas seeing the corresponding word does not.

Picture

Word Picture Word

Study modality

Figure 3 The picture superiority effect is not always seen on implicit memory tests. (Left) Overlap in perceptual features between the study and test phases influences priming on perceptual implicit memory tests. Robust priming occurs on a verbal perceptual implicit memory tests (e.g., word fragment completion) following visual presentation of words (e.g., seeing the word ''whistle'' primes solving the fragment ''w h _ _ _ l _''). However, seeing a picture corresponding to the concept of interest (e.g., a picture of a whistle) does not facilitate performance on word fragment completion. (Right) The converse set of results is seen for picture fragment identification. Seeing a picture primes the identification of its later fragmented form, whereas seeing the corresponding word does not.

Figure 4 The retrieval intentionality criterion. The retrieval intentionality criterion is met here because the same test cues are used (word fragments) with two different sets of test instructions, and different patterns of results are observed for the implicit and explicit tests. (Left) A reverse generation effect is seen on word fragment completion. Less priming results from generating a word from a clue relative to reading the word. The explanation for this pattern is that there is transfer with respect to the visual features of the word in the read condition but not the generate condition (i.e., the word is seen in the read case, but not the generate case). (Right) The typical generation effect is seen on word fragment cued recall (an explicit test in which people are given a word fragment and asked to complete it with a word from a previously studied list).

Figure 4 The retrieval intentionality criterion. The retrieval intentionality criterion is met here because the same test cues are used (word fragments) with two different sets of test instructions, and different patterns of results are observed for the implicit and explicit tests. (Left) A reverse generation effect is seen on word fragment completion. Less priming results from generating a word from a clue relative to reading the word. The explanation for this pattern is that there is transfer with respect to the visual features of the word in the read condition but not the generate condition (i.e., the word is seen in the read case, but not the generate case). (Right) The typical generation effect is seen on word fragment cued recall (an explicit test in which people are given a word fragment and asked to complete it with a word from a previously studied list).

effect is present depends on the exact type of test. If a picture-based test is given, the perceptual overlap between study and test will be enhanced for a picture study condition (relative to a word study condition). However, the opposite is true for a verbal perceptual implicit memory test: Encountering words in the study phase primes these more than pictures.

When different patterns are obtained on explicit and implicit tests, the tests are said to have been dissociated. The most convincing dissociation occurs when the only feature that differs between the implicit and explicit test is the instructions; when this occurs, it is said that the retrieval intentionality criterion is met, as described in the late 1980s by Dan Schacter, Jeffrey Bowers, and

could be given either with implicit instructions (fill in the blanks with the first word that comes to mind— word stem completion) or with explicit instructions (fill in the blanks to form a word that you encountered earlier in the experiment—word stem cued recall). The only feature that differs between the two cases is instructions: One instructional set requires that people intentionally retrieve, whereas the other does not.

When different patterns of results are obtained on different memory tests, one can argue that different forms of memory underlie the different tests. For example, examine Fig. 4. People either read words during the study phase (e.g., ''whistle'') or generated the word from conceptual cues (e.g., ''blow-w_'').

As discussed before, explicit tests exhibit a generation effect such that the generate condition enhances later retrieval relative to the read condition. This pattern is found for the explicit test of word fragment-cued recall in which people are given cues (e.g., ''w h___l _'') and asked to use the cue to create a word from the study list. However, if the same cues are given with a different set of instructions, in which people are simply asked to fill in the fragment with the first word that comes to mind, the opposite pattern is observed. The read condition leads to more priming than does the generate condition. In this case, the retrieval intentionality criterion is met; the only procedural difference between the word fragment-cued recall and word fragment completion tests is instructional; Instructions for the former ask subjects to recollect the past, whereas instructions for the latter ask people simply to perform a task to the best of their ability and no mention is made of the relevance of the recent past.

2. Conceptual Implicit Memory Tests

As discussed previously, conceptual implicit memory tests show many of the same patterns exhibited by most explicit memory tests. For example, they show greater priming following meaning-based processing relative to superficial processing (a level-of-processing effect), and they also demonstrate a generation effect. One surprising finding is that a picture superiority effect is not observed on these tests, and the reasons for this unexpected finding are not well understood. In general, however, conceptual priming demonstrates the same patterns of results seen on most explicit tests.

D. Neural Correlates

As alluded to previously, damage to regions in the medial temporal lobes does not produce a general impairment on perceptual implicit memory tests. Whether general impairments occur on conceptual implicit memory tests is more controversial. The finding of intact perceptual priming in patients with damage to the hippocampus and surrounding structures within the medial temporal lobe was reported by Elizabeth Warrington and Lawrence Weiskrantz in the late 1960s. This finding spurred interest in the phenomenon of priming. Prior to this finding, it had been thought that the medial temporal lobes were globally important in memory; however, this finding demonstrated that this was not the case.

If the medial temporal lobes are not critical for producing perceptual priming, then what brain regions are? The brain mechanisms that underlie priming effects differ as a function of the type of implicit memory test. In general terms, brain regions that are critical for performing a task in the unprimed state are less active in the primed state. This makes sense if one thinks about priming as facilitation; the brain regions critical to performing the task have to put forth less effort in the primed (facilitated) state.

Consider first perceptual implicit memory tests. Reading a visually presented word taxes the visual system. However, regions in extrastriate visual cortex (Fig. 1) show less activity in the primed state relative to the unprimed state, consistent with the idea that less neural effort is required to perform the task; the neural pathways necessary to accomplish the goal are facilitated. Although less well studied, on the basis of this logic we would expect auditory implicit memory tests (e.g., identifying an auditory word stem) to show less activation in regions of the brain responsible for auditory processing (relative to the unprimed state).

Conceptual implicit memory tests, however, are not sensitive to the match or mismatch in perceptual features between the study and test phases; hence, the neural manifestation is not at the perceptual level. Rather, facilitation is observed at higher level regions of the brain, which are concerned with the task at hand. Consider the case of generating an associate to a presented word (e.g., given the word "elephant," the person would respond with a related word, such as "tusk"). This task calls on many brain regions, and two critically important regions lie within the left inferior frontal cortex (Fig. 1, anterior and posterior inferior frontal gyri). These regions show diminished activation in the primed condition. Again, it can be seen that regions that are important for performing the task have to put forth less effort to accomplish the task at hand in the primed condition. The brain is more efficient in the primed case.

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