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The relationship between remembering and knowing: A cognitive neuroscience perspective
Acta Psychologica 98 (1998) 253±265
The relationship between remembering and knowing: A cognitive neuroscience perspective Barbara J. Knowlton
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Department of Psychology, University of California, UCLA, Los Angeles, CA 90095, USA
Abstract Cognitive neuroscience has provided strong support for the idea that there are multiple memory systems. Recent evidence suggests that remembering and knowing may be two types of recognition with dierent neural substrates. The remember/know distinction is not equivalent to the explicit/implicit distinction because both remembering and knowing are impaired after damage to medial temporal lobe structures. A number of converging lines of evidence suggest that the relationship between remembering and knowing is one of redundancy, with ``knowing'' processes also active during remembering. Remembering appears to depend additionally on frontal lobe functioning. Ó 1998 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2343; 2520 Keywords: Memory; Recollection; Cognitive neuroscience; Remember; Know
1. Introduction Memory can be measured in a number of ways including recall, recognition, and priming. Each of these measures has dierent properties, and dissociations have been demonstrated between each of them. For example, recall, recognition and priming are each aected dierently by dierent types of brain damage (for reviews see Schacter et al., 1993; Richardson-Klavehn and Bjork, 1988). Even within recognition memory, dissociations have been demonstrated between dierent measures. In this
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paper, I will focus on two dierent measures of recognition that have been dissociated, remembering and knowing. In particular, I will discuss evidence regarding the relationship between these two types of responses and how the processes underlying these two types of responses could be mapped onto brain systems. 2. The remember/know distinction The remember±know distinction was ®rst proposed by Tulving in 1985, and has been studied extensively since then by a number of researchers (see Gardiner and Java, 1993 for a review). Remembering refers to recognition that is accompanied by conscious recollection of seeing the item previously. These recollections could take the form of the subject remembering what he or she thought of when the item was ®rst presented, or being able to actually put oneself at the moment when the item was ®rst seen. Knowing refers to recognition in the absence of such recollections. Subjects are said to recognize an item because they simply know it has been presented before, probably because of familiarity. However, subjects do not actually remember the moment the item was presented. In a typical procedure to measure these responses, subjects study a set of items, generally words, and at test they circle all recognized items among a list of targets and distracters. Beside each con®dently recognized word, the subject indicates whether it was ``Remembered'' or ``Known'' by writing either an R or a K. It has been shown that these two types of recognition are not merely measures of con®dence and can be dissociated from guessing (Gardiner and Java, 1990; Gardiner et al., 1996). What is measured by a K response is probably quite sensitive to the procedure used. For example, asking subjects to use two steps, that is, circling con®dently recognized words and then indicating R or K for each one, typically results in low false alarm rates for each response. However, simply indicating R, K, or N for new beside each item may result in much higher false alarm rates for K responses than R responses, suggesting that subjects under these circumstances are operationalizing the responses as high and low con®dence (Sarfatti and Knowlton, in preparation). The two-step procedure appears to limit the R or K decision to only those items that are con®dently recognized. This methodological point may be critical in determining the type of memory subjects are experiencing when they give a K response, and dierences in procedure between laboratories may be contributing to the question of whether R and K dissociations are simply threshold eects (see Donaldson, 1996). 3. Remember/know and explicit vs. implicit memory As mentioned above, these two types of recognition are interesting in that they can be readily dissociated. For example, R responses are sensitive to levels-of-processing eects, whereas K responses are not (Gardiner, 1988). Also, R responses decrease with divided attention at study, whereas K responses do not (Gardiner and
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Parkin, 1990). Aging also seems to have a far greater eect on R response accuracy than on K response accuracy (Parkin and Walter, 1992). These dissociations are similar to those seen between explicit memory measures and implicit memory measures such as priming. Jacoby and colleagues have shown that K responses behave similarly to automatic memory under manipulations such as changes in modality or stimulus size between study and test (Jacoby et al., 1997; Kelley and Jacoby, 1998, Yonelinas and Jacoby, 1995). Thus, it is possible that K responses re¯ect an implicit memory contribution to recognition. Certainly, implicit memory can contribute to recognition under some circumstances. For example, perceptual ¯uency, which is thought to underlie the phenomenon of priming, can give rise to feelings of recognition (Graf and Mandler, 1984; Jacoby and Dallas, 1981; Johnston et al., 1985; Mandler, 1980). However, it is still debatable whether perceptual ¯uency can in¯uence recognition under normal circumstances. In general, subjects appear to use perceptual ¯uency information when explicit memory is poor or judgments are speeded (Johnston et al., 1991). Also, in tasks in which subjects are asked to make judgments based on ``gut feelings'' such as in the fame judgment task, or the arti®cial grammar learning task, subjects may draw extensively on perceptual ¯uency (Buchner, 1994; Jacoby et al., 1989). On standard recognition memory tests, it is unclear whether perceptual ¯uency has a great in¯uence. A number of researchers have argued that recognition, even when based on familiarity, is a type of explicit memory (Moscovitch and Bentin, 1993; Squire, 1994). The main argument against the idea that there is an implicit memory contribution to recognition comes from data from amnesic patients. Despite the severe memory de®cits exhibited by these patients, they show intact implicit memory, including intact priming. As such, one would expect that these patients should exhibit disproportionately spared recognition memory as compared to recall performance. The data suggest that this is not the case in patients with circumscribed memory de®cits in the absence of frontal lobe dysfunction (Haist et al., 1992). These ®ndings argue against the idea that perceptual ¯uency commonly in¯uences recognition performance. However, allowing subjects to give know responses might tap into implicit memory representations. Recently, this idea was put to the test by examining the performance of amnesic patients on a recognition test in which they were required to indicate whether each recognized response was remembered or known (Knowlton and Squire, 1995). The results did not support the idea that know responses re¯ect implicit memory, in that amnesic patients showed impaired accuracy relative to control subjects for these responses. (Naturally, their performance using R responses was impaired as well.) These data suggest that K responses materially depend on explicit memory and that perceptual ¯uency is not adequate to support these responses. These data are consistent with the view that both familiarity and recollection depend on the brain structures damaged in amnesia (i.e. the hippocampus and/or associated brain regions) (Moscovitch and Bentin, 1993; Squire, 1994). Besides the main eect of group, there was an interaction between group (control vs. amnesic) and response type (R vs. K) on performance. From these data alone, one could still argue that perceptual ¯uency is making some contribution to K
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responses because amnesic patients exhibited a disproportionate impairment for R responses (although there was still an impairment relative to controls on K responses). However, one would predict this result if remember responses depended on the convergence of two kinds of memory: There would be two ways that an amnesic patient could fail to ``remember'' an item, but only one way he or she could fail to ``know'' the item. Thus, the question of how the processes underlying these responses are related is relevant to the question of the contribution of implicit memory to recognition. 4. The relationship between remembering and knowing The results from amnesic patients suggest that these two responses are two dierent expressions of explicit memory. In addition to the question of whether there is an implicit component to know responses, an understanding of the relationship between these two types of responses will be necessary in order to map the underlying processes onto brain systems. I will next address the question of how the processes underlying these two types of response are related. Using a framework lucidly described by Jones (1987), the two processes could be related in three dierent ways: exclusivity, independence, or redundancy. R and K processes could be exclusive, in that one of the two processes, but not both, could be active for an item in memory. Obviously, the two responses are exclusive, since subjects may assign either an R or a K to an item, but not both. R and K responses refer to states of awareness that exist to the exclusion of each other. However, these two responses may or may not re¯ect exclusive underlying processes (Richardson-Klavehn et al., 1996). Another way these two responses could be related is independence. For each item either the process leading to an R response, or the process leading to a K response is active, or both are active. Presumably if both are active an R response would result. This view is adopted by Kelley and Jacoby (1998). According to another view of independence, there are two processes, perhaps a K process and a contextual memory process, with an R response occurring only when the two are simultaneously active. According to the fourth view, redundancy, the process leading to R responses is only active when the process leading to K responses is also active. This view ®ts Tulving's description of semantic and episodic memory in that episodic memory can only exist if semantic memory for the episode is also present (Tulving, 1985). According to this view, remembering is knowing with something extra added, the episodic character of the memory. How can we dierentiate between these alternatives? Kelley and Jacoby (1998) discuss the process dissociation procedure as a means to address this question. I will discuss a dierent approach that takes advantage of the fact that each view makes a dierent prediction about what should happen across time as forgetting occurs. Under exclusivity, as R items are forgotten across time there should be no accompanying increase in K responses. Each of the other views posits that for some or all R response items, the process underlying K responses is also active. Thus, these responses have the potential to become K responses if the R process is lost. For the
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exclusivity view, one would predict that across time the rate R to K conversions should be low and not dierent than the rate of K to R conversions. For the other views, the rate of R to K conversions would be predicted to be higher than the rate of K to R conversions across time. In the study by Knowlton and Squire (1995) (Experiment 3) data relevant to this question was collected. Subjects were presented with a series of words and then their recognition memory for the words was tested 10 min later using the R and K procedure previously described. These same subjects returned one week later and were again tested for their recognition of the study words using the R and K procedure. In this second test, a new set of distracters was used. The key ®nding of this experiment was that signi®cantly more items received an R response at 10 min and later received a K response than items that ®rst received a K response and then an R response. These data are inconsistent with the idea that R and K responses re¯ect exclusive underlying memory processes. Rather, these data suggest that some or all R responses re¯ect the contribution of two underlying processes, one of which by itself gives rise to K responses. Other than eliminating the exclusivity alternative, can the data lend support to one of the other views? Each view makes speci®c predictions about the rate of R to K conversions over time. Using the data from Knowlton and Squire (1995) we can examine the speci®c predictions made by each model for the rate of R to K conversions. In order to calculate these predictions I used the percent of target items given R or K responses at 10 min and at one week to calculate forgetting rates. I corrected these values for guessing by subtracting the false alarm rate for each type of response, yielding 31.0% of targets receiving R responses and 14.8% receiving K responses at 10 min. At one week, 15.8% of the targets received R responses while 12.7% of the targets received K responses. Thus, across the one week delay, 15.2% of the targets lost the R response. According to the redundancy view, some of these R responses became K responses because the R process was no longer active, and some became misses because they lost both R and K processes. Fig. 1 shows Venn diagrams describing the relationship between the remember and know processes according to the redundancy view. To determine how many target items should become K responses, one notes that 30.0% of the target items have the potential to elicit K responses according to this view (K process active for 14.8% of targets from the ®rst test +15.2% of targets that lost the R process). At the one-week test, 12.7% of the targets elicited K responses. Thus, 12.7/30 or 42.3% of the items that could potentially elicit K responses eventually did, while the others became misses. If we assume that items that had lost episodic memory over the one week (converted from R to K) were as likely to lose the K process (and become misses) as items that had initially received the K response, then 42.3% of the 15.2% of items that lost the R response should remain K responses, or 6.4% of the targets. Thus, the predicted rate of R to K conversions should be 6.4/31.0 or 20.7% of the targets that initially received R responses should later receive K responses according to the redundancy view. According to the ®rst view of independence, an R response will result when both an R process and a K process are present for a particular item, while a K response will result when only the K process is active. If the R process is active alone the items
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Fig. 1. Venn diagrams representing the redundancy view are shown. The areas labeled R and K are proportional to the number of items receiving each of the responses after a 10 min and a one week delay. According to the redundancy view, items receiving an R response are a subset of the items for which a K process is active.
will not be recognized. One way this view could be conceptualized is to imagine that the K process is semantic memory and the other process is contextual memory. An R response will result when only semantic and contextual memory are both active for that item, and an item will be missed if there is only contextual memory without semantic memory. According to this view, R responses can be lost because either contextual or semantic memory (or both) is lost. This view is similar to the redundancy view in that an R response cannot result unless the K process is active. Fig. 2 shows Venn diagrams representing the relationship between R and K processes under the ®rst view of independence.
Fig. 2. Venn diagrams representing the ®rst view of independence. According to this view, items for which a K process and a second contextual memory process are active will receive an R response.
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To determine the predicted rate of R to K conversions, I calculated the probability that an R item loses contextual memory yet retains semantic memory. The percent of targets for which semantic memory is available equals the percent of targets that receive K responses plus the percent of targets that receive R responses (45.8% at 10 min and 28.5% at one week). So 62.2% of the items retained semantic memory (the K process) over the delay. According to this view of independence, the probability that an item receives an R response is the product of the probabilities of contextual and semantic memory being present. We can determine the probability of a target having contextual memory by dividing the probability of an R response by the probability that semantic memory is available (at 10 min, 31:0=45:8 67:7%, and at 1 week, 15:8=28:5 35:4%). From these values one can determine that (67.7% ) 35.4%)/67.7% or 18.2% of the targets that had contextual memory at 10 min did not have it at 1 week. Finally, one can calculate the probability of retaining semantic memory and losing contextual memory as (0.622) (0.182) 0.113. Thus, 11.3% of the study items that initially received an R response should later receive a K response by this view. According to a second view of independence, an R response will occur if an R process is active, and a K response will occur if a K process is active. Unlike the exclusivity view, however, for some items both processes will be active. These items would presumably receive R responses. This view was discussed by Kelley and Jacoby (1998) as it is the view assumed by the process dissociation procedure. By this view, items can lose an R response by becoming misses, or because they lose the R process but retain the K process. Fig. 3 shows Venn diagrams that describe the relationship between the R and K processes under this view of independence. In order to calculate the predicted rate of R to K conversions, one must calculate the percent of targets for which both R and K process are active in order to determine the percent of R response items that could potentially become K responses. The probability that an R process is active at 10 min is 31.0%. The probability that a K process is active at 10 min equals the probability of a K response (14.8%) plus (0.310) times the probability of the K process being active. Thus, the K process is active for 21.4% of the targets. Similarly, it can be calculated that the K process was active for
Fig. 3. Venn diagrams representing a second view of independence. According to this view, there are independent R and K processess. Items for which both processes are active receive an R response.
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15.1% of the targets at the one week delay. At the 10 min delay, the R and K processes are both active for (0.310) (0.214) or 6.6% of the targets. These items have the potential to convert from R responses to K responses. In order to calculate how many of these items lose the R process yet retain the K process, these two probabilities are multiplied. We know that 0:31 ÿ 0:158=0:31 or 0.49 of the items in which the R process is active at 10 min lose it across the one-week delay. We know that 0:151=0:214 or 0.706 of the items for which the K process is active at 10 min retain it at the one-week delay. Thus, the probability that an item for which both processes are active at 10 min will lose the R process yet retain the K process is (0.49) (0.706) or 0.346. Therefore, of the 6.6% of the targets for which both the R and K processes are active at 10 min, 34.6% of them will lose the R process yet will retain the K process. This represents (0.346) (0.066) or 2.3% of all targets. Thus, 0:023=0:31 or 7.4% of the items that initially received an R response would later receive a K response. How do these values compare to the data obtained from subjects in Knowlton and Squire (1995)? We found that a full 28.5% of the items that elicited an R response at 10 min elicited a K response at one week. This value is higher than that predicted by any of the models. However, this value is probably in¯ated. A full 9.8% of the items missed at 10 min received K responses at one week. This value re¯ects the fact that in the second test, subjects may be recognizing some targets from the ®rst test and not the study list. Perhaps some of the items converting from R to K were actually ®rst forgotten from the study list during the one-week interval and then received a K response at the second test based on memory from the ®rst list. If we consider 9.8% to be the rate for items to be recognized from the ®rst test and given a K response on the second test, we can obtain a conservative estimate for R to K conversions by subtracting this baseline, thus obtaining a corrected value of 18.7% of R items converting to K items. This value is numerically closest to that predicted by the redundancy view (20.7%) although with a standard error of 5.1%, it is not signi®cantly higher than the prediction of the ®rst independence view (11.3%). The obtained R to K conversion rate is signi®cantly higher than what would be predicted by the second view of independence (7.4%) or by the exclusivity view (0). Thus, the conversion rate was signi®cantly higher than what would be expected from an independence relationship that is a key assumption of the process dissociation procedure (Kelley and Jacoby, 1998). The data are only consistent with views that hold that R responses result when the process underlying K responses and an additional process are concurrently active. This position is consistent with the way that Tulving originally characterized R and K responses, with R responses re¯ecting episodic memory, and K responses re¯ecting semantic memory, in that episodic memory can only exist when semantic memory is present for the episode. According to the redundancy view, changes in R responses may or may not be accompanied by changes in the accuracy of K responses. Any manipulation that increases the R process should increase R responses while K responses decrease, since the R response will be active for more of the items for which the K process is active. Conversely, a decrease in the ecacy of the R process would decrease R responses, but might also in¯ate K responses, since more would be ``unmasked''. Studying nonwords vs. words seems to be such a manipulation (Gardiner and Java, 1990). Sub-
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jects can form memory traces for nonwords, yet they may have diculty building around them a rich network of associations necessary to lead to episodic storage. It is also logically possible that a change in R responses could be accompanied by no change in K responses if the manipulation aected both the K process and the R process. For example, subjects who study words under the in¯uence of a sedative (lorazepam) exhibit decreased R responses with no signi®cant decrease in K responses (Curran et al., 1993). If one assumes that lorazepam aects both the R and K processes, the increase in K responses resulting from the impaired R process could be counteracted by a decrease in the ecacy of the K process. Again, this seems likely because of the general eects of such a drug. The redundancy view predicts that changes in K responses cannot occur without changes in R responses also occurring. Any increase in ecacy of the K process should increase the number of both K and R responses. Any decrease in the ecacy of this process should decrease both types of response. However, there have been a few reports of changes in K responses in the absence of changes in R responses. For example, Rajaram (1993) found that preceding an item by a masked repetition of the item lead to an increase in K responses in the absence of a signi®cant change in R responses. One possible explanation for these ®ndings is that the manipulation aected a bias to give K responses, rather than the underlying K process itself. In support of this idea is the fact that in the study by Rajaram (1993) study, the masked pre-presentation increased K responses to both targets and distracters roughly equivalently. In another study, Gregg and Gardiner (1994) found that K responses were aected by the congruency of study and test modality when study items were presented rapidly and subjects performed a very shallow perceptual encoding task (bluriness judgments). R responses were unaected by test modality under these conditions. In contrast to the Rajaram study, the eect on K responses was not due to bias, because it was apparent in hit rates corrected by subtracting false alarm rates. As mentioned earlier, perceptual ¯uency does appear to aect recognition under some circumstances. In the Gregg and Gardiner (1994) experiment, the study conditions made encoding extremely dicult, and thus subjects had little else to go on besides perceptual ¯uency. It appears that when perceptual ¯uency does contribute to recognition judgments, it results in K responses and that these K responses arise from processes that may be exclusive of the processes underlying other types of recognition. However, the K responses obtained under these conditions may be dierent than the familiarity-based K responses obtained under standard study conditions. Although more data is needed, it appears that in general K responses cannot be manipulated without accompanying changes in R responses. 5. Source memory and remembering In a majority of subjects we tested, remembering responses were accompanied by speci®c memories of the immediate context surrounding the item's presentation. In other words, remembering responses were generally accompanied by source memory. In fact, it has been suggested that R responses occur when source memory is
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present and K responses occur when it is absent (Johnson et al., 1993). Performance on recollection judgments and source monitoring has been shown to be quite similar (Donaldson et al., 1996). However, if source memory is de®ned broadly as memory for the source of a memory, it is certainly also present for known items as well as for remembered items. All of the words had undoubtedly been encountered before, so subjects were basing their judgments on recognition from the particular study list and not whether the words were at all familiar. It is possible that subjects are not directly representing source information for K responses: It may be that K judgments are based on trace strength with recently presented items having higher activation exceeding a threshold for recognition. However, this seems unlikely when subjects are accurately making know judgments across a one-week interval. It may be that source information can be represented at multiple levels: At a general level, which may include information about relative recency, and at a more speci®c level, which would include information surrounding the moment the memory was formed. 6. Remember/know and cognitive neuroscience The redundancy view of R and K responses is also consistent with the ®nding that amnesic patients exhibit impaired performance of both types of responses since both episodic and semantic memory are impaired in amnesia. Both remember and know responses depend on the brain structures damaged in amnesia, yet it appears that remember responses depend additionally on other brain regions. There are data that suggest that the frontal lobes may be this additional region. Parkin and Walter (1992) found that the probability of an R response correlated with performance on the Wisconsin Card Sorting task, a measure of frontal lobe function. Frontal lobe damage also results in source memory de®cits (Janowsky et al., 1989; Schacter et al., 1984), which would aect R responses in particular because they draw heavily on source memory. Data from a recent event-related potential study are consistent with the idea that the frontal lobes are critical for source monitoring (Johnson et al., 1997). Functional information from the human brain also supports the view that the process supporting K responses is also active for R responses. Using the Remember/ Know paradigm, Smith (1993) showed that event-related potentials for new and old items diered starting at about 400 ms after stimulus onset, while event-related potentials for remembered vs. known targets diered only after about 550 ms. These ®ndings are consistent with the idea that there is a neural system involved in retrieval that is activated for both R and K items and that an additional system becomes active if the products of retrieval merit an R response. Smith suggests that the hippocampal formation and the frontal lobes correspond to these two systems. By this view, the frontal lobes become involved after retrieval success. Another view concerns the type of retrieval that occurs. Although both R and K responses re¯ect conscious memory, they dier in that retrieval of information leading to a K response is unconscious, whereas retrieval of information leading to a conscious recollections seems to be volitional (Richardson-Klavehn et al., 1996). Thus, perhaps the role of the frontal lobes in R responses is related to volitional retrieval.
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More recent evidence using functional magnetic resonance imaging can potentially provide direct information about the brain systems underlying the two types of responses. Gabrieli et al. (1997) showed that one part of the medial temporal lobe system, the parahippocampal cortex, was activated more when subjects were viewing novel stimuli than when they were viewing familiar stimuli. These data suggest that the medial temporal lobe memory system plays an important role in familiarity judgments of the type that may correspond to K responses. These investigators also found activation in this system during active retrieval of previously presented stimuli, consistent with the idea that R responses also depend on medial temporal lobe activation. Interestingly, the activation in these two tasks occurred within dierent medial temporal lobe regions, suggesting that familiarity and recollection may be independent at some level. However, dierent stimuli were used in the familiarity and retrieval tasks which may have contributed to the activation dierence. Some recent studies have also found frontal lobe activation (chie¯y right lateralized) during memory recall tasks in which subjects are likely to be retrieving episodic information (Buckner et al., 1995; Buckner et al., 1996; see Fletcher et al., 1997 for a review; Nyberg et al., 1996). Neuroimaging data has also begun to address the question of whether this frontal activity is related to retrieval eort or retrieval success. In a recent study, activation in the prefrontal cortex as measured by positron emission tomography (PET) covaried with the proportion of old items that were presented, consistent with the idea that the prefrontal cortex operates on the products of memory retrieval, and is thus active after successful retrieval and not simply during retrieval eort (Rugg et al., 1996). Although the functional anatomy of memory is revealing itself to be very complex, it seems that it will make an important contribution to the formation of theoretical models. In summary, it appears that R and K responses re¯ect a dissociation between two types of explicit memory. One dierence between the two types of responses is that R responses depend on both frontal lobe and medial temporal lobe structures. The dissociation between the R and K responses that has been obtained with manipulations such as divided attention and levels-of-processing may re¯ect dierential eects on frontal lobe functioning. References Buchner, A., 1994. Indirect eects of synthetic grammar learning in an identi®cation task. Journal of Experimental Psychology: Learning, Memory, and Cognition 20, 550±566. Buckner, R.L., Petersen, S.E., Ojemann, J.G., Miezin, F.M., Squire, L.R., Raichle, M.E., 1995. Functional anatomical studies of explicit and implicit memory retrieval tasks. Journal of Neuroscience 15, 12±29. Buckner, R.L., Raichle, M.E., Miezin, F.M., Petersen, S.E., 1996. Functional anatomic studies of memory retrieval for auditory words and visual pictures. Journal of Neuroscience 16, 6219±6235. Curran, H.V., Gardiner, J.M., Java, R.I., Allen, D., 1993. Eects of lorazepam upon recollective experience in recognition memory. Psychopharmacology 110, 374±378. Donaldson, W., 1996. The role of decision processes in remembering and knowing. Memory and Cognition 24, 523±533. Donaldson, W., MacKenzie, T.M., Underhill, C.F., 1996. A comparison of recollective memory and source monitoring. Psychonomic Bulletin and Review 3, 486±490.
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Richardson-Klavehn, A., Bjork, R.A., 1988. Measures of memory. Annual Review of Psychology 39, 475±543. Richardson-Klavehn, A., Gardiner, J.M., Java, R.I., 1996. Memory: Task Dissociations, process dissociations, and dissociations of consciousness. In: Underwood, G. (Ed.), Implicit Cognition. Oxford University Press, New York, pp. 85±158. Rugg, M.D., Fletcher, P.C., Frith, C.D., Frackowiak, R.S., Dolan, R.J., 1996. Dierential activation of the prefrontal cortex in successful and unsuccessful memory retrieval. Brain 119, 2073±2083. Sarfatti, S. and Knowlton, B.J. The in¯uence of test instructions on remember/know-decisions. Manuscript in preparation. Schacter, D.L., Chiu, C.Y., Ochsner, K.N., 1993. Implicit memory: A selective review. Annual Review of Neuroscience 16, 159±182. Schacter, D.L., Harbluk, J.L., McLachlan, D.R., 1984. Retrieval without recollection: An experimental analysis of source amnesia. Journal of Verbal Learning and Verbal Behavior 23, 593±611. Smith, M.E., 1993. Neurophysiological manifestations of recollective experience during recognition memory judgments. Journal of Cognitive Neuroscience 5, 1±13. Squire, L.R., 1994. Declarative and nondeclarative memory: Multiple brain systems supporting learning and memory. In: Schacter, D.L., Tulving, E. (Eds.), Memory Systems 1994. MIT Press, Cambridge, MA, pp. 203±231. Tulving, E., 1985. Memory and consciousness. Canadian Psychologist 26, 1±12. Yonelinas, A.P., Jacoby, L.L., 1995. The relation between remembering and knowing as bases for recognition: Eects of size congruency. Journal of Memory and Language 34, 622±643.
The relationship between remembering and knowing: A cognitive neuroscience perspective Barbara J. Knowlton
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Department of Psychology, University of California, UCLA, Los Angeles, CA 90095, USA
Abstract Cognitive neuroscience has provided strong support for the idea that there are multiple memory systems. Recent evidence suggests that remembering and knowing may be two types of recognition with dierent neural substrates. The remember/know distinction is not equivalent to the explicit/implicit distinction because both remembering and knowing are impaired after damage to medial temporal lobe structures. A number of converging lines of evidence suggest that the relationship between remembering and knowing is one of redundancy, with ``knowing'' processes also active during remembering. Remembering appears to depend additionally on frontal lobe functioning. Ó 1998 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2343; 2520 Keywords: Memory; Recollection; Cognitive neuroscience; Remember; Know
1. Introduction Memory can be measured in a number of ways including recall, recognition, and priming. Each of these measures has dierent properties, and dissociations have been demonstrated between each of them. For example, recall, recognition and priming are each aected dierently by dierent types of brain damage (for reviews see Schacter et al., 1993; Richardson-Klavehn and Bjork, 1988). Even within recognition memory, dissociations have been demonstrated between dierent measures. In this
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Tel.: (310) 825-5917; e-mail: [email protected].
0001-6918/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 1 - 6 9 1 8 ( 9 7 ) 0 0 0 4 5 - 0
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paper, I will focus on two dierent measures of recognition that have been dissociated, remembering and knowing. In particular, I will discuss evidence regarding the relationship between these two types of responses and how the processes underlying these two types of responses could be mapped onto brain systems. 2. The remember/know distinction The remember±know distinction was ®rst proposed by Tulving in 1985, and has been studied extensively since then by a number of researchers (see Gardiner and Java, 1993 for a review). Remembering refers to recognition that is accompanied by conscious recollection of seeing the item previously. These recollections could take the form of the subject remembering what he or she thought of when the item was ®rst presented, or being able to actually put oneself at the moment when the item was ®rst seen. Knowing refers to recognition in the absence of such recollections. Subjects are said to recognize an item because they simply know it has been presented before, probably because of familiarity. However, subjects do not actually remember the moment the item was presented. In a typical procedure to measure these responses, subjects study a set of items, generally words, and at test they circle all recognized items among a list of targets and distracters. Beside each con®dently recognized word, the subject indicates whether it was ``Remembered'' or ``Known'' by writing either an R or a K. It has been shown that these two types of recognition are not merely measures of con®dence and can be dissociated from guessing (Gardiner and Java, 1990; Gardiner et al., 1996). What is measured by a K response is probably quite sensitive to the procedure used. For example, asking subjects to use two steps, that is, circling con®dently recognized words and then indicating R or K for each one, typically results in low false alarm rates for each response. However, simply indicating R, K, or N for new beside each item may result in much higher false alarm rates for K responses than R responses, suggesting that subjects under these circumstances are operationalizing the responses as high and low con®dence (Sarfatti and Knowlton, in preparation). The two-step procedure appears to limit the R or K decision to only those items that are con®dently recognized. This methodological point may be critical in determining the type of memory subjects are experiencing when they give a K response, and dierences in procedure between laboratories may be contributing to the question of whether R and K dissociations are simply threshold eects (see Donaldson, 1996). 3. Remember/know and explicit vs. implicit memory As mentioned above, these two types of recognition are interesting in that they can be readily dissociated. For example, R responses are sensitive to levels-of-processing eects, whereas K responses are not (Gardiner, 1988). Also, R responses decrease with divided attention at study, whereas K responses do not (Gardiner and
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Parkin, 1990). Aging also seems to have a far greater eect on R response accuracy than on K response accuracy (Parkin and Walter, 1992). These dissociations are similar to those seen between explicit memory measures and implicit memory measures such as priming. Jacoby and colleagues have shown that K responses behave similarly to automatic memory under manipulations such as changes in modality or stimulus size between study and test (Jacoby et al., 1997; Kelley and Jacoby, 1998, Yonelinas and Jacoby, 1995). Thus, it is possible that K responses re¯ect an implicit memory contribution to recognition. Certainly, implicit memory can contribute to recognition under some circumstances. For example, perceptual ¯uency, which is thought to underlie the phenomenon of priming, can give rise to feelings of recognition (Graf and Mandler, 1984; Jacoby and Dallas, 1981; Johnston et al., 1985; Mandler, 1980). However, it is still debatable whether perceptual ¯uency can in¯uence recognition under normal circumstances. In general, subjects appear to use perceptual ¯uency information when explicit memory is poor or judgments are speeded (Johnston et al., 1991). Also, in tasks in which subjects are asked to make judgments based on ``gut feelings'' such as in the fame judgment task, or the arti®cial grammar learning task, subjects may draw extensively on perceptual ¯uency (Buchner, 1994; Jacoby et al., 1989). On standard recognition memory tests, it is unclear whether perceptual ¯uency has a great in¯uence. A number of researchers have argued that recognition, even when based on familiarity, is a type of explicit memory (Moscovitch and Bentin, 1993; Squire, 1994). The main argument against the idea that there is an implicit memory contribution to recognition comes from data from amnesic patients. Despite the severe memory de®cits exhibited by these patients, they show intact implicit memory, including intact priming. As such, one would expect that these patients should exhibit disproportionately spared recognition memory as compared to recall performance. The data suggest that this is not the case in patients with circumscribed memory de®cits in the absence of frontal lobe dysfunction (Haist et al., 1992). These ®ndings argue against the idea that perceptual ¯uency commonly in¯uences recognition performance. However, allowing subjects to give know responses might tap into implicit memory representations. Recently, this idea was put to the test by examining the performance of amnesic patients on a recognition test in which they were required to indicate whether each recognized response was remembered or known (Knowlton and Squire, 1995). The results did not support the idea that know responses re¯ect implicit memory, in that amnesic patients showed impaired accuracy relative to control subjects for these responses. (Naturally, their performance using R responses was impaired as well.) These data suggest that K responses materially depend on explicit memory and that perceptual ¯uency is not adequate to support these responses. These data are consistent with the view that both familiarity and recollection depend on the brain structures damaged in amnesia (i.e. the hippocampus and/or associated brain regions) (Moscovitch and Bentin, 1993; Squire, 1994). Besides the main eect of group, there was an interaction between group (control vs. amnesic) and response type (R vs. K) on performance. From these data alone, one could still argue that perceptual ¯uency is making some contribution to K
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responses because amnesic patients exhibited a disproportionate impairment for R responses (although there was still an impairment relative to controls on K responses). However, one would predict this result if remember responses depended on the convergence of two kinds of memory: There would be two ways that an amnesic patient could fail to ``remember'' an item, but only one way he or she could fail to ``know'' the item. Thus, the question of how the processes underlying these responses are related is relevant to the question of the contribution of implicit memory to recognition. 4. The relationship between remembering and knowing The results from amnesic patients suggest that these two responses are two dierent expressions of explicit memory. In addition to the question of whether there is an implicit component to know responses, an understanding of the relationship between these two types of responses will be necessary in order to map the underlying processes onto brain systems. I will next address the question of how the processes underlying these two types of response are related. Using a framework lucidly described by Jones (1987), the two processes could be related in three dierent ways: exclusivity, independence, or redundancy. R and K processes could be exclusive, in that one of the two processes, but not both, could be active for an item in memory. Obviously, the two responses are exclusive, since subjects may assign either an R or a K to an item, but not both. R and K responses refer to states of awareness that exist to the exclusion of each other. However, these two responses may or may not re¯ect exclusive underlying processes (Richardson-Klavehn et al., 1996). Another way these two responses could be related is independence. For each item either the process leading to an R response, or the process leading to a K response is active, or both are active. Presumably if both are active an R response would result. This view is adopted by Kelley and Jacoby (1998). According to another view of independence, there are two processes, perhaps a K process and a contextual memory process, with an R response occurring only when the two are simultaneously active. According to the fourth view, redundancy, the process leading to R responses is only active when the process leading to K responses is also active. This view ®ts Tulving's description of semantic and episodic memory in that episodic memory can only exist if semantic memory for the episode is also present (Tulving, 1985). According to this view, remembering is knowing with something extra added, the episodic character of the memory. How can we dierentiate between these alternatives? Kelley and Jacoby (1998) discuss the process dissociation procedure as a means to address this question. I will discuss a dierent approach that takes advantage of the fact that each view makes a dierent prediction about what should happen across time as forgetting occurs. Under exclusivity, as R items are forgotten across time there should be no accompanying increase in K responses. Each of the other views posits that for some or all R response items, the process underlying K responses is also active. Thus, these responses have the potential to become K responses if the R process is lost. For the
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exclusivity view, one would predict that across time the rate R to K conversions should be low and not dierent than the rate of K to R conversions. For the other views, the rate of R to K conversions would be predicted to be higher than the rate of K to R conversions across time. In the study by Knowlton and Squire (1995) (Experiment 3) data relevant to this question was collected. Subjects were presented with a series of words and then their recognition memory for the words was tested 10 min later using the R and K procedure previously described. These same subjects returned one week later and were again tested for their recognition of the study words using the R and K procedure. In this second test, a new set of distracters was used. The key ®nding of this experiment was that signi®cantly more items received an R response at 10 min and later received a K response than items that ®rst received a K response and then an R response. These data are inconsistent with the idea that R and K responses re¯ect exclusive underlying memory processes. Rather, these data suggest that some or all R responses re¯ect the contribution of two underlying processes, one of which by itself gives rise to K responses. Other than eliminating the exclusivity alternative, can the data lend support to one of the other views? Each view makes speci®c predictions about the rate of R to K conversions over time. Using the data from Knowlton and Squire (1995) we can examine the speci®c predictions made by each model for the rate of R to K conversions. In order to calculate these predictions I used the percent of target items given R or K responses at 10 min and at one week to calculate forgetting rates. I corrected these values for guessing by subtracting the false alarm rate for each type of response, yielding 31.0% of targets receiving R responses and 14.8% receiving K responses at 10 min. At one week, 15.8% of the targets received R responses while 12.7% of the targets received K responses. Thus, across the one week delay, 15.2% of the targets lost the R response. According to the redundancy view, some of these R responses became K responses because the R process was no longer active, and some became misses because they lost both R and K processes. Fig. 1 shows Venn diagrams describing the relationship between the remember and know processes according to the redundancy view. To determine how many target items should become K responses, one notes that 30.0% of the target items have the potential to elicit K responses according to this view (K process active for 14.8% of targets from the ®rst test +15.2% of targets that lost the R process). At the one-week test, 12.7% of the targets elicited K responses. Thus, 12.7/30 or 42.3% of the items that could potentially elicit K responses eventually did, while the others became misses. If we assume that items that had lost episodic memory over the one week (converted from R to K) were as likely to lose the K process (and become misses) as items that had initially received the K response, then 42.3% of the 15.2% of items that lost the R response should remain K responses, or 6.4% of the targets. Thus, the predicted rate of R to K conversions should be 6.4/31.0 or 20.7% of the targets that initially received R responses should later receive K responses according to the redundancy view. According to the ®rst view of independence, an R response will result when both an R process and a K process are present for a particular item, while a K response will result when only the K process is active. If the R process is active alone the items
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Fig. 1. Venn diagrams representing the redundancy view are shown. The areas labeled R and K are proportional to the number of items receiving each of the responses after a 10 min and a one week delay. According to the redundancy view, items receiving an R response are a subset of the items for which a K process is active.
will not be recognized. One way this view could be conceptualized is to imagine that the K process is semantic memory and the other process is contextual memory. An R response will result when only semantic and contextual memory are both active for that item, and an item will be missed if there is only contextual memory without semantic memory. According to this view, R responses can be lost because either contextual or semantic memory (or both) is lost. This view is similar to the redundancy view in that an R response cannot result unless the K process is active. Fig. 2 shows Venn diagrams representing the relationship between R and K processes under the ®rst view of independence.
Fig. 2. Venn diagrams representing the ®rst view of independence. According to this view, items for which a K process and a second contextual memory process are active will receive an R response.
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To determine the predicted rate of R to K conversions, I calculated the probability that an R item loses contextual memory yet retains semantic memory. The percent of targets for which semantic memory is available equals the percent of targets that receive K responses plus the percent of targets that receive R responses (45.8% at 10 min and 28.5% at one week). So 62.2% of the items retained semantic memory (the K process) over the delay. According to this view of independence, the probability that an item receives an R response is the product of the probabilities of contextual and semantic memory being present. We can determine the probability of a target having contextual memory by dividing the probability of an R response by the probability that semantic memory is available (at 10 min, 31:0=45:8 67:7%, and at 1 week, 15:8=28:5 35:4%). From these values one can determine that (67.7% ) 35.4%)/67.7% or 18.2% of the targets that had contextual memory at 10 min did not have it at 1 week. Finally, one can calculate the probability of retaining semantic memory and losing contextual memory as (0.622) (0.182) 0.113. Thus, 11.3% of the study items that initially received an R response should later receive a K response by this view. According to a second view of independence, an R response will occur if an R process is active, and a K response will occur if a K process is active. Unlike the exclusivity view, however, for some items both processes will be active. These items would presumably receive R responses. This view was discussed by Kelley and Jacoby (1998) as it is the view assumed by the process dissociation procedure. By this view, items can lose an R response by becoming misses, or because they lose the R process but retain the K process. Fig. 3 shows Venn diagrams that describe the relationship between the R and K processes under this view of independence. In order to calculate the predicted rate of R to K conversions, one must calculate the percent of targets for which both R and K process are active in order to determine the percent of R response items that could potentially become K responses. The probability that an R process is active at 10 min is 31.0%. The probability that a K process is active at 10 min equals the probability of a K response (14.8%) plus (0.310) times the probability of the K process being active. Thus, the K process is active for 21.4% of the targets. Similarly, it can be calculated that the K process was active for
Fig. 3. Venn diagrams representing a second view of independence. According to this view, there are independent R and K processess. Items for which both processes are active receive an R response.
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15.1% of the targets at the one week delay. At the 10 min delay, the R and K processes are both active for (0.310) (0.214) or 6.6% of the targets. These items have the potential to convert from R responses to K responses. In order to calculate how many of these items lose the R process yet retain the K process, these two probabilities are multiplied. We know that 0:31 ÿ 0:158=0:31 or 0.49 of the items in which the R process is active at 10 min lose it across the one-week delay. We know that 0:151=0:214 or 0.706 of the items for which the K process is active at 10 min retain it at the one-week delay. Thus, the probability that an item for which both processes are active at 10 min will lose the R process yet retain the K process is (0.49) (0.706) or 0.346. Therefore, of the 6.6% of the targets for which both the R and K processes are active at 10 min, 34.6% of them will lose the R process yet will retain the K process. This represents (0.346) (0.066) or 2.3% of all targets. Thus, 0:023=0:31 or 7.4% of the items that initially received an R response would later receive a K response. How do these values compare to the data obtained from subjects in Knowlton and Squire (1995)? We found that a full 28.5% of the items that elicited an R response at 10 min elicited a K response at one week. This value is higher than that predicted by any of the models. However, this value is probably in¯ated. A full 9.8% of the items missed at 10 min received K responses at one week. This value re¯ects the fact that in the second test, subjects may be recognizing some targets from the ®rst test and not the study list. Perhaps some of the items converting from R to K were actually ®rst forgotten from the study list during the one-week interval and then received a K response at the second test based on memory from the ®rst list. If we consider 9.8% to be the rate for items to be recognized from the ®rst test and given a K response on the second test, we can obtain a conservative estimate for R to K conversions by subtracting this baseline, thus obtaining a corrected value of 18.7% of R items converting to K items. This value is numerically closest to that predicted by the redundancy view (20.7%) although with a standard error of 5.1%, it is not signi®cantly higher than the prediction of the ®rst independence view (11.3%). The obtained R to K conversion rate is signi®cantly higher than what would be predicted by the second view of independence (7.4%) or by the exclusivity view (0). Thus, the conversion rate was signi®cantly higher than what would be expected from an independence relationship that is a key assumption of the process dissociation procedure (Kelley and Jacoby, 1998). The data are only consistent with views that hold that R responses result when the process underlying K responses and an additional process are concurrently active. This position is consistent with the way that Tulving originally characterized R and K responses, with R responses re¯ecting episodic memory, and K responses re¯ecting semantic memory, in that episodic memory can only exist when semantic memory is present for the episode. According to the redundancy view, changes in R responses may or may not be accompanied by changes in the accuracy of K responses. Any manipulation that increases the R process should increase R responses while K responses decrease, since the R response will be active for more of the items for which the K process is active. Conversely, a decrease in the ecacy of the R process would decrease R responses, but might also in¯ate K responses, since more would be ``unmasked''. Studying nonwords vs. words seems to be such a manipulation (Gardiner and Java, 1990). Sub-
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jects can form memory traces for nonwords, yet they may have diculty building around them a rich network of associations necessary to lead to episodic storage. It is also logically possible that a change in R responses could be accompanied by no change in K responses if the manipulation aected both the K process and the R process. For example, subjects who study words under the in¯uence of a sedative (lorazepam) exhibit decreased R responses with no signi®cant decrease in K responses (Curran et al., 1993). If one assumes that lorazepam aects both the R and K processes, the increase in K responses resulting from the impaired R process could be counteracted by a decrease in the ecacy of the K process. Again, this seems likely because of the general eects of such a drug. The redundancy view predicts that changes in K responses cannot occur without changes in R responses also occurring. Any increase in ecacy of the K process should increase the number of both K and R responses. Any decrease in the ecacy of this process should decrease both types of response. However, there have been a few reports of changes in K responses in the absence of changes in R responses. For example, Rajaram (1993) found that preceding an item by a masked repetition of the item lead to an increase in K responses in the absence of a signi®cant change in R responses. One possible explanation for these ®ndings is that the manipulation aected a bias to give K responses, rather than the underlying K process itself. In support of this idea is the fact that in the study by Rajaram (1993) study, the masked pre-presentation increased K responses to both targets and distracters roughly equivalently. In another study, Gregg and Gardiner (1994) found that K responses were aected by the congruency of study and test modality when study items were presented rapidly and subjects performed a very shallow perceptual encoding task (bluriness judgments). R responses were unaected by test modality under these conditions. In contrast to the Rajaram study, the eect on K responses was not due to bias, because it was apparent in hit rates corrected by subtracting false alarm rates. As mentioned earlier, perceptual ¯uency does appear to aect recognition under some circumstances. In the Gregg and Gardiner (1994) experiment, the study conditions made encoding extremely dicult, and thus subjects had little else to go on besides perceptual ¯uency. It appears that when perceptual ¯uency does contribute to recognition judgments, it results in K responses and that these K responses arise from processes that may be exclusive of the processes underlying other types of recognition. However, the K responses obtained under these conditions may be dierent than the familiarity-based K responses obtained under standard study conditions. Although more data is needed, it appears that in general K responses cannot be manipulated without accompanying changes in R responses. 5. Source memory and remembering In a majority of subjects we tested, remembering responses were accompanied by speci®c memories of the immediate context surrounding the item's presentation. In other words, remembering responses were generally accompanied by source memory. In fact, it has been suggested that R responses occur when source memory is
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present and K responses occur when it is absent (Johnson et al., 1993). Performance on recollection judgments and source monitoring has been shown to be quite similar (Donaldson et al., 1996). However, if source memory is de®ned broadly as memory for the source of a memory, it is certainly also present for known items as well as for remembered items. All of the words had undoubtedly been encountered before, so subjects were basing their judgments on recognition from the particular study list and not whether the words were at all familiar. It is possible that subjects are not directly representing source information for K responses: It may be that K judgments are based on trace strength with recently presented items having higher activation exceeding a threshold for recognition. However, this seems unlikely when subjects are accurately making know judgments across a one-week interval. It may be that source information can be represented at multiple levels: At a general level, which may include information about relative recency, and at a more speci®c level, which would include information surrounding the moment the memory was formed. 6. Remember/know and cognitive neuroscience The redundancy view of R and K responses is also consistent with the ®nding that amnesic patients exhibit impaired performance of both types of responses since both episodic and semantic memory are impaired in amnesia. Both remember and know responses depend on the brain structures damaged in amnesia, yet it appears that remember responses depend additionally on other brain regions. There are data that suggest that the frontal lobes may be this additional region. Parkin and Walter (1992) found that the probability of an R response correlated with performance on the Wisconsin Card Sorting task, a measure of frontal lobe function. Frontal lobe damage also results in source memory de®cits (Janowsky et al., 1989; Schacter et al., 1984), which would aect R responses in particular because they draw heavily on source memory. Data from a recent event-related potential study are consistent with the idea that the frontal lobes are critical for source monitoring (Johnson et al., 1997). Functional information from the human brain also supports the view that the process supporting K responses is also active for R responses. Using the Remember/ Know paradigm, Smith (1993) showed that event-related potentials for new and old items diered starting at about 400 ms after stimulus onset, while event-related potentials for remembered vs. known targets diered only after about 550 ms. These ®ndings are consistent with the idea that there is a neural system involved in retrieval that is activated for both R and K items and that an additional system becomes active if the products of retrieval merit an R response. Smith suggests that the hippocampal formation and the frontal lobes correspond to these two systems. By this view, the frontal lobes become involved after retrieval success. Another view concerns the type of retrieval that occurs. Although both R and K responses re¯ect conscious memory, they dier in that retrieval of information leading to a K response is unconscious, whereas retrieval of information leading to a conscious recollections seems to be volitional (Richardson-Klavehn et al., 1996). Thus, perhaps the role of the frontal lobes in R responses is related to volitional retrieval.
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More recent evidence using functional magnetic resonance imaging can potentially provide direct information about the brain systems underlying the two types of responses. Gabrieli et al. (1997) showed that one part of the medial temporal lobe system, the parahippocampal cortex, was activated more when subjects were viewing novel stimuli than when they were viewing familiar stimuli. These data suggest that the medial temporal lobe memory system plays an important role in familiarity judgments of the type that may correspond to K responses. These investigators also found activation in this system during active retrieval of previously presented stimuli, consistent with the idea that R responses also depend on medial temporal lobe activation. Interestingly, the activation in these two tasks occurred within dierent medial temporal lobe regions, suggesting that familiarity and recollection may be independent at some level. However, dierent stimuli were used in the familiarity and retrieval tasks which may have contributed to the activation dierence. Some recent studies have also found frontal lobe activation (chie¯y right lateralized) during memory recall tasks in which subjects are likely to be retrieving episodic information (Buckner et al., 1995; Buckner et al., 1996; see Fletcher et al., 1997 for a review; Nyberg et al., 1996). Neuroimaging data has also begun to address the question of whether this frontal activity is related to retrieval eort or retrieval success. In a recent study, activation in the prefrontal cortex as measured by positron emission tomography (PET) covaried with the proportion of old items that were presented, consistent with the idea that the prefrontal cortex operates on the products of memory retrieval, and is thus active after successful retrieval and not simply during retrieval eort (Rugg et al., 1996). Although the functional anatomy of memory is revealing itself to be very complex, it seems that it will make an important contribution to the formation of theoretical models. In summary, it appears that R and K responses re¯ect a dissociation between two types of explicit memory. One dierence between the two types of responses is that R responses depend on both frontal lobe and medial temporal lobe structures. The dissociation between the R and K responses that has been obtained with manipulations such as divided attention and levels-of-processing may re¯ect dierential eects on frontal lobe functioning. References Buchner, A., 1994. Indirect eects of synthetic grammar learning in an identi®cation task. Journal of Experimental Psychology: Learning, Memory, and Cognition 20, 550±566. Buckner, R.L., Petersen, S.E., Ojemann, J.G., Miezin, F.M., Squire, L.R., Raichle, M.E., 1995. Functional anatomical studies of explicit and implicit memory retrieval tasks. Journal of Neuroscience 15, 12±29. Buckner, R.L., Raichle, M.E., Miezin, F.M., Petersen, S.E., 1996. Functional anatomic studies of memory retrieval for auditory words and visual pictures. Journal of Neuroscience 16, 6219±6235. Curran, H.V., Gardiner, J.M., Java, R.I., Allen, D., 1993. Eects of lorazepam upon recollective experience in recognition memory. Psychopharmacology 110, 374±378. Donaldson, W., 1996. The role of decision processes in remembering and knowing. Memory and Cognition 24, 523±533. Donaldson, W., MacKenzie, T.M., Underhill, C.F., 1996. A comparison of recollective memory and source monitoring. Psychonomic Bulletin and Review 3, 486±490.
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