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1 Behavioural Brain Research 000 (2001) Research report Perspectives on object-recognition memory following hippocampal damage: lessons from studies in rats Dave G. Mumby * Department of Psychology, DS-413, Concordia Uni ersity, 7141 Sherbrooke St. W., Montral, Que., Canada H4B 1R6 Received 11 February 2001; received in revised form 29 June 2001; accepted 25 July 2001 Abstract One of the routine memory abilities impaired in amnesic patients with temporal-lobe damage is object-recognition memory the ability to discriminate the familiarity of previously encountered objects. Reproducing this impairment has played a central role in animal models of amnesia during the past two decades, and until recent years most of the emphasis was on describing how hippocampal damage could impair object recognition. Today most investigators are looking outside the hippocampus to explain the impairment. This paper reviews studies of object-recognition memory in rats with hippocampal damage produced by ablation, fornix transection, or forebrain ischemia. Some new perspectives on previous findings reinforce the conclusion that damage to the hippocampus has little if any impact on the ability to recognize objects, while damage in some areas outside the hippocampus is far more effective. The few circumstances in which hippocampal damage can impair performance on object-recognition tasks are situations where ancillary abilities are likely to play a significant role in supporting task performance. Some of the factors that contributed to the origins and persistence of the hippocampalcentric view of object-recognition are considered, including lesion confounds, failure to distinguish between impaired task performance and impairment of a memory ability, and disproportionate attention to a few lesion studies in monkeys, even though the hypothesis was tested far more times in rats, under a greater variety of conditions, and rejected on nearly every occasion Published by Elsevier Science B.V. Keywords: Hippocampus; Fornix; Medial temporal lobe; Object recognition; Amygdala; Amnesia; Ischemia 1. Introduction One of the routine memory abilities impaired in amnesic patients is the capacity to recognize the people and objects they have encountered since the onset of their amnesia. Recognition memory can be described in different ways, but it is generally regarded as the ability to discriminate the familiarity of things previously encountered [3]. Recognition occurs in all sensory domains, but most research into the anatomical bases of recognition impairments in human amnesics is concerned with visual recognition. Attempts to reproduce visual-recognition impairments like those displayed by amnesic patients have played a central role in animal models of amnesia during the past two and a half decades, and until recent years most of the emphasis was on describing how damage to the hippocampal formation (HPC; including the dentate gyrus, Ammon s horn, and subiculum) could impair visual recognition, either alone or in combination with damage to other medial-temporal-lobe structures. Today, it is clear that the role of the HPC in recognition memory has been overemphasized, and most investigators are now looking elsewhere for an explanation of the recognition impairment displayed by amnesic patients. This includes many of those who study clinical populations and those who study animal models of amnesia. But there is still debate about the consequences of focal hippocampal damage for visual recognition. This review examines studies in which object-recognition memory, a subcategory of visual recognition, was assessed in rats after the HPC was made dysfunctional either by ablation or by fornix transection. It has two * Tel.: ; fax: address: mumby@vax2.concordia.ca (D.G. Mumby) /01/$ - see front matter 2001 Published by Elsevier Science B.V. PII: S (01)

2 2 D.G. Mumby / Beha ioural Brain Research 000 (2001) main goals: The first is to show that the findings from lesion studies in rats are fairly consistent in suggesting that HPC damage does not impair object-recognition memory. HPC damage might impair other memory abilities, but that issue will not be discussed. The second goal of this review accounts for the focus on lesion studies in rats, and it is to counter the tendency to give somewhat marginal consideration to findings in rats when making inferences about the neuroanatomical bases of amnesia in humans. With a few exceptions, most discussions in the literature concerning animal models of amnesia emphasize findings in monkeys, while corresponding studies in rats are either ignored entirely, or given only secondary consideration. When the relevant findings in rats are cited, it is often merely to demonstrate the generalizability of results obtained in monkeys, rather than to point out sources of original evidence. It is hoped this review will show that studies with rats can provide unique information not ascertained from studies in monkeys. Treatments that undoubtedly cause severe disruption of hippocampal function, including HPC ablation or fornix transection, have failed to impair performance on object-recognition tasks in rats under a wide range of conditions. Not surprisingly, there are a few exceptions. But upon close scrutiny, some of those appear to be illusory in the extent to which they reflect true impairments of object recognition. Together, the preponderance of evidence from rats suggests that the hippocampus plays, at most, a very limited role in object-recognition, and perhaps none at all. A similar conclusion was reached in recent reviews of neuroanatomical studies of spatial and nonspatial recognition in rats [3,34,101]. The present review clarifies some important evidence that was either addressed only partially by those earlier reviews, or excluded altogether. It discusses new findings that have come to light in recent months, and considers some new perspectives that were not reflected in the earlier reviews. It purposely excludes several converging lines of evidence from studies using nonlesion methods. These exclusions are made because the question asked in this review is whether damage to the HPC impairs the ability to recognize an object. Only lesion experiments can directly address this question. Broader reviews, like two of those just mentioned [3,101], serve the essential purpose of tying together converging evidence from different approaches to studying brain-memory relations, including experimental lesions, electrophysiological recording, brain imaging, psychopharmacology, neurogenetics, neuroanatomical studies, and neuropsychological assessments of clinical cases and populations. Most if not all investigators depend upon reviews like that to get an idea of how their own findings fit with those of other researchers working on similar problems. But as the number of published studies using each approach continues to grow at an accelerated rate, those undertaking the more general reviews have no choice but to deal with an enormous literature by giving only summary descriptions of the findings from the individual studies that use each particular approach. The references to previous findings are sometimes misleading, because they exclude qualifying details that would only come from a closer examination of what was originally reported. This review focuses on only one approach to studying brain memory relations, but the more limited scope allows for a more in-depth analysis of several key experiments. In this way, the present review differs from the earlier ones that addressed the broader questions concerning the role of the hippocampus in recognition memory [3,101]. Recognition of different visual aspects of an experience can be distinguished psychologically, and also in brain circuitry. The question addressed in this review is specific to object-recognition memory the ability to discriminate the familiarity of previously encountered objects. Thus, it excludes studies that examined the effects of HPC lesions on recognition of things like brightness cues, or visual features embedded in spatial arrays (e.g. distinctive patterns on the arms of a radial maze), or two-dimensional graphic images projected on a computer screen, notwithstanding the possibility that there may be important similarities in the brain circuitry required for recognizing these other types of visual stimuli and that underlying recognition of threedimensional objects. The controversy over the effects of HPC damage on object recognition is best appreciated in its historical context. It has origins in the 1950s with the discovery of global amnesia in humans after medial-temporallobe damage, and the subsequent development of animal models of amnesia in monkeys. The second main section of this paper provides a summary account of that story. During the past 15 years, a variety of behavioural methods for studying object-recognition memory in rats have been developed, thus providing a more accessible model and with it the means for more and larger-scale parametric studies on the effects of brain damage on object-recognition memory. The third main section describes the tasks that have been developed for rats and considers evidence that they involve recognition processes similar to those involved in the monkey versions. The Section 4 reviews the studies conducted in rats that are relevant to the question of whether HPC damage impairs object recognition. The Section 5 attempts to place the findings in rats into perspective, thus allowing a justified conclusion to be made about the consequences of HPC damage for object recognition.

3 D.G. Mumby / Beha ioural Brain Research 000 (2001) Object recognition and monkey models of amnesia The discovery of H.M. s amnesia came at a time when most psychologists accepted Karl Lashley s proposition that memory traces are widely distributed throughout the cortex, rather than localized within particular structures. Lashley s experiments with animals had appeared to demonstrate that the degree of impairment on learning tasks was proportional to the amount of cortex damaged, but unrelated to the particular area of damage. The occurrence of severe and selective memory impairment in H.M. after lesions that were restricted to medial-temporal-lobe structures led to renewed support for localisationist views of how mental functions are organized within the brain, and experimental psychologists began searching for the storage site for memory engrams. They began by focusing on the structures of the medial temporal lobes. H.M. s amnesia was originally attributed to the removal of his hippocampus for several reasons. Among them, there had been previous reports of patients who suffered from amnesia following bilateral hippocampal damage [19,45,108]. There also appeared to be a rough correlation between the extent of hippocampal damage and the severity of amnesia in a group of amnesic patients with bilateral medial-temporal-lobe resections [97]. Moreover, patients with bilateral damage that was largely limited to the amygdala did not have amnesia [97]. Still, the evidence from brain-damaged humans was far from conclusive. Horel pointed out that Scoville s surgical technique for removing medial-temporal-lobe structures in H.M. and other patients must also have damaged the temporal stem a fibre pathway that links the temporal cortex with other brain areas [52]. He hypothesized that bilateral temporal-stem damage, not bilateral hippocampal damage, was the cause of medial-temporal-lobe amnesia. This could account for the positive correlation noted above, because more extensive hippocampal resection would damage more of the temporal stem. In fact, an earlier study found that memory defects in a group of amnesic patients were more highly correlated with the extent of temporal neocortical damage than with the extent of hippocampal damage [47]. Not only was there little compelling evidence from amnesic patients to support the hippocampalcentric interpretation of medial-temporal-lobe amnesia, there was considerable contrary evidence from studies in nonhuman animals. Experiments with rats and monkeys that were conducted in the 1950s and 1960s found only inconsistent effects of HPC ablation on the performance of memory tasks. Monkeys with bilateral hippocampal removals were only slightly impaired in retention of preoperatively learned visual discriminations [66] and could learn new visual-discriminations normally, even when the trials were widely separated in time and the monkeys performed irrelevant discriminations during the intertrial intervals [85]. Monkeys with bilateral HPC lesions performed normally on delayedresponse tasks [[66,69,85], cf. [28]], and delayed matching-to-sample [27,32]. In some studies, monkeys with HPC lesions were impaired on a delayed-alternation task [87,85], while in others there were inconsistent results, and some subjects with large hippocampal lesions performed normally [28,112]. There was similar evidence that HPC ablation spared many memory abilities in rats. Although rats with HPC damage had difficulty learning mazes [57,60], and passive avoidance tasks [60] and delayed alternation [89], they had no difficulty performing brightness-discriminations [60], and their performance of active avoidance tasks was actually improved [54]. By the mid-1960s, the study of amnesic patients began to shed light on the nature of the spared memory abilities that followed medial-temporal-lobe damage. Evidence that procedural-learning is spared in amnesia provided an explanation for why bilateral hippocampal damage did not impair visual-discrimination learning in laboratory animals. On visual-discrimination tasks, it is not necessary for the subject to remember what happened on individual trials because the information that is relevant to successful performance is presented repeatedly over many trials. That is, visual discrimination tasks are not tests of explicit memory (declarative memory); rather, they are similar to the procedural-memory tasks that amnesic patients are capable of learning, and, therefore, they are unlikely to be very sensitive to amnesia. Thus, in early monkey experiments, the lesions may have produced amnesia that went undetected by the visual-discrimination tasks that were used to assess it. Evidence that short-term memory abilities are spared in patients with amnesia provided an explanation for why HPC damage did not impair the performance of laboratory animals in the earlier experiments using delay tasks. For example, the longest retention delays used to test delayed matching-to-sample or delayed-response performance in monkeys with hippocampal damage was only 12 s, or less [27,32], well within the range of short-term memory. Bilateral HPC damage may have produced amnesia that went undetected because the retention delays were too brief [31]. By the end of the 1960s, it was becoming apparent that the development of an animal model of amnesia would first require the development of memory tests for laboratory animals that amnesic patients would be expected to fail [43]. In the mid 1970s, such tasks were developed the trial-unique (or nonrecurring-items) delayed nonmatching-to-sample (DNMS) task [68], and its counterpart, delayed matching-to-sample (DMS) [43]. On each trial, a sample object is presented and the subject displaces it to retrieve a food reward hidden

4 4 D.G. Mumby / Beha ioural Brain Research 000 (2001) beneath. Following a delay during which the sample object is hidden from view, it is presented again, along with a novel object. For DNMS, the subject is rewarded for selecting the novel object from this pair, whereas for DMS the subject is rewarded for selecting the sample object. The important development was the use of large stimulus sets, which allowed for the provision of new items on every trial. Delayed matching- and nonmatching-to-sample procedures had been used with animals for decades before, but usually only a few items were used repeatedly over trials within each session [9,27,113]. When trial-unique stimuli are used, the subject has to discriminate between a sample object that, if remembered, would be familiar, and a novel object that had never been seen before and which must, therefore, be unfamiliar. In contrast, when a small number of items are repeatedly presented on several trials within a session, they all quickly become familiar and the task then becomes a way to assess recency memory, which shares some, but not all properties of recognition memory. For the remainder of this paper, references to DNMS or DMS will refer to the trial-unique or pseudo-trial-unique versions, unless otherwise specified. Pseudo-trial-unique refers to procedures in which stimulus items never recur within a session, but may recur across multiple sessions widely separated in time. For this review, it will be assumed that pseudo-trial-unique and truly trial-unique procedures engage the same recognition processes. The DNMS and DMS tasks are unique among memory tests for laboratory animals because they resemble human recognition-memory tasks. In a typical recognition-memory test for humans, a list of items is presented (e.g. a list of pictures, words, or nonsense syllables), and later, a test list that includes items from the first list and some new items is presented, and the subject must identify the items that appeared in the first list. The DMS and DNMS tasks involve the same principle, because the subject has to determine which test items were previously encountered. The memory demands on either task can be manipulated by controlling the duration of the retention delay, or by presenting lists of sample objects that must be retained over the delay. When a different pair of objects is used on each DNMS trial, the subject consistently receives a reward for displacing an object it has not encountered previously. Monkeys quickly master the DNMS task with brief retention delays of only a few seconds, and once they have done so, their performance is very good at delays of up to several minutes [68]. This is another important feature of the DNMS task, because it means that memory can be assessed using retention delays beyond those for which short-term memory span is capable of supporting performance. The trial-unique DNMS task continues to be the most widely used paradigm for assessing object recognition in primates. In the 1970s, two important findings suggested that the HPC plays a critical role in visual object recognition in primates: First, Gaffan [43] reported that monkeys with fornix lesions were impaired on a trial-unique DMS task, and later, Mishkin [67] reported that monkeys with bilateral amygdalo hippocampal lesions displayed delay-dependent deficits on the trial-unique DNMS task. Importantly, Mishkin also reported that monkeys with bilateral lesions restricted to either the hippocampus or amygdala were only mildly impaired. The severe impairment of DNMS performance in monkeys with amygdalo hippocampal lesions appeared to be a good animal model of medial-temporal-lobe amnesia for at least three reasons: because the brain damage in those monkeys was similar to that of patients with medial-temporal-lobe amnesia, because accurate DNMS performance requires a type of memory ability that is impaired in patients with medial-temporal-lobe amnesia, and because delay-dependant deficits are exactly what would be expected in amnesic patients, because intact short-term memory abilities could support performance at brief delays but not at longer delays. Indeed, it would later be shown that amnesic patients with temporal-lobe damage show similar delaydependent deficits on the DNMS task [105], and are also impaired on DMS tasks using visual stimuli [6,8,51]. After Mishkin s demonstration that bilateral amygdalo-hippocampal lesions disrupt DNMS performance in monkeys, several studies seemed to confirm the relatively minor effects of HPC lesions [13,63,123], and the synergistic effects of lesions involving both the HPC and amygdala [14,80,95,123]. These findings led to the proposal that the HPC and amygdala are critical links in parallel neural circuits involved in visual recognition, and that each circuit can partially compensate for damage in the other [48,106], but evidence contrary to this view came from studies that found severe DNMS deficits in monkeys with bilateral HPC lesions [63,124]. Starting in the late-1980s and throughout 1990, it became increasingly clear that the DNMS deficits produced in monkeys by removing the HPC and amygdala resulted from incidental damage to rhinal cortex (i.e. the perirhinal and entorhinal cortices). The conventional method for making medial-temporal lesions was tissue aspiration, and in order to gain access to the HPC and amygdala, it was necessary to remove portions of the overlying rhinal cortex and parahippocampal gyrus. Moreover, aspiration of the amygdala often damaged temporal fibers projecting from the rhinal cortical areas to the thalamus [49]. The greater extent of the collateral damage to these cortical areas, or to their efferent fibers, probably contributed to the more severe DNMS impairment observed following amygdalo

5 D.G. Mumby / Beha ioural Brain Research 000 (2001) hippocampal lesions than following separate lesions of either the HPC or amygdala. Monkeys with bilateral lesions of the entorhinal and perirhinal cortex [65] or of the perirhinal cortex alone [65,127] were severely impaired on the DNMS task. Consistent with these findings, monkeys displayed severe DMS deficits following either ablation or reversible cooling lesions of the inferior-temporal gyrus, which includes much of the perirhinal cortex [53]. Eventually, it was concluded that amygdala damage did not make a substantial contribution to the object-recognition deficits of monkeys with large medial-temporallobe lesions [49,79,126]. It is worth noting here that temporal-stem damage could account for the impairments in monkeys with large nonselective medial-temporal lesions. Although, one study found that monkeys with bilateral temporalstem lesions were unimpaired on the DNMS task [128], it was subsequently found that temporal-stem lesions placed more anterior produced a severe DMS impairment [23]. The more anterior lesions damaged the portions of the temporal stem that would be expected to be damaged in humans with extensive hippocampal resections, whereas the more posterior temporal-stem lesions did not. The anterior temporal stem lesions would have also disconnected more extensive portions of the rhinal cortex from efferent structures [49]. This is significant because there is now general agreement that medialtemporal-lobe damage is most likely to cause severe DNMS deficits in monkeys if it includes the rhinal cortex. A number of lesion studies conducted in rats have come to the same conclusion [15,46,75,115]. However, controversy remains over the question of whether lesions limited to the HPC are capable of producing a recognition impairment in monkeys that is comparable to the profound recognition impairment that is often displayed by patients with medial-temporal-lobe damage. The refinement of stereotaxic lesion techniques have enabled more selective lesions of the HPC in monkeys than were previously possible using aspiration, but this has done little to quell the controversy. Some investigators have found substantial object-recognition deficits in monkeys with bilateral radiofrequency or neurotoxic HPC lesions [10,18,131], whereas others have found no DNMS deficits [81]. One way of evaluating the data from a selection of these studies in monkeys even suggests an inverse relationship between the extent of damage to the HPC and the severity of the DNMS deficit [17]. Two points are interesting to note here. First, it seems that somewhere along the way the question motivating studies of medial-temporal lesions and DNMS performance in monkeys has changed in subtle but important ways. At first, it was assumed that reproducing a visual-recognition impairment like that displayed by amnesic patients would constitute an isomorphic animal model of the amnesic syndrome. Accordingly, the question that was addressed in the first few years of DNMS studies was, What medial-temporal-lobe structures must be damaged to produce the amnesic syndrome? As it became apparent that the severity of the memory impairment was related to the extent of the medial-temporal-lobe damage, including structures other than the HPC, the specific question about the HPC became, What contribution does HPC damage make to amnesia? Most investigators now acknowledge that the DNMS task does not represent all of the memory abilities that are impaired in amnesia. Today, the question that motivates studies of the effects of HPC lesions on DNMS performance in monkeys asks, Does damage limited to the HPC impair object-recognition memory? The questions that represent the different stages in the history of lesion studies and DNMS performance in monkeys reflect a movement away from attempts to develop isomorphic models of amnesia that reproduce all of the features of human amnesia, and toward models that reproduce only select aspects of amnesia. This trend acknowledges the evidence that global amnesia reflects multiple dissociable memory deficits, each resulting from damage in particular structures, or from specific combinations of damaged structures. This may explain why in recent years the term amnesia seems to be appearing less frequently in the descriptions of studies in animals that employ the DNMS or DMS tasks; it is being replaced by phrases like object-recognition deficit or impaired visual recognition. 3. Object-recognition tasks for rats 3.1. Delayed matching and nonmatching-to-sample During the latter half of the 1980s, several investigators were successful in adapting the DNMS task for use with rats [1,59,73,93]. This broadened the comparative basis for drawing inferences about the anatomical bases of object recognition, because now the same experiments can be conducted in rats and monkeys. This has practical advantages, because COMPARED WITH monkeys, the laboratory rat provides a more accessible model for most researchers, and therefore, facilitates the conduct of large-scale parametric experiments. One consequence has been that studies in rats have provided more data to test the hypothesis that HPC damage impairs object recognition. There have been more experiments in rats than in monkeys, and with only a few exceptions, those done with rats have included far more subjects than have been included in most experiments in monkeys.

6 6 D.G. Mumby / Beha ioural Brain Research 000 (2001) At least four different DNMS paradigms have been used to assess the effects of HPC damage in rats. The first was developed by Aggleton [1]. It uses a Y-maze in which distinctive goal boxes containing objects can be inserted into the arms of the maze to serve as test stimuli. At the beginning of a trial, the rat is confined for 20 s in a sample goal box containing distinctive objects, which is replaced with a featureless box for the retention delay, after which the rat is given access to the other two arms of the Y-maze. Each arm contains a distinctive goal box one of them matches the sample goal box, and the other one is novel, containing different objects. The rat is rewarded if it enters the novel goal box. This was the first demonstration that rats could perform well on an object-based DNMS task. After they mastered the task at short delays, the rats averaged approximately 80% correct at delays of up to 120 s. Rats were also able to master a DMS task in the Y-maze apparatus [1]. Within a few years, other DNMS tasks bearing even closer resemblance to the monkey version were developed by Rothblat and Hayes [93], Mumby and his colleagues [73], and Kesner [59]. Although the apparatuses and procedures are slightly different, they are also similar to each other and to the monkey DNMS paradigm in several key respects. For example, they use object stimuli similar to those used in the monkey task, and the rats select objects by displacing them from over food wells. The duration of exposure to the sample object is controlled by the rat, and is usually between 0.5 and 2 s (unpublished observations), which is more like the exposure durations in monkey studies than the 20 s of experimenter-controlled exposure in the Y-maze task. The Y-maze DNMS task uses a continuous-trials procedure (i.e. the correct stimulus on one trial serves as the sample for the next trial), whereas the monkey DNMS task and the other rat versions use discrete trials, each consisting of a sample phase and a choice phase, with two new objects on each trial. Regardless of the particular rat DNMS version, the typical procedure uses pseudo-trial-unique stimulus presentation that is, individual objects are used on only one trial per session, but they recur on other sessions. Thus, on later sessions, the rat has to make a discrimination based on relative familiarity, as both the sample and novel object will have been encountered before. Of course, the sample will have been encountered only moments ago, and the novel object will have been encountered days or weeks ago, so it is reasonable to assume that pseudo-trial-unique procedures engage recognition processes similar to those engaged by truly trial-unique procedures. In order to use the DNMS paradigm to study the neuroanatomy of visual object recognition, and to compare findings in rats, monkeys, and humans, there needs to be some degree of certainty that the different DNMS versions require similar memory abilities. There are no obvious reasons to suspect that any of the differences in general procedures for the various rat DNMS tasks would evoke different object-recognition processes, or that they would interact with lesion effects in a way that would obscure a true object-recognition deficit if one was present. Still, we must be cautious in assuming that rats, monkeys, and humans are solving the DNMS task in the same way, despite the superficial similarities between the versions that are administered to the different species. Since visual recognition is clearly needed for accurate DNMS performance in monkeys and humans, it is important to establish that the same is true for rats. On the rat DNMS task we used in our studies [73], the sample object is handled more recently than the novel object prior to the choice phase of each trial, and it has been suggested that rats do not solve the task using visual-recognition memory, but instead by making an olfactory discrimination based on the relative recency with which the sample and novel objects were touched by the experimenter [50]. However, a series of experiments clearly indicate that properly trained rats do not solve the task this way [70,72]. Rats mastered the DNMS task at a similar rate whether the sample was always touched last prior to the choice phase or the sample and novel object were each touched last on a random half of the trials. Moreover, there was no disruption in performance when rats trained under the former conditions were subsequently switched to the latter procedure [72]. Rats could learn to discriminate between two objects that differed only in that the experimenter touched one of them (S ) more recently than the other (S+), as long as they only had to wait 4 s between when S was touched and when they were allowed to attempt the discrimination, but the same rats failed the discrimination when the delay was increased to 15 s [72]. In contrast, rats achieve high levels of accuracy on the DNMS task with retention delays of up to several minutes [46,73 78,118], even when two identical objects are used as the sample on each trial one for the sample phase and the other for the test phase and the objects are positioned prior to the beginning of the trial [86]. Thus, experimenter odour confounds do not explain how rats learn the DNMS task with short retention delays or how they continue to perform well with longer delays. A different concern about possible olfactory solutions to the rat DNMS has been raised [62], but here the question is not whether the task requires object recognition, but whether the rats recognize the objects visual features or their olfactory features. Rats displace the objects with their snout (or, less frequently, with their forepaws), and, therefore, they have an opportunity to see, feel and smell the objects before making their selection. However, two frequent observations

7 D.G. Mumby / Beha ioural Brain Research 000 (2001) suggest that well-trained rats normally base their choices primarily on their memory of the visual properties of the sample objects: First, rats rarely contact an object on the choice phase without displacing it, and they rarely pause to palpate an object with their vibrissae, which suggests that they do not choose on the basis of tactual differences between the sample and novel objects. Second, well-trained rats tend to veer toward the correct object while they are approaching the goal area and still several centimetres away from it, further suggesting that on most trials they do not rely on tactual cues, and probably not olfactory cues. Other who have used the DNMS task have made the same observation [115]. In summary, empirical tests and incidental observations strongly suggest that performance on the rat DNMS task relies on visually-guided object recognition. Still, experimenters who use the task must remain vigilant to ensure that testing conditions do not inadvertently provide and encourage unintended solutions. Some of the methodological pitfalls of training rats on the DNMS task are discussed elsewhere [70] Spontaneous no elty preference Another paradigm used to assess object recognition in rats is often referred to as a spontaneous recognition test, because it is based on rats natural propensity to explore novel objects [38]. On the standard version of this task, a rat is placed in an open-field arena and allowed to explore two identical sample objects for a fixed period, usually a few minutes; alternatively, the rat may be allowed to explore the sample objects until it has spent a specific amount of cumulative time exploring them, usually 30 s, or so. The rat is then removed from the arena for a retention delay, after which it is returned to the arena with two new objects one is identical to the sample and the other is novel. Normal rats spend more time exploring the novel object during the first few minutes of the retention test, and when this bias is observed it is inferred that the rat remembers the sample object. With conventional procedures like those just described, rats typically are able to discriminate the sample and novel objects after retention delays lasting several minutes or hours, and up to 24 h or slightly more in some studies. The novelty-preference paradigm is considered to be analogous to the visual paired-comparison (VPC) task, which is sometimes used to assess visual-recognition in infant humans [41] and in monkeys [11]. However, there are some important differences between the primate VPC task and the rodent novelty-preference paradigm, which may, in some instances, mitigate against straightforward comparisons of findings in rats and monkeys on these two tasks. Although it is clear that differential-looking in the VPC task reflects some type of visual memory, we can be less certain of this in the case of differential exploration in the rodent novelty-preference paradigm. When rats explore an object, they sniff it, palpate it, and look at it. It seems likely that differential exploration of sample and novel objects reflects, to some extent, the rat s memory for tactual properties of the sample object, although visual properties may also be remembered and contribute to discrimination. If the objects are made out of different materials, memory of the sample object s olfactory features might also play a role. There is no way of knowing if all three types of information about an object are remembered at the time of the retention test. A similar question about the DNMS task was posed earlier, but in that case, incidental observations of how a rat behaves in that task enabled us to rule out the likelihood that olfactory or tactual cues are a primary basis for accurate DNMS performance. The rat s general behaviour when exploring an object does not allow us to make a reasoned guess about what type of information guides its exploratory preference. Thus, although the novelty-preference paradigm can be used to study object-recognition memory, it should not be viewed necessarily as a test of visual object-recognition. Accordingly, direct comparisons with findings in monkeys on the VPC task must be made with caution. As with any task, it is often a challenge to know what a performance deficit means on the novelty-preference task. When a treatment has no effect, and the animal continues to spend more time exploring novel objects than sample objects, it is a reasonably sound inference that the animal s object-recognition abilities have not been grossly impaired. When a treatment is effective, however, and even if perceptual impairments can be ruled out, there can still be several possible reasons for a failure to spend more time exploring the novel object. The usual interpretation is that the sample object has been forgotten and the two objects are no longer distinguishable on the basis of familiarity. But an equally plausible alternative is that the effective treatment has merely abolished, overshadowed, or altered the subject s natural bias for exploring novel objects. The potential for misinterpreting the basis for performance on this task is highlighted by the recent finding that rats with perirhinal cortex lesions actually displayed a significant exploratory preference for the i object after 5 min of familiarization followed by a 15 min retention delay, whereas sham-lesioned rats displayed the expected exploratory bias toward the novel object [71]. This result is puzzling, because it suggests that the rats with perirhinal cortex lesions recognized the sample object, but they now had a preference for familiar objects instead of the normal preference for novel objects. Such a scenario is not entirely implausible, and neither is one where an animal explores a

8 8 D.G. Mumby / Beha ioural Brain Research 000 (2001) sample and novel object equally, not because it has forgotten the sample, but because the natural bias has been altered or eliminated. The reliability of this unexpected result with perirhinal cortex lesions must be established before anything definite can be concluded from it, but it clearly shows there are interpretational difficulties with the novelty-preference paradigm, and these have not been discussed very much in the literature despite a recent proliferation of studies that have used it to index memory in mice, rats, and monkeys [100]. When this task was first described, the authors of that paper argued that deficits on the DNMS task can be difficult to interpret because they might reflect either failure of recognition, or failure to learn, remember, or apply the nonmatching rule [38]. This is true when performance is impaired even after brief retention delays. However, the novelty-preference paradigm has a similar shortcoming, as deficits might reflect either failure of recognition, or disruption of the instinctive propensity to explore novelty. It can help to rule out the second possibility if it is shown that subjects still display a preference for the novel object at short retention delays, as this would indicate that their instinctive preference for novelty has not been abolished, and that impaired performance at longer delays is more likely due to a failure of object recognition. Despite the interpretational challenges that arise when the normal preference for novel objects is not seen, it remains the case that preserved novelty preference in this paradigm can be taken as an indication that object recognition is intact. Accordingly, the noveltypreference paradigm can be used to test the hypothesis that object recognition is impaired by HPC damage. The easiest result to interpret would be one where novelty-preference is preserved after HPC dysfunction, thus disconfirming the hypothesis. As will be discussed in a later section, this is what has typically been observed after lesions of the HPC or fornix. 4. Object recognition following hippocampal damage in rats The sections that follow provide a review of several studies conducted in rats that either expressly or implicitly tested the hypothesis that hippocampal damage impairs object-recognition memory. Each one involved permanent disruption of hippocampal functions, either by HPC ablation, fornix lesion, or transient forebrain ischemia. To the best of my knowledge, this is a comprehensive list of the relevant studies that have been described in published reports or in recent published abstracts. Studies using the DNMS or DMS tasks are listed in Table 1, and they are discussed first. Studies involving the novelty-preference paradigm are discussed next, and are listed in Table 2. Studies that employed tasks with certain types of nonspatial visual stimuli are excluded, including those that involved matching or nonmatching to brightness cues, or visual features embedded within spatial arrays, such as the walls of a radial-maze arm. Such stimuli are visual, and the tasks might involve working-memory procedures and, therefore, bear some resemblance to tests of object recognition, but the stimuli themselves are not three-dimensional objects, and there is evidence to suggest that memory for objects involves different neural circuitry from that involved in memory for other kinds of visual cues. The general procedure followed in most experiments using DMS or DNMS involves first habituating the rat to the apparatus and shaping it to emit the required operant behaviour. Training on the DNMS task ensues, using a brief retention delay, until the rat reaches some criterion level of consistent accuracy. This establishes that the rat has learned the matching or nonmatching rule, and thereafter, recognition can be assessed with longer retention delays. In some experiments, all DNMS training is done postsurgery, and acquisition is measured by the number of trials or errors required to reach the performance criterion; subsequent performance at longer retention delays is expressed as percentage of trials correct at each delay. In other experiments the rats receive presurgery DNMS training; reacquisition is expressed as number trials or errors to retain criterion at the brief delay, and performance as percent-correct at longer delays Delayed matching and nonmatching to sample Explicit tests of the hypothesis In the first study to examine the effects of HPC ablation on DNMS performance in rats [4], aspiration lesions failed to disrupt either acquisition of the Y-maze task with a 0 s delay, or subsequent performance at delays of 20 or 60 s. A group of rats with HPC ablations that were trained on the DNMS task before surgery also showed normal reacquisition rates, and comparisons of their presurgery and postsurgery scores at the 20 and 60 s delays revealed no lesion effects. The results of this study were important, not only because they were the first to suggest that extensive HPC damage in rats does not impair object-recognition memory over delays of up to 60 s, but also because they contrasted with a previous report that fornix lesions impaired rats performance of a visual nonspatial working-memory task in which the stimuli were not objects, but instead were distinctive visual features of the arms in a radial maze [84]. Soon after, two more studies confirmed the effectiveness of HPC damage at impairing nonspatial working memory in rats when the stimuli were visual features of the alleys and goal areas in simple mazes [56,88]. As discussed below, there is now

9 D.G. Mumby / Beha ioural Brain Research 000 (2001) good evidence which suggests that HPC damage caused nonspatial working-memory deficits in those earlier studies because the visual stimuli were not actual objects [21,22]. The first report that HPC ablation in rats did not impair DNMS performance came at a time when there was wide support for Mishkin s proposal that objectrecognition memory was subserved by parallel circuits linking the HPC and amygdala with diencephalic structures [67]. This model was derived from lesion experiments in monkeys [67], and could account for the failure to observe DNMS deficits in rats with HPC damage because the amygdalo thalamic circuit would be able to compensate for damage in the hippocampal thalamic circuit. Following-up to their first study, Aggleton and his colleagues examined the effects of combined lesions of the HPC and amygdala [2], and fornix and amygdala [5], on DNMS performance in the Y-maze. In both studies, rats with combined lesions acquired the DNMS task at a normal rate with a 0-s delay, but were subsequently impaired when the delay was extended to 20 or 60 s, although the impairment appeared to be more severe after the amygdalo hippocampal lesions than after the fornix-plus-amygdala lesions [5]. Due to the intended extrahippocampal damage, these studies did not directly test the hypothesis that HPC damage impairs object recognition, but they were important in showing that extensive damage beyond the HPC that includes other structures that are normally damaged in humans with medial-temporal- Table 1 Studies using DMS or DNMS tasks to assess object recognition in rats with damage to the hippocampal formation (HPC) or fornix Study Lesion Delays Presurgery training Results [4] HPC 0, 20, 60 s One group was, another No impairment in acquisition, or performance [92] HPC, Fornix 10, 30 s was not Extensive, but only with No impairment in reacquisition or performance [59] HPC 4, 10, 20 s 10s delay Extensive, but only with No impairment in reacquisition or performance [55] HPC up to approx. 4-s delay Extensive/inclusive No impairment in performance 90 s [78] HPC 4, 15, 60, 120 s, Extensive/inclusive No impairment in reacquisition, possibly impaired at 600 s, 10 min depending on analysis a [72] HPC 4, 15, 30, 60, None No impairment in acquisition, possibly a mild impairment at 120 s 120 s delay b [102] HPC Up to approx. None Mild impairment in performance with lists of up to 32 objects 13 min [98] Fornix? None Better than control rats during acquisition. No impairment in performance [120] HPC, Fornix Approx. 3 5, 20 None No impairment with small boxes and objects, but impaired with large s boxes [90] HPC, Fornix Approx. 3 7 s None No impairment with small boxes and objects, but impaired with large boxes [21] Fornix Approx. 5 s Extensive No impairment with small boxes and objects, but impaired with large boxes [22] HPC Approx. 5 s Some, but not enough to No impairment with small boxes and objects, but impaired with large learn the task boxes [46] HPC 4, 15, 30, 60, Extensive/inclusive No impairment in reacquisition or performance 120 s [77] HPC 4, 15, 60, 120, Extensive/inclusive No impairment in performance, but a mild reacquisition deficit 300 s [119] HPC 4, 15, 30, 60 s None No impairment in acquisition or performance [33] HPC 4, 60, 120, 300 Extensive/inclusive, No impairment in acquisition, or performance s versus none [115] Fornix 5, 15, 30, 60, None Impaired at 30, 60, and 120 s delays 120 s [25] HPC 4, 30, 60, 120 s None Impaired at 60 s and 120 s delays Studies are only included if the HPC or fornix lesions were not combined with lesions of another structure. Studies are listed in the same order they are discussed in the text. Additional variables were manipulated in some studies see text for details. Presurgery training was extensive if rats were trained to some criterion level of good and consistent accuracy; it was inclusive if it included the same conditions under which postsurgery performance was later assessed. a Within-group comparisons of presurgery and postsurgery scores revealed no effect of HPC lesions. b DNMS scores of rats with HPC lesion were as good as those of control rats in more difficult conditions within the same experiment (e.g. with lists of up to seven objects and delays of 105 s).

10 10 D.G. Mumby / Beha ioural Brain Research 000 (2001) Table 2 Studies using the novelty-preference paradigm to assess object recognition in rats with damage to the hippocampal formation (HPC) or fornix Study Lesion Delays Results [36] Fornix l min, 15 min, 4 No impairment [37] Fornix h l min, 15 min No impairment [39] Fornix l min, 15 min No impairment [40] Fornix 1 min, 15 min No impairment [111] Fornix 15 min No impairment [24] HPC, Fornix 10, 60, 600 s, No impairment with fornix lesion; mixed results in two HPC groups at 10 min, 1 h, or 24 h 1h, 24 h Studies are listed in the same order they are discussed in the text. Studies are only included if the HPC or fornix lesions were not combined with lesions of another structure. lobe amnesia can produce DNMS deficits in rats. In discussing their results, the authors indicated the possibility that unintended damage to the piriform cortex or certain thalamic nuclei may also have contributed to the DNMS deficits following combined HPC-amygdala lesions [2]. The next published study on HPC damage and object recognition in rats found normal postsurgery reacquisition of the Rothblat-Hayes DNMS task in rats that had been first trained prior to receiving either fimbria-fornix lesions or aspiration of the ventral temporal area [92]. The lesions in the latter group -included much of the HPC, but also the entorhinal cortex, and portions of the amygdala. Both groups of rats continued to perform as accurately as controls when the retention delay was increased to 10 or 30 s. There was considerable extrahippocampal damage in the rats with ventral temporal area lesions, but given that this group showed no impairment, the extrahippocampal damage seems somewhat irrelevant to explaining the results. However, substantial portions of the dorsal HPC were spared in those rats, and it is possible that this might have spared some degree of hippocampal function. Still, together with the finding that fimbria-fornix lesions failed to produce even the slightest DNMS deficit, the results from the rats with direct HPC damage indicated that the ability of rats to recognize objects after delays of up to 30 s, does not depend upon the integrity of the HPC. The same conclusion was reached in a pair of studies using Kesner s DNMS paradigm. In one of them, rats were trained on the DNMS task before receiving HPC aspiration lesions. After surgery they remastered the task at a normal rate with a brief delay (approximately 3 or 4 s), and continued to perform like controls when the delay was increased to 10 or 20 s [59]. The HPC aspiration lesions were of intermediate extent, including most or all of the dorsal HPC, but sparing considerable portions of ventral HPC and subiculum. In the second study, a continuous recognition procedure was used and rats were first trained before surgery [55]. On successive trials, the rat encountered a single object at one end of the runway, and on some trials the object had occurred previously in that session. After displacing an object, the rat received a food reward, but only if the object had not been presented on a previous trial. The dependent measure was latency to displace the object. Rats with HPC lesions performed like control rats they had longer response latencies on trials with repeated objects, their latencies increased as a function of the lag between repeated occurrences of an object, and the latencies in the two groups were not significantly different. From the authors description, it is difficult to discern the actual interval between successive object presentations, but it appears to have been approximately equal to the sum of 10 s (the intertrial interval), plus, however, long it took rats to eat a half piece of breakfast cereal (say 5 s, on average), plus, however, long it took them to traverse the 1.2 m runway (say 3 s, on average). By this estimate, the delay over which a rat would have had to recognize a repeated object in the longest lag condition, four intervening objects, would have been approximately 90 s. Moreover, some degree of retroactive interference could be expected from the intervening objects, making this a more difficult task than discretetrial DNMS with an unfilled 90 s delay, although there are no data available to allow such a comparison. As was the case in the previous study [59], the HPC lesions included most of the dorsal HPC, but spared the ventral HPC. Within-group comparisons found that presurgery and postsurgery performance was equivalent for the rats with HPC lesions, and also for control rats. The next study was intended to emulate Mishkin s influential experiment in which he found severe DNMS deficits after combined amygdalo hippocampal lesions, with milder deficits following lesions of only the HPC or amygdala [78]. This study is often cited as showing that HPC lesions produce DNMS deficits. However, examining closely the details of what was found can lead to a somewhat different conclusion. Rats received presurgery training on the DNMS task at retention delays of 4, 15, 60, 120, and 600 s. After