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1 This article was downloaded by: [University of Cardiff] On: 17 November 2008 Access details: Access Details: [subscription number ] Publisher Psychology Press Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK The Quarterly Journal of Experimental Psychology Publication details, including instructions for authors and subscription information: Understanding anterograde amnesia: Disconnections and hidden lesions John P. Aggleton a a Cardiff University, Cardiff, UK First Published on: 31 July 2008 To cite this Article Aggleton, John P.(2008)'Understanding anterograde amnesia: Disconnections and hidden lesions',the Quarterly Journal of Experimental Psychology,61:10, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY 2008, 61 (10), EPS Mid-Career Award 2006 Understanding anterograde amnesia: Disconnections and hidden lesions John P. Aggleton Cardiff University, Cardiff, UK Three emerging strands of evidence are helping to resolve the causes of the anterograde amnesia associated with damage to the diencephalon. First, new anatomical studies have refined our understanding of the links between diencephalic and temporal brain regions associated with amnesia. These studies direct attention to the limited numbers of routes linking the two regions. Second, neuropsychological studies of patients with colloid cysts confirm the importance of at least one of these routes, the fornix, for episodic memory. By combining these anatomical and neuropsychological data strong evidence emerges for the view that damage to hippocampal mammillary body anterior thalamic interactions is sufficient to induce amnesia. A third development is the possibility that the retrosplenial cortex provides an integrating link in this functional system. Furthermore, recent evidence indicates that the retrosplenial cortex may suffer covert pathology (i.e., it is functionally lesioned) following damage to the anterior thalamic nuclei or hippocampus. This shared indirect lesion effect on the retrosplenial cortex not only broadens our concept of the neural basis of amnesia but may also help to explain the many similarities between temporal lobe and diencephalic amnesia. Keywords: Temporal lobe; Fornix; Memory; Subiculum; Entorhinal cortex; Hippocampus; Hypothalamus. Correspondence should be addressed to John Aggleton, School of Psychology, Cardiff University, Park Place, Cardiff, Wales, CF10 3AT, UK. aggleton@cf.ac.uk This review expands on the content of the Experimental Psychology Society (EPS) Mid-Career Award talk of the same title given to the EPS in The author wishes to thank the support of the Medical Research Council (MRC) and Wellcome Trust, as well as the valuable assistance of G. Poirier, S. D. Vann, and L. Woods. # 2008 The Experimental Psychology Society 1441 DOI: /

3 AGGLETON This review presents evidence for an integrated model of the neuroanatomy of anterograde amnesia, and it also suggests that the causes of amnesia might extend beyond the effects of visible pathology to include hidden or covert pathology. Findings from both clinical and animal studies are combined to try and answer a question that first emerged over 100 years ago (Gudden, 1896; Korsakoff, 1887) with the first formal descriptions of organic amnesia and its possible pathology. The question is whether we can identify the key structures that when damaged cause permanent amnesia and then infer why these structures are so important for memory. As there are multiple candidate structures in more than one brain region this task is unlikely to be straightforward. The focus of this review is on organic anterograde amnesia, a failure to learn new information following brain injury. Current definitions of anterograde amnesia (e.g., Parkin, 1997) emphasize thepresenceofsevereandpermanentdeficitsfor the recall of recent events (typically with poor recognition)thatcontrastwithintactshort-termmemory, IQ, semantic memory, skill learning, simple classical conditioning, perceptual learning, and priming. Consequently, the most striking effect is a loss of episodic memory. Given these dissociations it is not surprising that the study of anterograde amnesia has delivered important insights into the distinctions between long-term and short-term memory (Baddeley, 1990) and between explicit and implicit memory (Schacter, 1987). More recently, studies of amnesia have been at the forefront of the debate into whether recognition memory is a unitary process or whether it comprises multiple, distinct processes (Squire, Wixted, & Clark, 2007; Yonelinas, 2002). From a different perspective, uncovering the anatomical basis of anterograde amnesia provides the first line of targets for research into the physiological and molecular basis of recognition memory and episodic memory. Towards an integrated model of temporal lobe and diencephalic amnesia Pathology in two distinct brain regions, the medial temporal lobe and the medial diencephalon, is most consistently associated with anterograde amnesia. It is almost universally agreed that the critical region for temporal lobe amnesia is the hippocampal formation (Spiers, Maguire, & Burgess, 2001), although there remains much debate over the extent and nature of the contributions from pathology in the adjacent parahippocampal region (Witter & Wouterlood, 2002). Although diencephalic amnesia was first investigated long before temporal lobe amnesia, its neural basis remains far less certain. Within the diencephalon, which comprises the thalamus and hypothalamus, evidence exists to implicate a range of structures. These structures include the mammillary bodies and various thalamic nuclei, including the anterior thalamic nuclei, nucleus medialis dorsalis, nucleus parataenialis, nucleus lateralis dorsalis, and intralaminar nuclei (Aggleton & Sahgal, 1993; Gold & Squire, 2006; Harding, Halliday, Caine, & Kril, 2000; Kopelman, 2002; R. G. Mair, 1994; W. G. P. Mair, Warrington, & Weiskrantz, 1979; Markowitsch, 1982; Mayes, Meudell, Mann, & Pickering, 1988; Vann & Aggleton, 2004; Victor, Adams, & Collins, 1971, 1989). Classical neuropsychological studies have failed to provide definitive evidence concerning the neural basis of diencephalic amnesia. Such evidence would have to come from patients with a well-characterized amnesia that is associated with pathology restricted to just one structure confirmed by postmortem. Given the close proximity of the various thalamic nuclei and the presence of numerous white matter tracts running through the diencephalon, any convincing evidence may be a long time in coming. Fortunately, there are numerous clues that can be drawn from the neuropsychology of amnesia, from the neuroanatomical connections of the various candidate regions, and from animal models where the pathology can be highly selective. The most parsimonious overview of the neural basis of episodic memory would be that temporal lobe amnesia and diencephalic amnesia reflect dysfunctions in the same shared mnemonic system. This situation could occur if temporal lobe damage results in permanent dysfunctions in diencephalic function and vice versa that is, there is an integrated, interdependent medial 1442 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

4 UNDERSTANDING ANTEROGRADE AMNESIA temporal lobe medial diencephalic system. There are, however, at least three other models. One possibility is that the disruptive effects of medial temporal and medial diencephalic damage on memory are via quite independent actions. A second possibility is that damage in both medial temporal and medial diencephalic regionscausespermanentdysfunctioninathird common region, which is principally responsible for amnesia and, hence, unites these two regions indirectly. A weakness with this proposal is that it presupposes the existence of a third region primarily responsible for amnesia, yet anterograde amnesia is most consistently linked to the medial temporal lobe and the medial diencephalon. While there are some sites (e.g., medial prefrontal cortex and retrosplenial cortex) that have been implicated in anterograde amnesia and are connected directly to both regions, evidence for their preeminent importance for memory remains much weaker than that for either the medial temporal lobe or medial diencephalon. These factors do, however, suggest another possibility that is essentially a hybrid of two of the three previous models namely, that there is a common third site of dysfunction (capable of adding to the impact of either medial temporal or medial diencephalic damage) but this effect is additional to the shared integrated dysfunction that arises from either medial temporal lobe or diencephalic damage. It is this final model that is most strongly supported by the present review. Those accounts that closely link the medial temporal lobe and medial diencephalic substrates for amnesia are strengthened by both neuropsychological and neuroanatomical (next section) findings (Aggleton & Saunders, 1997). First, the core features of diencephalic and medial temporal lobe amnesia appear strikingly similar. It is, of course, inevitable that at some level the two syndromes must be very similar that is, a particularly severe and persistent loss of new episodic learning while other cognitive abilities (e.g., priming, procedural learning, and short-term memory) appear largely intact (Squire, 2004). For a while it was thought that temporal lobe amnesia, but not diencephalic amnesia, is associated with abnormally fast rates of forgetting (Huppert & Piercy, 1979). These original findings provoked much interest, but follow-up studies have repeatedly failed to find this dissociation (Freed & Corkin, 1988; Freed, Corkin, & Cohen, 1987; Kopelman, 2002; McKee & Squire, 1992). A further possible difference concerns the relative disruption of temporal contextual information (i.e., when an event occurred) in temporal lobe and diencephalic amnesia. There is evidence that patients with diencephalic amnesia are especially impaired at using or recalling temporal contextual information (Hunkin & Parkin, 1993; Hunkin, Parkin, & Longmore, 1994; Kopelman, Stanhope, & Kingsley, 1997; Parkin & Hunkin, 1993; Parkin, Leng, & Hunkin, 1990; Shimamura, Janowsky, & Squire, 1990). The significance of this dissociation is, however, weakened by the fact that nearly all of the studies on temporal context in diencephalic amnesia refer to patients with Korsakoff s disease (Hunkin & Parkin, 1993; Hunkin et al., 1994; Kopelman et al., 1997; Parkin et al., 1990; Shimamura et al., 1990). Because of the aetiology of Korsakoff s syndrome, this class of diencephalic amnesics is most likely to have additional frontal cortex impairments. This additional pathology is pertinent as prefrontal cortex damage is itself associated with difficulties in recalling temporal context (Kopelman, 2002; Kopelman et al., 1997; Mayes, Meudell, & Pickering, 1985; Shimamura et al., 1990). For this reason, arguably clearer evidence for an exaggerated deficit in the use of contextual information in diencephalic amnesia comes from those single case studies (e.g., Hunkin et al., 1994; Parkin & Hunkin, 1993) where direct prefrontal damage seems less likely. Given that nuclei in the midline, anterior, and dorsomedial thalamus all have reciprocal prefrontal cortex connections that will often be compromised in cases of diencephalic amnesia, it may prove that contextual deficits, via a disruption of prefrontal interconnections, are a more frequent feature of this form of amnesia. Even so, as frontal lobe damage can disrupt temporal contextual processing without causing amnesia (Shimamura et al., 1990), it could be argued that this impairment does not THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1443

5 AGGLETON constitute a core deficit. Consequently it appears that temporal lobe and diencephalic amnesia share many of the same features with remarkably little to separate them.thisconclusionbringsustotheissueofthe medial temporal lobe connections with the medial diencephalon and how these connections might help discriminate between the candidate regions for diencephalic amnesia. Medial temporal lobe medial diencephalic interconnections In order to describe the relevant neuroanatomy it is first necessary to consider briefly the various candidate sites for diencephalic amnesia (the mammillary bodies, anterior thalamic nuclei, nucleus medialis dorsalis, nucleus parataenialis, nucleus lateralis dorsalis, and intralaminar thalamic nuclei). Two of these sites, the mammillary bodies and anterior thalamic nuclei, stand out because of the conjunction of both neuropsychological and neuroanatomical findings. These structures are also notable because of their exceptionally close anatomical relationship; the principal efferent target of the mammillary bodies is the anterior thalamic nuclei, these projections forming the mammillothalamic tract (Vann, Saunders, & Aggleton, 2007). Furthermore, it is suspected that every cell in the mammillary bodies projects to the anterior thalamic nuclei (Allen & Hopkins, 1988; Vann et al., 2007), though this is a one-way relationship with no return projections. Pathology in the mammillary bodies and anterior thalamic nuclei has repeatedly been linked with diencephalic amnesia (Aggleton & Brown, 1999; Clarke et al., 1994; Dusoir, Kapur, Brynes, McKinstry, & Hoare, 1990; Gold & Squire, 2006; Harding et al., 2000; Hildebrandt, Mueller, Bussmann-Mork, Goebel, & Eilers, 2001; Kopelman, 1995; Malamut, Graff- Radford, Chawluk, Grossman, & Gur, 1992; Tsivilis et al., 2008; Vann & Aggleton, 2004; Victor et al., 1971). It is important to note that these diencephalic amnesics show the same defining features as those first described in the Introduction (e.g., Clarke et al., 1994; Dusoir et al., 1990; Malamut et al., 1992; Squire, Amaral, Zola-Morgan, Kritchevsky, & Press, 1989). The mammillary bodies have long been linked with diencephalic amnesia (Gudden, 1896) as these nuclei are always atrophied in the amnesic Korsakoff s syndrome. However, the presence of concurrent pathology in other sites (Harding et al., 2000; Victor et al., 1971, 1989) means that Korsakoff cases have failed to provide definitive evidence. Examples of people with mammillary body damage as a result of tumours or trauma (Dusoir et al., 1990; Hildebrandt et al., 2001; Tanaka, Miyazawa, Araoka, & Yamada, 1997) that do not appear to affect the other candidate diencephalic sites involved in Korsakoff s disease have proved to be very rare. Nevertheless, the few cases add support to the view that the mammillary bodies are necessary for normal episodic memory (Vann & Aggleton, 2004). At the same time, the memory deficits associated with mammillary body damage appear to be milder than those seen in amnesias where there is more widespread pathology (Kapur et al., 1998), suggesting the involvement of other areas. The anterior thalamic nuclei have been strongly implicated in amnesia by a detailed, stereological analysis of the pathology in Korsakoff s syndrome (Harding et al., 2000). In this study, neurodegeneration in the anterior thalamic nuclei was the only consistent pathology in alcoholic Korsakoff cases that differentiated them from alcoholics with Wernicke s encephalopathy (i.e., nonamnesic cases). Other evidence has come from neuropathological analyses of rostral thalamic vascular accidents that result in amnesia. In some cases there is direct damage to the anterior thalamic nuclei (Clarke et al., 1994; Ghika-Schmid & Bogousslavsky, 2000; Pepin & Auray-Pepin, 1993), though this damage is never restricted to the anterior thalamic nuclei. More typically there is damage to the mammillothalamic tract (Carlesimo et al., 2007; Malamut et al., 1992; Van der Werf, Jolles, Witter, & Uylings, 2003a; Van der Werf et al., 2003b; Van der Werf, Witter, Uylings, & Jolles, 2000; Von Cramon, Hebel, & Schuri, 1985). Indeed, damage to the mammillothalamic tract, which carries projections from the mammillary bodies to the anterior thalamic nuclei, appears to 1444 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

6 UNDERSTANDING ANTEROGRADE AMNESIA be the best predictor of memory deficits after thalamic strokes (Van der Werf et al., 2003a, 2003b; Van der Werf et al., 2000; Von Cramon et al., 1985). This finding may help to explain the amnesic case N.A. (Squire et al., 1989) who, following a brain-penetrating injury with a miniature fencing foil, sustained damage to the mammillothalamic tract along with the mammillary bodies, internal medullary lamina, and rostral midline and intralaminar thalamic nuclei (central medial, reuniens, paracentral, central lateral, and rhomboid). The mammillary bodies and the anterior thalamic nuclei stand out from the rest of the medial diencephalon for one other reason: Both receive direct, dense inputs from the hippocampal formation (Aggleton, Desimone, & Mishkin, 1986; Poletti & Creswell, 1977; Saunders, Mishkin, & Aggleton, 2005). These projections, which arise from the subicular complex and the entorhinal cortex, reach the diencephalon almost exclusively via the fornix (Aggleton et al., 1986; Aggleton, Vann, & Saunders, 2005b; Saunders et al., 2005). As shown in Figure 1, the only other medial diencephalic structures to receive direct innervations via the fornix are the thalamic nuclei reuniens, lateralis dorsalis, and paraventricularis, along with light projections to various hypothalamic nuclei. Aside from nucleus lateralis dorsalis there is little current evidence to link these other nuclei with diencephalic amnesia, although this could reflect the smallness of these nuclei and the resultant difficulty of isolating their contributions to memory. Nucleus lateralis dorsalis is of interest for several reasons. First, this nucleus shares many connections with the anterior thalamic nuclei, and it has sometimes been categorized as part of the anterior thalamic group (Bentivoglio, Kultas- Ilinsky, & Ilinsky, 1993; Van Groen & Wyss, 1992). Perhaps the major difference from the classic anterior thalamic nuclei is that lateralis dorsalis receives few, if any, inputs from the mammillary bodies (Vann et al., 2007). Interestingly, in contrast to the mammillary bodies and the anterior thalamic nuclei, nucleus lateralis dorsalis receives its direct hippocampal inputs via two parallel routes in the primate brain (Aggleton et al., 1986; Saunders et al., 2005). Not only does the subiculum project via the fornix but there is also Figure 1. Diagrammatic representation of the location of the fornix and its divisions. The dashed arrows show fornical connections that are solely efferent from the hippocampal formation, the narrow, solid arrows show fornical connections that are solely afferent to the hippocampal formation, and the wide, solid arrows show reciprocal connections within the fornix. Abbreviations: AC, anterior commissure; ATN, anterior thalamic nuclei; HYPOTH, hypothalamus; LC, locus coeruleus; LD, thalamic nucleus lateralis dorsalis; MB, mammillary bodies; RE, nucleus reuniens; SUM, supramammillary nucleus. THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1445

7 AGGLETON a second route via the temporopulvinar bundle of Arnold. This second route runs caudally around the pulvinar to reach lateralis dorsalis (Figure 2). Functional evidence linking lateralis dorsalis with the anterior thalamic nuclei and the mammillary bodies comes from electrophysiological studies showing the presence of head direction cells in all three areas in rats (Blair, Cho, & Sharp, 1998; Mizumori & Williams, 1993; Taube, 1995, 1998). These head direction cells fire when the head is pointed in a preferred direction, set by distal visual stimuli, vestibular stimuli, or both. As a consequence, nucleus lateralis dorsalis contributes to spatial navigation, along with the mammillary bodies and the anterior thalamic nuclei (Mizumori, Cooper, Leutgeb, & Pratt, 2001; Taube, 1998; Vertes, Hoover, & Viana Di Prisco, 2004). Both lateralis dorsalis and the anterior thalamic nuclei are also functionally linked by other electrophysiological studies, which show a common pattern of traininginduced activity during discriminative avoidance learning (Gabriel, 1993). Lesion studies in rats (Aggleton & Brown, 1999; Aggleton, Hunt, Nagle, & Neave, 1996; Byatt & Dalrymple- Alford, 1996; Mizumori, Miya, & Ward, 1994; Sziklas & Petrides, 1998; Van Groen, Kadish, & Wyss, 2002; Vann & Aggleton, 2003, 2004; Figure 2. Summary diagram showing the routes by which the hippocampus and parahippocampal region project to the thalamus. The thickness of the lines reflects the density of the projection while the ovals group together the connections within a particular tract. The figure includes previously published data concerning the efferents from the subiculum (Aggleton et al., 1986) and the parahippocampal region (Yeterian & Pandya, 1988). The perirhinal cortex consists of areas 35 and 36. Routes are not provided for the parahippocampal cortex as they have not yet been determined. Abbreviations: AD, anterior dorsal nucleus; AM, anterior medial nucleus; AV, anterior ventral nucleus; LD, nucleus lateralis dorsalis; MD, nucleus medialis dorsalis, including pars magnocellular (mc); PULV, pulvinar; TF, TH, parahippocampal cortex THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

8 UNDERSTANDING ANTEROGRADE AMNESIA Wilton, Baird, Muir, Honey, & Aggleton, 2001) have also shown that these same three structures (mammillary bodies, anterior thalamic nuclei, nucleus lateralis dorsalis) are required for the normal learning of spatial tasks that are sensitive to hippocampal lesions. Even though the severity of the impairments often do not match those seen after hippocampal lesions, this pattern of results is clearly consistent with a functional link with the hippocampus. At present, there is only limited neuropsychological evidence concerning nucleus lateralis dorsalis. In their review of postmortem cases of Korsakoff s syndrome, Victor et al. (1971) recorded lateralis dorsalis atrophy in 68% of their cases, placing it in the top three thalamic nuclei by frequency of pathology. Similarly, Brion and Mikol (1978) observed lateralis dorsalis abnormalities in 9 out of 11 Korsakoff cases, noting that it was the most frequent site of thalamic pathology in their sample. In both studies, mammillary body atrophy was seen in all amnesic cases (Brion & Mikol, 1978; Victor et al., 1971). There is a lack of other clinical data on nucleus lateralis dorsalis, although a single case (Q.X.) described by Edelstyn, Hunter, and Ellis (2006) is informative. Q.X. suffers from marked deficits in verbal memory as well as impaired recollection of the Rey complex figure, with more subtle deficits in recognition memory (Edelstyn, Ellis, Jenkinson, & Sawyer, 2002; Edelstyn et al., 2006). Recent magnetic resonance imaging (MRI) scans indicate (Edelstyn et al., 2006) unilateral damage in the medial dorsal thalamic nucleus but bilateral damage in lateralis dorsalis (called the dorsolateral thalamic nucleus in their paper). As this was the only site with suspected bilateral pathology the authors argued that this nucleus might also support memory. Both the anterior thalamic nuclei and lateralis dorsalis have dense, reciprocal connections with the retrosplenial cortex (areas 29, 30). This cortical region, within the posterior cingulate area, provides a potentially important, indirect route from the hippocampus to the anterior thalamic nuclei and lateralis dorsalis, as well as to the prefrontal cortex. The other key feature of these thalamic nuclei (anterior and lateralis dorsalis) is that they project directly back upon the hippocampal formation (Amaral & Cowan, 1980; De Vito, 1980). Once again, the retrosplenial cortex also provides a potential route for indirect projections from the medial diencephalon to the medial temporal lobe. For these neuropsychological and neuroanatomical reasons the anterior thalamic nuclei (and lateralis dorsalis) stand out from all of the other candidate regions implicated in diencephalic amnesia as they are most likely to function reciprocally with the hippocampus. To test this view directly it is necessary to turn to studies with animals. The first key finding is that the effects of selective anterior thalamic lesions and selective hippocampal lesions are alike in that they both disrupt tests of spatial learning and scene learning (Aggleton & Brown, 1999). Deficits have been found in both rats and monkeys on tasks thought to capture aspects of episodic-like memory namely, the ability to learn conjunctions between specific items and their locations (Parker & Gaffan, 1997a, 1997b; Wilton et al., 2001). Such learning tasks are of particular interest as they tax two (what? and where?) of the three key elements of episodic-like memory (Clayton & Dickinson, 1998). Direct evidence that the anterior thalamic nuclei and the hippocampus may be functionally interdependent comes from disconnection studies with rats. In these studies crossed-unilateral lesions were made in the hippocampus and anterior thalamic nuclei, producing spatial memory deficits similar to those seen after bilateral lesions in either structure (Henry, Petrides, St-Laurent,&Sziklas,2004;Warburton,Morgan, Baird, Muir, & Aggleton, 2001). An important test of this emerging medial temporal medial diencephalic model is to examine the effects of fornix damage on memory. The fornix stands out because it is the primary route from the medial temporal lobe to the medial diencephalon (Figures 1 and 2). An integrated model would have to predict that fornix damage is sufficient to induce marked memory deficits, unless the functional interrelationships between the two areas solely depend on the projections from the diencephalon to the hippocampal formation (which typically do not use the fornix). This THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1447

9 AGGLETON latter scenario seems most unlikely given (a) the high density of the termination sites of the hippocampal projections in the medial diencephalon, and (b) the reciprocal nature of many of these connections. Do fornix lesions produce amnesia? A seemingly definitive review that reported the outcome of 193 patients who had received surgeries in the region of the fornix concluded that there was no link between fornix damage and amnesia, reporting stereotaxic fornicotomies for epilepsy in which, to our knowledge, no persistent memory loss has been reported so far (Garcia- Bengochea & Friedman, 1987, p. 363). This reported lack of any association between fornix damage and amnesia was specifically noted by the editor who added that the authors are to be congratulated for bringing this correlated neuropathological red herring to our notice so well. In spite of this plaudit, the quality of the evidence used in their review was heavily criticized by Gaffan and Gaffan (1991), and this criticism has been followed by a spate of papers all describing how fornix pathology is strongly linked to anterograde amnesia (Aggleton et al., 2000; D Esposito, Verfaellie, Alexander, & Katz, 1995; Gaffan, Gaffan, & Hodges, 1991; Hodges & Carpenter, 1991; McMackin, Cockburn, Anslow, & Gaffan, 1995; Poreh et al., 2006; Tsivilis et al., 2008; Vann et al., in press). One valuable source of evidence has come from the study of patients with colloid cysts in the third ventricle (Figure 3). These benign tumours are surgically removed to control hydrocephalus, but an occasional consequence of these tumours is that the fornix is atrophied. In some cases, postsurgical MRIs reveal that the tract is completely interrupted in one or both hemispheres. Careful analysis of these patients has shown that such fornix interruption is associated with a clear anterograde amnesia (Aggleton et al., 2000; Gaffan et al., 1991; Gilboa et al., 2006; Hodges & Carpenter, 1991; McMackin et al., 1995; Poreh et al., 2006) and, in some cases, with retrograde amnesia as well (Gilboa et al., 2006). This Figure 3. Magnetic resonance imaging (MRI) scan (midline sagittal) of third ventricle colloid cyst. The colloid cyst (CC) lies immediately adjacent to the fornix. amnesia is evident in formal memory tests and markedly affects day-to-day life. Nevertheless, it is seemingly not as severe as that seen following extensive bilateral medial temporal lobe or medial diencephalic pathology. Evidence from other aetiologies (e.g., various tumours, trauma, vascular damage) also strongly suggests that fornix loss can impair memory (D Esposito et al., 1995; Park, Hahn, Kim, Na, & Huh, 2000; Tucker et al., 1988; Vann et al., in press; Yasuno et al., 1999). Unfortunately, there is not a single reported amnesic case in which selective fornix loss has occurred without concomitant damage elsewhere. Furthermore, many of the descriptions of fornix damage are hampered by the fact that they refer to single cases. Even the few group studies of colloid cyst cases have, until recently, only examined a maximum of 6 patients with bilateral loss of the fornix (McMackin et al., 1995) and a maximum of 12 colloid cyst cases, overall (Aggleton et al., 2000). Meanwhile, the presence of hydrocephalus in this condition, with its impact on other brain structures (Tsivilis et al., 2008) makes it very 1448 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

10 UNDERSTANDING ANTEROGRADE AMNESIA difficult to rule out a contribution from other affected regions. As a consequence it may not be possible to detect subtle cognitive deficits specifically linked to fornix damage. Furthermore, none of the neuropsychological studies has provided quantitative volume measurements of the fornix or any other structures. This shortcoming means that it has never been possible to test whether the memory loss in this condition is actually associated with the degree of fornix loss as opposed to the loss of any other structure. In order to address these shortcomings we recently assessed an unusually large cohort of colloid cyst patients (n ¼ 38) who received standardized psychometric (Tsivilis et al., 2008) and MRI volumetric (Denby et al., 2008, in press) measurement. All patients received surgery for cyst removal at least 12 months prior to taking part in the study. Memory testing including the Wechsler Memory Scale Third Edition (WMS- III), the Doors and People Test, and the Warrington Recognition Memory Test (WRMT), while the 3T MRI scanning protocols made it possible to estimate the volumes of key structures (the mammillary bodies, fornix, hippocampus, entorhinal cortex, perirhinal cortex, parahippocampal cortex, orbitomedial prefrontal cortex, orbitolateral prefrontal cortex, dorsomedial prefrontal cortex, dorsolateral prefrontal cortex, total temporal lobe, total hemisphere, and lateral ventricles). Memory deficits were widespread among the 38 patients, even though only 3 had complete bilateral interruption of the fornix. Careful analysis of the scans revealed that fornix atrophy (i.e., thinning without complete loss) was frequent, with over 40% of both the left and right fornix volumes at least one standard deviation below the mean fornix volume of a set of age-matched controls (Denby et al., in press). Of the other structures measured, the most consistent changes were found in the mammillary bodies, where over 50% of the left and right volumes were abnormally small (Denby et al., 2008). To assess the possible impact of these pathologies, correlations (n ¼ 38) were calculated between the volumes of the 13 target brain structures and the standard WMS-III memory indices (Auditory Immediate, Visual Immediate, Immediate Memory, Auditory Delayed, Visual Delayed, Auditory Recognition Delayed, General Memory, Working Memory it is important to note that the indices described as Immediate refer to supraspan tasks that primarily tax longterm memory while the Auditory Recognition index tests recall as well as recognition). A very clear pattern of results emerged: Mammillary body volume was significantly correlated with all of the above WMS-III Index scores, except for Working Memory (Tsivilis et al., 2008). In contrast, none of the other 12 structures correlated significantly with even a single WMS-III index score. For tests of recognition memory a very different pattern was seen (Tsivilis et al., 2008). Here, mammillary body volume and fornix volume correlated with just one out of six recognition tests (Word Recognition, WRMT). Closer inspection of the data from the Doors and People Test (which assesses both recall and recognition) was especially illuminating. Both left fornix volume and left mammillary body volume correlated significantly with the overall recall scores, but not with the recognition scores. Indeed, fornix volume and mammillary body volumes both correlated significantly with the Recall/Recognition difference for this test (i.e., the smaller the structure the greater the difference between these two forms of memory). While it is not clear whether the mammillary body atrophy in the colloid cyst cases solely reflected the loss of afferent hippocampal fibres (Loftus, Knight, & Amaral, 2000) or whether it was partially caused by direct damage to the structure, this study (Tsivilis et al., 2008) provides unusually strong evidence that hippocampal efferents within the fornix are vital for memory, with particular support for the contributions of the mammillary bodies. At the same time, both mammillary body volume and fornix volume were far more weakly correlated with recognition than with recall. This dissociation could be clearly seen when the colloid cyst patients who had been able to take all psychometric tests (n ¼ 34) were divided into those in the bottom third with THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1449

11 AGGLETON respect to mammillary body volume (n ¼ 11) and those in the top third that is, the cases with largest mammillary body volumes (n ¼ 11). There was a highly significant group difference for composite scores of recall performance (Figure 4). This group difference was found both for tests of supraspan ( immediate ) recall and for tests of delayed recall (Figure 4) that is, mammillary body atrophy was associated with poor recall. In contrast, combining the scores from the various recognition memory tests failed to reveal a difference between the two groups, resulting in a significant interaction between group and type of memory task (recall versus recognition). This pattern impaired recall and relatively spared recognition following fornix-mammillary body atrophy is predicted by a particular model of hippocampal medial diencephalic interactions (Aggleton & Brown, 1999). In that model it is argued that the extended hippocampal system comprising the hippocampus, fornix, mammillary Figure 4. Comparison between the 11 colloid cyst cases with the smallest mammillary bodies (small MB) and the 11 cases with the largest mammillary bodies (large MB). The bar graphs show the mean scaled scores (Y axis, population norm ¼ 10.0) for the tests of recall and recognition from the Wechsler Memory Scale Third Edition (WMS-III) and the Doors and People Test. Results are divided into immediate recall (I; 7 tests), delayed recall (D; 7 tests), and recognition (6 tests) see Tsivilis et al., The term immediate does not refer to tests of short-term memory. While those patients with the smallest mammillary bodies are significantly worse on both recall measures, this difference was not found for recognition (Warrington Recognition Memory Test, WRMT; Doors & People; Face Recognition tests from WMS-III). bodies, and anterior thalamic nuclei is vital for the encoding and, hence, subsequent recall of episodic information. This same model also assumes that recognition memory depends on two independent processes. One depends on the recollection of the event ( recollection ); the other depends on a signal of stimulus familiarity ( knowing ). Selective disruption of the extended hippocampal system (e.g., fornix damage) should, therefore disrupt recollective recognition as it relies on episodic memory. In contrast, familiarity-based recognition is spared because parahippocampal areas (e.g., perirhinal cortex) can still support this function. For this reason, the relative sparing of recognition in the colloid cyst cases would have been predicted. Following publication of the Aggleton and Brown (1999) model other neuropsychological studies have provided support for the central features of the model. Amnesics with selective hippocampal damage can show a relative sparing of recognition (Aggleton & Brown, 2006; Aggleton et al., 2005a; Baddeley, Vargha-Khadem, & Mishkin, 2001; Bastin et al., 2004; Holdstock et al., 2002; Mayes, Holdstock, Isaac, Hunkin, & Roberts, 2002; Skinner & Fernandes, 2007; Yonelinas, 2002), with additional evidence that this sparing reflects the preserved use of familiarity (Aggleton et al., 2005a; Bastin et al., 2004; Holdstock et al., 2002; Quamme, Yonelinas, Widaman, Kroll, & Sauvé, 2004; Turriziana, Fadda, Caltragirone, & Carlesimo, 2004; Yonelinas, 2002). Likewise, fornix damage can disproportionately impair recall (Aggleton et al., 2000; Gilboa et al., 2006; McMackin et al., 1995; Vann et al., in press), with evidence that familiarity-based recognition is spared (Aggleton et al., 2000; Gilboa et al., 2006). Similarly, mammillary body damage appears to spare recognition (Dusoir et al., 1990; Hildebrandt et al., 2001), and, while comparisons between familiarity-based and recollective-based recognition have yet to be reported in such cases, our data from the 38 colloid cyst patients would support such a distinction. In accord with this view, there is evidence that mammillothalamic tract damage also targets the recollective aspect of recognition with relative sparing of familiarity (Carlesimo et al., 2007) THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

12 UNDERSTANDING ANTEROGRADE AMNESIA It is, however, the case that not all patients with either fornix or hippocampal damage show a relative sparing of recognition (Cipolotti et al., 2006; Kopelman et al., 2007; Manns, Hopkins, Reed, Kitchener, & Squire, 2003; J. M. Reed & Squire, 1997). Furthermore, there remains much debate over the validity of two-process models of recognition (Squire et al., 2007). It has been argued that an alternative approach is to fit the data into a unitary model of recognition (Donaldson, 1996; Squire et al., 2007). In particular, it is suggested that a single continuum based around signal strength can account for current findings, such that strong memories are perceived as recollective while weak memories are only perceived as familiar (Squire et al., 2007). Because such models benefit from parsimonyitcouldbearguedthattheyshouldprovide the default explanation. A further, related issue concerns the validity of psychological tests purported to discriminate familiarity-based recognition from recollective-based recognition (Squire et al., 2007; Wais, Mickes, & Squire, 2008). This issue stems from the fact that any attempt to derive separate measures of these two putative aspects of recognition memory depends on making assumptions that are themselves difficult to prove independently. One solution might be to look for patient groups who show comparable levels of recall yet differ markedly on recognition. At the same time it would be necessary to match for task difficulty (recall vs. recognition) and to avoid ceiling or floor effects. Matching for task difficulty is, however, problematic because methods of testing recall and recognition are fundamentally different (e.g., two stimulus forced-choice recognition with a chance rate of 50%). One solution is to calibrate scores against population norms (e.g., Z scores) and then compare. With these constraints in mind it is informative to consider performance of the colloid cyst patients (Tsivilis et al., 2008) on the Doors and People Test. Not only does this test compare recall and recognition, but the recognition tests are sufficiently demanding to remove ceiling effects. Comparisons between the subgroups with large mammillary bodies (n ¼ 11) and with small mammillary bodies (n ¼ 11) showed that the latter group suffered a significant recall deficit based on the mean scaled scores, but there was no difference for recognition (Tsivilis et al., 2008). Furthermore, the mean recall scores (for the large mammillary subgroup) and the mean recognition scores (for both the large and the small mammillary body subgroups) did not differ from age-matched population norms for the Doors and People Test that is, the results were not compromised by scaling effects. This pattern of results (spared recognition but impaired recall) is difficult to reconcile with a unitary model as it would be supposed that a disproportionate loss of strong memories would impact on recognition as well as recall. A similar conclusion comes from a single case study of a man with bilateral fornix damage following tumour removal (Vann et al., in press). Despite having persistent problems in learning new episodic information he was able to show normal performance levels on the Warrington Recognition Memory Test even when a delay of 24 hr was imposed between sample presentation and test (normally the test phase immediately follows the sample phase). This manipulation makes the task appreciably harder, and so one would predict that a person more reliant on weak memories (as predicted by the unitary model) would make more errors, yet this pattern was not found (Vann et al., in press). Not surprisingly the Doors and People Test has previously been used with amnesics (e.g., Aggleton et al., 2000; Manns & Squire, 1999). In one of these studies (Manns & Squire, 1999) hippocampal damage following anoxia was associated with comparable recall and recognition deficits, a pattern of results qualitatively different to that described here after fornix damage associated with colloid cysts (Aggleton et al., 2000; Tsivilis et al., 2008). While this difference is not yet fully understood, it may reflect the complexity of fully describing the pathological changes following anoxia (see section Assumptions about lesion studies ). A consideration of the anatomy of the fornix provides other clues and predictions concerning this mnemonic system. While the primate fornix THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1451

13 AGGLETON contains a great many fibres originating in the hippocampal field CA3 (Saunders & Aggleton, 2007), the direct projections to the mammillary bodies and anterior thalamic nuclei arise almost entirely from the subicular region (Figure 5; Aggleton et al., 1986; Rosene & Van Hoesen, 1977; Saunders et al., 2005). If, as suggested, these particular efferents are vital for episodic memory, then bilateral pathology of the subicular cortices that spares the CA fields of the hippocampus should be sufficient to induce an anterograde amnesia. At present this prediction remains untested. The fornix also contains a great number of fibres running to sites other than the diencephalon that could potentially support memory and so explain why fornix damage might induce amnesia. Most notable among these projections are those to the prefrontal cortex and the septum/basal forebrain. The dense, reciprocal connections between the hippocampal formation and the septal/basal forebrain complex are of especial interest as the septum/diagonal band provides the principal source of cholinergic innervations to the hippocampus (Alonso, U, & Amaral, 1996). It has been assumed that these hippocampal afferents support mnemonic (Hasselmo, Wyble, & Wallenstein, 1996) and attentional (Baxter & Chiba, 1999) processes. One particular aspect of these hippocampal afferents that has received a great deal of attention is that both cholinergic and GABAergic (where GABA denotes gamma-aminobutyric acid) inputs from the septum appear to generate and control hippocampal theta rhythm (Vertes & Kocsis, 1997). Linked to this discovery is the growing consensus that theta is important for effective hippocampal encoding (Hasselmo, 2005; Hasselmo & Eichenbaum, 2005; Vertes, 2005). This functional description of the efferents from the basal forebrain to the hippocampus may initially seem to contradict the emphasis placed on the importance of medial temporal projections to the medial diencephalon throughout this review. In fact, these two viewpoints are not mutually exclusive. One possibility is that the functions of the septal/basal forebrain connections via the fornix are to optimize the activity of the hippocampus/entorhinal cortex prior to or after their interactions with the medial diencephalon. In other words, by having essentially different Figure 5. Source of fornix fibres from within the medial temporal lobe in the rhesus monkey (Macaca mulatta). The percentages refer to the proportion of retrogradely labelled cells following the implantation of horseradish peroxidase gel in different sites within the fornix. Abbreviation; MTT, mammillothalamic tract. (Data from Saunders & Aggleton, 2007.) 1452 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

14 UNDERSTANDING ANTEROGRADE AMNESIA roles these two sets of hippocampal connections do not replace each other, and both are likely to be needed for effective hippocampal functioning. A rather different proposal (Gaffan, 2002) is that dense medial temporal lobe amnesia arises from widespread disruptions of temporal cortical function, and that a key element of this syndrome is the loss of inputs from the basal forebrain and midbrain (Easton, Ridley, Baker, & Gaffan, 2002; Gaffan, 2002; Turchi, Saunders, & Mishkin, 2005). It is also argued that while fornix damage contributes to the full syndrome, it only produces a mild version as the loss of this tract is not sufficient to completely disrupt cortical plasticity (Gaffan, 2002). The same proposal also acknowledges the potential, additional contribution from efferents via the fornix to the medial diencephalon (Easton et al., 2002). The notion that fornix damage will not induce an amnesia as severe as that seen after extensive bilateral medial temporal damage (Gaffan, 2002) is, indeed, consistent with case descriptions (e.g., McMackin et al., 1995). Nevertheless, the loss of this tract is sufficient to induce the core features of the amnesic syndrome, except that recognition memory can be spared. This sparing presumably reflects a relative lack of impact of fornix loss on the parahippocampal cortices (Brown & Aggleton, 2001). Furthermore, the notion that the underlying deficit in amnesia is a loss of cortical plasticity (Gaffan, 2002) does accord with much of the present proposal as the impact of fornix loss is not just upon the medial diencephalon but is also indirectly upon regions such as the retrosplenial and medial prefrontal cortices (Aggleton & Brown, 2006; Garden et al., 2008; Garden et al., 2006; Vann, Brown, Erichsen, & Aggleton, 2000a, 2000b; see section Covert pathology, amnesia, and the retrosplenial cortex ). Points of difference would appear to be the greater emphasis that is placed on the importance of hippocampal dysfunction in the present model and the anatomical routes by which this can occur. A consideration of the anatomy of the extended hippocampal system shows that the anterior thalamic nuclei project back upon the hippocampus (not via the fornix), and so it is quite plausible that many of the critical hippocampal diencephalic interactions are reciprocal. This view fits with the concept of a closely integrated system. Little is known, however, about the ways in which the medial diencephalon might act back upon the hippocampal formation, and it is necessary to rely on experiments with animals where it is possible to produce selective manipulations among the many diencephalic nuclei. Studies with rats have shown that anterior thalamic lesions induce abnormalities in hippocampal activity as measured by immediate-early gene activity (Jenkins, Dias, Amin, & Aggleton, 2002a; Jenkins, Dias, Amin, Brown, & Aggleton, 2002b; see section Anterior thalamic lesions and covert pathology ). More diffuse thalamic lesions reduce the release of hippocampal acetyl choline (Savage, Chang, & Gold, 2003). There is also convincing lesion evidence to conclude that navigation mechanisms that rely on head direction cells are dependent on the projections from the mammillary bodies (lateral mammillary nucleus) to the anterior thalamic nuclei (anterior dorsal thalamic nucleus) and, thence, to the hippocampus (Bassett & Taube, 2005; Blair et al., 1998; Taube & Muller, 1998). These examples establish the principle that the medial temporal medial diencephalic interdependency may best be seen as reciprocal (Vann & Aggleton, 2004). As a consequence, the disconnection studies with rats (Henry et al., 2004; Warburton et al., 2001), which have shown that the hippocampus and anterior thalamic nuclei are interdependent for at least some aspects of spatial learning, cannot determine in which direction the effects are most critical. Finally, as alluded to earlier (in the section Towards an integrated model of temporal lobe and diencephalic amnesia ), there is a further, more complex way in which these regions might be functionally linked. There could be a third region that is anatomically connected with both the hippocampus and the anterior thalamic nuclei, and which functionally links these structures. One candidate region of considerable current interest is the retrosplenial cortex (see the section Covert pathology, amnesia, and the retrosplenial cortex ). Rather than argue that THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1453

15 AGGLETON the retrosplenial cortex is the sole common source of temporal and diencephalic anterograde amnesia, a much more plausible case can be made that dysfunctions in this cortical area contribute to both diencephalic and temporal lobe amnesias and, hence, increase their similarity. In order to consider the evidence for this novel notion it is necessary to first consider the ways in which lesion evidence is usually presented and interpreted. Assumptions about lesion studies The functional effects of brain damage never occur in isolation. At the same time, interpreting the outcome of selective brain injury remains one of the most powerful tools in the quest to understand the relationship between brain structure and function. It should, however, be remembered that lesion studies do not examine the functions of the structure removed rather they study the extent to which the brain can compensate in the absence of that structure. Even so, the traditional focus has been on describing overt structural damage for example, defining lesions by the extent of cytoarchitectonic damage as seen in Nissl stained sections. The implicit assumption behind this approach is that other intact structures can perform normally apart from any disconnection of information caused by the lesion (be it afferent or efferent). At this point, a critical distinction must be made. All researchers would accept that any lesion will cause a minimum of two other brain regions to behave abnormally for example, by a loss of afferent information and the loss of an efferent target. The question to be considered here is whether the distal effects of conventional lesions can be much more than just deafferentation. It should first be recognized that some lesions will cause overt pathology in sites outside the primary lesion target. There are numerous examples of anterograde or retrograde degeneration (e.g., prefrontal cortex lesions causing cell death and associated gliosis in the thalamic nucleus medialis dorsalis), but these effects are still overt. The issue here is whether covert or cryptic pathology can occur. The defining feature of such instances would be intact cytoarchitecture that is, no overt cellular changes or gliosis combined with a persistent abnormality in the way in which that structure processes all afferent or efferent information so that it is functionally rendered as lesioned. There is, in fact, a long history to the notion that damage to one part of the brain can have its effects at a distance (Finger, Koehler, & Jagella, 2004). Probably the most influential notion is that of diaschisis. This term is most closely linked with the work of Constantin von Monakow (Finger et al., 2004; von Monakow, 1911) although his ideas were predated by others from the 19th century. The diaschisis theory supposed that in addition to the temporary effects of trauma (oedema, changes in blood flow), the mere disconnection of an area could induce impairment of function in that area. The term diaschisis was usually applied to transient disruptions following acute injury, which von Monakow (1911) saw as an abolition of local excitability. Different rates of recovery could, however, be found in different regions (Finger et al., 2004), and while the emphasis was on recovery von Monakow also supposed that some remote diaschisis effects could be very long lasting ( diaschisis protractiva ). The concept of diaschisis was very important both in attempting to understand how recovery from brain insult may occur and in broadening our interpretations of lesions. The problem with this concept was how best to define and measure such hidden pathology. As a consequence, there still remains a debate over the existence of covert pathology and its relevance for amnesia. Covert pathology, amnesia, and the retrosplenial cortex The debate over whether covert pathology contributes to amnesia has up to now focused on temporal lobe amnesia (for opposing views see Bachevalier & Meunier, 1996; Squire & Zola, 1996). The term covert pathology is used to refer to an area that appears normal by standard histological means and yet is functionally lesioned. It is assumed that these changes are permanent and 1454 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

16 UNDERSTANDING ANTEROGRADE AMNESIA that they do not just reflect a disconnection of specific information (even though they are caused by a disconnection). The task would be to confirm that a region is rendered functionally unresponsive following distal damage, even when it is stimulated by surviving pathways. In the case of temporal lobe amnesia much of the debate has centred on the extent of functional brain damage following hypoxia or occlusion of the posterior cerebral artery. The latter procedure produces ischaemic damage in the hippocampus. It has been observed in monkeys that the effects of ischaemic hippocampal damage on tests of recognition memory are more disruptive than conventional lesions even though the apparent extent of cellular loss is comparable (Bachevalier & Meunier, 1996; but see Squire & Zola, 1996). Brain imaging studies have added weight to the idea that the functional lesion in anoxia might extend beyond the evident physical lesion (Grubb et al., 2000; Markowitsch, Weber-Luxemburger, Ewald, Kessler, & Heiss, 1997). Arguably the most striking result is that by Mumby and coworkers who first compared conventional hippocampal lesions with hippocampal lesions induced by vascular occlusion in rats (Mumby et al., 1996). While the former method had little or no effect on recognition (Mumby, 2001), the ischaemic preparation produced robust deficits (Mumby et al., 1996; Wood, Mumby, Pinel, & Phillips, 1993). While the latter method led to greater recognition deficits, it did not produce more hippocampal damage. The quite remarkable thing about this study was the discovery that conventional hippocampal lesions made prior to vascular occlusion resulted in a smaller recognition deficit than that seen after occlusion only (Mumby et al., 1996). This finding was interpreted as showing that hippocampal activity in response to the vascular insult normally provokes hidden chronic dysfunctions in extrahippocampal areas, but these effects are protected by first removing the hippocampus. A problem for this type of interpretation remains the difficulty of specify those sites suffering covert pathology, closely linked to the problem of finding a suitable marker for this putative effect. One possible site for covert pathology in both temporal amnesia and diencephalic amnesia is the retrosplenial cortex. It has long been appreciated that the retrosplenial cortex has dense, reciprocal connections with both the anterior thalamic nuclei and hippocampal formation. Neuropsychological studies of amnesia have also shown that damage to this area can cause anterograde amnesia (Maguire, 2001b; Rudge & Warrington, 1991; Valenstein et al., 1987; Yasuda, Watanabe, Tanaka, Tadashi, & Akiguchi, 1997) and topographical amnesia (Maguire, 2001b; Yasuda et al., 1997). Furthermore, functional neuroimaging studies repeatedly find raised retrosplenial activity during a range of memory (Cabeza & Nyberg, 2000; Maddock, Garrett, & Buonocore, 2001; Maguire, 2001a, 2001b) and spatial navigation (Epstein, Parker, & Feiler, 2007; Iaria, Chen, Guariglia, Ptito, & Petrides, 2007) tasks. It follows, therefore, that a hypothetical loss of retrosplenial function should contribute to the memory deficits caused by damage to a remote area (unless the functional effects are completely redundant). Lesion studies with rats also reveal the importance of the retrosplenial cortex for learning and memory, with the majority of studies confirming the importance of this region for spatial memory (Aggleton & Vann, 2004; Cooper & Mizumori, 2001; Sutherland & Hoesing, 1993; Whishaw, Maaswinkel, Gonzalez, & Kolb, 2001) including learning the locations of specific objects (Vann & Aggleton, 2002). These deficits are similar to those seen after both hippocampal and anterior thalamic lesions, consistent with the close anatomical relationships between these three regions. It is also noteworthy that the retrosplenial cortex is linked to spatial navigation in both rats and humans, which has itself been closely linked to episodic memory (Bird & Burgess, 2008). Evidence to suppose that the retrosplenial cortex might suffer covert pathology largely comes from animal studies. One of the first discoveries was that the retrosplenial cortex is abnormally sensitive to drugs that act upon glutamate receptors. Following systemic NMDA blockade, pyramidal and multipolar cells in layers III and IV of the retrosplenial cortex swell at low drug THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1455

17 AGGLETON doses, with cell death (overt pathology) at higher doses (Olney, Labruyere, & Price, 1989; Olney, Sesma, & Wozniak, 1993). These findings suggest that distal lesions that change glutamatergic activity in the retrosplenial cortex might bring about subtle pathological changes in this cortex. It is also known that the retrosplenial cortex is sensitive to local NMDA blockade in the anterior thalamus (Tomitaka, Tomitaka, Tolliver, & Sharp, 2000). Given that the dense inputs from the anterior thalamus to the retrosplenial cortex are primarily glutamatergic (Gonzalo-Ruiz, Sanz, Morte, & Lieberman, 1997), it might be predicted that the loss of these inputs could severely affect retrosplenial function. Anterior thalamic lesions and covert pathology Because covert pathology is, by definition, hidden it is necessary to look for a marker of neuronal function that could become abnormal, even though the neurons under investigation appear normal. The class of markers we have concentrated on in our research are known as immediate-early genes. It has long been known that neurons can influence other neurons via ion gated channels at the synapse to increase or decrease the likelihood of a postsynaptic action potential. It is also now clear that trans-synaptic activation can also stimulate slower, longer term changes in the postsynaptic neuron that involve the induction of programmes of gene expression. Genes that are responsive to trans-synaptic activation fall into two general classes: Immediate-early genes are activated rapidly (within minutes of neuronal stimulation) while late-response genes are induced (or repressed) more slowly and are dependent on new protein synthesis (Sheng & Greenberg, 1990). As immediate-early genes are activated without the need of a protein messenger they are seen as de novo. Their relatively early activation, combined with the fact that some immediate-early genes (e.g., c-fos) are inducible transcription factors that can orchestrate the transcription of other downstream genes places them as potential candidate markers of covert pathology. This is because their role at the head of a cascade of changes means that abnormal immediate-early gene activity is likely to signal a broad array of other changes. A further reason for measuring immediate-early gene activity is that some immediate-early genes (e.g., Arc, zif268, c-fos) have a role in synaptic processes thought to be central to plasticity and learning (Bozon, Davis, & Laroche, 2002; Countryman, Kaban, & Colombo, 2005; Davis, Bozon, & Laroche, 2003; Guzowski, Setlow, Wagner, & McGaugh, 2001; He, Yamada, & Nabeshima, 2002; Tischmeyer & Grimm, 1999; Vann et al., 2000a, 2000b). While immediate-early genes are not direct markers of neuronal activity, as these genes can have very different baseline levels of activity in different brain sites, and there are occasions when their activity does not parallel neuronal activity (Herdegen, 1996), these genes may be appropriate assays of retrosplenial cortex function. It is, for example, known that c-fos activity is increased in the retrosplenial cortex following the performance of spatial memory tasks that are sensitive to retrosplenial lesions (Vann & Aggleton, 2002). In addition, the retrosplenial cortex shows relatively high baseline levels of both c-fos and Zif268, and so any hypometabolism is unlikely to be hidden by floor effects. The first studies, therefore, looked at the immediate-early gene status of the rat retrosplenial cortex following anterior thalamic lesions. Over a series of studies we repeatedly found that that anterior thalamic lesions result in dramatic losses of retrosplenial immediate-early genes activity without overt pathology (Jenkins, Vann, Amin, & Aggleton, 2004; Figures 6, 7). Both bilateral and unilateral excitotoxic (NMDA) lesions of the rat anterior thalamic nuclei cause spectacular decreases in c-fos activity in the retrosplenial cortex (Jenkins et al., 2002a, 2002b, 2004). Our studies have shown that: (a) Anterior thalamic nuclei lesions cause massive reductions in c-fos levels (80% or more) that are most dramatic in the superficial layers of the granular retrosplenial cortex (Rga, Rgb; Figures 6, 7); (b) anterior thalamic nuclei lesions cause even greater c-fos depletions in 1456 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

18 UNDERSTANDING ANTEROGRADE AMNESIA Figure 6. Photomicrographs of coronal sections showing c-fos-positive cells (dark) in the rat retrosplenial cortex (granular B). The left section (A) is from a normal brain; the right section is from a brain in which there is a lesion in the anterior thalamic nuclei (ATN). The loss of c-fos in the superficial layers following the lesion is immediately apparent, while there is no change in deeper layers. Scale bar 200 ml. rats 10 months postsurgery, and that these depletions also become evident in the dysgranular retrosplenial cortex and the deeper laminae of the granular regions; (c) anterior thalamic nuclei lesions do not affect just one immediate-early gene as matching patterns of Zif268 depletions are also found (Jenkins et al., 2004); (d) the Fos changes are not specific to the lesion technique (both neurotoxic and radio-frequency lesions havethesameeffect)orstrainofrat;(e)this lesion-induced hypoactivity is not universal for all retrosplenial disconnections as lesions of the entorhinal cortex, a region that is reciprocally connected to the retrosplenial cortex (Burwell & Amaral, 1998), have little or no effect on Fos levels (Albasser, Poirier, Warburton, & Aggleton, 2007); (f) anterior thalamic lesions do not alter the appearance of the retrosplenial cortex, as determined by standard histological techniques (see also Van Groen, Vogt, & Wyss, 1993) or counts of Nissl stained cells (Jenkins et al., 2004) that is, the changes are covert. Figure 7. Bar graphs showing c-fos positive cell counts in retrosplenial cortex (Rga, Rgb, Rdg), as well as primary auditory cortex (AUD) insula cortex (AIP), motor cortex (MOP), and primary visual cortex (VISP) following excitotoxic lesions in the anterior thalamic nuclei (left: data from Jenkins et al., 2002b) and hippocampal formation (data from Albasser et al., 2007). Counts in each region are compared with those from a control brain (left) or control hemisphere (right), and so a score of 100 reflects an identical count in the lesioned and intact hemispheres. By distinguishing the retrosplenial counts for the superficial layers (I III) and for all layers it can be seen that anterior thalamic lesions have a much greater impact on superficial c-fos levels. THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1457

19 AGGLETON The anterior thalamic nuclei have direct connections with the retrosplenial cortex, and so it is important to consider why the observed retrosplenial immediate-early gene hypoactivity might reflect a more fundamental disturbance of cortical function than just the loss of one source of afferent information. Evidence against the mere disconnection view comes from the finding that layers II and upper III of retrosplenial cortex show by far the most extreme loss of Fospositive cells (80% plus) while the deeper layers can appear unaffected. This distribution does not reflect those laminae with thalamic inputs, as these projections terminate in a variety of layers (I, II, IV; Van Groen & Wyss, 2003). More importantly, the anterior thalamic lesions leave intact a wide array of other excitatory inputs (Gonzalo-Ruiz et al., 1997) to the superficial retrosplenial cortex (e.g., from the subiculum, postsubiculum, lateral dorsal thalamic nucleus, and entorhinal cortex; Figure 8). Even so, c-fos activity is still massively suppressed after anterior thalamic lesions under a wide array of behavioural conditions (Jenkins et al., 2004). More compelling evidence has come from the recent finding that cutting the mammillothalamic tract is sufficient to cause striking decreases of retrosplenial cortex immediate-early gene levels (Albasser & Vann, 2007). This surgery only indirectly disconnects the retrosplenial cortex that is, all direct inputs remain intact (Figure 8) yet the immediate-early gene loss is similar to that seen after anterior thalamic lesions. Thus, it seems increasingly unlikely that the immediate-early gene loss just reflects a decrease of neuronal activity as so many inputs are left intact. Other evidence that retrosplenial cortex function depends on the integrity of the anterior thalamus comes from a crossed lesion study in rats (Sutherland & Hoesing, 1993). Unilateral lesions of the anterior thalamic nuclei combined with unilateral retrosplenial cortex lesions in the contralateral hemisphere lead to spatial memory deficits in the Morris water maze. The covert pathology view would have to predict such a result, but more traditional explanations can also account for these results. Likewise, recording studies show that anterior thalamic lesions markedly disrupt retrosplenial function (Gabriel, 1993; Gabriel et al., 1983). Not only has it been found that the cells in the rabbit retrosplenial cortex show traininginduced activity that parallels acquisition of an avoidance task, but this plasticity does not develop following anterior thalamic lesions (Gabriel, 1993; Gabriel et al., 1983). Even more striking evidence that anterior thalamic inputs might be needed for the maintenance Figure 8. Schematic diagram showing sources of inputs to the retrosplenial cortex. The multiple afferents should ensure that when one source is removed the cortex is only partially deafferented. Abbreviations: AD, anterior dorsalis; AM anterior medialis; AV, anterior ventralis THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

20 UNDERSTANDING ANTEROGRADE AMNESIA of retrosplenial plasticity comes from a recent electrophysiological study of retrosplenial cortex slices (Garden et al., 2008; Garden et al., 2006). Unilateral anterior thalamic lesions were made in rats that were sacrificed five weeks after surgery. Comparisons were then made between the retrosplenial slices from the two hemispheres. While it was possible to induce long-term depression (LTD) in slices from the intact hemisphere, this was not the case for the slices from the hemisphere with a thalamic lesion that is, having a distal lesion in the thalamus disrupted retrosplenial plasticity (Figure 9). This loss of LTD was found in the superficial cell layers in the retrosplenial cortex and so corresponds to the cortical lamina showing the most pronounced immediate-early gene loss after anterior thalamic lesions. This loss of plasticity is especially informative as it is hard to ascribe it to an absence of afferent stimulation as stimulation was applied locally by the experimenter (Garden et al., 2008; Garden et al., 2006). Indeed, no changes in fast glutamatergic synaptic transmission were found in retrosplenial cortex slices following anterior thalamic lesions, nor any cell loss. For this reason, the LTD deficit cannot be due to the loss of actual afferent information, as matching patterns of stimulation were applied in both sets of slices. Therefore, by most standard measures this cortex appeared normal following anterior thalamic lesions. Nevertheless, there were intrinsic plasticity abnormalities within the cortex caused by the thalamic lesion several weeks earlier. A very different approach was then used to try and identify the nature of these intrinsic changes. All previous assessments of immediate-early gene activity provided only a very narrow picture of retrosplenial gene activity following anterior thalamic lesions as only two genes were studied in a single experiment. Microarray technology was next used in order to give a more global picture of gene activity change following anterior thalamic lesions (Poirier et al., in press). Rats with unilateral, anterior thalamic lesions were exposed to a novel environment for 20 min, and granular retrosplenial tissue was sampled from both hemispheres after 30 min, 2 hr, or 8 hr. Complementary analytical approaches revealed pervasive gene expression differences between the retrosplenial cortex ipsilateral to the thalamic lesion ( lesion ) and contralateral to the lesion ( intact ). The expression of many genes was reduced after thalamic lesions, and pathways that exhibited lower relative levels of specific mrnas included both energy metabolism and plasticityrelated pathways (signal transduction and transcript/protein regulation). These changes in functional gene expression may be driven by lesion-associated changes in the expression of multiple transcription factor genes, including brd8, c-fos, fra-2, klf5, nfat5, neurod1, nfix, nr4a1, RXRg, smad3, smarcc2, and zfp91 (Poirier et al., in press). These microarray findings confirm that the retrosplenial changes are not just confined to a few immediate-early genes but also provide insights into how reductions in metabolic activity and a loss of plasticity might follow remote thalamic lesions. Hippocampal lesions and retrosplenial activity Another major input to the retrosplenial cortex originates from the hippocampal formation (Wyss & Van Groen, 1992), raising the question of whether the loss of these inputs might also markedly disrupt retrosplenial function. Prior evidence comes from crossed disconnection lesion studies showing that the hippocampus and retrosplenial cortex make interdependent contributions to spatial memory (Sutherland & Hoesing, 1993). In order to examine this relationship we used the same methodology as that described above that is, measuring the impact of hippocampal lesions on retrosplenial cortex immediate-early gene levels (Albasser et al., 2007). Lesions of the rat hippocampus, whether made byradiofrequencyorbytheinjectionofneurotoxins, had very similar and consistent effects upon the retrosplenial cortex. Clear reductions in the expression of both c-fos and Zif268 were observed, so that counts of immediate-early gene positive cells were typically reduced by one half (Figure 10; Albasser et al., 2007). Unlike THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1459

21 AGGLETON Figure 9. Anterior thalamic lesions result in a layer-specific loss of long-term depression (LTD) in retrosplenial cortex. (a) Induction of homosynaptic LTD in normal retrosplenial cortex. (b) Following an anterior thalamic lesion there is a complete loss of LTD in restrosplenial slices ipsilateral to the lesion THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10)

22 UNDERSTANDING ANTEROGRADE AMNESIA Figure 10. Photomicrographs of neuronal density (NeuN) and c-fos levels in granular retrosplenial cortex (Rgb) in rats with either a sham surgery (control, A, B) or a bilateral ibotenate hippocampal lesion (C, D). The brightfield photomicrographs of horizontal sections show the comparable levels of neurons (A, C), which contrast with the striking loss of c-fos-positive cells following hippocampal lesions (D versus B). Scale bar, 100 mm. the effects of anterior thalamic lesions, this immediate-early gene hypoactivity was evident in both the superficial and the deep layers of the granular cortex. Thus, a clear loss of the c-fos signal was seen in layer V in addition to layers II and upper III (Figure 10; Albasser et al., 2007). Once again, there was no consistent evidence for a loss of neuronal numbers or changes in cellular morphology in the retrosplenial cortex that is, these changes were not detectable by standard methods. While more evidence is required, the pattern of results points to a hypersensitivity in the retrosplenial cortex following the loss of certain inputs (from the anterior thalamic nuclei and from the hippocampus). Not surprisingly, there may be some important clinical corollaries of these distal lesion effects upon the retrosplenial cortex. Of especial note is the repeated finding of posterior cingulate hypoactivity in Alzheimer s disease (Minoshima, Foster, & Kuhl, 1994; Minoshima et al., 1997). This hypoactivity is of particular interest as it is often the first metabolic change observed in positron emission tomography (PET) scans of the disease (Minoshima et al., 1994, 1997). Posterior cingulate hypoactivity, and in particular retrosplenial hypoactivity, is also found in subjects with mild cognitive impairment (Nestor, Fryer, Ikeda, & Hodges, 2003a; Nestor, Fryer, Smielewski, & Hodges, 2003b), which is often a prodromal stage of Alzheimer s disease. Both anterior thalamic and hippocampal pathology occur relatively early in the progression of Alzheimer s disease (Braak & Braak, 1991a, 1991b). A testable hypothesis is that pathology in the hippocampus, the anterior thalamic nuclei, or both could induce the posterior cingulate hypoactivity. The significance of this notion is not only that it provides a potential mechanism for cingulate hypoactivity but it also highlights how dysfunctions (some overt, some covert) THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (10) 1461