Selective hippocampal damage in rhesus monkeys impairs spatial memory in an open-field test

Working memory span capacity improved by a D2 but not D1 receptor family agonist

Patients with schizophrenia exhibit poor working memory (WM). Although several subcomponents of WM can be measured, evidence suggests the primary subcomponent affected in schizophrenia is span capacity (WMC). Indeed, the NIMH-funded MATRICS initiative recommended assaying the WMC when assessing the efficacy of a putative therapeutic for FDA approval. Although dopamine D1 receptor agonists improve delay-dependent memory in animals, evidence for improvements in WMC due to dopamine D1 receptor activation is limited. In contrast, the dopamine D2-family agonist bromocriptine improves WMC in humans. The radial arm maze (RAM) can be used to assess WMC, although complications due to ceiling effects or strategy confounds have limited its use. We describe a 12-arm RAM protocol designed to assess whether the dopamine D1-family agonist SKF 38393 (0, 1, 3, and 10 mg/kg) or bromocriptine (0, 1, 3, and 10 mg/kg) could improve WMC in C57BL/6N mice (n=12) in cross-over designs. WMC increased and strategy usage decreased with training. The dopamine D1 agonist SKF 38393 had no effect on WMC or long-term memory. Bromocriptine decreased WMC errors, without affecting long-term memory, consistent with human studies. These data confirm that WMC can be measured in mice and reveal drug effects that are consistent with reported effects in humans. Future research is warranted to identify the subtype of the D2-family of receptors responsible for the observed improvement in WMC. Finally, this RAM procedure may prove useful in developing animal models of deficient WMC to further assess putative treatments for the cognitive deficits in schizophrenia.

Alterations of white matter tracts following neurotoxic hippocampal lesions in macaque monkeys: a diffusion tensor imaging study

Diffusion tensor imaging (DTI) is a valuable tool for assessing presumptive white matter alterations in human disease and animal models. The current study used DTI to examine the effects of selective neurotoxic lesions of the hippocampus on major white matter tracts and anatomically related brain regions in macaque monkeys. Two years postlesion, structural MRI, and DTI sequences were acquired for each subject. Volumetric assessment revealed a substantial reduction in the size of the hippocampus in experimental subjects, averaging 72% relative to controls, without apparent damage to adjacent regions. DTI images were processed to yield measures of fractional anisotropy (FA), apparent diffusion coefficient (ADC), parallel diffusivity (lADC), and perpendicular diffusivity (tADC), as well as directional color maps. To evaluate potential changes in major projection systems, a region of interest (ROI) analysis was conducted including the corpus callosum, fornix, temporal stem, cingulum bundle, ventromedial prefrontal white matter, and optic radiations. Lesion-related abnormalities in the integrity of the fiber tracts examined were limited to known hippocampal circuitry, including the fornix and ventromedial prefrontal white matter. These findings are consistent with the notion that hippocampal damage results in altered interactions with multiple memory-related brain regions, including portions of the prefrontal cortex.

Recognition memory: material, processes, and substrates

The proposal that a system centering on the perirhinal cortex is responsible for familiarity discrimination, particularly for single items, whereas a system centering on the hippocampus is responsible for recollective and more complex associational aspects of recognition memory is reviewed in the light of recent findings. In particular, the proposal is reviewed in relation to recent animal work with rats and results from human clinical studies. Notably, progress has been made in determining potential neural memory substrate mechanisms within the perirhinal cortex in rats. Recent findings have emphasized the importance of specifying the type of material, the type of test, and the strategy used by subjects to solve recognition memory tests if substrates are to be accurately inferred. It is to be expected that the default condition is that both the hippocampal and perirhinal systems will contribute to recognition memory performance. Indeed, rat lesion experiments provide examples of where cooperation between both systems is essential. Nevertheless, there remain examples of the independent operation of the hippocampal and perirhinal systems. Overall, it is concluded that most, though not all, of the recent findings are in support of the proposal. However, there is also evidence that the systems involved in recognition memory need to include structures outside the medial temporal lobe: there are significant but as yet only partially defined roles for the prefrontal cortex and sensory association cortices in recognition memory processes.

Lateral but not medial telencephalic pallium ablation impairs the use of goldfish spatial allocentric strategies in a "hole-board" task

Strong evidence suggests that the ventral region of the lateral telencephalic pallium of teleost fish, a structure involved in allocentric spatial cognition, is homologous to the hippocampus of tetrapods. This homology was first proposed on basis of anatomical data, and subsequently confirmed by developmental, functional and behavioural studies. Nonetheless, Saito and Watanabe [30,32] claim that not the lateral but, rather, the medial pallium participates in goldfish spatial navigation and should be considered the homologue of the hippocampus. Here, we further investigate the effects of selective pallial lesions on the spatial cognition abilities of goldfish, trained in a "hole-board" analogue task, to find the baited feeder within a 5 x 5 feeder matrix surrounded by visual cues. The task in the present experiment is similar to that used by Saito and Watanabe, but including thorough probe tests that enabled to define clearly the spatial strategies employed by the animals, and, therefore, the spatial deficits caused by the pallial lesions. The results showed that the lateral, but not the medial pallium lesions, produced a dramatic impairment in the implementation of allocentric spatial strategies. Thus, only lateral pallium lesioned goldfish, like hippocampus lesioned tetrapods, failed to reach the goal when the cues in its proximity were excluded, indicating that they used a guidance strategy. These results do not replicate Saito and Watanabe's, but are consistent to previous data indicating a close functional similarity between the lateral pallium of teleost fish and the hippocampus of amniotes.

Selective involvement of the goldfish lateral pallium in spatial memory

The involvement of the main pallial subdivisions of the teleost telencephalic pallium in spatial cognition was evaluated in a series of three experiments. The first two compared the effects of lesions selective to the lateral (LP), medial (MP) and dorsal (DP) telencephalic pallium of goldfish, on the retention and the reversal learning of a spatial constancy task which requires the use of allocentric or relational strategies. The results showed that LP lesions produced a selective impairment on the capability of goldfish to solve the spatial task previously learned and on the reversal learning of the same procedure, whereas MP and DP lesions did not produce observable deficits. The third experiment evaluated, by means of the AgNOR stain, learning-dependent changes of the neuronal transcription activity in the pallium of goldfish trained in the spatial constancy task or in a cue version of the same procedure, which only differed on their spatial cognition demands. The results revealed that training in the spatial task produced an increment in the transcriptive activity which was selective to the neurons of the ventral lateral pallium, as indicated by increases in the size of the nucleolar organizing region (NOR), the nucleolar organelles associated with the synthesis of ribosomal proteins. In contrast, training in the cue version did not produced observable changes. These data, revealing a striking functional similarity between the lateral telencephalic pallium of the teleost fish and the amniote hippocampus, provide additional evidence regarding the homology of both structures.

What, if anything, can monkeys tell us about human amnesia when they can't say anything at all?

Despite a half century of development, the orthodox monkey model of human amnesia needs improvement, in part because of two problems inherent in animal models of advanced human cognition. First, animal models are perforce comparative, but the principles of comparative and evolutionary biology have not featured prominently in developing the orthodox model. Second, no one understands the relationship between human consciousness and cognition in other animals, but the orthodox model implicitly assumes a close correspondence. If we treat these two difficulties with the deference they deserve, monkeys can tell us a lot about human amnesia and memory. Three future contributions seem most likely: (1) an improved monkey model, one refocused on the hippocampus rather than on the medial temporal lobe as a whole; (2) a better understanding of cortical areas unique to primates, especially the granular prefrontal cortex; and (3), taking the two together, insight into prefrontal-hippocampal interactions. We propose that interactions among the granular prefrontal areas create the kind of cross-domain, analogical and self-referential knowledge that underlies advanced cognition in modern humans. When these products of frontal-lobe function interact with the hippocampus, and its ancestral function in navigation, what emerges is the human ability to embed ourselves in scenarios-real and imagined, self-generated and received-thereby creating a coherent, conscious life experience.

Circadian activity associated with spatial learning and memory in aging rhesus monkeys

In rodents, spatial learning and memory tests require navigation, whereas in nonhuman primates these tests generally do not involve a navigational component, thus assessing nonhomologous neural systems. To allow closer parallels between rodent and primate studies, we developed a navigational spatial learning and memory task for nonhuman primates and assessed the performance of elderly (19-25 years) female rhesus monkeys (Macaca mulatta). The animals were allowed to navigate in a room containing a series of food ports. After they learned to retrieve food from the ports, a single port was repeatedly baited and the animals were tested until they learned the correct location. The location of the baited port was then changed (shift position). We also determined whether test performance was associated with circadian activity measured with accelerometers. Performance measures included trials to criterion, search strategies, and several indices of circadian activity. Animals learned the task as reflected in their search strategies. Correlations were found between the number of initial or shift trials and circadian activity parameters including day activity, dark:light activity ratio, sleep latency, and wake bouts. Thus, disruptions in circadian rhythms in nonhuman primates are associated with poorer performance on this novel test. These data support the usefulness of this spatial navigational test to assess spatial learning and memory in rhesus monkeys and the importance of circadian activity in performance.

Spatial memory and the monkey hippocampus: not all space is created equal

Studies of the role of the monkey hippocampus in spatial learning and memory, however few, have reliably produced inconsistent results. Whereas the role of the hippocampus in spatial learning and memory has been clearly established in rodents, studies in nonhuman primates have made a variety of claims that range from the involvement of the hippocampus in spatial memory only at relatively longer memory delays, to no role for the hippocampus in spatial memory at all. In contrast, we have shown that selective damage restricted to the hippocampus (CA regions) prevents the learning or use of allocentric, spatial relational representations of the environment in freely behaving adult monkeys tested in an open-field arena. In this commentary, we discuss a unifying framework that explains these apparently discrepant results regarding the role of the monkey hippocampus in spatial learning and memory. We describe clear and strict criteria to interpret the findings from previous studies and guide future investigations of spatial memory in monkeys. Specifically, we affirm that, as in the rodent, the primate hippocampus is critical for spatial relational learning and memory, and in a time-independent manner. We describe how claims to the contrary are the result of experimental designs that fail to recognize, and control for, egocentric (hippocampus-independent) and allocentric (hippocampus-dependent) spatial frames of reference. Finally, we conclude that the available data demonstrate unequivocally that the central role of the hippocampus in allocentric, spatial relational learning and memory is conserved among vertebrates, including nonhuman primates.

Memory for spatial location and object-place associations are differently processed by the hippocampal formation, parahippocampal areas TH/TF and perirhinal cortex

To clarify the specific contribution of the medial temporal lobe structures in spatial memory, we tested monkeys (Macaca mulatta) with sham operations and with lesions of either the hippocampal formation, areas TH/TF or perirhinal cortex on two versions of the visual-paired comparison task, measuring Spatial Location, and Object-in-Place associations. In the Spatial Location version, the comparison was between two identical objects presented simultaneously in a familiar and a novel location. In the Object-in-Place version, the comparison was between an image consisting of five objects and another image showing the same five objects, but with the position of 2, 3, or 4 of the objects rearranged. Finally, a VPC-Control task was given to animals with hippocampal and perirhinal lesions, in which the comparison was between an image consisting of five objects and another image showing four of the five familiar objects and a new one. Perirhinal lesions yielded no deficit in the Spatial Location task and a deficit in the Object-in-Place task associated with a deficit in the VPC-control task, suggesting that this cortical area does not participate in spatial memory unless the stimuli have overlapping features. Areas TH/TF lesions produced a deficit in both Spatial Location and Object-in-Place tasks, whereas the hippocampal lesions resulted in a deficit of Object-in-Place associations only. The data showed that the hippocampal formation, areas TH/TF, and perirhinal cortex appear to contribute interactively to object and spatial memory processes.

Visual perception and memory: a new view of medial temporal lobe function in primates and rodents

The prevailing view of medial temporal lobe (MTL) function has two principal elements: first, that the MTL subserves memory but not perception, and second, that the many anatomically distinctive parts of the MTL function together in the service of declarative memory. Recent neuropsychological studies have, however, challenged both opinions. First, studies in rodents, nonhuman primates, and humans suggest that the perirhinal cortex represents information about objects for both mnemonic and perceptual purposes. Second, the idea that MTL components contribute to declarative memory in similar ways has also been contradicted. Whereas the perirhinal cortex plays an essential role in familiarity-based object recognition, the hippocampus contributes little, if at all, to this function. In both primates and rodents, the hippocampus contributes to the memory and perception of places and paths, whereas the perirhinal cortex does so for objects and the contents of scenes.

Measuring reward assessment in a semi-naturalistic context: the effects of selective amygdala, orbital frontal or hippocampal lesions

Studying the neural mechanisms underlying complex goal-directed behaviors, such as social behavior, reward seeking or punishment avoidance, has become increasingly tractable in humans, nonhuman primates and rodents. In most experiments, however, goal-directed behaviors are measured in a laboratory setting, which is vastly different from the context in which these behaviors naturally occur. This study adapted a reward assessment paradigm, previously conducted with rhesus monkeys (Macaca mulatta) in the controlled environment of a Wisconsin General Testing Apparatus (WGTA) [Machado CJ, Bachevalier J (2007) The effects of selective amygdala, orbital frontal cortex or hippocampal formation lesions on reward assessment in nonhuman primates. Eur J Neurosci 25:2885-2904], to a more naturalistic context. We used this new paradigm to examine the effects of bilateral amygdaloid, hippocampal or orbital frontal cortex lesions on established food and nonfood preferences. Behavioral modification following reinforcer devaluation was also measured. Consistent with our previous study, none of the lesions produced changes in preference for palatable foods relative to pre-surgery, but animals with amygdala lesions displayed heightened preference for unpalatable foods that control or other operated animals typically avoided. In contrast to several previous WGTA-based experiments, nonfood preference was not affected by any of the lesions. Finally, animals with orbital frontal cortex lesions continued to select preferred foods after satiation, but those with amygdala, hippocampal or sham lesions altered their foraging behavior appropriately and selected less of the sated food. These findings parallel food devaluation results obtained with these same animals when tested in the WGTA. Overall, this study stresses the importance of testing context when measuring decision-making abilities in nonhuman primates with selective brain lesions.

Activating the Medial Temporal Lobe during Oddity Judgment for Faces and Scenes

Impairments in visual discrimination beyond long-term declarative memory have been found in amnesic individuals, with hippocampal lesions resulting in deficits in scene discrimination and perirhinal cortex damage affecting object discrimination. To complement these findings, the present functional magnetic resonance imaging study found that in healthy participants oddity judgment for novel trial-unique scenes, compared with face or size oddity, was associated with increased posterior hippocampus and parahippocampal cortex activity. In contrast, perirhinal and anterior hippocampus activity was observed during unfamiliar trial-unique face oddity judgment, when contrasted with scene or size oddity tasks. Activity in all of these regions decreased as the stimuli were repeated across trials, reflecting the participants' increasing familiarity with the stimuli. This change was significant in all areas, with the exception of the perirhinal cortex, right anterior hippocampus, and to a lesser extent the left anterior hippocampus during face oddity judgment. One possibility is that the activity in these regions may not reflect entirely episodic memory encoding but, in part, processes beyond the mnemonic domain. Thus, the perirhinal cortex, and possibly anterior hippocampus, may play a more generic role in the discrimination and processing of objects.

Hippocampal lesion prevents spatial relational learning in adult macaque monkeys

The role of the hippocampus in spatial learning and memory has been extensively studied in rodents. Comparable studies in nonhuman primates, however, are few, and findings are often contradictory. This may be attributable to the failure to distinguish between allocentric and egocentric spatial representations in experimental designs. For this experiment, six adult monkeys received bilateral hippocampal ibotenic acid lesions, and six control subjects underwent sham surgery. Freely moving monkeys then foraged for food located in two arrays of three distinct locations among 18 locations distributed in an open-field arena. Multiple goals and four pseudorandomly chosen entrance points precluded the monkeys' ability to rely on an egocentric strategy to identify food locations. Monkeys were tested in two conditions. First, local visual cues marked the food locations. Second, no local cues marked the food locations, so that monkeys had to rely on an allocentric (spatial relational) representation of the environment to discriminate these locations. Both hippocampal-lesioned and control monkeys discriminated the food locations in the presence of local cues. However, in the absence of local cues, control subjects discriminated the food locations, whereas hippocampal-lesioned monkeys were unable to do so. Interestingly, histological analysis of the brain of one control monkey whose behavior was identical to that of the experimentally lesioned animals revealed a bilateral ischemic lesion restricted to the hippocampus. These findings demonstrate that the adult monkey hippocampal formation is critical for the establishment or use of allocentric spatial representations and that selective damage of the hippocampus prevents spatial relational learning in adult nonhuman primates.

Spatial relational memory in 9-month-old macaque monkeys

This experiment assesses spatial and nonspatial relational memory in freely moving 9-mo-old and adult (11-13-yr-old) macaque monkeys (Macaca mulatta). We tested the use of proximal landmarks, two different objects placed at the center of an open-field arena, as conditional cues allowing monkeys to predict the location of food rewards hidden in one of two sets of three distinct locations. Monkeys were tested in two different conditions: (1) when local visual cues marked the two sets of potentially baited locations, so that monkeys could use both local and spatial information to discriminate these locations from never-baited locations; and (2) when no local visual cues marked the two sets of potentially baited locations, so that monkeys had to rely on a spatial relational representation of the environment to discriminate these locations. No 9-mo-old or adult monkey associated the presence of the proximal landmarks, at the center of the arena, with the presence of food in one set of three distinct locations. All monkeys, however, discriminated the potentially baited locations in the presence of local visual cues, thus providing evidence of visual discrimination learning. More importantly, all 9-mo-old monkeys tested discriminated the potentially baited locations in absence of the local visual cues, thus exhibiting evidence of spatial relational learning. These findings indicate that spatial memory processes characterized by a relational representation of the environment are present as early as 9 mo of age in macaque monkeys.

No effect of hippocampal lesions on perirhinal cortex-dependent feature-ambiguous visual discriminations

Previous studies have shown that perirhinal cortex lesions in monkeys impair visual discriminations with a high degree of "feature ambiguity," a property of visual discriminations that can emerge when features are a part of both rewarded and unrewarded stimuli. The effects of damage to the hippocampus on these perirhinal-dependent feature-ambiguous tasks are, however, unknown. Prominent theories of medial temporal lobe function predict similar effects of perirhinal cortex and hippocampal lesions on cognitive tasks. In contrast, our hypothesis is that perirhinal cortex, and not the hippocampus, is important for nonspatial complex feature-ambiguous discriminations. We sought to distinguish between these competing theories in a straightforward way, by testing rhesus monkeys with hippocampal lesions on the same feature-ambiguous tasks shown previously to depend on perirhinal cortex. It was found that hippocampal lesions had no effects on any of these tasks. The findings support the perceptual-mnemonic/feature conjunction model of perirhinal cortex function, and provide further evidence for heterogeneity of function within the putative medial temporal lobe memory system.

Perirhinal cortex and its neighbours in the medial temporal lobe: contributions to memory and perception

As promised in the Introduction, this Special Issue presents several recurring themes concerning the perirhinal cortex and its neighbours within the medial temporal lobe (MTL). First, although orthodoxy insists that the diverse constituents of the MTL operate as a single functional entity, several papers presented here challenge that idea, although some defend it. Second, although many experts hold that the MTL subserves memory but not perception, several papers presented here point to a role for certain MTL structures in both. Third, although some researchers have invoked “species differences” to account for discrepant findings, several papers presented here document a striking convergence of findings in humans, nonhuman primates, and rodents. We close this Special Issue by high-lighting these recurring themes, acknowledging discrepant findings and pointing to future research that might resolve some current controversies.

Comparison of the effects of damage to the perirhinal and parahippocampal cortex on transverse patterning and location memory in rhesus macaques

Monkeys with damage to the parahippocampal (TH/TF) or perirhinal (PRh) cortex were tested on two sets of the transverse patterning (TP) problem (A+/B-, B+/C-, C+/A- and D+/E-, E+/F-, F+/D-) and delayed nonmatching-to-location paradigm (DNML), with delays ranging from 10 to 600 s. Damage to either area impaired acquisition and performance of TP but not of linear discriminations (e.g., A>B>C>X). Damage to areas TH/TF impaired performance of the DNML at all delays but only affected memory for objects at the longest delay, as measured by a delayed nonmatching-to-sample task (DNMS) (Nemanic et al., 2004). Damage to the PRh impaired performance of the DNMS but not of the DNML. The results present a dissociation in object and place memory for these two cortical regions and suggest a role for each in the cortical circuitry supporting configural/relational memory.

Dissociable contributions within the medial temporal lobe to encoding of object-location associations

The crucial role of the medial temporal lobe (MTL) in episodic memory is well established. Although there is little doubt that its anatomical subregions-the hippocampus, peri-, entorhinal and parahippocampal cortex (PHC)-contribute differentially to mnemonic processes, their specific functions in episodic memory are under debate. Data from animal, human lesion, and neuroimaging studies suggest somewhat contradictory perspectives on this functional specialization: a general participation in declarative memory, an exclusive involvement in associative mnemonic processes, and a specific contribution to spatial memory are reported for the hippocampus, adjacent cortices, and the PHC. A functional lateralization in humans dependent on the verbalizability of the material is also discussed herein. To further elucidate the differential contributions of the various MTL subregions to encoding, we employed an object-location association memory paradigm. The memory for each of the studied associations was tested twice: by the object, and by the location serving as retrieval cue. The memory accuracy in response to both cue types was also assessed parametrically. Brain activity during encoding which leads to different degrees of subsequent memory accuracy under the two retrieval conditions was compared. We found the bilateral posterior PHC to participate in encoding of both the object associated with a location and the location associated with an object. In contrast, activity in an area in the left anterior PHC and the right anterior MTL was only correlated with the memory for the location associated with an object.

The primate hippocampus: ontogeny, early insult and memory

Recent evidence suggests that in primates, as in rodents, the hippocampus shows a developmental continuum that affects memory abilities from infancy to adulthood. In primates relatively few hippocampal-dependent abilities (e.g. some aspects of recognition memory) are present in early infancy, whereas others (e.g. relational memory) begin to show adult-like characteristics around 2 years of age in monkeys and 5-7 years in humans. Profound and persistent memory loss resulting from insult to the hippocampus in infancy becomes evident in everyday behavior only later in childhood. This pattern of results suggests a maturational gradient within the medial temporal lobe memory system, with most abilities crucially dependent upon the hippocampus emerging in later stages of development, supporting a model of hierarchical organization of memory within the medial temporal lobe.

Perceptual deficits in amnesia: challenging the medial temporal lobe ‘mnemonic’ view

Recent animal studies suggest that the medial temporal lobe (MTL), which is thought to subserve memory exclusively, may support non-mnemonic perceptual processes, with the hippocampus and perirhinal cortex contributing to spatial and object perception, respectively. There is, however, no support for this view in humans, with human MTL lesions causing prominent memory deficits in the context of apparently normal perception. We assessed visual discrimination in amnesic cases to reveal that while selective hippocampal damaged patients could discriminate faces, objects, abstract art and colour, they were significantly poorer in discriminating spatial scenes. By contrast, patients with MTL damage, including perirhinal cortex, were significantly impaired in discriminating scenes, faces, and to a lesser extent objects, with relatively intact discrimination of art and colour. These novel observations imply that the human MTL subserves both perceptual and mnemonic functions, with the hippocampus and perirhinal cortex playing distinct roles in spatial and object discrimination, respectively.

Specialization in the medial temporal lobe for processing of objects and scenes

There has been considerable debate as to whether the hippocampus and perirhinal cortex may subserve both memory and perception. We administered a series of oddity tasks, in which subjects selected the odd stimulus from a visual array, to amnesic patients with either selective hippocampal damage (HC group) or more extensive medial temporal damage, including the perirhinal cortex (MTL group). All patients performed normally when the stimuli could be discriminated using simple visual features, even if faces or complex virtual reality scenes were presented. Both patient groups were, however, severely impaired at scene discrimination when a significant demand was placed on processing spatial information across viewpoint independent representations, while only the MTL group showed a significant deficit in oddity judgments of faces and objects when object viewpoint independent perception was emphasized. These observations provide compelling evidence that the human hippocampus and perirhinal cortex are critical to processes beyond long-term declarative memory and may subserve spatial and object perception, respectively.

What, if anything, is the medial temporal lobe, and how can the amygdala be part of it if there is no such thing?

Should the medial temporal lobe (MTL) of primates--which includes allocortical structures such as the hippocampus, neocortical structures such as the parahippocampal cortex, and nuclear structures such as the basolateral amygdala--be considered a single "thing"? According to the prevailing view, here termed the reification theory, the answer is yes. According to this theory, the MTL functions as an amalgamated entity that provides the neuronal mechanisms for declarative memory; the greater the damage to the MTL or any of its components, the greater the deleterious effects on declarative memory. A countervailing view, here called the balkanization theory, holds that the various components of the MTL process and store different kinds of information. According to this theory, damage to each part of the MTL causes a unique set of behavioral deficits-some involving memory, others involving perception, and yet others involving response selection. The empirical neuropsychological evidence favors the balkanization theory, as do some new concepts in theoretical neuroanatomy.