The Contribution of the Human Medial Temporal Lobe to Perception: Bridging the Gap between Animal and Human Studies

Prefrontal and Medial Temporal Lobe Cortical Contributions to Visual Short-Term Memory

A number of recent studies have indicated that the medial temporal lobe (MTL) plays a critical role in working memory (WM) and perception, but these results have been highly controversial given the traditional association of MTL with long-term memory. We review the research and highlight important factors that need to be considered in determining the role of MTL in WM including set-size of used stimuli and feature complexity and/or feature conjunctions/bindings embedded in those stimuli. These factors relate to hierarchical and, accordingly, domain-specific theories of functional organization within the temporal lobe. In addition, one must consider process-specific theories too, because two key processes commonly understood to contribute recognition memory, namely, recollection and familiarity, also have robust support from neurophysiological and neuroimaging research as to their functional dissociations within MTL. PFC has long been heavily implicated in WM; however, relatively less is known about how the PFC contributes to recollection and familiarity, although dynamic prefrontal coding models in WM may help to explain their neural mechanisms. The MTL and PFC are heavily interconnected and do not operate independently in underlying WM. We propose that investigation of the interactions between these two regions in WM, particularly their coordinated neural activities, and the modeling of such interactions, will be crucial for the advancing understanding of the neural mechanisms of WM.

The structure of hippocampal circuitry relates to rapid category learning in humans

Prior work suggests that complementary white matter pathways within the hippocampus (HPC) differentially support the learning of specific versus general information. In particular, while the trisynaptic pathway (TSP) rapidly forms memories for specific experiences, the monosynaptic pathway (MSP) slowly learns generalities. However, despite the theorized significance of such circuitry, characterizing how information flows within the HPC to support learning in humans remains a challenge. We leveraged diffusion-weighted imaging as a proxy for individual differences in white matter structure linking key regions along with TSP (HPC subfields CA₃ and CA₁ ) and MSP (entorhinal cortex and CA₁ ) and related these differences in hippocampal structure to category learning ability. We hypothesized that learning to categorize the "exception" items that deviated from category rules would benefit from TSP-supported mnemonic specificity. Participant-level estimates of TSP- and MSP-related integrity were constructed from HPC subfield connectomes of white matter streamline density. Consistent with theories of TSP-supported learning mechanisms, we found a specific association between the integrity of CA₃ -CA₁ white matter connections and exception learning. These results highlight the significant role of HPC circuitry in complex human learning.

Hormonal contraceptive use predicts decreased perseverance and therefore performance on some simple and challenging cognitive tasks

A growing body of research suggests that hormonal contraceptive (HC) use may be associated with lower self-control, as well as structural and functional differences in women's brains that could contribute to differences in perseverance on tasks requiring cognitive control. Here, we sought to extend this research by examining the relationship between HC use and college-aged women's perseverance (i.e., time spent) and performance on tasks requiring cognitive control. Across two studies, we find that, compared to naturally-cycling women, women using HCs display less perseverance on both simple (i.e., a spot-the-difference game) and challenging (i.e., Graduate Record Examination quantitative problems) tasks. Moreover, these differences in perseverance were found to predict performance decrements across tasks, with women taking HCs performing worse because they spent less time on the tasks. By demonstrating how HC use may influence perseverance and thereby performance, these results contribute to a growing body of research examining the unintended implications of HC use on cognition, learning, and memory.

Visual Object Discrimination Impairment as an Early Predictor of Mild Cognitive Impairment and Alzheimer's Disease

OBJECTIVE

Detection of cognitive impairment suggestive of risk for Alzheimer's disease (AD) progression is crucial to the prevention of incipient dementia. This study was performed to determine if performance on a novel object discrimination task improved identification of earlier deficits in older adults at risk for AD.

METHOD

In total, 135 participants from the 1Florida Alzheimer's Disease Research Center [cognitively normal (CN), Pre-mild cognitive impairment (PreMCI), amnestic mild cognitive impairment (aMCI), and dementia] completed a test of object discrimination and traditional memory measures in the context of a larger neuropsychological and clinical evaluation.

RESULTS

The Object Recognition and Discrimination Task (ORDT) revealed significant differences between the PreMCI, aMCI, and dementia groups versus CN individuals. Moreover, relative risk of being classified as PreMCI rather than CN increased as an inverse function of ORDT score.

DISCUSSION

Overall, the obtained results suggest that a novel object discrimination task improves the detection of very early AD-related cognitive impairment, increasing the window for therapeutic intervention. (JINS, 2019, 25, 688-698).

Left entorhinal cortex and object recognition

The present research explored the role of the medial temporal lobes in object memory in the unique patient MR, who has a selective lesion to her left lateral entorhinal cortex. Two experiments explored recognition memory for object identity and object location in MR and matched controls. The results showed that MR had intact performance in an object location task [MR=0.70, controls=0.69, t(6)=0.06, P>0.05], but was impaired in an object identity task [MR=0.62, controls=0.84, t(6)=-4.12, P<0.05]. No differences in correct recollection or familiarity emerged. These results suggest a differential role of the entorhinal cortex in object recognition memory. The current research is therefore the first patient study to show the role of the lateral entorhinal cortex in object identity recognition and suggests that current medial temporal lobe theoretical models on both object and recognition memory require a theoretical re-think to account for the contributions of the entorhinal cortex in these processes.

Category specificity in the medial temporal lobe: A systematic review

Theoretical accounts of medial temporal lobe (MTL) function ascribe different functions to subregions of the MTL including perirhinal, entorhinal, parahippocampal cortices, and the hippocampus. Some have suggested that the functional roles of these subregions vary in terms of their category specificity, showing preferential coding for certain stimulus types, but the evidence for this functional organization is mixed. In this systematic review, we evaluate existing evidence for regional specialization in the MTL for three categories of visual stimuli: faces, objects, and scenes. We review and synthesize across univariate and multivariate neuroimaging studies, as well as neuropsychological studies of cases with lesions to the MTL. Neuroimaging evidence suggests that faces activate the perirhinal cortex, entorhinal cortex, and the anterior hippocampus, while scenes engage the parahippocampal cortex and both the anterior and posterior hippocampus, depending on the contrast condition. There is some evidence for object-related activity in anterior MTL regions when compared to scenes, and in posterior MTL regions when compared to faces, suggesting that aspects of object representations may share similarities with face and scene representations. While neuroimaging evidence suggests some hippocampal specialization for faces and scenes, neuropsychological evidence shows that hippocampal damage leads to impairments in scene memory and perception, but does not entail equivalent impairments for faces in cases where the perirhinal cortex remains intact. Regional specialization based on stimulus categories has implications for understanding the mechanisms of MTL subregions, and highlights the need for the development of theoretical models of MTL function that can accommodate the differential patterns of specificity observed in the MTL.

Spared perception of object geometry and object components after hippocampal damage

We tested the proposal that medial temporal lobe (MTL) structures support not just memory but also high-level object perception. In one task, participants decided whether a line drawing could represent an object in three-dimensional space and, in another task, they saw the components of an object and decided what object could be formed if the components were assembled. Patients with hippocampal lesions were intact, indicating that the hippocampus is not needed for perceiving the structural coherence of objects or appreciating the relations among object parts. Patients with large MTL lesions were moderately impaired, likely due to damage outside the MTL.

Encoding-related brain activity and accelerated forgetting in transient epileptic amnesia

The accelerated forgetting of newly learned information is common amongst patients with epilepsy and, in particular, in the syndrome of transient epileptic amnesia (TEA). However, the neural mechanisms underlying accelerated forgetting are poorly understood. It has been hypothesised that interictal epileptiform activity during longer retention intervals disrupts normally established memory traces. Here, we tested a distinct hypothesis-that accelerated forgetting relates to the abnormal encoding of memories. We studied a group of 15 patients with TEA together with matched, healthy control subjects. Despite normal performance on standard anterograde memory tasks, patients showed accelerated forgetting of a word list over one week. We used a subsequent memory paradigm to compare encoding-related brain activity in patients and controls. Participants studied a series of visually presented scenes whilst undergoing functional MRI scanning. Recognition memory for these scenes was then probed outside the scanner after delays of 45 min and of 4 days. Patients showed poorer memory for the scenes compared with controls. In the patients but not the controls, subsequently forgotten stimuli were associated with reduced hippocampal activation at encoding. Furthermore, patients demonstrated reduced deactivation of posteromedial cortex regions upon viewing subsequently remembered stimuli as compared to subsequently forgotten ones. These data suggest that abnormal encoding-related activity in key memory areas of the brain contributes to accelerated forgetting in TEA. We propose that abnormally encoded memory traces may be particularly vulnerable to interference from subsequently encountered material and hence be forgotten more rapidly. Our results shed light on the mechanisms underlying memory impairment in epilepsy, and offer support to the proposal that accelerated forgetting may be a useful marker of subtle dysfunction in memory-related brain systems.

Viewpoints: how the hippocampus contributes to memory, navigation and cognition

The hippocampus serves a critical function in memory, navigation, and cognition. Nature Neuroscience asked John Lisman to lead a group of researchers in a dialog on shared and distinct viewpoints on the hippocampus.

There has been a long history of studying the hippocampus, but recent work has made it possible to study the cellular and network basis of defined operations—operations that include cognitive processes that have been otherwise difficult to study (see Box 1 for useful terminology). These operations deal with the context-dependent representation of complex memories, the role of mental exploration based on imagined rather than real movements, and the use of recalled information for navigation and decision-making. The progress that has been made in understanding the hippocampus has motivated the study of other brain regions that provide hippocampal input or receive hippocampal output; the hippocampus is thus serving as a nucleating point for the larger goal of understanding the neural codes that allow inter-regional communication and more generally, understanding how memory-guided behavior is achieved by large scale integration of brain regions. In generating a discussion among experts in the study of the cognitive processes of the hippocampus, the editors and I have posed questions that probe important principles of hippocampal function. We hope that the resulting discussion will make clear to readers the progress that has been made, while also identifying issues where consensus has not yet been achieved and that should be pursued in future research. – John Lisman

Representational specializations of the hippocampus in phylogenetic perspective

In a major evolutionary transition that occurred more than 520 million years ago, the earliest vertebrates adapted to a life of mobile, predatory foraging guided by distance receptors concentrated on their heads. Vision and olfaction served as the principal sensory systems for guiding their search for nutrients and safe haven. Among their neural innovations, these animals had a telencephalon that included a homologue of the hippocampus. Experiments on goldfish, turtles, lizards, rodents, macaque monkeys and humans have provided insight into the initial adaptive advantages provided by the hippocampus homologue. These findings indicate that it housed specialized map-like representations of odors and sights encountered at various locations in an animal's home range, including the order and timing in which they should be encountered during a journey. Once these representations emerged in early vertebrates, they also enabled a variety of behaviors beyond navigation. In modern rodents and primates, for example, the specialized representations of the hippocampus enable the learning and performance of tasks involving serial order, timing, recency, relations, sequences of events and behavioral contexts. During primate evolution, certain aspects of these representations gained particular prominence, in part due to the advent of foveal vision in haplorhines. As anthropoid primates-the ancestors of monkeys, apes and humans-changed from small animals that foraged locally into large ones with an extensive home range, they made foraging choices at a distance based on visual scenes. Experimental evidence shows that the hippocampus of monkeys specializes in memories that reflect the representation of such scenes, rather than spatial processing in a general sense. Furthermore, and contrary to the idea that the hippocampus functions in memory to the exclusion of perception, brain imaging studies and lesion effects in humans show that its specialized representations support both the perception and memory of scenes and sequences.

Deciding what is possible and impossible following hippocampal damage in humans

There is currently much debate about whether the precise role of the hippocampus in scene processing is predominantly constructive, perceptual, or mnemonic. Here, we developed a novel experimental paradigm designed to control for general perceptual and mnemonic demands, thus enabling us to specifically vary the requirement for constructive processing. We tested the ability of patients with selective bilateral hippocampal damage and matched control participants to detect either semantic (e.g., an elephant with butterflies for ears) or constructive (e.g., an endless staircase) violations in realistic images of scenes. Thus, scenes could be semantically or constructively 'possible' or 'impossible'. Importantly, general perceptual and memory requirements were similar for both types of scene. We found that the patients performed comparably to control participants when deciding whether scenes were semantically possible or impossible, but were selectively impaired at judging if scenes were constructively possible or impossible. Post-task debriefing indicated that control participants constructed flexible mental representations of the scenes in order to make constructive judgements, whereas the patients were more constrained and typically focused on specific fragments of the scenes, with little indication of having constructed internal scene models. These results suggest that one contribution the hippocampus makes to scene processing is to construct internal representations of spatially coherent scenes, which may be vital for modelling the world during both perception and memory recall. © 2016 The Authors. Hippocampus Published by Wiley Periodicals, Inc.

Super-Memorizers Are Not Super-Recognizers

Humans have a natural expertise in recognizing faces. However, the nature of the interaction between this critical visual biological skill and memory is yet unclear. Here, we had the unique opportunity to test two individuals who have had exceptional success in the World Memory Championships, including several world records in face-name association memory. We designed a range of face processing tasks to determine whether superior/expert face memory skills are associated with distinctive perceptual strategies for processing faces. Superior memorizers excelled at tasks involving associative face-name learning. Nevertheless, they were as impaired as controls in tasks probing the efficiency of the face system: face inversion and the other-race effect. Super memorizers did not show increased hippocampal volumes, and exhibited optimal generic eye movement strategies when they performed complex multi-item face-name associations. Our data show that the visual computations of the face system are not malleable and are robust to acquired expertise involving extensive training of associative memory.

Anterior hippocampus: the anatomy of perception, imagination and episodic memory

The brain creates a model of the world around us. We can use this representation to perceive and comprehend what we see at any given moment, but also to vividly re-experience scenes from our past and imagine future (or even fanciful) scenarios. Recent work has shown that these cognitive functions--perception, imagination and recall of scenes and events--all engage the anterior hippocampus. In this Opinion article, we capitalize on new findings from functional neuroimaging to propose a model that links high-level cognitive functions to specific structures within the anterior hippocampus.

Extent of hippocampal atrophy predicts degree of deficit in recall

Which specific memory functions are dependent on the hippocampus is still debated. The availability of a large cohort of patients who had sustained relatively selective hippocampal damage early in life enabled us to determine which type of mnemonic deficit showed a correlation with extent of hippocampal injury. We assessed our patient cohort on a test that provides measures of recognition and recall that are equated for difficulty and found that the patients' performance on the recall tests correlated significantly with their hippocampal volumes, whereas their performance on the equally difficult recognition tests did not and, indeed, was largely unaffected regardless of extent of hippocampal atrophy. The results provide new evidence in favor of the view that the hippocampus is essential for recall but not for recognition.

Hippocampal contribution to implicit configuration memory expressed via eye movements during scene exploration

Although hippocampus unequivocally supports explicit/declarative memory, fewer findings have demonstrated its role in implicit expressions of memory. We tested for hippocampal contributions to an implicit expression of configural/relational memory for complex scenes using eye-movement tracking during functional magnetic resonance imaging (fMRI) scanning. Participants studied scenes and were later tested using scenes that resembled study scenes in their overall feature configuration but comprised different elements. These configurally similar scenes were used to limit explicit memory, and were intermixed with new scenes that did not resemble studied scenes. Scene configuration memory was expressed through eye movements reflecting exploration overlap (EO), which is the viewing of the same scene locations at both study and test. EO reliably discriminated similar study-test scene pairs from study-new scene pairs, was reliably greater for similarity-based recognition hits than for misses, and correlated with hippocampal fMRI activity. In contrast, subjects could not reliably discriminate similar from new scenes by overt judgments, although ratings of familiarity were slightly higher for similar than new scenes. Hippocampal fMRI correlates of this weak explicit memory were distinct from EO-related activity. These findings collectively suggest that EO was an implicit expression of scene configuration memory associated with hippocampal activity. Visual exploration can therefore reflect implicit hippocampal-related memory processing that can be observed in eye-movement behavior during naturalistic scene viewing.

Hippocampus, perirhinal cortex, and complex visual discriminations in rats and humans

Structures in the medial temporal lobe, including the hippocampus and perirhinal cortex, are known to be essential for the formation of long-term memory. Recent animal and human studies have investigated whether perirhinal cortex might also be important for visual perception. In our study, using a simultaneous oddity discrimination task, rats with perirhinal lesions were impaired and did not exhibit the normal preference for exploring the odd object. Notably, rats with hippocampal lesions exhibited the same impairment. Thus, the deficit is unlikely to illuminate functions attributed specifically to perirhinal cortex. Both lesion groups were able to acquire visual discriminations involving the same objects used in the oddity task. Patients with hippocampal damage or larger medial temporal lobe lesions were intact in a similar oddity task that allowed participants to explore objects quickly using eye movements. We suggest that humans were able to rely on an intact working memory capacity to perform this task, whereas rats (who moved slowly among the objects) needed to rely on long-term memory.

The end point of the ventral visual stream: face and non-face perceptual deficits following unilateral anterior temporal lobe damage

While it has been claimed that the ventral visual stream ends in the inferior aspects of the anterior temporal lobe (ATL), little is known about whether this region is important for visual perception. Here the performance of two patients with unilateral ATL damage was assessed across four visual perception tasks that parametrically varied stimulus similarity. Patients performed normally on difficult judgments of circle size or face age but were impaired on face identity and dot pattern matching tasks. Portions of the ATL, most likely the ventral surface, may have a functional role in visual perception tasks requiring detailed configural processing, most commonly used to discern facial identity.

Mechanisms for widespread hippocampal involvement in cognition

The quintessential memory system in the human brain--the hippocampus and surrounding medial temporal lobe--is often treated as a module for the formation of conscious, or declarative, memories. However, growing evidence suggests that the hippocampus plays a broader role in memory and cognition and that theories organizing memory into strictly dedicated systems may need to be updated. We first consider the historical evidence for the specialized role of the hippocampus in declarative memory. Then, we describe the serendipitous encounter that motivated the special section in this issue, based on parallel research from our labs that suggested a more pervasive contribution of the hippocampus to cognition beyond declarative memory. Finally, we develop a theoretical framework that describes 2 general mechanisms for how the hippocampus interacts with other brain systems and cognitive processes: the memory modulation hypothesis, in which mnemonic representations in the hippocampus modulate the operation of other systems, and the adaptive function hypothesis, in which specialized computations in the hippocampus are recruited as a component of both mnemonic and nonmnemonic functions. This framework is consistent with an emerging view that the most fertile ground for discovery in cognitive psychology and neuroscience lies at the interface between parts of the mind and brain that have traditionally been studied in isolation.

Reversible information flow across the medial temporal lobe: the hippocampus links cortical modules during memory retrieval

A simple cue can be sufficient to elicit vivid recollection of a past episode. Theoretical models suggest that upon perceiving such a cue, disparate episodic elements held in neocortex are retrieved through hippocampal pattern completion. We tested this fundamental assumption by applying functional magnetic resonance imaging (fMRI) while objects or scenes were used to cue participants' recall of previously paired scenes or objects, respectively. We first demonstrate functional segregation within the medial temporal lobe (MTL), showing domain specificity in perirhinal and parahippocampal cortices (for object-processing vs scene-processing, respectively), but domain generality in the hippocampus (retrieval of both stimulus types). Critically, using fMRI latency analysis and dynamic causal modeling, we go on to demonstrate functional integration between these MTL regions during successful memory retrieval, with reversible signal flow from the cue region to the target region via the hippocampus. This supports the claim that the human hippocampus provides the vital associative link that integrates information held in different parts of cortex.

The hippocampus supports high-resolution binding in the service of perception, working memory and long-term memory

It is well established that the hippocampus plays a critical role in our ability to recollect past events. A number of recent studies have indicated that the hippocampus may also play a critical role in working memory and perception, but these results have been highly controversial because other similar studies have failed to find evidence for hippocampal involvement. Thus, the precise role that the hippocampus plays in cognition is still debated. In the current paper, I propose that the hippocampus supports the generation and utilization of complex high-resolution bindings that link together the qualitative aspects that make up an event; these bindings are essential for recollection, and they can also contribute to performance across a variety of tasks including perception and working memory. An examination of the existing patient literature provides support for this proposal by showing that hippocampal damage leads to impairments on perception and working memory tasks that require complex high-resolution bindings. Conversely, hippocampal damage is much less likely to lead to impairments on tasks that require only low-resolution or simple associations/relations. The current proposal can be distinguished from earlier accounts of hippocampal function, and it generates a number of novel predictions that can be tested in future studies.

Multiple cognitive abilities from a single cortical algorithm

One strong claim made by the representational-hierarchical account of cortical function in the ventral visual stream (VVS) is that the VVS is a functional continuum: the basic computations carried out in service of a given cognitive function, such as recognition memory or visual discrimination, might be the same at all points along the VVS. Here, we use a single-layer computational model with a fixed learning mechanism and set of parameters to simulate a variety of cognitive phenomena from different parts of the functional continuum of the VVS: recognition memory, categorization of perceptually related stimuli, perceptual learning of highly similar stimuli, and development of retinotopy and orientation selectivity. The simulation results indicate--consistent with the representational-hierarchical view--that the simple existence of different levels of representational complexity in different parts of the VVS is sufficient to drive the emergence of distinct regions that appear to be specialized for solving a particular task, when a common neurocomputational learning algorithm is assumed across all regions. Thus, our data suggest that it is not necessary to invoke computational differences to understand how different cortical regions can appear to be specialized for what are considered to be very different psychological functions.

Visual discrimination performance, memory, and medial temporal lobe function

We evaluated recent proposals that structures in the medial temporal lobe (MTL)--in particular, perirhinal cortex--support not just memory but certain kinds of perceptual abilities as well. Specifically, it has been suggested that the perirhinal cortex supports the perceptual abilities needed to accomplish visual discrimination performance when the stimuli have complex features and overlapping elements. However, the tasks that have been studied are quite challenging. Stimulus features must be held in working memory while attention shifts among the several parts of the display. When working memory capacity is exceeded, performance must depend on retrieval from long-term memory. Five patients with limited hippocampal lesions and one patient with large MTL lesions were asked to identify the unique object among twin pairs of objects that had a high degree of feature overlap and perceptual similarity. The patient groups performed similarly to controls when there were few objects and features in the displays, but exhibited abrupt declines in performance when the displays contained more objects and more features. Notably, the impairment was observed in memory-impaired patients with hippocampal lesions, not only in association with large MTL lesions that included perirhinal cortex. The pattern of performance suggested that patients encountered difficulty because working memory capacity was exceeded in the more difficult conditions such that performance needed to depend at least in part on long-term memory. Furthermore, when the burden on working memory was removed entirely, the patient with large MTL lesions performed as well as controls. Accordingly, we suggest that deficits on difficult discrimination tasks reported for patients with MTL lesions are due to impaired memory rather than impaired perception.

A role for perirhinal cortex in memory for novel object-context associations

It is debated whether functional divisions between structures in the medial temporal lobe (MTL), in particular the perirhinal cortex (PrC) and hippocampus (HC), are best conceptualized according to memory process (Diana et al., 2007; Ranganath, 2010; Wixted et al., 2010) or stimulus category (Graham et al., 2010). In the former account, PrC is critical for item familiarity but not recollection of associations between items and their contexts (which is instead dependent upon the HC; Ranganath et al., 2004). In the latter theory, complex object representations in PrC are capable of supporting memory for objects as well as for object-context associations, particularly when there is a demand to discriminate between highly visually similar objects (Cowell et al., 2010). To adjudicate between these accounts, human participants were scanned while making two different judgments about visually presented objects (is the object common or uncommon, or does the object have more edges or curves). In a subsequent, unscanned, retrieval phase, participants made item (old/new) followed by context (encoding task) judgments about previously seen and novel objects. Neural activity at encoding was separated according to the accuracy of the retrieval judgments. PrC activity predicted successful item-context judgments, a result that remained when item-memory strength was equated across objects for which the context was remembered or forgotten. These data imply that the function of PrC goes beyond processing item-based memory information, contributing additionally to memory for item-context associations when the stimuli are objects (Graham et al., 2010).

Structured cognition and neural systems: from rats to language

Much of animal and human cognition is compositional in nature: higher order, complex representations are formed by (rule-governed) combination of more primitive representations. We review here some of the evidence for compositionality in perception and memory, motivating an approach that takes ideas and techniques from computational linguistics to model aspects of structural representation in cognition. We summarize some recent developments in our work that, on the one hand, use algorithms from computational linguistics to model memory consolidation and the formation of semantic memory, and on the other hand use insights from the neurobiology of memory to develop a neurally inspired model of syntactic parsing that improves over existing (not cognitively motivated) models in computational linguistics. These two theoretical studies highlight interesting analogies between language acquisition, semantic memory and memory consolidation, and suggest possible neural mechanisms, implemented in computational algorithms that may underlie memory consolidation.

The hippocampus and visual perception

In this review, we will discuss the idea that the hippocampus may be involved in both memory and perception, contrary to theories that posit functional and neuroanatomical segregation of these processes. This suggestion is based on a number of recent neuropsychological and functional neuroimaging studies that have demonstrated that the hippocampus is involved in the visual discrimination of complex spatial scene stimuli. We argue that these findings cannot be explained by long-term memory or working memory processing or, in the case of patient findings, dysfunction beyond the medial temporal lobe (MTL). Instead, these studies point toward a role for the hippocampus in higher-order spatial perception. We suggest that the hippocampus processes complex conjunctions of spatial features, and that it may be more appropriate to consider the representations for which this structure is critical, rather than the cognitive processes that it mediates.

Teasing apart tangrams: testing hippocampal pattern separation with a collaborative referencing paradigm

The hippocampus and the medial temporal lobe cortex [medial temporal lobe cortices (MTLC)] both contribute to long-term memory. Although their contributions are thought to be dissociable, the nature of the representations that each region supports remains unclear. The Complementary Learning Systems (CLS) modeling approach suggests that hippocampus represents overlapping information in a sparser and therefore more separated fashion than MTLC. We tested this prediction using a collaborative referencing paradigm whereby hippocampal amnesic patients and a partner work together to develop and use unique labels for a set of abstract visual stimuli (tangrams) across extended interactions. Previously, we reported that amnesic patients demonstrate intact learning when the tangrams are conceptually dissimilar. Here, we manipulated the degree of visual similarity; half of the stimuli were dissimilar to one another (e.g., camel and giraffe), and half were similar (e.g., birds). We hypothesized that while patients would have little difficulty with the dissimiliar tangrams (quickly arriving at unique and concise labels), they would be unable to rapidly form distinct representations of highly similar visual patterns. Consistent with this prediction, patients and both healthy and brain-damaged comparison participants showed similar rates of learning for dissimilar tangrams, but the similar tangrams proved more difficult for hippocampal patients as reflected in the greater number of words they used to describe each similar card. This result supports the CLS model's central claim of hippocampal specialization for pattern separation and suggests that our collaborative referencing paradigm may be a useful tool for observing extended encoding of complex representations.

Intact performance on feature-ambiguous discriminations in rats with lesions of the perirhinal cortex

We developed a behavioral paradigm for the rat that made it possible to separate the evaluation of memory functions from the evaluation of perceptual functions. Animals were given extensive training on an automated two-choice discrimination task and then maintained their memory performance at a high level while interpolated probe trials tested visual perceptual ability. The probe trials systematically varied the degree of feature ambiguity between the stimuli, such that perceptual functions could be tested across 14 different levels of difficulty. As feature ambiguity increased, performance declined in an orderly, monotonic manner (from 87% correct to chance, 50% correct). Bilateral lesions of the perirhinal cortex fully spared the capacity to make feature-ambiguous discriminations and the performance of lesioned and intact animals was indistinguishable at every difficulty level. In contrast, the perirhinal lesions did impair recognition memory. The findings suggest that the perirhinal cortex is important for memory and not for perceptual functions.

Memory, visual discrimination performance, and the human hippocampus

We evaluated recent proposals that the hippocampus supports certain kinds of visual discrimination performance, for example, when spatial processing is required and the stimuli have a high degree of feature overlap. Patients with circumscribed hippocampal lesions tried to discriminate between images of similar faces or images of similar scenes. In one condition, elements of the stimulus display repeated from trial to trial, and in another condition every trial was unique. In the repeated condition for both faces and scenes, controls gradually improved their performance across testing. In the trial-unique condition, no improvement occurred. The patients were impaired for both faces and scenes in the repeated condition where controls could benefit from learning. However, the patients were fully intact in the trial-unique condition. The results suggest that previous reports of impaired discrimination performance after medial temporal lobe damage may reflect impaired learning rather than impaired visual perception. The findings support the fundamental idea that memory is a distinct cerebral function separable from other perceptual and cognitive abilities.

Perception and conception: temporal lobe activity during complex discriminations of familiar and novel faces and objects

Recent studies indicate that medial-temporal lobe (MTL) damage, either from focal lesions or neurodegenerative disease (e.g., semantic dementia), impairs perception as well as long-term declarative memory. Notably, however, these two patient groups show different performance for meaningful versus unfamiliar stimuli. In amnesics with nonprogressive MTL lesions, the use of meaningful stimuli, compared with unfamiliar items, boosted discrimination performance. In semantic dementia, a condition characterized by progressive deterioration of conceptual knowledge in the context of anterolateral temporal lobe damage, performance for meaningful stimuli was equivalent to that for unfamiliar items. To further investigate these findings, we scanned healthy volunteers while they performed odd-one-out discriminations involving familiar (i.e., meaningful/famous) and unfamiliar (i.e., novel) objects and faces and a baseline task of size oddity. Outside the scanner, volunteers' recognition memory was assessed. We found above baseline activity in the perirhinal cortex and hippocampus for all object and face discriminations and above baseline activity in the temporal pole for all face discriminations. The comparison of meaningful, relative to novel, faces and objects, revealed increased activity in the perirhinal cortex and hippocampus. In the temporal pole, we also found activity related to meaningfulness for faces but not for objects. Importantly, these meaningfulness effects were evident even for discriminations that were not subsequently well remembered, suggesting that the difference between meaningful and novel stimuli reflects perceptual or conceptual processes rather than solely incidental encoding into long-term memory. The results provide further evidence that the MTL is recruited during complex perceptual discrimination and additionally suggest that these structures are recruited in semantic processing of objects and faces.

Borders, extent, and topography of human perirhinal cortex as revealed using multiple modern neuroanatomical and pathological markers

Despite rapidly increasing interests in specific contributions of different components of human medial temporal lobe (MTL) to memory and memory impairments in normal aging and in many abnormal conditions such as Alzheimer's disease and Pick's disease, few modern neuroanatomical studies are available about the borders, extent, and topography of human perirhinal areas 35 and 36, which are important components of the MTL memory system. By a combined use of several cellular, neurochemical, and pathological markers, which mainly include neuronal nuclear antigen, calcium-binding proteins (parvalbumin and calbindin-D28k), nonphosphorylated neurofilament protein (SMI-32), Wisteria floribunda agglutinin, and abnormally phosphorylated tau (AT8), this study has revealed that the borders of human perirhinal areas 35 and 36 are significantly different from those defined with conventional Nissl staining. In general, areas 35 and 36 occupy the ventromedial temporopolar and rhinal sulcal regions, the collateral sulcal region, and the anterior two-thirds of fusiform gyrus or occipitotemporal gyrus. Furthermore, the precise borders, extent, and topography of human areas 35 and 36 and adjoining entorhinal cortex were marked at different anteroposterior levels of the MTL with reference to variations of rhinal and collateral sulci and other useful landmarks. These findings would provide reliable neuroanatomical base for the great and yet rapidly increasing number of neuroimaging studies of the human MTL structures in healthy and many abnormal conditions.

A single-trace dual-process model of episodic memory: a novel computational account of familiarity and recollection

Dual-process theories of episodic memory state that retrieval is contingent on two independent processes: familiarity (providing a sense of oldness) and recollection (recovering events and their context). A variety of studies have reported distinct neural signatures for familiarity and recollection, supporting dual-process theory. One outstanding question is whether these signatures reflect the activation of distinct memory traces or the operation of different retrieval mechanisms on a single memory trace. We present a computational model that uses a single neuronal network to store memory traces, but two distinct and independent retrieval processes access the memory. The model is capable of performing familiarity and recollection-based discrimination between old and new patterns, demonstrating that dual-process models need not to rely on multiple independent memory traces, but can use a single trace. Importantly, our putative familiarity and recollection processes exhibit distinct characteristics analogous to those found in empirical data; they diverge in capacity and sensitivity to sparse and correlated patterns, exhibit distinct ROC curves, and account for performance on both item and associative recognition tests. The demonstration that a single-trace, dual-process model can account for a range of empirical findings highlights the importance of distinguishing between neuronal processes and the neuronal representations on which they operate.

[Anterograde declarative memory and its models]

INTRODUCTION

Patient H.M.'s recent death provides the opportunity to highlight the importance of his contribution to a better understanding of the anterograde amnesic syndrome. The thorough study of this patient over five decades largely contributed to shape the unitary model of declarative memory. This model holds that declarative memory is a single system that cannot be fractionated into subcomponents. As a system, it depends mainly on medial temporal lobes structures. The objective of this review is to present the main characteristics of different modular models that have been proposed as alternatives to the unitary model. It is also an opportunity to present different patients, who, although less famous than H.M., helped make signification contribution to the field of memory.

STATE OF THE ART

The characteristics of the five main modular models are presented, including the most recent one (the perceptual-mnemonic model). The differences as well as how these models converge are highlighted.

PERSPECTIVES

Different possibilities that could help reconcile unitary and modular approaches are considered.

CONCLUSION

Although modular models differ significantly in many aspects, all converge to the notion that memory for single items and semantic memory could be dissociated from memory for complex material and context-rich episodes. In addition, these models converge concerning the involvement of critical brain structures for these stages: Item and semantic memory, as well as familiarity, are thought to largely depend on anterior subhippocampal areas, while relational, context-rich memory and recollective experiences are thought to largely depend on the hippocampal formation.

Going beyond LTM in the MTL: a synthesis of neuropsychological and neuroimaging findings on the role of the medial temporal lobe in memory and perception

Studies in rats and non-human primates suggest that medial temporal lobe (MTL) structures play a role in perceptual processing, with the hippocampus necessary for spatial discrimination, and the perirhinal cortex for object discrimination. Until recently, there was little convergent evidence for analogous functional specialisation in humans, or for a role of the MTL in processes beyond long-term memory. A recent series of novel human neuropsychological studies, however, in which paradigms from the animal literature were adapted and extended, have revealed findings remarkably similar to those seen in rats and monkeys. These experiments have demonstrated differential effects of distinct stimulus categories on performance in tasks for which there was no explicit requirement to remember information across trials. There is also accruing complementary evidence from functional neuroimaging that MTL structures show differential patterns of activation for scenes and objects, even on simple visual discrimination tasks. This article reviews some of these key studies and discusses the implications of these new findings for existing accounts of memory. A non-modular view of memory is proposed in which memory and perception depend upon the same anatomically distributed representations (emergent memory account). The limitations and criticisms of this theory are discussed and a number of outstanding questions proposed, including key predictions that can be tested by future studies.

Integrating visual and tactile information in the perirhinal cortex

By virtue of its widespread afferent projections, perirhinal cortex is thought to bind polymodal information into abstract object-level representations. Consistent with this proposal, deficits in cross-modal integration have been reported after perirhinal lesions in nonhuman primates. It is therefore surprising that imaging studies of humans have not observed perirhinal activation during visual–tactile object matching. Critically, however, these studies did not differentiate between congruent and incongruent trials. This is important because successful integration can only occur when polymodal information indicates a single object (congruent) rather than different objects (incongruent). We scanned neurologically intact individuals using functional magnetic resonance imaging (fMRI) while they matched shapes. We found higher perirhinal activation bilaterally for cross-modal (visual–tactile) than unimodal (visual–visual or tactile–tactile) matching, but only when visual and tactile attributes were congruent. Our results demonstrate that the human perirhinal cortex is involved in cross-modal, visual–tactile, integration and, thus, indicate a functional homology between human and monkey perirhinal cortices.

Perception and the medial temporal lobe: evaluating the current evidence

A dominant view in the learning and memory literature states that a subset of anatomically related structures within the medial temporal lobe (MTL), including the hippocampus, entorhinal, perirhinal, and parahippocampal cortices, forms a functionally related system specialized for declarative memory but not for perception. However, recent reports challenge this view, suggesting instead that the medial temporal lobe is not only important for memory, but also critical for certain forms of perception. In this review, I argue that little or no conclusive evidence currently exists to support the latter view. Experimental studies that have examined the perceptual functions of the MTL in monkeys are inconclusive because they fail to isolate perceptual from mnemonic task demands. Evaluation of conflicting results from studies in human amnesic patients suggests that extraneous damage to extra-MTL areas may underlie the reported perceptual deficits in the group of amnesic patients at the heart of this debate. See the related Review from Baxter, "Involvement of Medial Temporal Lobe Structures in Memory and Perception," in this issue of Neuron.

Auditory trace fear conditioning requires perirhinal cortex

The hippocampus is well-known to be critical for trace fear conditioning, but nothing is known about the importance of perirhinal cortex (PR), which has reciprocal connections with hippocampus. PR damage severely impairs delay fear conditioning to ultrasonic vocalizations (USVs) and discontinuous tones (pips), but has no effect on delay conditioning to continuous tones. Here we demonstrate that trace auditory fear conditioning also critically depends on PR function. The trace interval between the CS offset and the US onset was 16s. Pre-training neurotoxic lesions were produced through multiple injections of N-methyl-D-aspartate along the full length of PR, which was directly visualized during the injections. Control animals received injections with phosphate-buffered saline. Three-dimensional reconstructions of the lesion volumes demonstrated that the neurotoxic damage was well-localized to PR and included most of its anterior-posterior extent. Automated video analysis quantified freezing behavior, which served as the conditional response. PR-damaged rats were profoundly impaired in trace conditioning to either of three different CSs (a USV, tone pips, and a continuous tone) as well as conditioning to the training context. Within both the lesion and control groups, the type of cue had no effect on the mean CR. The overall PR lesion effect size was 2.7 for cue conditioning and 3.9 for context conditioning. We suggest that the role of PR in trace fear conditioning may be distinct from some of its more perceptual functions. The results further define the essential circuitry underlying trace fear conditioning to auditory cues.

Addition of fornix transection to frontal-temporal disconnection increases the impairment in object-in-place memory in macaque monkeys

Both frontal-inferotemporal disconnection and fornix transection (Fx) in the monkey impair object-in-place scene learning, a model of human episodic memory. If the contribution of the fornix to scene learning is via interaction with or modulation of frontal-temporal interaction − that is, if they form a unitary system − then Fx should have no further effect when added to frontal-temporal disconnection. However, if the contribution of the fornix is to some extent distinct, then fornix lesions may produce an additional deficit in scene learning beyond that caused by frontal-temporal disconnection. To distinguish between these possibilities, we trained three male rhesus monkeys on the object-in-place scene-learning task. We tested their learning on the task following frontal-temporal disconnection, achieved by crossed unilateral aspiration of the frontal cortex in one hemisphere and the inferotemporal cortex in the other, and again following the addition of Fx. The monkeys were significantly impaired in scene learning following frontal-temporal disconnection, and furthermore showed a significant increase in this impairment following the addition of Fx, from 32.8% error to 40.5% error (chance = 50%). The increased impairment following the addition of Fx provides evidence that the fornix and frontal-inferotemporal interaction make distinct contributions to episodic memory.