Selective hippocampal lesions yield nonspatial memory impairments in rhesus monkeys

Closed-Loop Theta Stimulation in the Orbitofrontal Cortex Prevents Reward-Based Learning

Neuronal oscillations in the frontal cortex have been hypothesized to play a role in the organization of high-level cognition. Within the orbitofrontal cortex (OFC), there is a prominent oscillation in the theta frequency (4-8 Hz) during reward-guided behavior, but it is unclear whether this oscillation has causal significance. One methodological challenge is that it is difficult to manipulate theta without affecting other neural signals, such as single-neuron firing rates. A potential solution is to use closed-loop control to record theta in real time and use this signal to control the application of electrical microstimulation to the OFC. Using this method, we show that theta oscillations in the OFC are critically important for reward-guided learning and that they are driven by theta oscillations in the hippocampus (HPC). The ability to disrupt OFC computations via spatially localized and temporally precise stimulation could lead to novel treatment strategies for neuropsychiatric disorders involving OFC dysfunction.

Nonhuman primate models of hippocampal development and dysfunction

Nonhuman primates provide highly valuable animal models that have significantly advanced our understanding of numerous behavioral and biological phenomena in humans. Here, we reviewed a series of developmental neuropsychological studies that informed us on the timing of development of the hippocampus and of hippocampal-dependent cognitive functions in primates. Data indicate that, in primates, the emergence of adult-like proficiency on behavioral tasks sensitive to hippocampal dysfunction is a stepwise process and reflects the gradual maturation of different hippocampal circuits and their connections with other neural structures. Profound and persistent memory loss resulting from insult to the hippocampus in infancy was absent in early infancy but became evident later in childhood and persisted in adulthood, indicating very little sparing or recovery of function. Finally, the early hippocampal insult resulted in both adaptive and maladaptive neuroplasticity: i.e., sparing contextual memory, but affecting working memory processes as well as emotional reactivity and hypothalamic-pituitary-adrenal (HPA) axis functioning. The results provide significant information on the emergence of hippocampal-dependent functions in humans, on the time course of memory impairment in human cases with early hippocampal insult, and on the clinical implication of the hippocampus in developmental neuropsychiatric disorders.

Neonatal hippocampal lesions facilitate biconditional contextual discrimination learning in monkeys

This study examined whether selective neonatal hippocampal lesions in monkeys (Macaca mulatta), which left the surrounding cortical areas (parahippocampal cortex) intact, affect contextual learning and memory compared with controls. Monkeys were tested with an automated touch-screen apparatus so that stimuli and contextual cues could be manipulated independently of one another. The data suggest that animals with neonatal hippocampal lesions have sparing of function with regard to contextual learning and memory when (a) contextual information is irrelevant or (b) relevant for good discrimination performance, and (c) when transferring a contextual rule to new discriminations. These findings are at odds with studies examining contextual learning and memory in monkeys with selective adult-onset hippocampal lesions, and those with nonselective neonatal hippocampal lesions, which have demonstrated impairment in contextual learning and memory. Therefore, the sparing of function seen in this study may be attributable to the early nature of the damage and the plastic nature of the infant brain, as well as the intact medial temporal lobe cortical areas as a result of the lesion methodology. Specifically, by removing the hippocampus early in life, before it has begun to function, the parahippocampal (TH/TF) and perirhinal cortices and its interactions with the lateral prefrontal cortex may be able to support context processing throughout life. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

The Rhesus Monkey Hippocampus Critically Contributes to Scene Memory Retrieval, But Not New Learning

Humans can recall a large number of memories years after the initial events. Patients with amnesia often have lesions to the hippocampus, but human lesions are imprecise, making it difficult to identify the anatomy underlying memory impairments. Rodent studies enable great precision in hippocampal manipulations, but not investigation of many interleaved memories. Thus it is not known how lesions restricted to the hippocampus affect the retrieval of multiple sequentially encoded memories. Furthermore, disagreement exists as to whether hippocampal inactivations lead to temporally graded or ungraded amnesia, which could be a consequence of differences between rodent and human studies. In the current study, rhesus monkeys of both sexes received either bilateral neurotoxic hippocampal lesions or remained unoperated controls and were tested on recognition and new learning of visual object-in-place scenes. Monkeys with hippocampal lesions were significantly impaired at remembering scenes that were encoded before the lesion. We did not observe any temporal gradient effect of the lesion on memory recognition, with recent and remote memories being equally affected by the lesion. Monkeys with hippocampal lesions showed no deficits in learning new scenes. Thus, the hippocampus, like other cortical regions, may be engaged in the acquisition and storage of new memories, but the role of the damaged hippocampus can be taken over by spared hippocampal tissue or extra-hippocampal regions following a lesion. These findings illustrate the utility of experimental paradigms for studying retrograde and anterograde amnesia that make use of the capacity of nonhuman primates to rapidly acquire many distinct visual memories.SIGNIFICANCE STATEMENT Recalling old memories, creating new memories, and the process by which memories transition from temporary to permanent storage all may rely on the hippocampus. Whether the hippocampus is necessary for encoding and retrieval of multiple related visual memories in primates is not known. Monkeys that learned many visual memory problems before precise lesions of the hippocampus were impaired at recalling those memories after hippocampal damage regardless of when the memories were formed, but could learn new memory problems at a normal rate. This suggests the hippocampus is normally vital for retrieval of complex visual memories regardless of their age, and also points to the importance of investigating mechanisms by which memories may be acquired in the presence of hippocampal damage.

The Role of the Perirhinal Cortex and Hippocampus in Learning, Memory, and Perception

One traditional and long-held view of medial temporal lobe (MTL) function is that it contains a system of structures that are exclusively involved in memory, and that the extent of memory loss following MTL damage is simply related to the amount of MTL damage sustained. Indeed, human patients with extensive MTL damage are typically profoundly amnesic whereas patients with less extensive brain lesions centred upon the hippocampus typically exhibit only moderately severe anterograde amnesia. Accordingly, the latter observations have elevated the hippocampus to a particularly prominent position within the purported MTL memory system. This article reviews recent lesion studies in macaque monkeys in which the behavioural effects of more highly circumscribed lesions (than those observed to occur in human patients with MTL lesions) to different subregions of the MTL have been examined. These studies have reported new findings that contradict this concept of a MTL memory system. First, the MTL is not exclusively involved in mnemonic processes; some MTL structures, most notably the perirhinal cortex, also contribute to perception. Second, there are some forms of memory, including recognition memory, that are not always affected by selective hippocampal lesions. Third, the data support the idea that regional functional specializations exist within the MTL. For example, the macaque perirhinal cortex appears to be specialized for processing object identity whereas the hippocampus may be specialized for processing spatial and temporal relationships.

Neurons in the Primate Medial Basal Forebrain Signal Combined Information about Reward Uncertainty, Value, and Punishment Anticipation

It has been suggested that the basal forebrain (BF) exerts strong influences on the formation of memory and behavior. However, what information is used for the memory-behavior formation is unclear. We found that a population of neurons in the medial BF (medial septum and diagonal band of Broca) of macaque monkeys encodes a unique combination of information: reward uncertainty, expected reward value, anticipation of punishment, and unexpected reward and punishment. The results were obtained while the monkeys were expecting (often with uncertainty) a rewarding or punishing outcome during a Pavlovian procedure, or unexpectedly received an outcome outside the procedure. In vivo anterograde tracing using manganese-enhanced MRI suggested that the major recipient of these signals is the intermediate hippocampal formation. Based on these findings, we hypothesize that the medial BF identifies various contexts and outcomes that are critical for memory processing in the hippocampal formation.

The influence of context on recognition memory in monkeys: effects of hippocampal, parahippocampal and perirhinal lesions

This study further investigated the specific contributions of the medial temporal lobe structures to contextual recognition memory. Monkeys (Macaca mulatta) with either neurotoxic lesions of the hippocampus, aspiration lesions of the perirhinal cortex and parahippocampal areas TH/TF, or sham operations were tested on five conditions of a visual-paired comparison (VPC) task in which 3-dimensional objects were presented over multicolored backgrounds. In two conditions (Conditions 1 and 2: Context-changes), the sample object was presented on a new background during the retention tests, whereas in the three others (Conditions 3-5: No-context-changes) the sample object was presented over its familiar background. Novelty preference scores of control animals were weaker, but still significantly different from chance, in the Context-changes conditions than on the No-context-changes conditions. Animals in the three experimental groups showed strong preference for novelty on the No-context-change conditions, but weaker novelty preference on the Context-change conditions than controls. Thus, animals in all three lesion types had greater difficulty recognizing an object when its background was different from that used during encoding. The data are consistent with the view that the hippocampal formation, areas TH/TF, and perirhinal cortex contribute interactively to contextual memory processes.

Contextual behavior and neural circuits

Animals including humans engage in goal-directed behavior flexibly in response to items and their background, which is called contextual behavior in this review. Although the concept of context has long been studied, there are differences among researchers in defining and experimenting with the concept. The current review aims to provide a categorical framework within which not only the neural mechanisms of contextual information processing but also the contextual behavior can be studied in more concrete ways. For this purpose, we categorize contextual behavior into three subcategories as follows by considering the types of interactions among context, item, and response: contextual response selection, contextual item selection, and contextual item–response selection. Contextual response selection refers to the animal emitting different types of responses to the same item depending on the context in the background. Contextual item selection occurs when there are multiple items that need to be chosen in a contextual manner. Finally, when multiple items and multiple contexts are involved, contextual item–response selection takes place whereby the animal either chooses an item or inhibits such a response depending on item–context paired association. The literature suggests that the rhinal cortical regions and the hippocampal formation play key roles in mnemonically categorizing and recognizing contextual representations and the associated items. In addition, it appears that the fronto-striatal cortical loops in connection with the contextual information-processing areas critically control the flexible deployment of adaptive action sets and motor responses for maximizing goals. We suggest that contextual information processing should be investigated in experimental settings where contextual stimuli and resulting behaviors are clearly defined and measurable, considering the dynamic top-down and bottom-up interactions among the neural systems for contextual behavior.

Autobiographical memory in temporal lobe epilepsy: role of hippocampal and temporal lateral structures

The present study was aimed at investigating the impact of hippocampal and temporal cortical lesions on remote autobiographical memories in temporal lobe epilepsy (TLE). Episodic specificity, episodic richness, and personal semantic memory from different life periods were assessed using a modified version of the Autobiographical Memory Interview (AMI) (M.D. Kopelman, A.E. Wilson, A. Baddeley, The autobiographical memory interview. Bury St. Edmunds: Thames Valley Test Co.; 1990) in 47 patients with unilateral mesial or lateral TLE and 38 healthy controls. Patients with TLE performed significantly more poorly than controls. Patients with left and right mTLE were equally moderately impaired, but patients with left lateral TLE had the most severe episodic memory deficits, particularly for childhood memories. With respect to personal semantic memory, patients with left TLE were significantly more impaired than those with right TLE, most pronounced for childhood memories. Both autobiographical memory aspects, episodic and personal semantic memory, were significantly intercorrelated, but both did not correlate with anterograde memory, indicating a structural dissociation between both functions.

From vice to virtue: insights from sensitization in the nonhuman primate

Repeated, intermittent administration of psychomotor stimulants, or D1 agonists in dopamine-deficient states, induces behavioral sensitization, characterized by an enhanced response to a subsequent acute low dose challenge, which may be manifested in form of altered behavior or cognitive function. Amphetamine sensitization in the nonhuman primate encompasses profound and enduring changes to similar neuronal and neurochemical substrates that occur in rodents. The process of sensitization in the monkey also results in a long-lasting depression in baseline behavioral responding, as well as emergence of hallucinatory-like behaviors reminiscent of human psychosis in response to an acute challenge. Nonhuman primates show a reduction in spine density and dendritic length in prefrontal neurons and a marked reduction in basal dopamine turnover in both prefrontal cortex and striatum. A major hallmark of amphetamine sensitization in both nonhuman primates and rodents is the manifestation of deficits in executive function and working memory which rely upon the integrity of prefrontal cortex and thereby, may yield significant insights into the cognitive dysfunction associated with addiction. Together with evidence from human and rodent studies, it can be concluded that repeated exposure to psychomotor stimulants can lead to a corruption of neuroadaptive systems in the brain by an extraordinary influence on synaptic plasticity, learning, and memory. Actively harnessing this same process by repeated, intermittent D1 agonist administration may be the key to improved working memory and decision making in addiction and other dopamine dysfunctional states, such as schizophrenia.

Cortical efferents of the entorhinal cortex and the adjacent parahippocampal region in the monkey (Macaca fascicularis)

Entorhinal cortex (EC) relays information from the hippocampus to the cerebral cortex. The origin of this entorhino-cortical pathway was studied semiquantitatively and topographically with the use of 23 retrograde tracer injections in cortical areas of the frontal, temporal, and parietal lobes of the monkey. To assess possible alternative, parallel pathways, the parahippocampal region, comprised of temporal pole (TP), perirhinal (PRC), and posterior parahippocampal cortices (PPH), was included in the study. The majority of the cortical areas receive convergent projections from EC and the parahippocampal region. Strong EC layer V output is directed to temporal pole, medial frontal and orbitofrontal cortices, and the rostral part of the polysensory area of the superior temporal sulcus (sts). Moderate EC output is directed to the caudal superior temporal gyrus, area TE, and parietal cortex, and little to none to the lateral frontal cortex. With the exception of the projection to the medial frontal cortex, output from TP, PRC, and PPH surpassed that from EC, although with regional differences. TP layers II-III, V-VI project strongly to all areas injected except parietal cortex and caudal superior temporal gyrus, while PRC layers III/V-VI send strong projections to rostral parts of area TE and sts. PPH layers III/V-VI project heavily to parietal cortex and caudal superior temporal gyrus. These results suggest that the medial temporal output is primarily organized hierarchically, but at the same time, it has multiple exits of information. These parallel, alternative routes may influence local circuitry in the cerebral cortex and participate in the consolidation of declarative memory.

THE MEDIAL TEMPORAL LOBE

The medial temporal lobe includes a system of anatomically related structures that are essential for declarative memory (conscious memory for facts and events). The system consists of the hippocampal region (CA fields, dentate gyrus, and subicular complex) and the adjacent perirhinal, entorhinal, and parahippocampal cortices. Here, we review findings from humans, monkeys, and rodents that illuminate the function of these structures. Our analysis draws on studies of human memory impairment and animal models of memory impairment, as well as neurophysiological and neuroimaging data, to show that this system (a) is principally concerned with memory, (b) operates with neocortex to establish and maintain long-term memory, and (c) ultimately, through a process of consolidation, becomes independent of long-term memory, though questions remain about the role of perirhinal and parahippocampal cortices in this process and about spatial memory in rodents. Data from neurophysiology, neuroimaging, and neuroanatomy point to a division of labor within the medial temporal lobe. However, the available data do not support simple dichotomies between the functions of the hippocampus and the adjacent medial temporal cortex, such as associative versus nonassociative memory, episodic versus semantic memory, and recollection versus familiarity.

Learning and Retrieval of Concurrently Presented Spatial Discrimination Tasks: Role of the Fornix

In macaque monkeys (Macaco mulatta), memory for scenes presented on touch screens is fornix dependent. However, scene learning is not a purely spatial task, and existing direct evidence for a fornix role in spatial memory comes exclusively from tasks involving learning about food-reward locations. Here the authors demonstrate that fornix transection impairs learning about spatial stimuli presented on touch screens. Using a new concurrent spatial discrimination learning task, they found that fornix transection did not impair recall of preoperatively learned problems. Relearning, on the other hand, was mildly impaired, and new learning was strongly impaired. New learning of smaller sets of harder problems was also markedly impaired, as was spatial configured learning. This pattern supports a functional specialization according to stimulus domain in the medial temporal lobe.

Unilateral hippocampal and inferotemporal cortex lesions in opposite hemispheres impair learning of single-pair visual discriminations as well as visuovisual conditional tasks in monkeys

Monkeys with unilateral ablations of the inferotemporal (IT) cortex were not impaired on learning or retention of single-pair object discriminations or visuovisual conditional tasks. Addition of an excitotoxic hippocampal lesion to the hemisphere opposite to the IT ablation impaired retention and acquisition of single-pair object discriminations and visuovisual conditional tasks. Histology revealed no areas of bilaterally symmetrical damage. Previous experiments have shown that bilateral excitotoxic hippocampal lesions do not impair single-pair object discriminations although they do produce a substantial impairment on visuovisual conditional tasks. Bilateral IT ablations produce impairment on single-pair object discrimination tasks. It is argued that the hippocampus in the hemisphere with the IT ablation is deprived of feed-forward visual input and that this, in addition to the contralateral hippocampal lesion, accounts for the impairment on the visuovisual conditional tasks. It is also argued that feed-back projections from the hippocampus to the IT cortex influence the learning of single-pair object discriminations. This influence may be difficult to demonstrate by the addition of hippocampal lesions to IT lesions because of the substantial effect of the IT lesion alone. It may be difficult to demonstrate by bilateral hippocampal lesions alone since the effect may be below that which generates an observable impairment. Nonetheless, an effect may be seen when a hippocampal lesion is made in monkeys with some IT damage, as in this experiment, as well as by the general observation that large lesions of the temporal lobes produce larger perceptuo-mnemonic impairments than lesions confined to the hippocampus or temporal neocortex in monkeys and man.

Use of excitotoxins to lesion the hippocampus: update

Although many of the problems associated with the use of conventional lesion techniques (aspiration, electrolytic, radiofrequency) can be avoided by employing focal injections of excitotoxins, experience gained over the past 12 years has shown that considerable care must be exercised with this newer method, to limit the cell loss to the intended area or structure. Of the toxins that have been used most often to selectively destroy the cells that comprise the hippocampus, ibotenic acid (IBO) and N-methyl-D-aspartate (NMDA) have proved to be nonspecific in their effects on different cell types and these toxins do not cause seizures. In contrast, focal injections of kainate (KA) and quisqualate result in damage that centers primarily in the CA3 pyramidal cell field and hilar cells in the dentate gyrus. In addition, there are obvious seizures and secondary distant damage involving a number of structures and areas associated with mediating seizure activity. Intrahippocampal injections of the toxin colchicine result in a preferential destruction of dentate granule cells but usually also lead to additional cell loss in adjacent areas. Attempts to limit cell loss to specific hippocampal subfields, using different toxins, have met with mixed success. Both the dosage of the agent and the volume injected are important in determining the extent of cell loss, but the volume of the toxin injected has been shown to be especially important in limiting the damage to the intended area. With the development of newer procedures (e.g., immunotoxins, gene knockouts, antisense) that permit more selective cell loss, it should be possible in the future to achieve a level of lesion control that has been lacking in the past. As with the use of excitotoxins, these newer approaches will require special care to limit the damage to the intended area and interpret the results obtained properly.

Storage, recall, and novelty detection of sequences by the hippocampus: elaborating on the SOCRATIC model to account for normal and aberrant effects of dopamine

In order to understand how the molecular or cellular defects that underlie a disease of the nervous system lead to the observable symptoms, it is necessary to develop a large-scale neural model. Such a model must specify how specific molecular processes contribute to neuronal function, how neurons contribute to network function, and how networks interact to produce behavior. This is a challenging undertaking, but some limited progress has been made in understanding the memory functions of the hippocampus with this degree of detail. There is increasing evidence that the hippocampus has a special role in the learning of sequences and the linkage of specific memories to context. In the first part of this paper, we review a model (the SOCRATIC model) that describes how the dentate and CA3 hippocampal regions could store and recall memory sequences in context. A major line of evidence for sequence recall is the "phase precession" of hippocampal place cells. In the second part of the paper, we review the evidence for theta-gamma phase coding. According to a framework that incorporates this form of coding, the phase precession is interpreted as cued recall of a discrete sequence of items from long-term memory. The third part of the paper deals with the issue of how the hippocampus could learn memory sequences. We show that if multiple items can be active within a theta cycle through the action of a short-term "buffer," NMDA-dependent plasticity can lead to the learning of sequences presented at realistic item separation intervals. The evidence for such a buffer function is reviewed. An important underlying issue is whether the hippocampal circuitry is configured differently for learning and recall. We argue that there are indeed separate states for learning and recall, but that both involve theta oscillations, albeit in possibly different forms. This raises the question of how neuromodulatory input might switch the hippocampus between learning and recall states and more generally how different neuromodulatory inputs reconfigure the hippocampus for different functions. In the fifth part of this paper we review our studies of dopamine and dopamine/NMDA interactions in the control of synaptic function. Our results show that dopamine dramatically reduces the direct cortical input to CA1 (the perforant path input), while having little effect on the input from CA3. In order to interpret the functional consequences of this pathway-specific modulation, it is necessary to understand the function of CA1 and the role of dopaminergic input from the ventral tegmental area (VTA). In the sixth part of this paper we consider several possibilities and address the issue of how dopamine hyperfunction or NMDA hypofunction, abnormalities that may underlie schizophrenia, might lead to the symptoms of the disease. Relevant to this issue is the demonstrated role of the hippocampus in novelty detection, a function that is likely to depend on sequence recall by the hippocampus. Novelty signals are generated when reality does not match the expectations generated by sequence recall. One possible site for computing mismatch is CA1, since it receives predictions from CA3 and sensory "reality" via the perforant path. Our data suggest that disruption of this comparison would be expected under conditions of dopamine hyperfunction or NMDA hypofunction. Also relevant is the fact that the VTA, which fires in response to novelty, may both depend on hippocampal-dependent novelty detection processes and, in turn, affect hippocampal function. Through large-scale modeling that considers both the processes performed by the hippocampus and the neuromodulatory loops in which the hippocampus is embedded, it is becoming possible to generate working hypotheses that relate synaptic function and malfunction to behavior.

Opposite relationship of hippocampal and rhinal cortex damage to delayed nonmatching-to-sample deficits in monkeys

Three recent studies in macaque monkeys that examined the effects on memory of restricted hippocampal lesions (Murray and Mishkin, J Neurosci 1998;18:6568-6582; Beason-Held et al., Hippocampus 1999;9:562-574; Zola et al., J Neurosci 2000;20:451-463) differed in their conclusions about the involvement of the hippocampus in recognition memory. Because these experiments used a common behavioral procedure, trial-unique visual delayed nonmatching-to-sample (DNMS), a quantitative synthesis ("meta-analysis") was performed to determine whether hippocampal lesions produced a reliable net impairment in DNMS performance, and whether this impairment was related to the magnitude of hippocampal damage. A similar analysis was performed on data from monkeys with perirhinal or rhinal cortex damage (Meunier et al., J Neurosci 1993;13:5418-5432; Buffalo et al., Learn Mem 1999;6:572-599). DNMS performance scores were transformed to d' values to permit comparisons across studies, and a loss in d' score, a measure of the magnitude of the recognition deficit relative to the control group, was calculated for each operated monkey. Two main findings emerged. First, the loss in d' following hippocampal damage was reliably larger than zero, but was smaller than that found after lesions limited to the perirhinal cortex. Second, the correlation of loss in d' with extent of hippocampal damage was large and negative, indicating that greater impairments were associated with smaller hippocampal lesions. This relationship was opposite to that between loss in d' and rhinal cortex damage, for which larger lesions were associated with greater impairment. These findings indicate that damage to the hippocampus and to the rhinal cortex affects recognition memory in different ways. Furthermore, they provide a framework for understanding the seemingly disparate effects of hippocampal damage on recognition memory in monkeys, and by extension, for interpreting the conflicting reports on the effects of such damage on recognition memory abilities in amnesic humans.

Cognitive functions of the temporal lobe in the dog: a review

1 The current paper reviews the role of temporal lobe structures in learning and different kinds of memory, with an emphasis on behavioral tasks that re auditory stimuli. 2 The effects of lesions to structures in the temporal lobe were examined in separate groups of dogs, which were trained on an auditory spatial delayed response, or in a trial-unique auditory delayed match to sample recognition task. 3 Spatial memory was impaired after bilateral hippocampal lesions. On the other hand, neither an anterior temporal lesion or rhinal cortical injury nor combined lesion to the hippocampus and the anterior temporal lobe, affected postoperative retraining and performance of the spatial task. 4 Auditory recognition memory task was not impaired after a hippocampal and/or rhinal cortex lesion. However, postoperative retraining of the task was impaired after a lesion to auditory association areas. 5 These results confirm the role of the hippocampus in spatial memory in the dog. On the other hand, the organization of auditory recognition functions within the temporal lobe appears to be different from those described for visual recognition functions.

Contrasting effects on discrimination learning after hippocampal lesions and conjoint hippocampal-caudate lesions in monkeys

Eighteen monkeys with lesions of the hippocampal region (the hippocampus proper, the dentate gyrus, and the subiculum) made by an ischemic procedure, radio frequency, or ibotenic acid were tested on a simple, two-choice object discrimination learning task that has been shown to be sensitive to large lesions of the medial temporal lobe. The monkeys were also tested on two other discrimination tasks (pattern discrimination and eight-pair concurrent discrimination) that can be learned normally by monkeys with large medial temporal lobe lesions. All of the lesion groups were impaired at learning the simple object discrimination task. Seven of the monkeys who had sustained damage to the hippocampal region also sustained damage to the tail of the caudate nucleus. These seven monkeys, but not the other 11 monkeys with hippocampal lesions, were impaired on pattern discrimination and concurrent discrimination learning. The results suggest that the hippocampal region is important for learning easy, two-choice discriminations, whereas the caudate nucleus is necessary for the normal learning of more difficult, gradually acquired discrimination tasks. The findings support the distinction between declarative memory, which depends on the hippocampus and related medial temporal lobe structures, and habit learning, which depends on the caudate nucleus.

Contextual cueing of visual attention

Visual context information constrains what to expect and where to look, facilitating search for and recognition of objects embedded in complex displays. This article reviews a new paradigm called contextual cueing, which presents well-defined, novel visual contexts and aims to understand how contextual information is learned and how it guides the deployment of visual attention. In addition, the contextual cueing task is well suited to the study of the neural substrate of contextual learning. For example, amnesic patients with hippocampal damage are impaired in their learning of novel contextual information, even though learning in the contextual cueing task does not appear to rely on conscious retrieval of contextual memory traces. We argue that contextual information is important because it embodies invariant properties of the visual environment such as stable spatial layout information as well as object covariation information. Sensitivity to these statistical regularities allows us to interact more effectively with the visual world.

An evaluation of the concurrent discrimination task as a measure of habit learning: performance of amnesic subjects

Habit learning has been defined as an association between a stimulus and a response that develops slowly and automatically through repeated reinforcement. Concurrent discrimination (CD) learning, in which subjects learn to choose the rewarded objects in a series of pairs, is believed to be an example of habit learning in monkeys. Studies of human amnesic subjects, however, have produced equivocal results, revealing impaired or absent learning on the same CD tasks that monkeys with medial temporal-lobe (MTL) lesions learn normally. One possible explanation for impaired performance in human amnesic subjects is that, unlike monkeys, human subjects use explicit memory to solve CD problems. To test this hypothesis, we administered a 10-object pair CD learning task to two amnesic subjects, HM and PN, and normal control subjects (NCS). Both amnesic subjects have severe anterograde amnesia with little ability to form explicit memories. On the CD task, they demonstrated little or no learning and acquired no explicit knowledge of the task procedures or reward contingencies. In contrast, NCS learned the task quickly and easily using explicit memory strategies. These results suggest that CD tasks cannot be learned by habit in human subjects, and emphasize the discrepancies between the human and monkey literature on habit learning.

Memory deficits for implicit contextual information in amnesic subjects with hippocampal damage

The role of the hippocampus and adjacent medial temporal lobe structures in memory systems has long been debated. Here we show in humans that these neural structures are important for encoding implicit contextual information from the environment. We used a contextual cuing task in which repeated visual context facilitates visual search for embedded target objects. An important feature of our task is that memory traces for contextual information were not accessible to conscious awareness, and hence could be classified as implicit. Amnesic patients with medial temporal system damage showed normal implicit perceptual/skill learning but were impaired on implicit contextual learning. Our results demonstrate that the human medial temporal memory system is important for learning contextual information, which requires the binding of multiple cues.

Role of the hippocampal system in conditional motor learning: mapping antecedents to action

Macaque monkeys can learn arbitrary mappings between stimuli and spatially directed actions (often termed conditional motor learning), and, after the development of a strong learning set, can do so in just a few trials. Ablation studies have shown that the hippocampus plus subjacent cortex is necessary for this rapid and highly flexible type of learning. We consider evidence that the arbitrary mapping function of the hippocampal system may be more general and fundamental than currently accepted and what limitations there may be, if any, on the information that it can map. Removal of the hippocampal system yields a pattern of deficits and preserved abilities that correlates remarkably closely with that found in human global amnesics, such as patient H.M., on a variety of declarative memory tasks. Thus, the rapid acquisition of arbitrary visuomotor mappings may represent an example of declarative memory in nonhuman primates.