Crossed unilateral lesions of the medial forebrain bundle and either inferior temporal or frontal cortex impair object-reward association learning in Rhesus monkeys

Amyloid-β deposition in basal frontotemporal cortex is associated with selective disruption of temporal mnemonic discrimination

Cerebral amyloid-beta (Aβ) accumulation, a hallmark pathology of Alzheimer's disease (AD), precedes clinical impairment by two to three decades. However, it is unclear whether Aβ contributes to subtle memory deficits observed during the preclinical stage. The heterogenous emergence of Aβ deposition may selectively impact certain memory domains, which rely on distinct underlying neural circuits. In this context, we tested whether specific domains of mnemonic discrimination, a neural computation essential for episodic memory, exhibit specific deficits related to early Aβ deposition. We tested 108 cognitively unimpaired human older adults (66% female) who underwent 18F-florbetapir positron emission tomography (Aβ-PET), and a control group of 35 young adults, on a suite of mnemonic discrimination tasks taxing object, spatial, and temporal domains. We hypothesized that Aβ pathology would be selectively associated with temporal discrimination performance due to Aβ's propensity to accumulate in the basal frontotemporal cortex, which supports temporal processing. Consistent with this hypothesis, we found a dissociation in which generalized age-related deficits were found for object and spatial mnemonic discrimination, while Aβ-PET levels were selectively associated with deficits in temporal mnemonic discrimination. Further, we found that higher Aβ-PET levels in medial orbitofrontal and inferior temporal cortex, regions supporting temporal processing, were associated with greater temporal mnemonic discrimination deficits, pointing to the selective vulnerability of circuits related to temporal processing early in AD progression. These results suggest that Aβ accumulation within basal frontotemporal regions may disrupt temporal mnemonic discrimination in preclinical AD, and may serve as a sensitive behavioral biomarker of emerging AD progression.

Prefrontal Cortex Represents Long-Term Memory of Object Values for Months

As a central hub for cognitive control, prefrontal cortex (PFC) is thought to utilize memories. However, unlike working or short-term memory, the neuronal representation of long-term memory in PFC has not been systematically investigated. Using single-unit recordings in macaques, we show that PFC neurons rapidly update and maintain responses to objects based on short-term reward history. Interestingly, after repeated object-reward association, PFC neurons continue to show value-biased responses to objects even in the absence of reward. This value-biased response is retained for several months after training and is resistant to extinction and to interference from new object-reward learning for many complex objects (>90). Accordingly, the monkeys remember the values of the learned objects for several months in separate testing. These findings reveal that in addition to flexible short-term and low-capacity memories, primate PFC represents stable long-term and high-capacity memories, which could prioritize valuable objects far into the future.

Methods matter: A primer on permanent and reversible interference techniques in animals for investigators of human neuropsychology

The study of patients with brain lesions has contributed greatly to our understanding of the biological bases of human cognition, but this approach also has several unavoidable limitations. Research that uses animal models complements and extends human neuropsychology by addressing many of these limitations. In this review, we provide an overview of permanent and reversible animal lesion techniques for researchers of human neuropsychology, with the aim of highlighting how these methods provide a valuable adjunct to behavioural, neuroimaging, physiological, and clinical investigations in humans. Research in animals has provided important lessons about how the limitations of one or more techniques, or differences in their mechanism of action, has impacted upon the understanding of brain organisation and function. These cautionary tales highlight the importance of striving for a thorough understanding of how any intereference technique works (whether in animal or human), and for how to best use animal research to clarify the precise mechanisms underlying temporary lesion methods in humans.

Limitations of the rat medial forebrain lesion model to study prefrontal cortex mediated cognitive tasks in Parkinson's disease

Parkinson's Disease (PD) is a progressive movement disorder characterized by the loss of dopaminergic neurons in the midbrain. Besides motor impairment, PD patients exhibit non-motor symptoms that negatively impact their quality of life and often manifest prior to motor deficits. One such symptom is mild cognitive impairment (PD-MCI), which is comprised of deficits in executive function such as working memory, attention, cognitive flexibility, and spatial memory. The 6-hydroxydopamine (6-OHDA) induced unilateral medial forebrain bundle (MFB) lesion animal model successfully recapitulates PD motor impairment but is also used to assess non-motor deficits. The present study utilizes a unilateral 6-OHDA induced MFB lesion rodent model to investigate prefrontal cortex (PFC)-mediated cognitive processes that are impaired in PD patients. In a test of attentional set shifting, PD rodents demonstrated deficits in simple discrimination, but not in rule reversal or extradimensional shifts. PD rodents also exhibited deficits in a temporal order memory task but had no deficits in novel/spatial object recognition or object-in-place tasks. These results reveal limitations of the 6-OHDA induced unilateral MFB lesion model to completely recapitulate PD-MCI symptoms suggesting a need for better lesion models to study PD-MCI.

Evidence for Mediodorsal Thalamus and Prefrontal Cortex Interactions during Cognition in Macaques

It is proposed that mediodorsal thalamus contributes to cognition via interactions with prefrontal cortex. However, there is relatively little evidence detailing the interactions between mediodorsal thalamus and prefrontal cortex linked to cognition in primates. This study investigated these interactions during learning, memory, and decision-making tasks in rhesus monkeys using a disconnection lesion approach. Preoperatively, monkeys learned object-in-place scene discriminations embedded within colorful visual backgrounds. Unilateral neurotoxic lesions to magnocellular mediodorsal thalamus (MDmc) impaired the ability to learn new object-in-place scene discriminations. In contrast, unilateral ablations to ventrolateral and orbital prefrontal cortex (PFv+o) left learning intact. A second unilateral MDmc or PFv+o lesion in the contralateral hemisphere to the first operation, causing functional MDmc–PFv+o disconnection across hemispheres, further impaired learning object-in-place scene discriminations, although object discrimination learning remained intact. Adaptive decision-making after reward satiety devaluation was also reduced. These data highlight the functional importance of interactions between MDmc and PFv+o during learning object-in-place scene discriminations and adaptive decision-making but not object discrimination learning. Moreover, learning deficits observed after unilateral removal of MDmc but not PFv+o provide direct behavioral evidence of the MDmc role influencing more widespread regions of the frontal lobes in cognition.

Functional localization within the prefrontal cortex: missing the forest for the trees?

Anatomical and functional studies of the prefrontal cortex (PFC) have identified multiple PFC subregions. We argue that the PFC is involved in cognitive functions exceeding the sum of specific functions attributed to its subregions. These can be revealed either by lesions of the whole PFC, or more specifically by selective disconnection of the PFC from certain types of information (for example, visual) allowing the investigation of PFC function in toto. Recent studies in macaque monkeys using the latter approach lead to a second conclusion: that the PFC, as a whole, could be fundamentally specialized for representing events that are extended in time. The representation of temporally complex events might underlie PFC involvement in general intelligence, decision-making, and executive function.

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.

Critical role of the cholinergic system for object-in-place associative recognition memory

Object-in-place memory, which relies on the formation of associations between an object and the place in which it was encountered, depends upon a neural circuit comprising the perirhinal (PRH) and medial prefrontal (mPFC) cortices. This study examined the contribution of muscarinic cholinergic neurotransmission within this circuit to such object-in-place associative memory. Intracerebral administration of scopolamine in the PRH or mPFC impaired memory acquisition, but not retrieval and importantly we showed that unilateral blockade of muscarinic receptors simultaneously in both regions in opposite hemispheres, significantly impaired performance. Thus, object-in-place associative memory depends upon cholinergic modulation of neurones within the PRH-PFC circuit.

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.

Prefrontal cortex function in the representation of temporally complex events

The frontal cortex and inferior temporal cortex are strongly functionally interconnected. Previous experiments on prefrontal function in monkeys have shown that a disconnection of prefrontal cortex from inferior temporal cortex impairs a variety of complex visual learning tasks but leaves simple concurrent object-reward association learning intact. We investigated the possibility that temporal components of visual learning tasks determine the sensitivity of those tasks to prefrontal-temporal disconnection by adding specific temporal components to the concurrent object-reward association learning task. Monkeys with crossed unilateral lesions of prefrontal cortex and inferior temporal cortex were impaired compared with unoperated controls at associating two-item sequences of visual objects with reward. The impairment was specific to the learning of visual sequences, because disconnection was without effect on object-reward association learning for an equivalent delayed reward. This result was replicated in monkeys with transection of the uncinate fascicle, thus determining the anatomical specificity of the dissociation. Previous behavioral results suggest that monkeys represent the two-item serial compound stimuli in a configural manner, similar to the way monkeys represent simultaneously presented compound stimuli. The representation of simultaneously presented configural stimuli depends on the perirhinal cortex. The present experiments show that the representation of serially presented compound stimuli depends on the interaction of prefrontal cortex and inferior temporal cortex. We suggest that prefrontal-temporal disconnection impairs a wide variety of learning tasks because in those tasks monkeys lay down similar temporally complex representations.

Perirhinal cortical contributions to object perception

The traditional theory of the medial temporal lobe (MTL) memory system asserts that the primate MTL (hippocampus, perirhinal, entorhinal and parahippocampal cortices) is exclusively involved in consolidating declarative memories. However, several recent reports have directly challenged this dogma by arguing that MTL structures also contribute to perception. Controversy remains as many of the behavioural tasks used have confounded memory with perception. We review the evidence here and highlight new studies in humans and macaques that indicate a perceptual role for MTL in the absence of such confounds. We argue that the challenge to MTL memory system theory is substantiated and that the implications are considerable, namely that most psychologists and neuroscientists have held a fundamentally flawed view of how memory is implemented in the brain.

Opposing effects of amygdala and orbital prefrontal cortex lesions on the extinction of instrumental responding in macaque monkeys

Extinction is a well-known behavioural phenomenon that allows organisms to respond flexibly to a changing environment. Although recent work implicates the amygdala and orbital prefrontal cortex (PFo) in extinction of Pavlovian conditioned fear and aversion, much less is known about the neural bases of instrumental extinction. To explore the contribution of the macaque amygdala to flexible responding in the face of changing reward contingency, we tested the effects of selective, excitotoxic lesions of the amygdala on extinction of an instrumental response. For comparison, we evaluated the effects of ablation of PFo on the same task. Amygdala lesions facilitated the extinction of instrumental responses, whereas lesions of PFo had the opposite effect.

Conditional motor learning in the nonspatial domain: effects of errorless learning and the contribution of the fornix to one-trial learning

Conditional motor learning contributes importantly to behavioral flexibility. In previous work, the authors found that fornix transections impaired the ability of macaque monkeys (Macaca mulatta) to learn conditional motor associations between the nonspatial features of visual stimuli and nonspatially differentiated responses. In the present study, they found that significant 1-trial learning of such associations also depended on the fornix. Furthermore, removal of the hippocampus, subiculum, and subjacent parahippocampal cortex, added to fornix transection, had no effect, thus demonstrating that fornix transections eliminated the contribution of the hippocampal system. In addition, the authors examined the effect of errorless learning and found, in control monkeys, that errors made prior to the 1st correct response retarded 1-trial learning.

Further analysis of the effects of immunotoxic lesions of the basal nucleus of Meynert reveals substantial impairment on visual discrimination learning in monkeys

In this paper we undertake a combined analysis of several studies in which marmoset monkeys received immunotoxic lesions of the cortical cholinergic projections from the basal nucleus of Meynert (NBM) bilaterally and/or in combination with immunotoxic lesions of other parts of the cholinergic system or ablations of the target inferotemporal neocortical area. Analysis of the mean learning scores across all visual discriminations learning tasks for each lesion combination revealed highly significant impairments where the NBM was lesioned bilaterally or where an NBM lesion in one hemisphere was crossed with an inferotemporal cortical ablation in the other hemisphere. This demonstrates that the cholinergic projection from the NBM to the major target area of neocortex involved in visual discrimination learning, i.e. the inferotemporal cortex, makes an important contribution to the perceptuo-mnemonic processes necessary for this type of learning. A new study demonstrates a significant effect of a subtotal bilateral cholinergic lesion confined to the NBM on a concurrent object-reward association task using black objects which is perceptually and mnemonically demanding. These results do not preclude the possibility that cholinergic projections from the NBM to other parts of the neocortex make a contribution to other cortical functions which are not mnemonic. It is well established that lesions of the cholinergic projection from the diagonal band of Broca disrupts the mnemonic functions of the hippocampus. The results described here suggest that degeneration of the cholinergic projections in Alzheimer's disease and other dementias will contribute to the loss of those mnemonic functions which are dependent on the neocortex.

The effect of cingulate lesions on social behaviour and emotion

Functional and structural neuroimaging of the human cingulate cortex has identified this region with emotion and social cognition and suggested that cingulate pathology may be associated with emotional and social behavioural disturbances. The importance of the cingulate cortex for emotion and social behaviour, however, has not been clear from lesion studies. Bilateral lesions in the cingulate cortex were made in three macaques and their social interactions were compared with those of controls. Subsequently, cingulate lesions were made in the three controls and their behaviour was compared before and after surgery. Cingulate lesions were associated with decreases in social interactions, time spent in proximity with other individuals, and vocalisations but an increase in manipulation of an inanimate object. The results are consistent with a cingulate role in social behaviour and emotion.

Neural mechanisms for detecting and remembering novel events

The ability to detect and respond to novel events is crucial for survival in a rapidly changing environment. Four decades of neuroscientific research has begun to delineate the neural mechanisms by which the brain detects and responds to novelty. Here, we review this research and suggest how changes in neural processing at the cellular, synaptic and network levels allow us to detect, attend to and subsequently remember the occurrence of a novel event.

Insights into the nature of fronto-temporal interactions from a biconditional discrimination task in the monkey

Previous work in monkeys has shown that both frontal and inferior temporal cortices are required to solve visual learning tasks. When communication between these cortical areas is prevented within the same hemisphere by crossed lesions of the frontal cortex in one hemisphere and the inferior temporal cortex in the opposite hemisphere, most learning tasks are impaired, but learning of object-reward associations is unimpaired. The current experiment aims to understand further the role of the interaction between the frontal and inferior temporal cortices in learning tasks. We trained monkeys on a biconditional discrimination task, in which different visual cues guided behaviour towards choice objects. One visual cue predicted immediate delivery of reward to a correct response, the other visual cue predicted a delayed delivery of reward to a correct response. Pre-operative behavioural data clearly shows that the monkeys form expectations of the reward outcome for the individual cues and choice objects. Crossed lesions of frontal and inferior temporal cortices, however, produce no impairment on this task. The result suggests (in combination with previous experiments) that task difficulty does not determine the reliance of a task on interactions between the frontal cortex and the inferior temporal cortex within the same hemisphere. Instead, we propose that tasks that can be solved by using expectation of the reward outcome do not require interaction of frontal and inferior temporal cortices within the same hemisphere. The results are discussed in the context of other data on frontal interactions with inferior temporal cortex in learning tasks.

Interaction of inferior temporal cortex with frontal cortex and basal forebrain: double dissociation in strategy implementation and associative learning

Macaque monkeys learned a strategy task in which two groups of visual objects needed to be treated differently, one with persistent and one with sporadic object choices, to obtain food rewards. After preoperative training, they were divided into two surgical groups of three monkeys each. One group received crossed unilateral removals of frontal cortex and inferior temporal cortex (IT x FC) and were severely impaired in performing the strategy task. The other group received bilateral transection of anterior temporal stem, amygdala, and fornix (TS+AM+FX) and were unimpaired in performing the strategy task. Subsequently the same animals were tested in visual object-reward association learning. Here, confirming previous results, group IT x FC was unimpaired, whereas group TS+AM+FX was severely impaired. The results show that the amnesic effects of TS+AM+FX cannot be generally attributed to the partial temporal-frontal disconnection that this lesion creates, and therefore support the hypothesis that the amnesic effects of this lesion are caused primarily by the disconnection of temporal cortex from ascending inputs from the basal forebrain. The results also show that temporal-frontal interaction in strategy implementation does not require those routes of temporal-frontal interaction that are interrupted in TS+AM+FX, and therefore support the hypothesis that projections to other posterior cortical areas allow temporal and frontal cortex to interact with each other by multisynaptic corticocortical routes in strategy implementation.

Against memory systems

The medial temporal lobe is indispensable for normal memory processing in both human and non-human primates, as is shown by the fact that large lesions in it produce a severe impairment in the acquisition of new memories. The widely accepted inference from this observation is that the medial temporal cortex, including the hippocampal, entorhinal and perirhinal cortex, contains a memory system or multiple memory systems, which are specialized for the acquisition and storage of memories. Nevertheless, there are some strong arguments against this idea: medial temporal lesions produce amnesia by disconnecting the entire temporal cortex from neuromodulatory afferents arising in the brainstem and basal forebrain, not by removing cortex; the temporal cortex is essential for perception as well as for memory; and response properties of temporal cortical neurons make it impossible that some kinds of memory trace could be stored in the temporal lobe. All cortex is plastic, and it is possible that the same rules of plasticity apply to all cortical areas; therefore, memory traces are stored in widespread cortical areas rather than in a specialized memory system restricted to the temporal lobe. Among these areas, the prefrontal cortex has an important role in learning and memory, but is best understood as an area with no specialization of function.

The amygdala and reward

The amygdala -- an almond-shaped group of nuclei at the heart of the telencephalon -- has been associated with a range of cognitive functions, including emotion, learning, memory, attention and perception. Most current views of amygdala function emphasize its role in negative emotions, such as fear, and in linking negative emotions with other aspects of cognition, such as learning and memory. However, recent evidence supports a role for the amygdala in processing positive emotions as well as negative ones, including learning about the beneficial biological value of stimuli. Indeed, the amygdala's role in stimulus-reward learning might be just as important as its role in processing negative affect and fear conditioning.

What is a memory system? Horel's critique revisited

J.A. Horel's critique of what he termed "the hippocampal memory hypothesis" turns out, 23 years later, to have been remarkably discerning and prophetic. There is now an overwhelming weight of evidence to confirm his four key proposals: that selective destruction of the hippocampus or fornix does not produce dense global amnesia; that the effects of hippocampal or fornix lesions are not primarily a memory impairment, but an impairment in processing spatial information; that damage to the anterior temporal stem is part of the explanation of dense temporal lobe amnesia; and that the interaction of temporal cortex with prefrontal cortex is essential in memory. This review summarizes the modern evidence that reinforces each of these four proposals. A final section argues that, not only in the case of the hippocampus but also in the case of other temporal and frontal cortical areas that are involved in normal memory, the concept of a "memory system" is harmful.

Theories of episodic memory

Theories of episodic memory need to specify the encoding (representing), storage, and retrieval processes that underlie this form of memory and indicate the brain regions that mediate these processes and how they do so. Representation and re-representation (retrieval) of the spatiotemporally linked series of scenes, which constitute an episode, are probably mediated primarily by those parts of the posterior neocortex that process perceptual and semantic information. However, some role of the frontal neocortex and medial temporal lobes in representing aspects of context and high-level visual object information at encoding and retrieval cannot currently be excluded. Nevertheless, it is widely believed that the frontal neocortex is mainly involved in coordinating episodic encoding and retrieval and that the medial temporal lobes store aspects of episodic information. Establishing where storage is located is very difficult and disagreement remains about the role of the posterior neocortex in episodic memory storage. One view is that this region stores all aspects of episodic memory ab initio for as long as memory lasts. This is compatible with evidence that the amygdala, basal forebrain, and midbrain modulate neocortical storage. Another view is that the posterior neocortex only gradually develops the ability to store some aspects of episodic information as a function of rehearsal over time and that this information is initially stored by the medial temporal lobes. A third view is that the posterior neocortex never stores these aspects of episodic information because the medial temporal lobes store them for as long as memory lasts in an increasingly redundant fashion. The last two views both postulate that the medial temporal lobes initially store contextual markers that serve to cohere featural information stored in the neocortex. Lesion and functional neuroimaging evidence still does not clearly distinguish between these views. Whether the feeling that an episodic memory is familiar depends on retrieving an association between a retrieved episode and this feeling, or by an attribution triggered by a priming process, is unclear. Evidence about whether the hippocampus and medial temporal lobe cortices play different roles in episodic memory is conflicting. Identifying similarities and differences between episodic memory and both semantic memory and priming will require careful componential analysis of episodic memory.

Exploring the neural bases of episodic and semantic memory: the role of structural and functional neuroimaging

Exploration of the neural bases of episodic and semantic memory is best pursued through the combined examination of the effects of identified lesions on memory and functional neuroimaging of both normal people and patients when they engage in memory processing of various kinds. Both structural and functional neuroimaging acquisition and analysis techniques have developed rapidly and will continue to do so. This review briefly outlines the history of neuroimaging as it impacts on memory research. Next, what has been learned so far from lesion-based research is outlined with emphasis on areas of disagreement as well as agreement. What has been learned from functional neuroimaging, particularly emission tomography and functional magnetic resonance imaging, is then discussed, and some stress is placed on topics where the interpretation of imaging studies has so far been unclear. Finally, how functional and structural imaging techniques can be optimally used to help resolve three areas of disagreement in the lesion literature will be discussed. These disagreements concern what the hippocampus and perirhinal cortex contribute to memory; whether any form of priming depends on the medial temporal lobes; and whether remote episodic as well as semantic memories cease to depend on the medial temporal lobes. Although the discussion will show the value of imaging techniques, it will also emphasize some of the limitations of current neuroimaging studies.

Crossed unilateral lesions of medial forebrain bundle and either inferior temporal or frontal cortex impair object recognition memory in Rhesus monkeys

In monkeys, section of the fornix, amygdala and anterior temporal stem results in a severe anterograde amnesia. Immunolesions of the cholinergic cells of the basal forebrain suggest that this amnesia is a result of isolating the inferior temporal cortex and medial temporal lobe from their cholinergic basal forebrain afferents. In this experiment, six monkeys were trained in a delayed match-to-sample task and then received a section of the medial forebrain bundle in one hemisphere and an ablation of either the frontal or inferior temporal cortex in the opposite hemisphere. All the animals were severely impaired in the performance of this task following this surgery, and the severity of the impairment was independent of the cortical area from which the medial forebrain bundle was disconnected. These results support a model of fronto-temporal interaction via the basal forebrain in new learning, in which midbrain sites related to reward modulate the cholinergic basal forebrain activity.