Aspiration removal of orbitofrontal cortex disrupts cholinergic fibers of passage to anterior cingulate cortex in rhesus macaques.

The study of anthropoid nonhuman primates has provided valuable insights into frontal cortex function in humans, as these primates share similar frontal anatomical subdivisions (Murray et al. 2011). Causal manipulation studies have been instrumental in advancing our understanding of this area. One puzzling finding is that macaques with bilateral aspiration removals of orbitofrontal cortex (OFC) are impaired on tests of cognitive flexibility and emotion regulation, whereas those with bilateral excitotoxic lesions of OFC are not (Rudebeck et al. 2013). This discrepancy is attributed to the inadvertent disruption of fibers of passage by aspiration lesions but not by excitotoxic lesions. Which fibers of passage are responsible for the impairments observed? One candidate is cholinergic fibers originating in the nucleus basalis magnocellularis (NBM) and passing nearby or through OFC on their way to other frontal cortex regions (Kitt et al. 1987). To investigate this possibility, we performed unilateral aspiration lesions of OFC in three macaques, and then compared cholinergic innervation of the anterior cingulate cortex (ACC) between hemispheres. Histological assessment revealed diminished cholinergic innervation in the ACC of hemispheres with OFC lesions relative to intact hemispheres. This finding indicates that aspiration lesions of the OFC disrupt cholinergic fibers of passage, and suggests the possibility that loss of cholinergic inputs to ACC contributes to the impairments in cognitive flexibility and emotion regulation observed after aspiration but not excitotoxic lesions of OFC.

Bacterial community structure and response to nitrogen amendments in Lake Shenandoah (VA, USA).

Microbial processes are critical to the function of freshwater ecosystems, yet we still do not fully understand the factors that shape freshwater microbial communities. Furthermore, freshwater ecosystems are particularly susceptible to effects of environmental change, including influx of exogenous nutrients such as nitrogen and phosphorus. To evaluate the impact of nitrogen loading on the microbial community structure of shallow freshwater lakes, water samples collected from Lake Shenandoah (Virginia, USA) were incubated with two concentrations of either ammonium, nitrate, or urea as a nitrogen source. The potential impact of these nitrogen compounds on the bacterial community structure was assessed via 16S rRNA amplicon sequencing. At the phylum level, the dominant taxa in Lake Shenandoah were comprised of Actinobacteria and Proteobacteria, which were not affected by exposure to the various nitrogen treatments. Overall, there was not a significant shift in the diversity of the bacterial community of Lake Shenandoah with the addition of nitrogen sources, indicating this shallow system may be constrained by other environmental factors.

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

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

Rhesus monkeys with orbital prefrontal cortex lesions can learn to inhibit prepotent responses in the reversed reward contingency task.

Monkeys with lesions of the orbital prefrontal cortex (PFo) are impaired on behavioral tasks that require the ability to respond flexibly to changes in reward contingency (e.g., object reversal learning and extinction). These and related findings in rodents and humans have led to the suggestion that PFo is critical for the inhibitory control needed to overcome prepotent responses. To test this idea, we trained rhesus monkeys with PFo lesions and unoperated controls on acquisition of the reversed reward contingency task. In this task, selecting the smaller of 2 food quantities (1 half peanut [1P]) leads to receipt of the larger quantity (4 half peanuts [4P]) and vice versa. Choice of a larger quantity of food is a reliable prepotent response, and, accordingly, all monkeys initially selected 4P rather than one. With experience, however, all monkeys learned to select 1P in order to receive 4. Surprisingly, monkeys with PFo lesions learned as quickly as unoperated controls. Thus, PFo lesions do not yield a deficit in all tests that require the inhibition of a prepotent response.

Role of the hippocampal system in associative learning beyond the spatial domain.

Expert opinion remains divided on the issue of whether the hippocampal system functions exclusively in spatial information processing, e.g. in navigation or in understanding spatial relations, or whether it plays a more general role in higher brain function. Previous work on monkeys and rats has tended to support the former view, whereas observations in the clinic point to the latter, including functions as diverse as declarative knowledge, episodic memory, word learning, and understanding relations among objects. One influential theory posits a general role for the hippocampal system in associative learning, with emphasis on associations learned rapidly and recently. The results presented here are consistent with this theory, along with previous clinical and theoretical studies indicating that the hippocampal system is necessary for associative learning even if no component of the association relies on spatial information. In the study reported here, rhesus monkeys learned a series of conditional stimulus-response associations involving complex visual stimuli presented on a video monitor. Each stimulus instructed one of three responses: tapping the stimulus with the hand, steady hand contact with the stimulus for a brief period of time, or steady contact for a longer time. Fornix transection impaired the learning of these associations, even though both the stimuli and the responses were nonspatially differentiated, and this deficit persisted for at least 2 years. This finding indicates that the hippocampal system plays an important role in associative learning regardless of the relevance of spatial information to any aspect of the association. Fornix-transected monkeys were impaired in learning new stimulus-response associations even when the stimuli were highly familiar. Thus, the deficit was one of associating each stimulus with a response, as opposed to problems in distinguishing the stimuli from each other. In contrast to these effects, fornix transection did not impair performance when familiar stimuli instructed a response according to an already-learned association, which shows that the deficit was one of learning new associations rather than one of retention or retrieval of previously learned ones. Taken together, these results show that fornix transection causes a long-lasting impairment in associative learning outside of the spatial domain, in a manner consistent with theories of hippocampal-system function that stress a general role in the rapid acquisition of associative knowledge.

The role of ventral and orbital prefrontal cortex in conditional visuomotor learning and strategy use in rhesus monkeys (Macaca mulatta).

Four rhesus monkeys (Macaca mulatta) were trained to learn novel sets of visuomotor associations in 50 trials or less, within single test sessions. After bilateral ablation of the orbital and ventral prefrontal cortex, the monkeys lost the ability to learn these associations within a session, although they could learn them when given several daily sessions. Thus, relatively slow, across-session visuomotor learning depends on neither the ventral nor orbital prefrontal cortex, but rapid, within-session learning does. The ablations also eliminated at least 2 response strategies, repeat-stay and lose-shift, which might account, in part, for the deficit in rapid learning. The deficit is unlikely to result from a failure of visual discriminative ability or working memory: The monkeys could discriminate similar stimulus material within a session, and reducing the working memory load did not improve within-session learning.

Neural substrates of crossmodal association memory in monkeys: the amygdala versus the anterior rhinal cortex.

Nine rhesus monkeys were trained on visual, tactual, and crossmodal (tactual-visual) versions of delayed nonmatching-to-sample (DNMS). They then received bilateral aspiration lesions of the anterior rhinal cortex or bilateral excitotoxic lesions of the amygdala or were retained as unoperated controls. Monkeys with anterior rhinal cortex lesions displayed a persistent deficit on crossmodal DNMS as well as a deficit on tactual DNMS. In contrast, monkeys with amygdala lesions exhibited only a transient impairment on crossmodal DNMS, and their difficulty appeared to be related to inadvertent damage to the anterior rhinal cortex. The present findings support the idea that the rhinal cortex is important for the formation and retrieval of stimulus-stimulus associations across sensory modalities.

Impairments in visual discrimination learning and recognition memory produced by neurotoxic lesions of rhinal cortex in rhesus monkeys.

Much work on the cognitive functions of the primate rhinal (i.e. entorhinal plus perirhinal) cortex has been based on aspiration lesions of this structure, which might disrupt fibres passing nearby and through the rhinal cortex in addition to removing the cell bodies of the rhinal cortex itself. To determine whether damage limited to the cell bodies of the rhinal cortex is sufficient to impair visual learning and memory, four rhesus monkeys (Macaca mulatta) were preoperatively trained on a battery of visual learning and memory tasks, including single-pair discrimination learning for primary reinforcement, single-pair discrimination reversals, concurrent discrimination learning and reversal, and delayed matching-to-sample. Following acquisition of these tasks and a preoperative performance test, ibotenic acid was injected bilaterally into the rhinal cortex, and the monkeys were retested. Consistent with the results of studies using aspiration lesions, the monkeys were impaired on single-pair discrimination learning as well as recognition memory performance postoperatively, although reliable reversal learning impairments were not observed. The magnitude of postoperative impairment in discrimination learning was not correlated with the magnitude of postoperative impairment in recognition memory, suggesting a possible dissociation between these functions within the rhinal cortex. The correspondence of behavioural deficits following aspiration and neurotoxic lesions of the rhinal cortex validates the attribution of various cognitive functions to this structure, based on the results of studies with aspiration lesions.

Role of perirhinal cortex in object perception, memory, and associations.

The perirhinal cortex plays a key role in acquiring knowledge about objects. It contributes to at least four cognitive functions, and recent findings provide new insights into how the perirhinal cortex contributes to each: first, it contributes to recognition memory in an automatic fashion; second, it probably contributes to perception as well as memory; third, it helps identify objects by associating together the different sensory features of an object; and fourth, it associates objects with other objects and with abstractions.

Consolidation and the medial temporal lobe revisited: methodological considerations.

It is widely believed that new memories are stored in the medial temporal lobe structures in the short term, but then are reorganized over time as the neocortex gradually comes to support stable long-term storage. On this view, the medial temporal lobe structures play a time-limited role in information storage. This putative process of reorganization, known as consolidation, is supported by some clinical findings in humans and by some data from nonhuman animals. Here we review prospective studies of retrograde memory in nonhuman animals, with particular emphasis on experimental design. In considering the evidence for a time-limited role for the medial temporal lobe in information storage, we note that there are alternative interpretations for at least some of the findings typically cited in support of the consolidation process. In addition, we suggest that some studies arguing against the consolidation view should probably be given more weight than they have so far received. Finally, we observe that different structures in the medial temporal lobe are unlikely to operate together as a single functional unit mediating a single consolidation process. Although evidence for a time-limited role for medial temporal lobe structures in memory is at present equivocal, future studies that consider some of the alternative accounts we and others have identified will provide a clearer picture of the mechanisms underlying information storage and retrieval in the brain.

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.

Learning motivational significance of visual cues for reward schedules requires rhinal cortex.

The limbic system is necessary to associate stimuli with their motivational and emotional significance. The perirhinal cortex is directly connected to this system, and neurons in this region carry signals related to a monkey's progress through visually cued reward schedules. This task manipulates motivation by displaying different visual cues to indicate the amount of work remaining until reward delivery. We asked whether rhinal (that is, entorhinal and perirhinal) cortex is necessary to associate the visual cues with reward schedules. When faced with new visual cues in reward schedules, intact monkeys adjusted their motivation in the schedules, whereas monkeys with rhinal cortex removals failed to do so. Thus, the rhinal cortex is critical for forming associations between visual stimuli and their motivational significance.

Role of prefrontal cortex in a network for arbitrary visuomotor mapping.

In arbitrary visuomotor mapping, an object instructs a particular action or target of action, but does so in a particular way. In other forms of visuomotor control, the object is either the target of action (termed standard mapping) or its location provides the information needed for targeting (termed transformational mapping). By contrast, in arbitrary mapping, the object's location bears no systematic spatial relationship with the action. Neuropsychological and neurophysiological investigation has, in large part, identified the neural network that underlies the rapid acquisition and performance of arbitrary visuomotor mappings. This network consists of parts of the premotor (PM) and prefrontal (PF) cortex, the hippocampal system (HS), and the basal ganglia (BG). Here, we propose specialized contributions of the network's different components to its overall function. To do so, we invoke the concept of distributed information-processing architectures, or modules, which may involve a variety of neural structures. According to this view, recurrent neural networks involving cortex, basal ganglia, and thalamus operate largely in parallel. Each of these interacting networks can be termed a cortical-BG module. A large number of these modules include PM neurons, and they can be termed PM cortical-BG modules. A comparable number include PF neurons, termed PF cortical-BG modules. We propose that PM and PF cortical-BG modules compute specific object-to-action mappings, in which the network learns the action associated with a given input. These mappings serve as specific solutions to arbitrary visuomotor mapping problems. However, they are also exemplars of more abstract rules, such as the knowledge that nonspatial visual information (e.g., color) can guide the choice of action. We propose that PF cortical-BG modules subserve abstract rules of this kind, along with other problem-solving strategies. This view should not be taken to imply that the PF network lacks the capacity to compute specific mappings, but rather that it has higher-order mapping functions in addition to its lower-order ones. Furthermore, it seems likely that PF provides PM with pertinent sensory information. The hippocampal system appears to play a role parallel to that of both neocortical-BG networks discussed here. However, in accord with several models, it operates mainly in the intermediate term, pending the consolidation of the relevant information in those neocortical-BG networks.

The parahippocampal region and object identification.

The hippocampus has long been thought to be critical for memory, including memory for objects. However, recent neuropsychological studies in nonhuman primates have indicated that other regions within the medial temporal lobe, specifically, structures in the parahippocampal region, are primarily responsible for object recognition and object identification. This article reviews the behavioral effects of removal of structures within the parahippocampal region in monkeys, and cites relevant work in rodents as well. It is argued that the perirhinal cortex, in particular, contributes to object identification in at least two ways: (i) by serving as the final stage in the ventral visual cortical pathway that represents stimulus features, and (ii) by operating as part of a network for associating together sensory inputs within and across sensory modalities.

Arbitrary associations between antecedents and actions.

The arbitrary linkage of sensory cues to actions and goals represents one of the most-flexible capabilities in the behavioral repertoire of mammals. This ability has been termed 'conditional motor learning', 'conditional discrimination' or, more recently, 'arbitrary visuomotor mapping'. Unlike other forms of visuomotor guidance, in arbitrary mapping the location of the sensory cue lacks any systematic spatial relationship with the action or its goal. Recent work has identified much of the neural network that underlies this behavior. It consists of parts of the frontal cortex, hippocampal system and basal ganglia, each of which has neurons whose activity undergoes systematic evolution during learning.

Control of response selection by reinforcer value requires interaction of amygdala and orbital prefrontal cortex.

Goal-directed actions are guided by expected outcomes of those actions. Humans with bilateral damage to ventromedial prefrontal cortex, or the amygdala, are deficient in their ability to use information about positive and negative outcomes to guide their choice behavior. Similarly, rats and monkeys with orbital prefrontal or amygdala damage have been found to be impaired in their responses to changing values of outcomes. In the present study, we tested whether direct, functional interaction between the amygdala and the orbital prefrontal cortex is necessary for guiding behavior based on expected outcomes. Unlike control monkeys, rhesus monkeys with surgical disconnection of these two structures, achieved by crossed unilateral lesions of the amygdala in one hemisphere and orbital prefrontal cortex in the other, combined with forebrain commissurotomy, were unable to adjust their choice behavior after a change in the outcome (here, a reduction in the value of a particular reinforcer). The lesions did not affect motivation to work for a food reinforcer, or food preferences, per se. Hence, the amygdala and orbital prefrontal cortex act as part of an integrated neural system guiding decision-making and adaptive response selection.

Effects of aspiration versus neurotoxic lesions of the amygdala on emotional responses in monkeys.

All previous reports describing alterations in emotional reactivity after amygdala damage in monkeys were based on aspiration or radiofrequency lesions which likely disrupted fibres of passage coursing to and from adjacent ventral and medial temporal cortical areas. To determine whether this associated indirect damage was responsible for some or all of the changes described earlier, we compared the changes induced by aspiration of the amygdala with those induced by fibre-sparing neurotoxic lesions. Four different stimuli, two with and two without a social component, were used to evaluate the expression of defence, aggression, submission and approach responses. In unoperated controls, defence and approach behaviours were elicited by all four stimuli, 'social' and inanimate alike, whereas aggression and submission responses occurred only in the presence of the two 'social' stimuli. Furthermore, all defence reactions were reduced with an attractive inanimate item, while freezing was selectively increased with an aversive one. Relative to controls, monkeys with neurotoxic amygdala lesions showed the same array of behavioural changes as those with aspiration lesions, i.e. reduced fear and aggression, increased submission, and excessive manual and oral exploration. Even partial neurotoxic lesions involving less than two-thirds of the amygdala significantly altered fear and manual exploration. These findings convincingly demonstrate that the amygdala is crucial for the normal regulation of emotions in monkeys. Nevertheless, because some of the symptoms observed after neurotoxic lesions were less marked than those seen after aspiration lesions, the emotional disorders described earlier after amygdalectomy in monkeys were likely exacerbated by the attendant fibre damage.

Increased expression of nitric oxide synthase and dendritic injury in simian immunodeficiency virus encephalitis.

OBJECTIVES: Widespread dendritic injury may be one mechanism involved in the neurologic impairment that occurs in HIV-1 infection. The objectives of this study were to quantitate the extent of dendritic injury in a primate model of central nervous system (CNS) infection, investigate the role of nitric oxide (NO) as a mediator of neuropathologic changes, and evaluate the relation of these changes to cognitive and motor function. STUDY DESIGN/METHODS: Cognitive and motor function was assessed in rhesus macaque monkeys infected with simian immunodeficiency virus (SIV). In situ hybridization, immunohistochemistry, and quantitative image analysis were employed to assess the relations among productive infection, NO synthase (iNOS), and dendritic injury. RESULTS: Productive infection of cells of the macrophage lineage in CNS is associated with inflammation, increased expression of iNOS, and dendritic injury. The tests of cognitive and motor function employed were abnormal in both animals that had evidence of productive infection and those that did not. CONCLUSIONS: Increased NO accompanying productive infection and encephalitis may be one cause of neuronal injury in lentivirus infections of the CNS. Extension of tests of cognitive and motor function to late-stage AIDS in rhesus monkeys is needed to assess the potential role of NO-induced dendritic damage in lentiviral encephalopathy/AIDS dementia complex.

Rhinal cortex lesions produce mild deficits in visual discrimination learning for an auditory secondary reinforcer in rhesus monkeys.

Aspiration, but not neurotoxic, lesions of the amygdala impair performance on a visual discrimination learning task in which an auditory secondary reinforcer signals which of 2 stimuli will be reinforced with food. Because aspiration lesions of the amygdala interrupt projections of the rhinal cortex traveling close to the amygdala, it was hypothesized that damage to the rhinal cortex would severely impair learning in this task. Rhesus monkeys (Macaca mulatta) were trained to solve visual discrimination problems based on an auditory secondary reinforcer, were given lesions of the rhinal cortex or the perirhinal cortex alone, and were then retested. The monkeys displayed a reliable, albeit mild, deficit in postoperative performance. It is concluded that the aspiration lesions of the amygdala that produced a severe impairment did so because they interrupted connections of temporal cortical fields beyond the rhinal cortex that are also involved in learning in this task.