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.

Lessons from assembling UCEs: A comparison of common methods and the case of Clavinomia (Halictidae)

Sequence data assembly is a foundational step in high-throughput sequencing, with untold consequences for downstream analyses. Despite this, few studies have interrogated the many methods for assembling phylogenomic UCE data for their comparative efficacy, or for how outputs may be impacted. We study this by comparing the most commonly used assembly methods for UCEs in the under-studied bee lineage Nomiinae and a representative sampling of relatives. Data for 63 UCE-only and 75 mixed taxa were assembled with five methods, including ABySS, HybPiper, SPAdes, Trinity and Velvet, and then benchmarked for their relative performance in terms of locus capture parameters and phylogenetic reconstruction. Unexpectedly, Trinity and Velvet trailed the other methods in terms of locus capture and DNA matrix density, whereas SPAdes performed favourably in most assessed metrics. In comparison with SPAdes, the guided-assembly approach HybPiper generally recovered the highest quality loci but in lower numbers. Based on our results, we formally move Clavinomia to Dieunomiini and render Epinomia once more a subgenus of Dieunomia. We strongly advise that future studies more closely examine the influence of assembly approach on their results, or, minimally, use better-performing assembly methods such as SPAdes or HybPiper. In this way, we can move forward with phylogenomic studies in a more standardized, comparable manner.

The Chalcidoidea bush of life: evolutionary history of a massive radiation of minute wasps

Chalcidoidea are mostly parasitoid wasps that include as many as 500 000 estimated species. Capturing phylogenetic signal from such a massive radiation can be daunting. Chalcidoidea is an excellent example of a hyperdiverse group that has remained recalcitrant to phylogenetic resolution. We combined 1007 exons obtained with Anchored Hybrid Enrichment with 1048 ultra-conserved elements (UCEs) for 433 taxa including all extant families, >95% of all subfamilies, and 356 genera chosen to represent the vast diversity of the superfamily. Going back and forth between the molecular results and our collective knowledge of morphology and biology, we detected bias in the analyses that was driven by the saturation of nucleotide data. Our final results are based on a concatenated analysis of the least saturated exons and UCE datasets (2054 loci, 284 106 sites). Our analyses support an expected sister relationship with Mymarommatoidea. Seven previously recognized families were not monophyletic, so support for a new classification is discussed. Natural history in some cases would appear to be more informative than morphology, as illustrated by the elucidation of a clade of plant gall associates and a clade of taxa with planidial first-instar larvae. The phylogeny suggests a transition from smaller soft-bodied wasps to larger and more heavily sclerotized wasps, with egg parasitism as potentially ancestral for the entire superfamily. Deep divergences in Chalcidoidea coincide with an increase in insect families in the fossil record, and an early shift to phytophagy corresponds with the beginning of the "Angiosperm Terrestrial Revolution". Our dating analyses suggest a middle Jurassic origin of 174 Ma (167.3-180.5 Ma) and a crown age of 162.2 Ma (153.9-169.8 Ma) for Chalcidoidea. During the Cretaceous, Chalcidoidea may have undergone a rapid radiation in southern Gondwana with subsequent dispersals to the Northern Hemisphere. This scenario is discussed with regard to knowledge about the host taxa of chalcid wasps, their fossil record and Earth's palaeogeographic history.

The evolutionary history of bees in time and space

Bees are the most significant pollinators of flowering plants. This partnership began ca. 120 million years ago, but the uncertainty of how and when bees spread across the planet has greatly obscured investigations of this key mutualism. We present a novel analysis of bee biogeography using extensive new genomic and fossil data to demonstrate that bees originated in Western Gondwana (Africa and South America). Bees likely originated in the Early Cretaceous, shortly before the breakup of Western Gondwana, and the early evolution of any major bee lineage is associated with either the South American or African land masses. Subsequently, bees colonized northern continents via a complex history of vicariance and dispersal. The notable early absences from large landmasses, particularly in Australia and India, have important implications for understanding the assembly of local floras and diverse modes of pollination. How bees spread around the world from their hypothesized Southern Hemisphere origin parallels the histories of numerous flowering plant clades, providing an essential step to studying the evolution of angiosperm pollination syndromes in space and time.

Gene Tree Estimation Error with Ultraconserved Elements: An Empirical Study on Pseudapis Bees

Summarizing individual gene trees to species phylogenies using two-step coalescent methods is now a standard strategy in the field of phylogenomics. However, practical implementations of summary methods suffer from gene tree estimation error, which is caused by various biological and analytical factors. Greatly understudied is the choice of gene tree inference method and downstream effects on species tree estimation for empirical data sets. To better understand the impact of this method choice on gene and species tree accuracy, we compare gene trees estimated through four widely used programs under different model-selection criteria: PhyloBayes, MrBayes, IQ-Tree, and RAxML. We study their performance in the phylogenomic framework of $>$800 ultraconserved elements from the bee subfamily Nomiinae (Halictidae). Our taxon sampling focuses on the genus Pseudapis, a distinct lineage with diverse morphological features, but contentious morphology-based taxonomic classifications and no molecular phylogenetic guidance. We approximate topological accuracy of gene trees by assessing their ability to recover two uncontroversial, monophyletic groups, and compare branch lengths of individual trees using the stemminess metric (the relative length of internal branches). We further examine different strategies of removing uninformative loci and the collapsing of weakly supported nodes into polytomies. We then summarize gene trees with ASTRAL and compare resulting species phylogenies, including comparisons to concatenation-based estimates. Gene trees obtained with the reversible jump model search in MrBayes were most concordant on average and all Bayesian methods yielded gene trees with better stemminess values. The only gene tree estimation approach whose ASTRAL summary trees consistently produced the most likely correct topology, however, was IQ-Tree with automated model designation (ModelFinder program). We discuss these findings and provide practical advice on gene tree estimation for summary methods. Lastly, we establish the first phylogeny-informed classification for Pseudapis s. l. and map the distribution of distinct morphological features of the group. [ASTRAL; Bees; concordance; gene tree estimation error; IQ-Tree; MrBayes, Nomiinae; PhyloBayes; RAxML; phylogenomics; stemminess].

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.

Viral transmission in honey bees and native bees, supported by a global black queen cell virus phylogeny

In recent decades, we have realized that honey bee viruses are not, in fact, exclusive to honey bees. The potential impact of Apis-affiliated viruses on native pollinators is prompting concern. Our research addresses the issue of virus crossover between honey bees and native bees foraging in the same localities. We measured the presence of black queen cell virus (BQCV), deformed wing virus (DWV) and sacbrood virus (SBV) in managed Apis mellifera (honey bees) and native Andrena spp. (subgenus Melandrena) bee populations in five commercial orchards. We identified viral presence across sites and bees and related these data to measures of bee community diversity. All viruses were found in both managed and native bees, and BQCV was the most common virus in each. To establish evidence for viral crossover between taxa, we undertook an additional examination of BQCV where 74 samples were sequenced and placed in a global phylogenic framework of hundreds of BQCV strains. We demonstrate pathogen sharing across managed honey bees and distantly related wild bees. This phylogenetic analysis contributes to growing evidence for host switching and places local incidence patterns in a worldwide context, revealing multispecies viral transmission.

Pollinivory and the diversification dynamics of bees

Pollinivory—the consumption of pollen rather than arthropod prey—is a defining feature of bees (Anthophila; the flower lovers). In virtually all bee species, larvae consume a diet composed of pollen mixed with nectar or floral oils. Bees arose from within a group of solitary, carnivorous, apoid wasps in the Early to Mid-Cretaceous, coincident with the rapid rise of flowering plants. It is assumed that the switch from carnivory to pollen-feeding was a key innovation that led to the rapid diversification of bees, but this has never been examined empirically. Here, we explore the hypothesis that pollinivory led to the increased diversification of bees. In contrast to common perception, we find that the switch to pollen-feeding per se does not explain their extensive diversification. Rather, our results indicate that pollinivory was a necessary but not sufficient condition for diversification, and that other complementary innovations, such as a broadening of host-plant diet, allowed the diversification of the major bee lineages. Our results have broad implications for understanding tempo and mode of bee diversification dynamics in light of their floral resources.

Combining transcriptomes and ultraconserved elements to illuminate the phylogeny of Apidae

Two increasingly popular approaches to reconstruct the Tree of Life involve whole transcriptome sequencing and the target capture of ultraconserved elements (UCEs). Both methods can be used to generate large, multigene datasets for analysis of phylogenetic relationships in non-model organisms. While targeted exon sequencing across divergent lineages is now a standard method, it is still not clear if UCE data can be readily combined with published transcriptomes. In this study, we evaluate the combination of UCEs and transcriptomes in a single analysis using genome-, transcriptome-, and UCE data for 79 bees in the largest and most biologically diverse bee family, Apidae. Using existing tools, we first developed a workflow to assemble phylogenomic data from different sources and produced two large nucleotide matrices of combined data. We then reconstructed the phylogeny of the Apidae using concatenation- and coalescent-based methods, and critically evaluated the resulting phylogenies in the context of previously published genetic, genomic, and morphological data sets. Our estimated phylogenetic trees are robustly supported and largely congruent with previous molecular hypotheses, from deep nodes to shallow species-level phylogenies. Moreover, the combined approach allows us to resolve controversial nodes of the apid Tree of Life, by clarifying the relationships among the genera of orchid bees (Euglossini) and the monophyly of the Centridini. Additionally, we present novel phylogenetic evidence supporting the monophyly of the diverse clade of cleptoparasitic Apidae and the placement of two enigmatic, oil-collecting genera (Ctenoplectra and Tetrapedia). Lastly, we propose a revised classification of the family Apidae that reflects our improved understanding of apid higher-level relationships.

Functional Imbalance of Anterolateral Entorhinal Cortex and Hippocampal Dentate/CA3 Underlies Age-Related Object Pattern Separation Deficits

The entorhinal cortex (EC) is among the earliest brain areas to deteriorate in Alzheimer's disease (AD). However, the extent to which functional properties of the EC are altered in the aging brain, even in the absence of clinical symptoms, is not understood. Recent human fMRI studies have identified a functional dissociation within the EC, similar to what is found in rodents. Here, we used high-resolution fMRI to identify a specific hypoactivity in the anterolateral EC (alEC) commensurate with major behavioral deficits on an object pattern separation task in asymptomatic older adults. Only subtle deficits were found in a comparable spatial condition, with no associated differences in posteromedial EC between young and older adults. We additionally linked this condition to dentate/CA3 hyperactivity, and the ratio of activity between the regions was associated with object mnemonic discrimination impairment. These results provide novel evidence of alEC-dentate/CA3 circuit dysfunction in cognitively normal aged humans.

The impact of GC bias on phylogenetic accuracy using targeted enrichment phylogenomic data

The field of sequence based phylogenetic analyses is currently being transformed by novel hybrid-based targeted enrichment methods, such as the use of ultraconserved elements (UCEs). Rather than analyzing relationships among organisms using a small number of genes, these methods now allow us to evaluate relationships with many hundreds to thousands of individual gene loci. However, the inclusion of thousands of loci does not necessarily overcome the long-standing challenge of incongruence among phylogenetic trees derived from different genes or gene regions. One factor that impacts the level of incongruence in phylogenomic data sets is the level of GC bias. GC rich gene regions are prone to higher recombination rates than AT rich regions, driven by a process referred to as "GC biased gene conversion". As a result, high GC content can be negatively associated with phylogenetic accuracy, but the extent to which this impacts incongruence among UCEs is currently unstudied. We investigated the impact of GC content on phylogeny reconstruction using in silico captured UCE data for the corbiculate bees (Hymenoptera: Apidae). The phylogeny of this group has been the subject of extensive study, and incongruence among gene trees is thought to be a source of phylogenetic error. We conducted coalescent- and concatenation-based analyses of 810 individual gene loci from all 13 currently available bee genomes, including 8 corbiculate taxa. Both coalescent- and concatenation-based methods converged on a single topology for the corbiculate tribes. In contrast to concatenation, the coalescent-based methods revealed significant topological conflict at nodes involving the orchid bees (Euglossini) and honeybees (Apini). Partitioning the loci by GC content reveals decreasing support for the inferred topology with increasing GC bias. Based on the results of this study, we report the first evidence that GC biased gene conversion may contribute to topological incongruence in studies based on ultraconserved elements.

Disruption of amygdala-entorhinal-hippocampal network in late-life depression

Episodic memory deficits are evident in late-life depression (LLD) and are associated with subtle synaptic and neurochemical changes in the medial temporal lobes (MTL). However, the particular mechanisms by which memory impairment occurs in LLD are currently unknown. We tested older adults with (DS+) and without (DS-) depressive symptoms using high-resolution fMRI that is capable of discerning signals in hippocampal subfields and amygdala nuclei. Scanning was conducted during performance of an emotional discrimination task used previously to examine the relationship between depressive symptoms and amygdala-mediated emotional modulation of hippocampal pattern separation in young adults. We found that hippocampal dentate gyrus (DG)/CA3 activity was reduced during correct discrimination of negative stimuli and increased during correct discrimination of neutral items in DS+ compared to DS- adults. The extent of the latter increase was correlated with symptom severity. Furthermore, DG/CA3 and basolateral amygdala (BLA) activity predicted discrimination performance on negative trials, a relationship that depended on symptom severity. The impact of the BLA on depressive symptom severity was mediated by the DG/CA3 during discrimination of neutral items, and by the lateral entorhinal cortex (LEC) during false recognition of positive items. These results shed light on a novel mechanistic account for amygdala-hippocampal network changes and concurrent alterations in emotional episodic memory in LLD. The BLA-LEC-DG/CA3 network, which comprises a key pathway by which emotion modulates memory, is specifically implicated in LLD. © 2017 Wiley Periodicals, Inc.

Repetition reveals ups and downs of hippocampal, thalamic, and neocortical engagement during mnemonic decisions

The extent to which current information is consistent with past experiences and our capacity to recognize or discriminate accordingly are key factors in flexible memory-guided behavior. Despite a wealth of evidence linking hippocampal and neocortical computations to these phenomena, many important factors remain poorly understood. One such factor is repeated encoding of learned information. In this experiment, participants completed a task in which study stimuli were incidentally encoded either once or three separate times during high-resolution fMRI scanning. We asked how repetition influenced recognition and discrimination memory judgments, and how this affects engagement of hippocampal and neocortical regions. Repetition revealed shifts in engagement in an anterior (ventral) CA1-thalamic-medial prefrontal network related to true and false recognition. Conversely, repetition revealed shifts in a posterior (dorsal) dentate/CA3-parahippocampal-restrosplenial network related to accurate discrimination. These differences in engagement were accompanied by task-related correlations in respective anterior and posterior networks. In particular, the anterior thalamic region observed during recognition judgments is functionally and anatomically consistent with nucleus reuniens in humans, and was found to mediate correlations between the anterior CA1 and medial prefrontal cortex. These findings offer new insights into how repeated experience affects memory and its neural substrates in hippocampal-neocortical networks. © 2016 Wiley Periodicals, Inc.

Age-related individual variability in memory performance is associated with amygdala-hippocampal circuit function and emotional pattern separation

While aging is generally associated with episodic memory decline, not all older adults exhibit memory loss. Furthermore, emotional memories are not subject to the same extent of forgetting and appear preserved in aging. We conducted high-resolution fMRI during a task involving pattern separation of emotional information in older adults with and without age-related memory impairment (characterized by performance on a word-list learning task: low performers: LP vs. high performers: HP). We found signals consistent with emotional pattern separation in hippocampal dentate (DG)/CA3 in HP but not in LP individuals, suggesting a deficit in emotional pattern separation. During false recognition, we found increased DG/CA3 activity in LP individuals, suggesting that hyperactivity may be associated with overgeneralization. We additionally observed a selective deficit in basolateral amygdala-lateral entorhinal cortex-DG/CA3 functional connectivity in LP individuals during pattern separation of negative information. During negative false recognition, LP individuals showed increased medial temporal lobe functional connectivity, consistent with overgeneralization. Overall, these results suggest a novel mechanistic account of individual differences in emotional memory alterations exhibited in aging.

Differences in Perceptual-Motor Measures in Children with Good and Poor Handwriting

The concept that handwriting is primarily a perceptual-motor act is held by various authors (Chapman & Wedell, 1972; Furner, 1969; Sovick, 1975; Ziviani, Hayes, & Chant, 1990). However, the assumption that poor handwriters would perform less well than good handwriters on perceptual-motor tests has not yet been well researched. The purpose of this study was to examine this assumption as well as the relationship of perceptual-motor abilities to the legibility of handwriting. One hundred forty-three Chinese children in grades 3 through 5 served as subjects. Perceptual-motor tests that measured the abilities proposed to be subskills of handwriting were administered along with a handwriting test. Results showed that poor handwriters scored more poorly than good handwriters on most of the perceptual-motor tests. Regression analysis revealed that among the perceptual-motor measures visual-motor integration, as measured by the Developmental Test of Visual-Motor Integration, and eye-hand coordination, as measured by the Motor Accuracy Test, contributed most to the legibility of handwriting for the total group of handwriters. However, for poor handwriters, results of a stepwise regression analysis revealed that motor planning, as measured by the Finger Position Imitation Test, contributed the most to the legibility of handwriting. In contrast, for good handwriters, visual perception, as measured by the Test of Visual Perceptual Skills, contributed most to the legibility of handwriting.

The Effect of Strategy on the Recall of the Rey-Osterrieth Complex Figure in Children with or without Learning Disabilities

This study was designed to examine the effect of two copying strategies on children's recall of the Rey-Osterrieth Complex Figure (ROCF). The study included 75 children with learning disabilities (LD) and 75 typical children (NC). Children were assigned to either a three-step strategy (condition A), a seven-step strategy (condition B), or a no-strategy condition (condition C), with 25 children in the LD and NC groups in each condition. Children copied the ROCF and then drew it from memory immediately after. The results showed that the NC group performed better than the LD group on recall accuracy, and the groups receiving a strategy performed better than groups without a strategy. For recall organization, children in conditions A and B did better than children in condition C. However, there was no significant difference between the NC and the LD groups in condition A. For recall style, more drawings were “configurational” when a strategy was provided for both the LD and NC groups. This study confirmed that breaking down the steps of a complex task may facilitate its learning.

Positivity effect specific to older adults with subclinical memory impairment

Numerous studies have suggested that older adults preferentially remember positive information (“positivity effect”), however others have reported mixed results. One potential source of conflict is that aging is not a unitary phenomenon and individual differences exist. We modified a standard neuropsychological test to vary emotional content and tested memory at three time points (immediate/20 min/1 wk). Cognitively normal older adults were stratified into those with and without subclinical memory impairment. We found that the positivity effect was limited to those with subclinical memory impairment, suggesting that consideration of subclinical memory impairment is necessary for understanding age-related emotional memory alterations.

Greater loss of object than spatial mnemonic discrimination in aged adults

Previous studies across species have established that the aging process adversely affects certain memory-related brain regions earlier than others. Behavioral tasks targeted at the function of vulnerable regions can provide noninvasive methods for assessing the integrity of particular components of memory throughout the lifespan. The present study modified a previous task designed to separately but concurrently test detailed memory for object identity and spatial location. Memory for objects or items is thought to rely on perirhinal and lateral entorhinal cortices, among the first targets of Alzheimer's related neurodegeneration. In line with prior work, we split an aged adult sample into "impaired" and "unimpaired" groups on the basis of a standardized word-learning task. The "impaired" group showed widespread difficulty with memory discrimination, whereas the "unimpaired" group showed difficulty with object, but not spatial memory discrimination. These findings support the hypothesized greater age-related impacts on memory for objects or items in older adults, perhaps even with healthy aging. © 2015 Wiley Periodicals, Inc.

Dose-response effects of the text4baby mobile health program: randomized controlled trial

BACKGROUND

Mobile health (mHealth) is growing rapidly, but more studies are needed on how to optimize programs, including optimal timing of messaging, dose of exposure, and value of interactive features. This study evaluates final outcomes of text4baby (a text message service for pregnant and postpartum women) from a randomized trial performed in a population of pregnant female soldiers and family members.

OBJECTIVE

The study aims were to evaluate (1) treatment effects and (2) dose-response effects of text4baby on behavioral outcomes compared to control (no text4baby) condition.

METHODS

The study was a randomized trial of text4baby at Madigan Army Medical Center. Female military health beneficiaries who met inclusion criteria were eligible for the study. Participants provided consent, completed a baseline questionnaire, and then were randomized to enroll in text4baby or not. They were followed up at 3 time points thereafter through delivery of their baby. Generalized estimating equation models were used to evaluate outcomes. We examined treatment effects and the effects of higher doses of text4baby messages on outcomes.

RESULTS

We report descriptive statistics including dosage of text messages delivered. The main finding was a significant effect of high exposure to text4baby on self-reported alcohol consumption postpartum (OR 0.212, 95% CI 0.046-0.973, P=.046), as measured by the question "Since you found out about your pregnancy, have you consumed alcoholic beverages?" The text4baby participants also reported lower quantities of alcohol consumed postpartum.

CONCLUSIONS

Studies of text4baby have helped to build the mHealth evidence base. The effects of text4baby offer lessons for future scalable mHealth programs and suggest the need to study dose-response effects of these interventions.

Ancient host shifts followed by host conservatism in a group of ant parasitoids

While ant colonies serve as host to a diverse array of myrmecophiles, few parasitoids are able to exploit this vast resource. A notable exception is the wasp family Eucharitidae, which is the only family of insects known to exclusively parasitize ants. Worldwide, approximately 700 Eucharitidae species attack five subfamilies across the ant phylogeny. Our goal is to uncover the pattern of eucharitid diversification, including timing of key evolutionary events, biogeographic patterns and potential cophylogeny with ant hosts. We present the most comprehensive molecular phylogeny of Eucharitidae to date, including 44 of the 53 genera and fossil-calibrated estimates of divergence dates. Eucharitidae arose approximately 50 Ma after their hosts, during the time when the major ant lineages were already established and diversifying. We incorporate host association data to test for congruence between eucharitid and ant phylogenies and find that their evolutionary histories are more similar than expected at random. After a series of initial host shifts, clades within Eucharitidae maintained their host affinity. Even after multiple dispersal events to the New World and extensive speciation within biogeographic regions, eucharitids remain parasitic on the same ant subfamilies as their Old World relatives, suggesting host conservatism despite access to a diverse novel ant fauna.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Monkeys with rhinal cortex damage or neurotoxic hippocampal lesions are impaired on spatial scene learning and object reversals

Rhesus monkeys (Macaca mulatta) with lesions of the rhinal cortex or parahippocampal gyrus (made by aspiration) or hippocampus (made with ibotenic acid) and unoperated controls were tested on object discrimination and reversal, place discrimination and reversal, and spatial scene learning to determine the contribution of these temporal lobe structures to these forms of learning and memory. Rhinal cortex lesions produced a severe deficit in object reversal learning; hippocampal lesions produced a milder deficit. Monkeys with rhinal cortex removals and those with hippocampal lesions were equally impaired on spatial scene learning. None of the lesions impaired place discrimination or reversal. These results argue against the idea that the mnemonic contributions of the rhinal cortex and hippocampus are limited to object and spatial domains, respectively.

The SIV-infected rhesus monkey model for HIV-associated dementia and implications for neurological diseases

The neuropathogenesis of human immunodeficiency virus (HIV)-associated dementia has remained elusive, despite identification of HIV as the causal agent. Although a number of contributing factors have been identified, the series of events that culminate in motor and cognitive impairments after HIV infection of the central nervous system (CNS) are still not known. Rhesus monkeys infected with simian immunodeficiency virus (SIV) manifest immunosuppression and CNS disease that is pathologically [L. R. Sharer et al. (1991) J. Med. Primatol. 20, 211-217] and behaviorally [E. A. Murray et al. (1992) Science 255, 1246-1249] similar to humans. The SIV model of HIV-associated dementia (HAD) is widely recognized as a highly relevant model in which to investigate neuropathogenesis. With better understanding of neuropathogenesis comes the opportunity to interrupt progression and to design better treatments for HAD. This becomes increasingly important as patients live longer yet still harbor HIV-infected cells in the CNS. The use of the SIV model has allowed the identification of neurochemical markers of neuropathogenesis important not only for HAD, but also for other inflammatory neurological diseases.

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.

Rhinal cortex ablations fail to disrupt reinforcer devaluation effects in rhesus monkeys (Macaca mulatta)

Studies have shown that excitotoxic lesions of the amygdala attenuate reinforcer devaluation effects in monkeys and rats. Because the rhinal (i.e., entorhinal and perirhinal) cortex has prominent reciprocal connections with the amygdala and has been suggested to store knowledge about objects, it is possible that it too composes part of the critical circuitry subserving learning about objects and their associated reinforcement value. To test this possibility, rhesus monkeys with rhinal cortex removals as well as unoperated controls were tested using a reinforcer devaluation procedure. Monkeys with rhinal cortex removals and controls, unlike those with amygdala lesions, tended to avoid displacing objects overlying a devalued food. These results indicate that the rhinal cortex is not a critical part of the neural circuitry mediating the effects of reinforcer devaluation.

Monkeys with rhinal cortex damage or neurotoxic hippocampal lesions are impaired on spatial scene learning and object reversals

Rhesus monkeys (Macaca mulatta) with lesions of the rhinal cortex or parahippocampal gyrus (made by aspiration) or hippocampus (made with ibotenic acid) and unoperated controls were tested on object discrimination and reversal, place discrimination and reversal, and spatial scene learning to determine the contribution of these temporal lobe structures to these forms of learning and memory. Rhinal cortex lesions produced a severe deficit in object reversal learning; hippocampal lesions produced a milder deficit. Monkeys with rhinal cortex removals and those with hippocampal lesions were equally impaired on spatial scene learning. None of the lesions impaired place discrimination or reversal. These results argue against the idea that the mnemonic contributions of the rhinal cortex and hippocampus are limited to object and spatial domains, respectively.

Selective hippocampal lesions yield nonspatial memory impairments in rhesus monkeys

Monkeys with removals of medial temporal lobe (MTL) structures are widely recognized as valid models of human global anterograde amnesia, a syndrome that arises consequent to damage to a finite set of brain structures situated in the medial temporal lobe and/or medial diencephalon. However, a comparison of memory deficits in human and nonhuman primates with MTL damage has presented a long-standing puzzle. Whereas amnesic patients are impaired in learning object discrimination problems, monkeys with MTL damage are typically not. One possible explanation for this difference is that object discrimination tasks for humans and monkeys differ in that the former but not the latter requires the use of contextual information. If this analysis is correct, monkeys with MTL damage might be disadvantaged in learning to discriminate similar objects presented in different contexts. To test this possibility, we evaluated the effects of excitotoxic lesions of one of the MTL structures, the hippocampus, on the rate of learning of discrimination problems embedded within unique contexts. Monkeys with hippocampal lesions were impaired relative to controls in learning object discrimination problems of this type. These findings strongly support the idea that the difference in the effect on object memory of MTL damage in human and nonhuman primates is due to a difference in the opportunity to employ contextual cues rather than to a difference in the organization of memory.

Object recognition and location memory in monkeys with excitotoxic lesions of the amygdala and hippocampus

Earlier work indicated that combined but not separate removal of the amygdala and hippocampus, together with the cortex underlying these structures, leads to a severe impairment in visual recognition. More recent work, however, has shown that removal of the rhinal cortex, a region subjacent to the amygdala and rostral hippocampus, yields nearly the same impairment as the original removal. This raises the possibility that the earlier results were attributable to combined damage to the rostral and caudal portions of the rhinal cortex rather than to the combined amygdala and hippocampal removal. To test this possibility, we trained rhesus monkeys on delayed nonmatching-to-sample, a measure of visual recognition, gave them selective lesions of the amygdala and hippocampus made with the excitotoxin ibotenic acid, and then assessed their recognition abilities by using increasingly longer delays and list lengths, including delays as long as 40 min. Postoperatively, monkeys with the combined amygdala and hippocampal lesions performed as well as intact controls at every stage of testing. The same monkeys also were unimpaired relative to controls on an analogous test of spatial memory, delayed nonmatching-to-location. It is unlikely that unintended sparing of target structures can account for the lack of impairment; there was a significant positive correlation between the percentage of damage to the hippocampus and scores on portions of the recognition performance test, suggesting that, paradoxically, the greater the hippocampal damage, the better the recognition. The results show that, within the medial temporal lobe, the rhinal cortex is both necessary and sufficient for visual recognition.

Rhinal cortex ablations fail to disrupt reinforcer devaluation effects in rhesus monkeys (Macaca mulatta)

Studies have shown that excitotoxic lesions of the amygdala attenuate reinforcer devaluation effects in monkeys and rats. Because the rhinal (i.e., entorhinal and perirhinal) cortex has prominent reciprocal connections with the amygdala and has been suggested to store knowledge about objects, it is possible that it too composes part of the critical circuitry subserving learning about objects and their associated reinforcement value. To test this possibility, rhesus monkeys with rhinal cortex removals as well as unoperated controls were tested using a reinforcer devaluation procedure. Monkeys with rhinal cortex removals and controls, unlike those with amygdala lesions, tended to avoid displacing objects overlying a devalued food. These results indicate that the rhinal cortex is not a critical part of the neural circuitry mediating the effects of reinforcer devaluation.

Aspiration lesions of the amygdala disrupt the rhinal corticothalamic projection system in rhesus monkeys

In macaque monkeys, aspiration but not excitotoxic lesions of the medial temporal lobe limbic structures, the amygdala and hippocampus, produce a severe impairment in visual recognition memory. Furthermore, certain ventromedial cortical regions, namely the rhinal (i.e., entorhinal and perirhinal) cortex, are now known to be critical for visual recognition memory. Because the route taken by temporal cortical efferent fibers, especially perirhinal efferents, passes nearby the amygdala, it is possible that inadvertent damage to these fibers is produced by the aspirative but not the excitotoxic process, thereby accounting at least in part for the different behavioral outcomes of the two types of lesion. To test this idea, we assessed the integrity of the rhinal corticothalamic projection system after aspiration lesions of the amygdala. Three rhesus monkeys with unilateral amygdala removals received bilaterally symmetrical injections of a retrograde fluorescent tracer into the medial portion of the mediodorsal nucleus of the thalamus. Retrogradely labeled cells were identified using conventional fluorescence microscopy techniques. In all three cases, the rhinal cortex of the intact hemispheres contained moderate numbers of retrogradely labeled cells. By contrast, the rhinal cortex of the amygdalectomized hemispheres consistently contained few retrogradely labeled cells, and a direct comparison of the two hemispheres showed this difference to be statistically significant. A similar asymmetric pattern was observed for area TE but not for the cortex lining the dorsal bank of the superior temporal sulcus, nor for the rostral cingulate motor area, which was examined as a control. The results indicate that aspiration lesions of the amygdala not only remove the cell bodies of the amygdala, as intended, but also inadvertently disrupt projection fibers arising from cells in the rhinal cortex and area TE that pass nearby or through the amygdala en route to the thalamus. Behavioral studies examining the effects of aspiration lesions of the amygdala in nonhuman primates need to take these findings into consideration.

Rhinal cortex removal produces amnesia for preoperatively learned discrimination problems but fails to disrupt postoperative acquisition and retention in rhesus monkeys

To test whether the rhinal cortex (i.e., entorhinal and perirhinal cortex) plays a time-limited role in information storage, eight rhesus monkeys were trained to criterion on two sets of 60 object discrimination problems, one set at each of two different time periods separated by 15 weeks. After the monkeys had learned both sets, two groups balanced for preoperative acquisition rates were formed. One group received bilateral ablation of the rhinal cortex (n = 4), and the other was retained as an unoperated control group (n = 4). After a 2 week rest period, monkeys were assessed for retention of the object discrimination problems. Retention was significantly poorer in monkeys with removals of the rhinal cortex relative to the controls (68 vs 91%). Although both groups showed slightly better retention of problems from the more recently learned set, there was no evidence of a differential effect of the cortical removal across sets (i.e., no temporal gradient). In addition, the monkeys with rhinal cortex lesions subsequently learned three new sets of 10 object discrimination problems as quickly as the controls did, thus ruling out the possibility of a gross impairment in visual perception or discrimination abilities. Furthermore, they retained these postoperatively learned object discriminations as well as the controls did. The findings indicate that the rhinal cortex is critical for the storage and/or retrieval of object discrimination problems that were learned up to 16 weeks before rhinal cortex ablation; however, in the absence of the rhinal cortex, efficient learning and retention of new discrimination problems can still occur.