Deficits In Pattern Separation Research Articles

Dentate Gyrus Integrity Is Necessary for Behavioral Pattern Separation But Not Statistical Learning.

Pattern separation, the creation of distinct representations of similar inputs, and statistical learning, the rapid extraction of regularities across multiple inputs, have both been linked to hippocampal processing. It has been proposed that there may be functional differentiation within the hippocampus, such that the trisynaptic pathway (entorhinal cortex > dentate gyrus > CA3 > CA1) supports pattern separation, whereas the monosynaptic pathway (entorhinal cortex > CA1) supports statistical learning. To test this hypothesis, we investigated the behavioral expression of these two processes in B. L., an individual with highly selective bilateral lesions in the dentate gyrus that presumably disrupt the trisynaptic pathway. We tested pattern separation with two novel auditory versions of the continuous mnemonic similarity task, requiring the discrimination of similar environmental sounds and trisyllabic words. For statistical learning, participants were exposed to a continuous speech stream made up of repeating trisyllabic words. They were then tested implicitly through a RT-based task and explicitly through a rating task and a forced-choice recognition task. B. L. showed significant deficits in pattern separation on the mnemonic similarity tasks and on the explicit rating measure of statistical learning. In contrast, B. L. showed intact statistical learning on the implicit measure and the familiarity-based forced-choice recognition measure. Together, these results suggest that dentate gyrus integrity is critical for high-precision discrimination of similar inputs, but not the implicit expression of statistical regularities in behavior. Our findings offer unique new support for the view that pattern separation and statistical learning rely on distinct neural mechanisms.

A Single Administration of Psilocybin Persistently Rescues Cognitive Deficits Caused by Adolescent Chronic Restraint Stress Without Long-Term Changes in Synaptic Protein Gene Expression in a Rat Experimental System with Translational Relevance to Depression

Introduction: Psilocybin has shown long-lasting antidepressant effects in preclinical and clinical trials, but the mechanisms responsible are unclear. As both passive coping strategies and pattern separation deficits are characteristics of major depression, we used adult female rats subjected to adolescent chronic restraint stress (aCRS) to investigate the effects of psilocybin on forced swim test (FST) and object pattern separation (OPS) behaviors 5 weeks after a single administration. Methods: Adolescent rats were randomly assigned to one of four treatment groups—not restrained/saline, not restrained/psilocybin, restrained/saline, and restrained/psilocybin. Restrained group rats were restrained for 1 h daily from day 1 through day 14. Saline and psilocybin were administered on day 21, OPS was evaluated on days 51–55, forced swim behavior was evaluated on day 57 or 58, and animals were sacrificed on day 63. Brains were removed and the medial prefrontal cortex, dorsal dentate gyrus, dorsal CA3 hippocampal area, and ventral hippocampus were microdissected out and prepared for mRNA analysis of a panel of genes relevant to synaptic plasticity using quantitative polymerase chain reaction. Results: Psilocybin rescued cognitive function in aCRS rats in both assays, but did not affect either measure in nonstressed rats. Immobility in the FST was correlated with impaired discrimination ability in the OPS. No differences in mRNA expression for a panel of genes related to structural synaptic proteins were observed between groups, although stress was a significant contributor to variability of the gene for glutamate metabotropic receptor 2 (Grm2) in two hippocampal regions. Conclusions: Our data indicate that aCRS and OPS represent a powerful system with translational relevance to study depression, and that a single treatment with psilocybin has long-lasting antidepressant-like effects without long-term alterations of mRNA related to synaptic density in brain areas relevant to depression.

APOE Genotype Moderates the Relationship Between Plasma Phosphorylated‐tau181 and Pattern Separation Performance in Non‐demented Adults

AbstractBackgroundThe apolioprotein e (APOE) e4 allele enhances risk for Alzheimer’s disease (AD), particularly an amnestic AD phenotype with medial temporal lobe tau pathology. Pattern separation, the ability to discern novel yet similar information from previously learned information, is a cognitive marker of hippocampal function. APOE e4 carriers often exhibit pattern separation deficits prior to conversion to AD dementia, however the impact of APOE on tau‐related pattern separation performance is unknown. We examined the relationship between plasma P‐tau181 and pattern separation performance in non‐demented adults, and tested whether this association was dependent on APOE e4 genotype.MethodsPlasma P‐tau181 levels were measured (Quanterix Simoa) in 171 non‐demented adults (139 cognitively unimpaired, 32 mild cognitive impairment; 29% APOE e4+, 54% female, aged 52‐92) who completed a validated behavioral task of visual pattern separation recognition. The lure discrimination index (LDI), reflecting ability to discriminate similar ‘lure’ items from old items, was modeled as the primary outcome. Linear regression examined LDI scores as a function of plasma P‐tau181, APOE4 genotype, and their interaction, controlling for age, sex, and education.ResultAPOE e4+ had higher plasma P‐tau181 than e4‐ (Cohen’s d=0.64, p<0.001), but APOE4 groups did not differ on LDI scores (Cohen’s d=0.27, p=0.124). In the full sample, higher plasma P‐tau181 significantly related to lower (worse) LDI scores (β=‐0.17, p=0.028). APOE4 status significantly moderated the relationship between plasma P‐tau181 and LDI scores (β=‐0.29, p=0.030) such that higher P‐tau181 related to lower LDI scores in e4+ (β=‐0.38, p=0.004) but not in e4‐ individuals (β=‐0.02, p=0.824).ConclusionsAPOE e4+ individuals had elevated plasma P‐tau181, which in turn related to worse pattern separation performance only within e4+ individuals. These data suggest that APOE4‐related elevations in plasma P‐tau181 may be particularly sensitive to hippocampal functioning outcomes prior to frank dementia, which could reflect underlying hippocampal hyperexcitability due to e4‐mediated GABAergic interneuron loss. Future work incorporating imaging markers of brain structure and function may help elucidate whether P‐tau181‐related alterations in pattern separation among e4+ individuals are mediated by hippocampal‐specific mechanisms.

Plasticity impairment alters community structure but permits successful pattern separation in a hippocampal network model.

Patients who suffer from traumatic brain injury (TBI) often complain of learning and memory problems. Their symptoms are principally mediated by the hippocampus and the ability to adapt to stimulus, also known as neural plasticity. Therefore, one plausible injury mechanism is plasticity impairment, which currently lacks comprehensive investigation across TBI research. For these studies, we used a computational network model of the hippocampus that includes the dentate gyrus, CA3, and CA1 with neuron-scale resolution. We simulated mild injury through weakened spike-timing-dependent plasticity (STDP), which modulates synaptic weights according to causal spike timing. In preliminary work, we found functional deficits consisting of decreased firing rate and broadband power in areas CA3 and CA1 after STDP impairment. To address structural changes with these studies, we applied modularity analysis to evaluate how STDP impairment modifies community structure in the hippocampal network. We also studied the emergent function of network-based learning and found that impaired networks could acquire conditioned responses after training, but the magnitude of the response was significantly lower. Furthermore, we examined pattern separation, a prerequisite of learning, by entraining two overlapping patterns. Contrary to our initial hypothesis, impaired networks did not exhibit deficits in pattern separation with either population- or rate-based coding. Collectively, these results demonstrate how a mechanism of injury that operates at the synapse regulates circuit function.

Loss of functional heterogeneity along the CA3 transverse axis in aging

Age-related deficits in pattern separation have been postulated to bias the output of hippocampal memory processing toward pattern completion, which can cause deficits in accurate memory retrieval. Although the CA3 region of the hippocampus is often conceptualized as a homogeneous network involved in pattern completion, growing evidence demonstrates a functional gradient in CA3 along the transverse axis, as pattern-separated outputs (dominant in the more proximal CA3) transition to pattern-completed outputs (dominant in the more distal CA3). We examined the neural representations along the CA3 transverse axis in young (Y), aged memory-unimpaired (AU), and aged memory-impaired (AI) rats when different changes were made to the environment. Functional heterogeneity in CA3 was observed in Y and AU rats when the environmental similarity was high (altered cues or altered environment shapes in the same room), with more orthogonalized representations in proximal CA3 than in distal CA3. In contrast, AI rats showed reduced orthogonalization in proximal CA3 but showed normal (i.e., generalized) representations in distal CA3, with little evidence of a functional gradient. Under experimental conditions when the environmental similarity was low (different rooms), representations in proximal and distal CA3 remapped in all rats, showing that CA3 of AI rats is able to encode distinctive representations for inputs with greater dissimilarity. These experiments support the hypotheses that the age-related bias toward hippocampal pattern completion is due to the loss in AI rats of the normal transition from pattern separation to pattern completion along the CA3 transverse axis.

Neurogenesis and chronic neurobehavioral outcomes are partially improved by vagus nerve stimulation in a mouse model of Gulf War illness

Gulf War illness (GWI) is a chronic, multi-symptom disorder that has impacted approximately one third of Gulf War veterans. GWI and its symptoms have been linked to the exposure to neurological chemicals, including the anti-nerve gas drug pyridostigmine bromide (PB) and the insecticide permethrin (PER), among others. Mouse models utilizing these chemicals have reported symptomology analogous to human GWI. These changes include behavioral and cognitive impairment, neuroinflammation and hippocampal pathogenesis. Disease modifying interventions that target these pathological components are desperately needed. Vagus nerve stimulation (VNS) is FDA approved for drug-resistant epilepsy and depression. VNS has also been used off-label to target a myriad of symptoms, some of which are encompassed within the Kansas and CDC definitions of clinical GWI symptomology. A GWI animal model in which mice are exposed to a daily injection of PB and PER for 10 consecutive days has been shown to exhibit cognitive impairment and hippocampal pathology. The purpose of this study was to determine if 2–4 weeks of continuous vagus nerve stimulation initiated at 32 weeks after exposure to PB and PER would improve cognitive performance and hippocampal pathology. The results of the study revealed that exposure to PB and PER produces long-term cognitive deficits and reduced hippocampal neurogenesis. The results also showed that the VNS treatment was anxiolytic, improved some aspects of pattern separation deficits, and mitigated the reduced hippocampal neurogenesis. Thus, VNS improves outcomes in a mouse model of GWI and should be examined as a potential therapeutic strategy for mitigating some symptomology associated with GWI.

Pattern separation and pattern completion for human face identity recognition in prodromal Alzheimer’s disease

AbstractBackgroundThe integrity of face identity recognition (FIR) in prodromal Alzheimer’s disease (prAD) is inconsistent in the literature, and the cognitive processes underpinning a potential impairment remain elusive. The concepts of pattern separation (PS) and pattern completion (PC) offer an interesting insight in this respect, since efficient FIR implies that facial identities are stored in at least partly distinct representations (corresponding to a PS process) and to fill‐in the currently perceived facial pattern based on these mnemonic representations (corresponding to the PC process). The goal of our study was therefore to investigate FIR in prAD patients with a PC and PS paradigm. In addition, we aimed to verify that prAD patients were able to individuate faces in a task that does not involve any long‐term memory component.MethodThirty‐two healthy older participants and 17 prAD patients were familiarized with 40 facial identities, through a picture‐matching task (Fig. 1.A). Participants then performed a forced‐choice recognition task aiming at assessing and strengthening the memory encoding of these 40 faces (Fig. 1.B). They then underwent separate yes/no recognition tasks involving parametrically controlled blurred or morphed faces (Fig. 2), designed to solicit PC and PS processes respectively (Fig. 1.C‐D). Finally, in two separate discrimination tasks (Fig. 1.E‐F), participants were instructed to determine whether two simultaneously displayed faces were identical or not. These discrimination tasks included the blurred and morphed faces displayed in the PC and PS recognition tasks respectively.ResultsprAD patients showed significantly lower performance than controls in both the PC and PS recognition tasks (Fig. 3). However, this group difference was not significant when the recognition accuracy index for the non‐manipulated faces (not blurred nor morphed) displayed in these tasks was included as a covariate. In addition, group differences were unexpectedly found for the familiarization and discrimination tasks.ConclusionOur study failed to highlight specific PC or PS deficits in prAD patients. However, it showed that prAD patients performed poorly on tasks that required comparing and matching simultaneously presented facial patterns, in the absence of any long‐term memory component. Future research should investigate whether this impairment could represent an early cognitive marker of AD that could be easily assessed in clinical practice.

Hippocampal sub-networks exhibit distinct spatial representation deficits in Alzheimer’s disease model mice

Not much is known about how the dentate gyrus (DG) and hippocampal CA3 networks, critical for memory and spatial processing, malfunction in Alzheimer's disease (AD). While studies of associative memory deficits in AD have focused mainly on behavior, here, we directly measured neurophysiological network dysfunction. We asked what the pattern of deterioration of different networks is during disease progression. We investigated how the associative memory-processing capabilities in different hippocampal subfields are affected by familial AD (fAD) mutations leading to amyloid-β dyshomeostasis. Specifically, we focused on the DG and CA3, which are known to be involved in pattern completion and separation and are susceptible to pathological alterations in AD. To identify AD-related deficits in neural-ensemble dynamics, we recorded single-unit activity in wild-type (WT) and fAD model mice (APPSwe+PSEN1/ΔE9) in a novel tactile morph task, which utilizes the extremely developed somatosensory modality of mice. As expected from the sub-network regional specialization, we found that tactile changes induced lower rate map correlations in the DG than in CA3 of WT mice. This reflects DG pattern separation and CA3 pattern completion. In contrast, in fAD model mice, we observed pattern separation deficits in the DG and pattern completion deficits in CA3. This demonstration of region-dependent impairments in fAD model mice contributes to understanding of brain networks deterioration during fAD progression. Furthermore, it implies that the deterioration cannot be studied generally throughout the hippocampus but must be researched at a finer resolution of microcircuits. This opens novel systems-level approaches for analyzing AD-related neural network deficits.

Deficits in pattern separation and dentate gyrus proliferation after rodent lateral fluid percussion injury.

It has been demonstrated that adult born granule cells are generated after traumatic brain injury (TBI). There is evidence that these newly generated neurons are aberrant and are poised to contribute to poor cognitive function after TBI. Yet, there is also evidence that these newly generated neurons are important for cognitive recovery. Pattern separation is a cognitive task known to be dependent on the function of adult generated granule cells. Performance on this task and the relation to dentate gyrus dysfunction after TBI has not been previously studied. Here we subjected Sprague Dawley rats to lateral fluid percussion injury or sham and tested them on the dentate gyrus dependent task pattern separation. At 2 weeks after injury, we examined common markers of dentate gyrus function such as GSK3ß phosphorylation, Ki-67 immunohistochemistry, and generation of adult born granule cells. We found that injured animals have deficits in pattern separation. We additionally found a decrease in proliferative capacity at 2 weeks indicated by decreased phosphorylation of GSK3ß and Ki-67 immunopositivity as compared to sham animals. Lastly we found an increase in numbers of new neurons generated during the pattern separation task. These findings provide evidence that dentate gyrus dysfunction may be an important contributor to TBI pathology.

Neurocognitive mechanisms of spatial pattern separation in older adults with and without subclinical memory impairment

AbstractBackgroundDecline in episodic memory is a hallmark feature of Alzheimer’s disease (AD). Episodic memories can be operationalized as detailed conjunctions of items and their spatiotemporal context. Our past work has uncovered deficits in pattern separation of similar object memories in nondemented older adults. However, separation of similar spatial locations was only slightly impaired in this population. We hypothesized that these deficits may be more readily observable in individuals with subclinical memory impairment (assessed by verbal list recall). We also sought to determine the neural mechanisms by which spatial pattern separation deficits may manifest in older adults, focusing on the posterior parietal network which is involved in spatial processing and implicated in AD.MethodsTwo cohorts were included in this analysis. The first is a group of older adults with subclinical memory impairment (aged impaired – AI, age range: 61‐91; mean MMSE: 28.3 ± 1.50; mean RAVLT delayed recall: 9.66 ± 4.05). The second is a group of non‐demented older adults without memory impairment (aged unimpaired – AU; age range: 61‐86; mean MMSE: 28.1 ± 1.59; RAVLT delayed recall: 11.4 ± 3.01). Participants underwent whole brain high‐resolution resting state functional magnetic resonance imaging (1.5 x 1.5 x 2 mm resolution). They were also administered a battery of neuropsychological tests including tests of object and spatial pattern separation. We used performance on the RAVLT as the outcome predicted by performance on the discrimination tests. Specifically, we used delayed recall (DR), retroactive interference (RI), and percent forgetting (PF).ResultsBehaviorally, spatial, but not item, pattern separation performance predicted RAVLT DR, RI and PF scores in the AI group, but not the AU group. Functional connectivity analyses revealed that the connectivity of regions involved in spatial processing was significantly related to spatial pattern separation performance in the AI group, but not the AU group.ConclusionThese results validate the neurobiological basis of spatial pattern separation and suggest that such tasks can be used to detect early stages of AD, which has important implications for future diagnostics and treatment approaches.

Genetic Alzheimer’s Disease Risk Affects the Neural Mechanisms of Pattern Separation in Hippocampal Subfields

The hippocampal subfields perform distinct operations during acquisition, differentiation, and recollection of episodic memories, and deficits in pattern separation are among the first symptoms of Alzheimer's disease (AD). We investigated how hippocampal subfields contribute to pattern separation and how this is affected by Apolipoprotein-E (APOE), the strongest AD genetic risk factor. Using ultra-high-field (7T) functional magnetic resonance imaging (fMRI), APOE-ε3-ε3 carriers predominantly recruited cornu ammonis 3 (CA3) during a spatial mnemonic discrimination task, whereas APOE-ε3-ε4 and APOE-ε3-ε2 carriers engaged CA3 and dentate gyrus (DG) to the same degree. Specifically, APOE-ε3-ε4 carriers showed reduced pattern separation in CA3, whereas APOE-ε3-ε2 carriers exhibited increased effects in DG and pattern separation-related functional connectivity between DG and CA3. Collectively, these results demonstrate that AD genetic risk alters hemodynamic responses in young pre-symptomatic individuals, paving the way for development of biomarkers for preclinical AD.

The enhancing effect of 200 mg caffeine on mnemonic discrimination is at best small

ABSTRACT Mnemonic discrimination is the ability to discriminate among similar memories, which requires separable representations of similar information. The neurocomputational process that assumedly decorrelates representations during encoding and consolidation is referred to as pattern separation. Deficits in pattern separation contribute to age-related declines in mnemonic functioning, which has motivated the development of targeted interventions. We followed-up a recent report that one 200 mg-dose of caffeine administered post-study enhances mnemonic discrimination [Borota, D., Murray, E., Keceli, G., Chang, A., Watabe, J. M., Ly, M., Toscano, J. P., & Yassa, M. A. (2014). Post-study caffeine administration enhances memory consolidation in humans. Nature Neuroscience, 17(2), 201–203. https://doi.org/10.1038/nn.3623]. To test whether the reported enhancements are an artifact of performance-impairing withdrawal symptoms in the control group, we did not restrict preexperimental caffeine intake and statistically adjusted treatment effects for habitual caffeine consumption. We detected no effects of caffeine and nonsuperiority testing ruled out medium and large enhancements in both average (1200 mg per week) and low-consumers (50 mg per week). Our results raise doubts about a caffeine-mediated enhancement of mnemonic discrimination on two counts: If the effect exists, it (1) is substantially smaller than originally reported and (2) may reflect an offset of performance-impairing withdrawal symptoms rather than genuinely enhanced consolidation. We recommend that future studies employ an alternating exposure-abstinence protocol, use an active control group, and verify posttreatment caffeine abstinence via saliva or blood samples.

Genetic Alzheimer's Disease Risk Affects the Neural Mechanisms of Pattern Separation in Hippocampal Subfields

The hippocampal subfields perform distinct operations during acquisition, differentiation and recollection of episodic memories, and deficits in pattern separation are among the first symptoms of Alzheimer’s disease (AD). We investigated how hippocampal subfields contribute to pattern separation and how this is affected by Apolipoprotein-E (APOE), the strongest AD genetic risk factor. We scanned young healthy participants at average, high, and low AD-risk during a spatial mnemonic discrimination task using ultra-high field (7T) functional magnetic resonance imaging (fMRI). Average AD-risk carriers predominantly recruited CA3 for the task, while high and low AD-risk carriers engaged CA3 and dentate gyrus (DG) to the same degree. Specifically, high AD-risk carriers showed reduced pattern separation in CA3, while low AD-risk carriers exhibited increased effects in DG and pattern separation-related CA3-DG functional connectivity. These results demonstrate that AD genetic risk alters hemodynamic responses in young asymptomatic individuals, paving way for development of biomarkers for preclinical AD.

One Dose of Psilocybin in Late Adolescence Mitigates Deleterious Effects of Developmental Stress on Cognition and Behavioral Despair in Adult Female Rats

IntroductionPsilocybin (PSI) has persistent antidepressant efficacy in human trials. We have shown one dose of PSI to significantly decrease depressive‐like behavior in male Wistar‐Kyoto (WKY) rats for at least five weeks without losing efficacy. However, the outcome assay we used to evaluate depressive‐like behavior, the forced swim test (FST), has been criticized for not providing circuit‐specific endpoint data. Further, rodent strains like WKY selectively bred for face validity in modeling depression have lower translational value than chronic/developmental stress models.Pattern separation is a function of the dentate gyrus (DG) and CA3 region of the hippocampus. Pattern separation deficits are measurable in a number of psychological and neurological disorders where the DG‐CA3 circuit is impaired, including major depressive disorder, schizophrenia, and age‐related dementias. The object pattern separation (OPS) task is a new paradigm to measure DG‐CA3 function in rodents, proposed as a cognitive‐based outcome measure for depression‐like phenotypes, but so far only used in healthy male animals. Whether OPS performance correlates with established measures of behavioral despair, or can be normalized by human pharmacotherapies, is unknown.ObjectivesEvaluate long‐term antidepressant‐like effects of PSI in the stress‐based adolescent chronic restraint stress (aCRS) model for depression, establish face and predictive validity of the OPS task, and determine whether OPS correlates with FST immobility in aCRS rats.MethodsAdolescent female Sprague Dawley rats were assigned to groups: not restrained‐saline (NRS), not restrained‐PSI (NRP), restrained‐saline (RS), and restrained‐PSI (RP). RS and RP rats were restrained 1 hr/day post‐natal days (PND) 32–45. Rats received IP PSI (1 mg/kg) or saline PND 52. The OPS task was performed nightly PND 82 (0 cm) – 86 (24 cm) in a 66 cm diameter arena containing two identical objects, one of which was moved from 0 cm to 6, 12, 18, and 24 cm from 0 following an hour intertrial interval. FST was performed PND 89–90.ResultsSignificant discrimination in the OPS at 24 cm was observed in NRS (p<0.001), NRP (p<0.0001), and RP (p<0.01), but RS rats were unable to discriminate between moved and stationary objects. RS rats were significantly more immobile in the FST than NRS rats (p<0.0001). No differences were observed between NRS, NRP, and RP rats. Immobility in the FST was inversely correlated to discrimination in the OPS at 24 cm (r=−0.402, p=0.023).ConclusionsThe OPS is a translationally relevant outcome with face and predictive validity, correlates with an established measure of depressive‐like behavior, and can be used in female rats.One dose of PSI in late adolescence mitigates cognitive and behavioral deficiencies characteristic of the developmentally‐stressed adult aCRS rat model for depression.Our aCRS model is uniquely suitable for elucidating the antidepressant mechanisms of PSI, which has translationally relevant antidepressant‐like effects that can be measured using the OPS task, many weeks after administration.Support or Funding InformationThis work supported by Eleusis, PBC.

Pilot investigation of the relationship between hippocampal volume and pattern separation deficits in multiple sclerosis

Memory impairment and hippocampal atrophy are common in multiple sclerosis (MS). In the present pilot study, we investigate whether the mnemonic process of pattern separation is impaired and a predictor of hippocampal volume in relapsing remitting MS. MS participants and healthy controls completed the Mnemonic Similarities Task (MST) along with traditional neurocognitive assessments of memory. 3T structural magnetic resonance imaging was used to estimate whole hippocampal volumes (main aim) and hippocampal subfield volumes (exploratory aim). Results revealed that pattern separation performance was worse for MS participants compared to healthy controls (Cohen's d = 0.96). For MS participants, hippocampal volume was more strongly related to pattern separation performance (r = 0.83) than a traditional neurocognitive measure of visual memory (r = 0.65); hippocampal volume was not related to a traditional neurocognitive measure of verbal memory in this sample. No brain-behavior correlations were significant in the healthy control group. Results of the exploratory analysis revealed relationships between pattern separation performance and subiculum, molecular layer, and CA1 hippocampal subfield volumes for MS participants only. Pattern separation assessed using the MST may be a sensitive cognitive-biomarker of memory dysfunction and changes in hippocampal volume in relapsing-remitting MS.

The location discrimination reversal task in mice is sensitive to deficits in performance caused by aging, pharmacological and other challenges.

Deficits in hippocampal-mediated pattern separation are one aspect of cognitive function affected in schizophrenia (SZ) or Alzheimer's disease (AD). To develop novel therapies, it is beneficial to explore this specific aspect of cognition preclinically. The location discrimination reversal (LDR) task is a hippocampal-dependent operant paradigm that evaluates spatial learning and cognitive flexibility using touchscreens. Here we assessed baseline performance as well as multimodal disease-relevant manipulations in mice. Mice were trained to discriminate between the locations of two images where the degree of separation impacted performance. Administration of putative pro-cognitive agents was unable to improve performance at narrow separation. Furthermore, a range of disease-relevant manipulations were characterized to assess whether performance could be impaired and restored. Pertinent to the cholinergic loss in AD, scopolamine (0.1 mg/kg) produced a disruption in LDR, which was attenuated by donepezil (1 mg/kg). Consistent with NMDA hypofunction in cognitive impairment associated with SZ, MK-801 (0.1 mg/kg) also disrupted performance; however, this deficit was not modified by rolipram. Microdeletion of genes associated with SZ (22q11) resulted in impaired performance, which was restored by rolipram (0.032 mg/kg). Since aging and inflammation affect cognition and are risk factors for AD, these aspects were also evaluated. Aged mice were slower to acquire the task than young mice and did not reach the same level of performance. A systemic inflammatory challenge (lipopolysaccharide (LPS), 1 mg/kg) produced prolonged (7 days) deficits in the LDR task. These data suggest that LDR task is a valuable platform for evaluating disease-relevant deficits in pattern separation and offers potential for identifying novel therapies.

APOE ε4 status in healthy older African Americans is associated with deficits in pattern separation and hippocampal hyperactivation.

African Americans are 1.4 times more likely than European Americans to carry the apolipoprotein E (APOE) ε4 allele, a risk factor for Alzheimer's disease (AD). However, little is known about the neural correlates of cognitive function in older African Americans and how they relate to genetic risk for AD. In particular, no past study on African Americans has examined the effect of APOE ε4 status on pattern separation-mnemonic discrimination performance and its corresponding neural computations in the hippocampus. Previous work using the mnemonic discrimination paradigm has localized increased activation in the DG/CA3 hippocampal subregions as being correlated with discrimination deficits. In a case-control high-resolution functional magnetic resonance imaging study of 30 healthy African Americans, aged 60 years and older, we observed APOE ε4-related impairments in mnemonic discrimination, coincident with dysfunctional hyperactivation in the DG/CA3, and CA1 regions, despite no evidence of structural differences in the hippocampus between carriers and noncarriers. Our results add to the growing body of evidence that deficits in pattern separation may be an early marker for AD-related neuronal dysfunction.

Age-related vulnerability of pattern separation in C57BL/6J mice

Aging is associated with impaired performance in behavioral pattern separation (PS) tasks based on similarities in object features and in object location. These deficits have been attributed to functional alterations in the dentate gyrus (DG)-CA3 region. Animal studies suggested a role of adult-born DG neurons in PS performance. The present study investigated the effect of aging in C57BL/6J mice performing PS tasks based on either object features or object location. At the age of 18 months or more, performance was severely impaired in both tasks. Spatial PS performance declined gradually over adult lifespan from 3 to 21 months. Subchronic treatment with the cognitive enhancer D-serine fully rescued spatial PS performance in 18-month-old mice and induced a modest increase in the number of 4-week-old adult-born cells in the DG. Performance of mice in these PS tasks shows an age dependence, which appears to translate well to that found in humans. This model should help in deciphering physiological changes underlying PS deficits and in identifying future therapeutic targets.

Pattern separation and goal-directed behavior in the aged canine.

The pattern separation task has recently emerged as a behavioral model of hippocampus function and has been used in several pharmaceutical trials. The canine is a useful model to evaluate a multitude of hippocampal-dependent cognitive tasks that parallel those in humans. Thus, this study was designed to evaluate the suitability of pattern separation task(s) for detecting age-related changes in canines. We also assessed the dogs’ ability to show pattern separation and discrimination reversal, which provides a novel extension of the pattern separation learning literature. Our data show that aged dogs are impaired on a complex pattern separation task (six-well task) relative to easier tasks (four-well or six-well pattern discrimination task), and that the age-related deficits are due to loss of perceptual and inhibitory control in addition to the loss of spatial discrimination and pattern separation ability. Our data also suggest that aged animals show pattern separation deficits when the objects are brought progressively closer together while changing the location of both correct and incorrect objects. However, if the location of any one object is fixed the animals tend to use alternate strategies. Overall, these data provide important insight into age-related pattern separation deficits in a higher animal model and offers additional means for evaluating the impact of lifestyle and pharmaceutical interventions on episodic memory in preclinical trials.

Dendrites of dentate gyrus granule cells contribute to pattern separation by controlling sparsity.

ABSTRACTThe hippocampus plays a key role in pattern separation, the process of transforming similar incoming information to highly dissimilar, nonverlapping representations. Sparse firing granule cells (GCs) in the dentate gyrus (DG) have been proposed to undertake this computation, but little is known about which of their properties influence pattern separation. Dendritic atrophy has been reported in diseases associated with pattern separation deficits, suggesting a possible role for dendrites in this phenomenon. To investigate whether and how the dendrites of GCs contribute to pattern separation, we build a simplified, biologically relevant, computational model of the DG. Our model suggests that the presence of GC dendrites is associated with high pattern separation efficiency while their atrophy leads to increased excitability and performance impairments. These impairments can be rescued by restoring GC sparsity to control levels through various manipulations. We predict that dendrites contribute to pattern separation as a mechanism for controlling sparsity. © 2016 The Authors Hippocampus Published by Wiley Periodicals, Inc.