The operation of pattern separation and pattern completion processes associated with different attributes or domains of memory

Volume of hippocampal subfields and episodic memory in childhood and adolescence

Episodic memory critically depends on the hippocampus to bind the features of an experience into memory. Episodic memory develops in childhood and adolescence, and hippocampal changes during this period may contribute to this development. Little is known, however, about how the hippocampus contributes to episodic memory development. The hippocampus is comprised of several cytoarchitectural subfields with functional significance for episodic memory. However, hippocampal subfields have not been assessed in vivo during child development, nor has their relation with episodic memory been assessed during this period. In the present study, high-resolution T2-weighted images of the hippocampus were acquired in 39 children and adolescents aged 8 to 14 years (M=11.30, SD=2.38), and hippocampal subfields were segmented using a protocol previously validated in adult populations. We first validated the method in children and adolescents and examined age-related differences in hippocampal subfields and correlations between subfield volumes and episodic memory. Significant age-related increases in the subfield volume were observed into early adolescence in the right CA3/DG and CA1. The right CA3/DG subfield volumes were positively correlated with accurate episodic memory for item-color relations, and the right CA3/DG and subiculum were negatively correlated with item false alarm rates. Subfield development appears to follow a protracted developmental trajectory, and likely plays a pivotal role in episodic memory development.

Human neuroimaging studies on the hippocampal CA3 region - integrating evidence for pattern separation and completion

Human functional magnetic resonance imaging (fMRI) studies have long investigated the hippocampus without differentiating between its subfields, even though theoretical models and rodent studies suggest that subfields support different and potentially even opposite functions. The CA3 region of the hippocampus has been ascribed a pivotal role both in initially forming associations during encoding and in reconstructing a memory representation based on partial cues during retrieval. These functions have been related to pattern separation and pattern completion, respectively. In recent years, studies using high-resolution fMRI in humans have begun to separate different hippocampal subregions and identify the role of the CA3 subregion relative to the other subregions. However, some of these findings have been inconsistent with theoretical models and findings from electrophysiology. In this review, we describe selected recent studies and highlight how their results might help to define different processes and functions that are presumably carried out by the CA3 region, in particular regarding the seemingly opposing functions of pattern separation and pattern completion. We also describe how these subfield-specific processes are related to behavioral, functional and structural alterations in patients with mild cognitive impairment and Alzheimer’s disease. We conclude with discussing limitations of functional imaging and briefly outline possible future developments of the field.

Asymmetric effects of emotion on mnemonic interference

Emotional experiences can strengthen memories so that they can be used to guide future behavior. Emotional arousal, mediated by the amygdala, is thought to modulate storage by the hippocampus, which may encode unique episodic memories via pattern separation--the process by which similar memories are stored using non-overlapping representations. While prior work has examined mnemonic interference due to similarity and emotional modulation of memory independently, examining the mechanisms by which emotion influences mnemonic interference has not been previously accomplished in humans. To this end, we developed an emotional memory task where emotional content and stimulus similarity were varied to examine the effect of emotion on fine mnemonic discrimination (a putative behavioral correlate of hippocampal pattern separation). When tested immediately after encoding, discrimination was reduced for similar emotional items compared to similar neutral items, consistent with a reduced bias towards pattern separation. After 24h, recognition of emotional target items was preserved compared to neutral items, whereas similar emotional item discrimination was further diminished. This suggests a potential mechanism for the emotional modulation of memory with a selective remembering of gist, as well as a selective forgetting of detail, indicating an emotion-induced reduction in pattern separation. This can potentially increase the effective signal-to-noise ratio in any given situation to promote survival. Furthermore, we found that individuals with depressive symptoms hyper-discriminate negative items, which correlated with their symptom severity. This suggests that utilizing mnemonic discrimination paradigms allows us to tease apart the nuances of disorders with aberrant emotional mnemonic processing.

Spatial discrimination deficits as a function of mnemonic interference in aged adults with and without memory impairment

It is well established that aging is associated with declines in episodic memory. In recent years, an emphasis has emerged on the development of behavioral tasks and the identification of biomarkers that are predictive of cognitive decline in healthy as well as pathological aging. Here, we describe a memory task designed to assess the accuracy of discrimination ability for the locations of objects. Object locations were initially encoded incidentally, and appeared in a single space against a 5 × 7 grid. During retrieval, subjects viewed repeated object-location pairings, displacements of 1, 2, 3, or 4 grid spaces, and maximal corner-to-opposite-corner displacements. Subjects were tasked with judging objects in this second viewing as having retained their original location, or having moved. Performance on a task such as this is thought to rely on the capacity of the individual to perform hippocampus-mediated pattern separation. We report a performance deficit associated with a physically healthy aged group compared to young adults specific to trials with low mnemonic interference. Additionally, for aged adults, performance on the task was correlated with performance on the delayed recall portion of the Rey Auditory Verbal Learning Test (RAVLT), a neuropsychological test sensitive to hippocampal dysfunction. In line with prior work, dividing the aged group into unimpaired and impaired subgroups based on RAVLT Delayed Recall scores yielded clearly distinguishable patterns of performance, with the former subgroup performing comparably to young adults, and the latter subgroup showing generally impaired memory performance even with minimal interference. This study builds on existing tasks used in the field, and contributes a novel paradigm for differentiation of healthy from possible pathological aging, and may thus provide an avenue for early detection of age-related cognitive decline.

Fear conditioning of SCR but not the startle reflex requires conscious discrimination of threat and safety

There is conflicting evidence as to whether awareness is required for conditioning of the skin conductance response (SCR). Recently, Schultz and Helmstetter (2010) reported SCR conditioning in contingency unaware participants by using difficult to discriminate stimuli. These findings are in stark contrast with other observations in human fear conditioning research, showing that SCR predominantly reflects contingency learning. Therefore, we repeated the study by Schultz and Helmstetter and additionally measured conditioning of the startle response, which seems to be less sensitive to declarative knowledge than SCR. While we solely observed SCR conditioning in participants who reported awareness of the contingencies (n = 16) and not in the unaware participants (n = 18), we observed startle conditioning irrespective of awareness. We conclude that SCR but not startle conditioning depends on conscious discriminative fear learning.

Memory-related eye movements challenge behavioral measures of pattern completion and pattern separation

The hippocampus creates distinct episodes from highly similar events through a process called pattern separation and can retrieve memories from partial or degraded cues through a process called pattern completion. These processes have been studied in humans using tasks where participants must distinguish studied items from perceptually similar lure items. False alarms to lures (incorrectly reporting a perceptually similar item as previously studied) are thought to reflect pattern completion, a retrieval-based process. However, false alarms to lures could also result from insufficient encoding of studied items, leading to impoverished memory of item details and a failure to correctly reject lures. The current study investigated the source of lure false alarms by comparing eye movements during the initial presentation of items to eye movements made during the later presentation of item repetitions and similar lures in order to assess mnemonic processing at encoding and retrieval, respectively. Relative to other response types, lure false alarms were associated with fewer fixations to the initially studied items, suggesting that false alarms result from impoverished encoding. Additionally, lure correct rejections and lure false alarms garnered more fixations than hits, denoting additional retrieval-related processing. The results suggest that measures of pattern separation and completion in behavioral paradigms are not process-pure.

Glycogen synthase kinase-3 inhibitors reverse deficits in long-term potentiation and cognition in fragile X mice

BACKGROUND

Identifying feasible therapeutic interventions is crucial for ameliorating the intellectual disability and other afflictions of fragile X syndrome (FXS), the most common inherited cause of intellectual disability and autism. Hippocampal glycogen synthase kinase-3 (GSK3) is hyperactive in the mouse model of FXS (FX mice), and hyperactive GSK3 promotes locomotor hyperactivity and audiogenic seizure susceptibility in FX mice, raising the possibility that specific GSK3 inhibitors may improve cognitive processes.

METHODS

We tested if specific GSK3 inhibitors improve deficits in N-methyl-D-aspartate receptor-dependent long-term potentiation at medial perforant path synapses onto dentate granule cells and dentate gyrus-dependent cognitive behavioral tasks.

RESULTS

GSK3 inhibitors completely rescued deficits in long-term potentiation at medial perforant path-dentate granule cells synapses in FX mice. Furthermore, synaptosomes from the dentate gyrus of FX mice displayed decreased inhibitory serine-phosphorylation of GSK3β compared with wild-type littermates. The potential therapeutic utility of GSK3 inhibitors was further tested on dentate gyrus-dependent cognitive behaviors. In vivo administration of GSK3 inhibitors completely reversed impairments in several cognitive tasks in FX mice, including novel object detection, coordinate and categorical spatial processing, and temporal ordering for visual objects.

CONCLUSIONS

These findings establish that synaptic plasticity and cognitive deficits in FX mice can be improved by intervention with inhibitors of GSK3, which may prove therapeutically beneficial in FXS.

The form and function of hippocampal context representations

Context is an essential component of learning and memory processes, and the hippocampus is critical for encoding contextual information. However, connecting hippocampal physiology with its role in context and memory has only recently become possible. It is now clear that contexts are represented by coherent ensembles of hippocampal neurons and new optogenetic stimulation studies indicate that activity in these ensembles can trigger the retrieval of context appropriate memories. We interpret these findings in the light of recent evidence that the hippocampus is critically involved in using contextual information to prevent interference, and propose a theoretical framework for understanding contextual influence on memory retrieval. When a new context is encountered, a unique hippocampal ensemble is recruited to represent it. Memories for events that occur in the context become associated with the hippocampal representation. Revisiting the context causes the hippocampal context code to be re-expressed and the relevant memories are primed. As a result, retrieval of appropriate memories is enhanced and interference from memories belonging to other contexts is minimized.

Dentate gyrus mediates cognitive function in the Ts65Dn/DnJ mouse model of Down syndrome

In the Ts65Dn/DnJ mouse model of Down syndrome (DS), hippocampal deficits of learning and memory are the most robust features supporting this mouse as a valid cognitive model of DS. Although dentate gyrus (DG) dysfunction is suggested by excessive GABAergic inhibition, its role in perturbing DG functions in DS is unknown. We hypothesize that in the Ts65Dn/DnJ mouse, the specific role of the DG is disturbed in its support of contextual and spatial information. Support for this hypothesis comes from rats with DG lesions that show similar deficits. In order to test this hypothesis, we have developed a novel series of spontaneous exploratory tasks that emphasize the importance of recognizing spatial and contextual cues and that involve DG function. The results with this exploratory battery show that Ts65Dn/DnJ mice are impaired in DG-dependent short-term recognition tests involving object recognition with contextual cues, in place recognition and in metric distance recognition relative to wild type littermate controls. Further, whereas Ts65Dn/DnJ mice can recognize object novelty in the absence of contextual cues after a 5-min delay, they cannot do so after a delay of 24 h, suggesting a problem with CA1-mediated consolidation. The results also show that Ts65Dn/DnJ mice are not impaired in tasks (object recognition and configural object recognition) that are mediated by the perirhinal cortex (PRh). These results implicate the DG as a specific therapeutic target and the PRh as a potential therapeutic strength for future research to ameliorate learning and memory in DS.

Adult neurogenesis in the mammalian hippocampus: why the dentate gyrus?

In the adult mammalian brain, newly generated neurons are continuously incorporated into two networks: interneurons born in the subventricular zone migrate to the olfactory bulb, whereas the dentate gyrus (DG) of the hippocampus integrates locally born principal neurons. That the rest of the mammalian brain loses significant neurogenic capacity after the perinatal period suggests that unique aspects of the structure and function of DG and olfactory bulb circuits allow them to benefit from the adult generation of neurons. In this review, we consider the distinctive features of the DG that may account for it being able to profit from this singular form of neural plasticity. Approaches to the problem of neurogenesis are grouped as "bottom-up," where the phenotype of adult-born granule cells is contrasted to that of mature developmentally born granule cells, and "top-down," where the impact of altering the amount of neurogenesis on behavior is examined. We end by considering the primary implications of these two approaches and future directions.

Selective importance of the rat anterior thalamic nuclei for configural learning involving distal spatial cues

To test potential parallels between hippocampal and anterior thalamic function, rats with anterior thalamic lesions were trained on a series of biconditional learning tasks. The anterior thalamic lesions did not disrupt learning two biconditional associations in operant chambers where a specific auditory stimulus (tone or click) had a differential outcome depending on whether it was paired with a particular visual context (spot or checkered wall-paper) or a particular thermal context (warm or cool). Likewise, rats with anterior thalamic lesions successfully learnt a biconditional task when they were reinforced for digging in one of two distinct cups (containing either beads or shredded paper), depending on the particular appearance of the local context on which the cup was placed (one of two textured floors). In contrast, the same rats were severely impaired at learning the biconditional rule to select a specific cup when in a particular location within the test room. Place learning was then tested with a series of go/no-go discriminations. Rats with anterior thalamic nuclei lesions could learn to discriminate between two locations when they were approached from a constant direction. They could not, however, use this acquired location information to solve a subsequent spatial biconditional task where those same places dictated the correct choice of digging cup. Anterior thalamic lesions produced a selective, but severe, biconditional learning deficit when the task incorporated distal spatial cues. This deficit mirrors that seen in rats with hippocampal lesions, so extending potential interdependencies between the two sites.

The mechanisms for pattern completion and pattern separation in the hippocampus

The mechanisms for pattern completion and pattern separation are described in the context of a theory of hippocampal function in which the hippocampal CA3 system operates as a single attractor or autoassociation network to enable rapid, one-trial, associations between any spatial location (place in rodents, or spatial view in primates) and an object or reward, and to provide for completion of the whole memory during recall from any part. The factors important in the pattern completion in CA3 together with a large number of independent memories stored in CA3 include a sparse distributed representation which is enhanced by the graded firing rates of CA3 neurons, representations that are independent due to the randomizing effect of the mossy fibers, heterosynaptic long-term depression as well as long-term potentiation in the recurrent collateral synapses, and diluted connectivity to minimize the number of multiple synapses between any pair of CA3 neurons which otherwise distort the basins of attraction. Recall of information from CA3 is implemented by the entorhinal cortex perforant path synapses to CA3 cells, which in acting as a pattern associator allow some pattern generalization. Pattern separation is performed in the dentate granule cells using competitive learning to convert grid-like entorhinal cortex firing to place-like fields. Pattern separation in CA3, which is important for completion of any one of the stored patterns from a fragment, is provided for by the randomizing effect of the mossy fiber synapses to which neurogenesis may contribute, by the large number of dentate granule cells each with a sparse representation, and by the sparse independent representations in CA3. Recall to the neocortex is achieved by a reverse hierarchical series of pattern association networks implemented by the hippocampo-cortical backprojections, each one of which performs some pattern generalization, to retrieve a complete pattern of cortical firing in higher-order cortical areas.

Temporal associations for spatial events: the role of the dentate gyrus

Previous research suggests that the dorsal dentate gyrus (DG) hippocampal subregion mediates spatial processing functions. However, a novel role for the DG in temporal processing for spatial information has begun to emerge based on the development of a computational model of neurogenesis. According to this model, adult born granule cells in the DG contribute to a temporal associative integration process for events presented closer in time. Currently, there is a paucity of behavioral evidence to support the temporal integration theory. Therefore, we developed a novel behavioral paradigm to investigate the role of the dDG in temporal integration for proximal and distal spatial events. Male Long-Evans rats were randomly assigned to a control group or to receive bilateral intracranial infusions of colchicine into the dDG. Following recovery from surgery, each rat was tested on a cued-recall of sequence paradigm. In this task, animals were allowed to explore identical objects placed in designated spatial locations on a cheeseboard maze across 2 days (e.g., Day 1: A and B; Day 2: C and D). One week later, animals were given a brief cue (A or C) followed by a preference test between spatial location B and D. Control animals had a significant preference for the spatial location previously paired with the cue (the temporal associate) whereas dDG lesioned animals failed to show a preference. These findings suggest that selective colchicine-induced dDG lesions are capable of disrupting the formation of temporal associations between spatial events presented close in time.

Pattern separation and pattern completion in Alzheimer's disease: evidence of rapid forgetting in amnestic mild cognitive impairment

Over the past four decades, the characterization of memory loss associated with Alzheimer's disease (AD) has been extensively debated. Recent iterations have focused on disordered encoding versus rapid forgetting. To address this issue, we used a behavioral pattern separation task to assess the ability of the hippocampus to create and maintain distinct and orthogonalized visual memory representations in patients with amnestic mild cognitive impairment (aMCI) and mild AD. We specifically used a lag-based continuous recognition paradigm to determine whether patients with aMCI and mild AD fail to encode visual memory representations or whether these patients properly encode representations that are rapidly forgotten. Consistent with the rapid forgetting hypothesis of AD, we found that patients with aMCI demonstrated decreasing pattern separation rates as the lag of interfering objects increased. In contrast, patients with AD demonstrated consistently poor pattern separation rates across three increasingly longer lags. We propose a continuum that reflects underlying hippocampal neuropathology whereby patients with aMCI are able to properly encode information into memory but rapidly lose these memory representations, and patients with AD, who have extensive hippocampal and parahippocampal damage, cannot properly encode information in distinct, orthogonal representations. Our results also revealed that whereas patients with aMCI demonstrated similar behavioral pattern completion rates to healthy older adults, patients with AD showed lower pattern completion rates when we corrected for response bias. Finally, these behavioral pattern separation and pattern completion results are discussed in terms of the dual process model of recognition memory.

Competitive Trace Theory: A Role for the Hippocampus in Contextual Interference during Retrieval

Much controversy exists regarding the role of the hippocampus in retrieval. The two dominant and competing accounts have been the Standard Model of Systems Consolidation (SMSC) and Multiple Trace Theory (MTT), which specifically make opposing predictions as to the necessity of the hippocampus for retrieval of remote memories. Under SMSC, memories eventually become independent of the hippocampus as they become more reliant on cortical connectivity, and thus the hippocampus is not required for retrieval of remote memories, only recent ones. MTT on the other hand claims that the hippocampus is always required no matter the age of the memory. We argue that this dissociation may be too simplistic, and a continuum model may be better suited to address the role of the hippocampus in retrieval of remote memories. Such a model is presented here with the main function of the hippocampus during retrieval being "recontextualization," or the reconstruction of memory using overlapping traces. As memories get older, they are decontextualized due to competition among partially overlapping traces and become more semantic and reliant on neocortical storage. In this framework dubbed the Competitive Trace Theory (CTT), consolidation events that lead to the strengthening of memories enhance conceptual knowledge (semantic memory) at the expense of contextual details (episodic memory). As a result, remote memories are more likely to have a stronger semantic representation. At the same time, remote memories are also more likely to include illusory details. The CTT is a novel candidate model that may provide some resolution to the memory consolidation debate.

The role of the dorsal dentate gyrus in object and object-context recognition

The aim of this study was to determine the role of the dorsal dentate gyrus (dDG) in object recognition memory using a black box and object-context recognition memory using a clear box with available cues that define a spatial context. Based on a 10 min retention interval between the study phase and the test phase, the results indicated that dDG lesioned rats are impaired when compared to controls in the object-context recognition test in the clear box. However, there were no reliable differences between the dDG lesioned rats and the control group for the object recognition test in the black box. Even though the dDG lesioned rats were more active in object exploration, the habituation gradients did not differ. These results suggest that the dentate gyrus lesioned rats are clearly impaired when there is an important contribution of context. Furthermore, based on a 24 h retention interval in the black box the dDG lesioned rats were impaired compared to controls.

Hippocampal and subicular efferents and afferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

Available evidence suggests there is functional differentiation among hippocampal and parahippocampal subregions and along the dorsoventral (septotemporal) axis of the hippocampus. The aim of this study was to characterize and compare the efferent and afferent connections of perirhinal areas 35 and 36, postrhinal cortex, and the lateral and medial entorhinal areas (LEA and MEA) with dorsal and ventral components of the hippocampal formation (dentate gyrus, hippocampus cornu ammonis fields, and subiculum) as well as the presubiculum, and the parasubiculum. The entorhinal connections were also characterized with respect to the LEA and MEA dentate gyrus-projecting bands. In general, the entorhinal connections with the hippocampal formation are much stronger than the perirhinal and postrhinal connections. The entorhinal cortex projects strongly to all components of the hippocampal formation, whereas the perirhinal and postrhinal cortices project weakly and only to CA1 and the subiculum. In addition, the postrhinal cortex preferentially targets the dorsal CA1 and subiculum, whereas the perirhinal cortex targets ventral subiculum. Similarly, the perirhinal cortex receives more input from ventral hippocampal formation structures and the postrhinal cortex receives more input from dorsal hippocampal structures. The LEA and the MEA medial band are more strongly interconnected with ventral hippocampal structures, whereas the MEA lateral band is more interconnected with dorsal hippocampal structures. With regard to the presubiculum and parasubiculum, the postrhinal cortex and the MEA lateral band receive stronger input from the dorsal presubiculum and caudal parasubiculum. In contrast, the LEA and MEA medial bands receive stronger input from the ventral presubiculum and rostral parasubiculum.

A quantitative theory of the functions of the hippocampal CA3 network in memory

A quantitative computational theory of the operation of the hippocampal CA3 system as an autoassociation or attractor network used in episodic memory system is described. In this theory, the CA3 system operates as a single attractor or autoassociation network to enable rapid, one-trial, associations between any spatial location (place in rodents, or spatial view in primates) and an object or reward, and to provide for completion of the whole memory during recall from any part. The theory is extended to associations between time and object or reward to implement temporal order memory, also important in episodic memory. The dentate gyrus (DG) performs pattern separation by competitive learning to produce sparse representations suitable for setting up new representations in CA3 during learning, producing for example neurons with place-like fields from entorhinal cortex grid cells. The dentate granule cells produce by the very small number of mossy fiber (MF) connections to CA3 a randomizing pattern separation effect important during learning but not recall that separates out the patterns represented by CA3 firing to be very different from each other, which is optimal for an unstructured episodic memory system in which each memory must be kept distinct from other memories. The direct perforant path (pp) input to CA3 is quantitatively appropriate to provide the cue for recall in CA3, but not for learning. Tests of the theory including hippocampal subregion analyses and hippocampal NMDA receptor knockouts are described, and support the theory.

Hippocampal inactivation with TTX impairs long-term spatial memory retrieval and modifies brain metabolic activity

Functional inactivation techniques enable studying the hippocampal involvement in each phase of spatial memory formation in the rat. In this study, we applied tetrodotoxin unilaterally or bilaterally into the dorsal hippocampus to evaluate the role of this brain structure in retrieval of memories acquired 28 days before in the Morris water maze. We combined hippocampal inactivation with the assessment of brain metabolism using cytochrome oxidase histochemistry. Several brain regions were considered, including the hippocampus and other related structures. Results showed that both unilateral and bilateral hippocampal inactivation impaired spatial memory retrieval. Hence, whereas subjects with bilateral hippocampal inactivation showed a circular swim pattern at the side walls of the pool, unilateral inactivation favoured swimming in the quadrants adjacent to the target one. Analysis of cytochrome oxidase activity disclosed regional differences according to the degree of hippocampal functional blockade. In comparison to control group, animals with bilateral inactivation showed increased CO activity in CA1 and CA3 areas of the hippocampus during retrieval, while the activity of the dentate gyrus substantially decreased. However, unilateral inactivated animals showed decreased CO activity in Ammon's horn and the dentate gyrus. This study demonstrated that retrieval recruits differentially the hippocampal subregions and the balance between them is altered with hippocampal functional lesions.

A process analysis of the CA3 subregion of the hippocampus

From a behavioral perspective, the CA3a,b subregion of the hippocampus plays an important role in the encoding of new spatial information within short-term memory with a duration of seconds and minutes. This can easily be observed in tasks that require rapid encoding, novelty detection, one-trial short-term or working memory, and one-trial cued recall primarily for spatial information. These are tasks that have been assumed to reflect the operations of episodic memory and require interactions between CA3a,b and the dentate gyrus (DG) via mossy fiber inputs into the CA3a,b. The CA3a,b is also important for encoding of spatial information requiring the acquisition of arbitrary and relational associations. All these tasks are assumed to operate within an autoassociative network function of the CA3 region. The CA3a,b also supports retrieval of short-term memory information based on a spatial pattern completion process. Based on afferent inputs into CA3a,b from the DG via mossy fibers and afferents from the entorhinal cortex into CA3a,b as well as reciprocal connections with the septum, CA3a,b can bias the process of encoding utilizing the operation of spatial pattern separation and the process of retrieval utilizing the operation of pattern completion. The CA3a,b also supports sequential processing of information in cooperation with CA1 based on the Schaffer collateral output from CA3a,b to CA1. The CA3c function is in part based on modulation of the DG in supporting pattern separation processes.

Cholinergic modulation of olfactory pattern separation

Pattern separation plays an important role in perception and memory. In olfaction, pattern separation is critical component of piriform cortical odor processing contributing to behavioral perception of overlapping odor mixtures. Previous work has demonstrated that odor discrimination ability is modulated by acetylcholine. Here, we extended this previous work by using a distinct, well characterized complex odor stimulus set that has been shown to differentially involve pattern separation processes within piriform cortex. We find that the cholinergic muscarinic receptor agonist oxotremorine facilitates the acquisition of odor discrimination. Furthermore, the muscarinic receptor antagonist scopolamine impairs acquisition of odor discrimination even if the antagonist is limited to the piriform cortex. Finally, acetylcholine effects are most robust during discrimination acquisition, with minimal effects during expression.

Essential role of the perirhinal cortex in complex tactual discrimination tasks in rats

We designed a battery of tactual discrimination tasks to study whether rats with perirhinal cortex (Prh) lesions had any deficit in resolving complex/ambiguous tactual tasks in the dark. Animals had to discriminate among 3 stimuli simultaneously exposed in 3 arms of a 4-arm plus-shaped maze. Rats with Prh lesions showed a profound impairment in a texture discrimination learning task when the stimuli had a high or intermediate degree of feature ambiguity (experiments 1a and 1b), but not when they had a low degree of feature ambiguity (experiment. 1c). Hippocampal lesions, however, did not cause any impairment in task acquisition even when the stimuli had a high degree of feature ambiguity (experiment 2). Experiments 3a, 3b, and 4 showed that perirhinal and control rats performed the task similarly when the animals had to discriminate on the basis of simple/individual, nonoverlapping features of the stimuli (size) with different levels of difficulty. Finally, to isolate the task's memory functions from its perceptual functions, a reversal learning task revealed a profound deficit in the initial learning phase, but unimpaired learning in the reversal phase with identical stimuli (experiment 5). The findings suggest that the Prh plays an essential role in somatosensory perceptual functions.

Functional circuits of new neurons in the dentate gyrus

The hippocampus is crucial for memory formation. New neurons are added throughout life to the hippocampal dentate gyrus (DG), a brain area considered important for differential storage of similar experiences and contexts. To better understand the functional contribution of adult neurogenesis to pattern separation processes, we recently used a novel synapse specific trans-neuronal tracing approach to identify the (sub) cortical inputs to new dentate granule cells (GCs). It was observed that newly born neurons receive sequential innervation from structures important for memory function. Initially, septal-hippocampal cells provide input to new neurons, including transient innervation from mature GCs as well as direct feedback from area CA3 pyramidal neurons. After about 1 month perirhinal (PRH) and lateral entorhinal cortex (LEC), brain areas deemed relevant to integration of novel sensory and environmental information, become substantial input to new GCs. Here, we review the developmental time-course and proposed functional relevance of new neurons, within the context of their unique neural circuitry.

Spatial pattern completion deficits in older adults

Aging may have an impact on the CA3 autoassociative network of the hippocampus, posited by computational models as supporting pattern completion. Twenty-five young (YAs) and 25 older adults (OAs) performed a spatial pattern completion task using a computerized navigational paradigm analogous to a rodent pattern completion task reliant on the CA3. Participants identified a previously seen goal location, and the availability of distal cues in the environment was manipulated such that 0, 2, or 4 cues were missing. Performance in both groups declined as a function of decreased cue availability. However, controlling for age differences in task performance during a pre-experimental baseline task, OAs performed equivalently to YAs when all cues were available, but worse than YAs as the number of cues decreased. These findings suggest spatial pattern completion may be impaired in OAs. We discuss these findings in the context of a growing body of literature suggesting age-related imbalances in pattern separation vs. pattern completion.