A limited role for the hippocampus in the modulation of novel-object preference by contextual cues

Sex differences in the context dependency of episodic memory

Context contributes to multiple aspects of human episodic memory including segmentation and retrieval. The present studies tested if, in adult male and female mice, context influences the encoding of odors encountered in a single unsupervised sampling session of the type used for the routine acquisition of episodic memories. The three paradigms used differed in complexity (single vs. multiple odor cues) and period from sampling to testing. Results show that males consistently encode odors in a context-dependent manner: the mice discriminated novel from previously sampled cues when tested in the chamber of initial cue sampling but not in a distinct yet familiar chamber. This was independent of the interval between cue encounters or the latency from initial sampling to testing. In contrast, female mice acquired both single cues and the elements of multi-cue episodes, but recall of that information was dependent upon the surrounding context only when the cues were presented serially. These results extend the list of episodic memory features expressed by rodents and also introduce a striking and unexpected sex difference in context effects.

Memory capacity and prioritization in female mice

Our brain's capacity for memory storage may be vast but is still finite. Given that we cannot remember the entirety of our experiences, how does our brain select what to remember and what to forget? Much like the triage of a hospital's emergency room, where urgent cases are prioritized and less critical patients receive delayed or even no care, the brain is believed to go through a similar process of memory triage. Recent salient memories are prioritized for consolidation, which helps create stable, long-term representations in the brain; less salient memories receive a lower priority, and are eventually forgotten if not sufficiently consolidated (Stickgold and Walker in Nat Neurosci 16(2):139-145, 2013). While rodents are a primary model for studying memory consolidation, common behavioral tests typically rely on a limited number of items or contexts, well within the memory capacity of the subject. A memory test allowing us to exceed an animal's memory capacity is key to investigating how memories are selectively strengthened or forgotten. Here we report a new serial novel object recognition task designed to measure memory capacity and prioritization, which we test and validate using female mice.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Assessing mild cognitive impairment using object-location memory in immersive virtual environments

Pathological changes in the medial temporal lobe (MTL) are found in the early stages of Alzheimer's disease (AD) and aging. The earliest pathological accumulation of tau colocalizes with the areas of the MTL involved in object processing as part of a wider anterolateral network. Here, we sought to assess the diagnostic potential of memory for object locations in iVR environments in individuals at high risk of AD dementia (amnestic mild cognitive impairment [aMCI] n = 23) as compared to age-related cognitive decline. Consistent with our primary hypothesis that early AD would be associated with impaired object location, aMCI patients exhibited impaired spatial feature binding. Compared to both older (n = 24) and younger (n = 53) controls, aMCI patients, recalled object locations with significantly less accuracy (p < .001), with a trend toward an impaired identification of the object's correct context (p = .05). Importantly, these findings were not explained by deficits in object recognition (p = .6). These deficits differentiated aMCI from controls with greater accuracy (AUC = 0.89) than the standard neuropsychological tests. Within the aMCI group, 16 had CSF biomarkers indicative of their likely AD status (MCI+ n = 9 vs. MCI- n = 7). MCI+ showed lower accuracy in the object-context association than MCI- (p = .03) suggesting a selective deficit in object-context binding postulated to be associated with anterior-temporal areas. MRI volumetric analysis across healthy older participants and aMCI revealed that test performance positively correlates with lateral entorhinal cortex volumes (p < .05) and hippocampus volumes (p < .01), consistent with their hypothesized role in binding contextual and spatial information with object identity. Our results indicate that tests relying on the anterolateral object processing stream, and in particular requiring successful binding of an object with spatial information, may aid detection of pre-dementia AD due to the underlying early spread of tau pathology.

Lateral entorhinal cortex lesions impair odor-context associative memory in male rats

Lateral entorhinal cortex (LEC) has been hypothesized to process nonspatial, item information that is combined with spatial information from medial entorhinal cortex to form episodic memories within the hippocampus. Recent studies, however, have demonstrated that LEC has a role in integrating features of episodic memory prior to the hippocampus. While the precise role of LEC is still unclear, anatomical studies show that LEC is ideally placed to be a hub integrating multisensory information. The current study tests whether the role of LEC in integrating information extends to long-term multimodal item-context associations. In Experiment 1, male rats were trained on a context-dependent odor discrimination task, where two different contexts served as the cue to the correct odor. Rats were pretrained on the task and then received either bilateral excitotoxic LEC or sham lesions. Following surgery, rats were tested on the previously learned odor-context associations. Control rats showed good memory for the previously learned association but rats with LEC lesions showed significantly impaired performance relative to both their own presurgery performance and to control rats. Experiment 2 went on to test whether impairments in Experiment 1 were the result of LEC lesions impairing either odor or context memory retention alone. Male rats were trained on simple odor and context discrimination tasks that did not require integration of features to solve. Following surgery, both LEC and control rats showed good memory for previously learned odors and contexts. These data show that LEC is critical for long-term odor-context associative memory.

Innate Preferences Affect Results of Object Recognition Task in Wild Type and Alzheimer's Disease Mouse Models

BACKGROUND

Object recognition task (ORT) is a widely used behavioral paradigm to assess memory in rodent models, due to its easy technical execution, the lack of aversive stressful stimuli, and the possibility to repeat the test on the same animals. However, mouse exploration might be strongly influenced by a variety of variables.

OBJECTIVE

To study whether innate preferences influenced exploration in male and female wild type mice and the Alzheimer's disease (AD) model 3xTg.

METHODS

We first evaluated how object characteristics (material, size, and shape) influence exploration levels, latency, and exploration modality. Based on these findings, we evaluated whether these innate preferences biased the results of ORT performed in wild type mice and AD models.

RESULTS

Assessment of Exploration levels, i.e., the time spent in exploring a certain object in respect to the total exploration time, revealed an innate preference for objects made in shiny materials, such as metal and glass. A preference for bigger objects characterized by higher affordance was also evident, especially in male mice. When performing ORT, exploration was highly influenced by these innate preferences. Indeed, both wild type and AD mice spent more time in exploring the metal object, regardless of its novelty. Furthermore, the use of objects with higher affordance such as the cube was a confounding factor leading to "false" results that distorted ORT interpretation.

CONCLUSION

When designing exploration-based behavioral experiments aimed at assessing memory in healthy and AD mice, object characteristics should be carefully evaluated to improve scientific outcomes and minimize possible biases.

Unfolding of spatial representation at systems level in infant rats

Spatial representations enable navigation from early life on. However, the brain regions essential to form spatial representations, like the hippocampus, are considered functionally immature before weaning. Here, we examined the formation of representations of space in rat pups on postnatal day (PD) 16, using a simple habituation paradigm where the pups were exposed to an arena on three occasions, separated by ~140 min. Whereas on the first two occasions the arena was the same, on the third "test" occasion either proximal cues (Prox group), or distal cues (Dist group), or proximal and distal cues (Prox-Dist group), or no cues (No-change group) were rearranged. Locomotion (distance traveled) was used as behavioral measure of habituation, and c-Fos expression to measure regional brain activity at test. Locomotion generally decreased across the first two occasions. At test, it reached a minimum in the No-change group, indicating familiarity with the spatial conditions. By contrast, the Prox-Dist group displayed a significant increase in locomotion which was less robust in the Prox group and absent in the Dist group, a pattern suggesting that the pups relied more on proximal than distal cues during spatial exploration. c-Fos activity in the No-change group was significantly suppressed in the hippocampus (CA1, CA3, dentate gyrus) but simultaneously enhanced in the prelimbic area (PL) of the medial prefrontal cortex, compared with untreated Home-cage controls, pointing to a possible involvement of the PL in regulating locomotion in familiar spaces. By contrast, in both Prox-Dist and Prox groups c-Fos activity was enhanced in hippocampal CA1 and CA3 regions, suggesting these regions might be particularly involved in regulating exploration of spatial novelty. Our findings show that functional representations of space at a systems level are formed already in pre-weanling rats.

Loss of α7 nicotinic acetylcholine receptors in GABAergic neurons causes sex-dependent decreases in radial glia-like cell quantity and impairments in cognitive and social behavior

The dentate gyrus (DG) is a unique brain structure in that neurons can be generated postnatally and integrated within existing circuitry throughout life. The maturation process of these newly generated neurons (granule cells) is modulated by nicotinic acetylcholine receptors (nAChRs) through a variety of mechanisms such as neural stem pool proliferation, cell survival, signal modulation, and dendritic integration. Disrupted nAChR signaling has been implicated in neuropsychiatric and neurodegenerative disorders, potentially via alterations in DG neurogenesis. GABAergic interneurons are known to express nAChRs, predominantly the α7 subtype, and have been shown to shape development, integration, and circuit reorganization of DG granule cells. Therefore, we examined histological and behavioral effects of knocking out α7 nAChRs in GABAergic neurons. Deletion of α7 nAChRs resulted in a reduction of radial glia-like cells within the subgranular zone of the DG and a concomitant trend towards decreased immature neurons, specifically in male mice, as well as sex-dependent changes in several behaviors, including social recognition and spatial learning. Overall, these findings suggest α7 nAChRs expressed in GABAergic neurons play an important role in regulating the adult neural stem cell pool and behavior in a sex-dependent manner. This provides important insight into the mechanisms by which cholinergic dysfunction contributes to the cognitive and behavioral changes associated with neurodevelopmental and neurodegenerative disorders.

Spontaneous object-location memory based on environmental geometry is impaired by both hippocampal and dorsolateral striatal lesions

We examined the role of the hippocampus and the dorsolateral striatum in the representation of environmental geometry using a spontaneous object recognition procedure. Rats were placed in a kite-shaped arena and allowed to explore two distinctive objects in each of the right-angled corners. In a different room, rats were then placed into a rectangular arena with two identical copies of one of the two objects from the exploration phase, one in each of the two adjacent right-angled corners that were separated by a long wall. Time spent exploring these two objects was recorded as a measure of recognition memory. Since both objects were in different locations with respect to the room (different between exploration and test phases) and the global geometry (also different between exploration and test phases), differential exploration of the objects must be a result of initial habituation to the object relative to its local geometric context. The results indicated an impairment in processing the local geometric features of the environment for both hippocampus and dorsolateral striatum lesioned rats compared with sham-operated controls, though a control experiment showed these rats were unimpaired in a standard object recognition task. The dorsolateral striatum has previously been implicated in egocentric route-learning, but the results indicate an unexpected role for the dorsolateral striatum in processing the spatial layout of the environment. The results provide the first evidence that lesions to the hippocampus and dorsolateral striatum impair spontaneous encoding of local environmental geometric features.

The medial prefrontal cortex - hippocampus circuit that integrates information of object, place and time to construct episodic memory in rodents: Behavioral, anatomical and neurochemical properties

Rats and mice have been demonstrated to show episodic-like memory, a prototype of episodic memory, as defined by an integrated memory of the experience of an object or event, in a particular place and time. Such memory can be assessed via the use of spontaneous object exploration paradigms, variably designed to measure memory for object, place, temporal order and object-location inter-relationships. We review the methodological properties of these tests, the neurobiology about time and discuss the evidence for the involvement of the medial prefrontal cortex (mPFC), entorhinal cortex (EC) and hippocampus, with respect to their anatomy, neurotransmitter systems and functional circuits. The systematic analysis suggests that a specific circuit between the mPFC, lateral EC and hippocampus encodes the information for event, place and time of occurrence into the complex episodic-like memory, as a top-down regulation from the mPFC onto the hippocampus. This circuit can be distinguished from the neuronal component memory systems for processing the individual information of object, time and place.

Paternal morphine self-administration produces object recognition memory deficits in female, but not male offspring

RATIONALE

Parental drug use around or before conception can have adverse consequences for offspring. Historically, this research has focused on the effects of maternal substance use on future generations but less is known about the influence of the paternal lineage. This study focused on the impact of chronic paternal morphine exposure prior to conception on behavioral outcomes in male and female progeny.

OBJECTIVES

This study sought to investigate the impact of paternal morphine self-administration on anxiety-like behavior, the stress response, and memory in male and female offspring.

METHODS

Adult, drug-naïve male and female progeny of morphine-treated sires and controls were evaluated for anxiety-like behavior using defensive probe burying and novelty-induced hypophagia paradigms. Hypothalamic-pituitary-adrenal (HPA) axis function was assessed by measuring plasma corticosterone levels following a restraint stressor in male and female progeny. Memory was probed using a battery of tests including object location memory, novel object recognition, and contextual fear conditioning.

RESULTS

Paternal morphine exposure did not alter anxiety-like behavior or stress-induced HPA axis activation in male or female offspring. Morphine-sired male and female offspring showed intact hippocampus-dependent memory: they performed normally on the long-term fear conditioning and object location memory tests. In contrast, paternal morphine exposure selectively disrupted novel object recognition in female, but not male, progeny.

CONCLUSIONS

Our findings demonstrate that paternal morphine taking produces sex-specific and selective impairments in object recognition memory while leaving hippocampal function largely intact.

Assessing object-recognition memory in rats: Pitfalls of the existent tasks and the advantages of a new test

Studies of object-recognition memory in lab rats began in the late 1980s, using variants of the trial-unique delayed nonmatching-to-sample (DNMS) task. By the end of the 20th century, most investigators who wanted to study object-recognition in rodents had abandoned the DNMS task in favor of the novel-object-preference (NOP) test, mainly because the latter test is relatively easy to employ, whereas conventional DNMS tasks are not. Some concerns have been raised, however, about the internal validity of the NOP test as a method of measuring object-recognition abilities. We describe two experiments using a new DNMS procedure which requires considerably less training than the DNMS tasks of the 1980s and 1990s, and which cannot be subject to the same criticisms that have been leveled at the NOP test. In Experiment 1, rats were trained on the new modified-DNMS (mDNMS) task using short delays. Rats successfully learned the nonmatching rule in fewer than 25 trials, and they made accurate choices with retention intervals of up to 10 min. Experiment 2 examined a different group of rats' performance on the mDNMS task following long retention intervals (72 h, 3 weeks, and ~45 weeks). Rats made accurate choices on all retention intervals, even the longest retention interval of ~45 weeks. Overall, the findings demonstrate some benefits of an alternative approach to assess object-recognition memory in rats.

Effects of perirhinal cortex and hippocampal lesions on rats' performance on two object-recognition tasks

The effects of hippocampal (HPC) damage on rats' novel object preference (NOP) performance have been rather consistent, in that HPC lesions do not disrupt novelty preferences on the test. Conversely, there have been inconsistent findings regarding the effects of perirhinal cortex (PRh) lesions on rats' novel-object preferences. Given the concerns that have been raised regarding the internal validity of the NOP test, viz. that the magnitude of the novel-object preference does not necessarily reflect the strength in memory for an object, it could explain the discrepant findings. The goal of the present experiment was to examine the effects of PRh and HPC lesions on rats' object-recognition memory using a new modified delayed nonmatching-to-sample (mDNMS) task, as it circumvents the interpretational problems associated with the NOP test. Rats received PRh, HPC, or Sham lesions and were trained on the mDNMS task using a short delay (∼30 s). Both PRh and HPC rats acquired the task at the same rate as Sham rats, and reached a similar level of accuracy, indicating intact object-recognition. Thereafter, rats were tested on the NOP test using a 180-s delay. Rats with HPC lesions exhibited significant novel-object preferences, however, both the PRh and Sham rats failed to show a novelty preference. The discrepancy in both the PRh and Sham rats' performance on the mDNMS task and NOP test raises concerns regarding the internal validity of the NOP test, in that the magnitude of a rat's novel-object preference does not accurately reflect the persistence or accuracy of a rat's memory for the sample object.

Dissociable involvement of estrogen receptors in perirhinal cortex-mediated object-place memory in male rats

Estrogens and the estrogen receptors (ER) - ERα, ERβ, and the G-protein coupled estrogen receptor (GPER) - are implicated in various forms of hippocampus (HPC)-dependent memory. However, the involvement of ER-related mechanisms in perirhinal cortex (PRh), which is necessary for object memory, remains much less clear. Moreover, there is a paucity of data assessing ER contributions to cognition in males,despite documented sex differences at the cellular level.We hypothesized that estrogens in PRh are important for object memory in males, assessingthe role of 17-βestradiol (E2), ERα, ERβ, GPER, and their downstream signaling pathways, in PRh-mediated object-in-place (OiP) memory in gonadally-intact male rats. Intra-PRh administration of E2 enhanced both long-term memory (LTM; 24 h) and short-term memory (STM; 20 min). Conversely, aromatase inhibition with letrozole impaired LTM and STM. The semi-selective ER inhibitor ICI 182780 impaired LTM, but not STM. This effect may be due to inhibition of ERβ, as the ERβagonist DPN, but not ERαagonist PPT, enhanced LTM. GPER was also found to be necessary in PRh, as the antagonist G15 impaired both LTM and STM. Western blot analyses demonstrated that phosphorylation levels of the extracellular signal-related kinase (ERK2 isoform), awell-establisheddownstream signaling pathway activated by estrogens through ERα/ERβ, was elevated in PRh 5 min following OiP learning.We also reportincreased levels of c-Jun N-terminal kinase (JNK; p46 and p54 isoforms) phosphorylation in PRh 5 min following learning,consistent with recent research linking GPER activation and JNK signaling in the HPC. This effect was abolished by intra-PRh administration of G15, but not letrozole, suggesting that JNK signaling is triggered via GPER activation during OiP learning, and is possibly E2-independent, similar to findings in the HPC. These results, therefore, reveal interesting dissociations between the roles of various ERs, possibly involving both estrogen-dependent and independent mechanisms, in PRh-mediated object-place learning in male rats.

The hippocampus is crucial for forming non-hippocampal long-term memory during sleep

There is a long-standing division in memory research between hippocampus-dependent memory and non-hippocampus-dependent memory, as only the latter can be acquired and retrieved in the absence of normal hippocampal function^1,2. Consolidation of hippocampus-dependent memory, in particular, is strongly supported by sleep^3-5. Here we show that the formation of long-term representations in a rat model of non-hippocampus-dependent memory depends not only on sleep but also on activation of a hippocampus-dependent mechanism during sleep. Rats encoded non-hippocampus-dependent (novel-object recognition^6-8) and hippocampus-dependent (object-place recognition) memories before a two-hour period of sleep or wakefulness. Memory was tested either immediately thereafter or remotely (after one or three weeks). Whereas object-place recognition memory was stronger for rats that had slept after encoding (rather than being awake) at both immediate and remote testing, novel-object recognition memory profited from sleep only three weeks after encoding, at which point it was preserved in rats that had slept after encoding but not in those that had been awake. Notably, inactivation of the hippocampus during post-encoding sleep by intrahippocampal injection of muscimol abolished the sleep-induced enhancement of remote novel-object recognition memory. By contrast, muscimol injection before remote retrieval or memory encoding had no effect on test performance, confirming that the encoding and retrieval of novel-object recognition memory are hippocampus-independent. Remote novel-object recognition memory was associated with spindle activity during post-encoding slow-wave sleep, consistent with the view that neuronal memory replay during slow-wave sleep contributes to long-term memory formation. Our results indicate that the hippocampus has an important role in long-term consolidation during sleep even for memories that have previously been considered hippocampus-independent.

Gynostemma pentaphyllum is neuroprotective in a rat model of cardiopulmonary resuscitation

Asphyxial cardiac arrest (ACA)-induced ischemia results in acute and delayed neuronal cell death. The early reperfusion phase is critical for the outcome. Intervention strategies directed to this period are promising to reduce ACA/resuscitation-dependent impairments. This study focused on the evaluation of the protective potential of an extract from Gynostemma pentaphyllum (GP), a plant used in traditional medicine with antioxidative, glucose lowering and neuroprotective activities, in an ACA rat model. We tested the following parameters: i) Basic systemic parameters such as pCO₂ and blood glucose value within the first 30 min post-ACA; ii) mitochondrial response by determining activities of citrate synthase, respiratory chain complexes I + III and II + III, and the composition of cardiolipin 6 and 24 h post-ACA; iii) neuronal vitality of the CA1 hippocampal region by immunohistochemistry 24 h and 7 days post-ACA; and iv) cognitive function by a novel object recognition test 7 days post-ACA. GP, administered after reaching spontaneous circulation, counteracted the following: i) ACA-mediated increases in arterial CO₂ tension and blood glucose values; ii) transient increase in the activity of the respiratory chain complexes II + III; iii) elevation in cardiolipin content; iv) hippocampal CA1 neurodegeneration, and v) loss of normal novelty-object seeking. The protective effects of GP were accompanied by side effects of the vehicle DMSO, such as the stimulation of citrate synthase activity in control animals, inhibition of cardiolipin synthesis in ACA animals and complex II + III activity in both control and ACA animals. The results emphasize the importance of the early post-resuscitation phase for the neurological outcome after ACA/resuscitation, and demonstrated the power of GP substitution as neuroprotective intervention. Moreover, the results underline the need of a careful handling of the popular vehicle DMSO.

An analysis of dentate gyrus function (an update)

In this review there will be a description of the dentate gyrus (DG) neural circuitry that mediates the operation of a variety of mnemonic processes associated with dorsal and ventral DG function in rats. Dysfunction of the dorsal DG can be shown to mediate mnemonic processing of spatially based information including a) the operation of conjunctive encoding of multiple sensory inputs to determine spatial representations, b) pattern separation based on reducing interference between similar spatial locations and spatial contexts for horizontal distance between objects, vertical distance for height of objects, slope or angle of motor movements, c) importance of spatial context in object recognition and processing of shades of grey associated with the walls of the box d) temporal integration in the creation of remote memory based in part on DG neurogenesis and function of the CA3 subregion of the hippocampus. Dysfunction of the ventral DG can be shown to mediate mnemonic processing of odor and reward value based information including a) pattern separation for odors and reward value, and b) social recognition.

Dissociable roles for histone acetyltransferases p300 and PCAF in hippocampus and perirhinal cortex-mediated object memory

The importance of histone acetylation for certain types of memory is now well established. However, the specific contributions of the various histone acetyltransferases to distinct memory functions remain to be determined; therefore, we employed selective histone acetyltransferase protein inhibitors and short-interference RNAs to evaluate the roles of CREB-binding protein (CBP), E1A-binding protein (p300) and p300/CBP-associated factor (PCAF) in hippocampus and perirhinal cortex (PRh)-mediated object memory. Rats were tested for short- (STM) and long-term memory (LTM) in the object-in-place task, which relies on the hippocampus and PRh for spatial memory and object identity processing, respectively. Selective inhibition of these histone acetyltransferases by small-interfering RNA and pharmacological inhibitors targeting the HAT domain produced dissociable effects. In the hippocampus, CBP or p300 inhibition impaired long-term but not short-term object memory, while inhibition of PCAF impaired memory at both delays. In PRh, HAT inhibition did not impair STM, and only CBP and PCAF inhibition disrupted LTM; p300 inhibition had no effects. Messenger RNA analyses revealed findings consistent with the pattern of behavioral effects, as all three enzymes were upregulated in the hippocampus (dentate gyrus) following learning, whereas only CBP and PCAF were upregulated in PRh. These results demonstrate, for the first time, the necessity of histone acetyltransferase activity for PRh-mediated object memory and indicate that the specific mnemonic roles of distinctive histone acetyltransferases can be dissociated according to specific brain regions and memory timeframe.

Ontogeny of object-in-context recognition in the rat

The object-in-context recognition (OiC) task [19] is a spontaneous exploration task that serves as an index of incidental contextual learning and memory. During the test phase, rats prefer to explore the object mismatched to the testing context based on previous object-context pairings experienced during training. The mechanisms of OiC memory have been explored in adult rats [12,35]; however, little is known about its determinants during development. Thus, the present study examined the ontogeny of the OiC task in preweanling through adolescent rats. We demonstrate that postnatal day (PD) 17, 21, 26, and 31 rats can perform the OiC task (Experiment 1) and that preference for the novel target is eliminated when rats are tested in an alternate context not encountered during training (Experiment 2). Lastly, we show that PD26 but not PD17 rats can perform the OiC task when the training contexts only differed by distal spatial cues (Experiment 3). These data demonstrate for the first time that PD17 rats can acquire and retain short-term OiC memory, which involves associative learning of object and context information. However, we also provide evidence that preweanling rats' ability to utilize certain aspects of a context (i.e., distal spatial cues) in the OiC task is not equivalent to that of their older counterparts. Implications for the development of contextual memory and its related neural substrates are discussed.

D2 dopamine receptor regulation of learning, sleep and plasticity

Dopamine and sleep have been independently linked with hippocampus-dependent learning. Since D2 dopaminergic transmission is required for the occurrence of rapid-eye-movement (REM) sleep, it is possible that dopamine affects learning by way of changes in post-acquisition REM sleep. To investigate this hypothesis, we first assessed whether D2 dopaminergic modulation in mice affects novel object preference, a hippocampus-dependent task. Animals trained in the dark period, when sleep is reduced, did not improve significantly in performance when tested 24h after training. In contrast, animals trained in the sleep-rich light period showed significant learning after 24h. When injected with the D2 inverse agonist haloperidol immediately after the exploration of novel objects, animals trained in the light period showed reduced novelty preference upon retesting 24h later. Next we investigated whether haloperidol affected the protein levels of plasticity factors shown to be up-regulated in an experience-dependent manner during REM sleep. Haloperidol decreased post-exploration hippocampal protein levels at 3h, 6h and 12h for phosphorylated Ca(2+)/calmodulin-dependent protein kinase II, at 6h for Zif-268; and at 12h for the brain-derived neurotrophic factor. Electrophysiological and kinematic recordings showed a significant decrease in the amount of REM sleep following haloperidol injection, while slow-wave sleep remained unaltered. Importantly, REM sleep decrease across animals was strongly correlated with deficits in novelty preference (Rho=0.56, p=0.012). Altogether, the results suggest that the dopaminergic regulation of REM sleep affects learning by modulating post-training levels of calcium-dependent plasticity factors.

Differential contributions of de novo and maintenance DNA methyltransferases to object memory processing in the rat hippocampus and perirhinal cortex--a double dissociation

Epigenetic mechanisms are increasingly acknowledged as major players in memory formation. Specifically, DNA methylation is necessary for the formation of long-term memory in various brain regions, including the hippocampus (HPC); however, its role in the perirhinal cortex (PRh), a structure critical for object memory, has not been characterized. Moreover, the mnemonic effects of selective DNA methyltransferase (DNMT) inhibition have not yet been investigated systematically, despite distinct roles for de novo (DNMT3a, 3b) and maintenance (DNMT1) methyltransferases. Consequently, we assessed the effects of various DNMT inhibitors within the HPC and PRh of rats using the object-in-place paradigm, which requires both brain regions. The non-nucleoside DNA methyltransferase inhibitor RG-108 impaired long-term object-in-place memory in both regions. Furthermore, intracranial administration of Accell short-interference RNA sequences to inhibit the expression of individual DNMTs implicated DNMT3a and DNMT1 in the HPC and PRh effects, respectively. mRNA expression analyses revealed a complementary pattern of results, as only de novo DNMT3a and DNMT3b mRNA was upregulated in the HPC (dentate gyrus) following object-in-place learning, whereas DNMT1 mRNA was selectively upregulated in the PRh. These results reinforce the established functional double dissociation between the HPC and PRh and imply the operation of different epigenetic mechanisms in brain regions dedicated to long-term memory processing for different types of information.

Role of the dentate gyrus in mediating object-spatial configuration recognition

In the present study the effects of dorsal dentate gyrus (dDG) lesions in rats were tested on recognition memory tasks based on the interaction between objects, features of objects, and spatial features. The results indicated that the rats with dDG lesions did not differ from controls in recognition for a change within object feature configuration and object recognition tasks. In contrast, there was a deficit for the dDG lesioned rats relative to controls in recognition for a change within object-spatial feature configuration, complex object-place feature configuration and spatial recognition tasks. It is suggested that the dDG subregion of the hippocampus supports object-place and complex object-place feature information via a conjunctive encoding process.

The influence of context on recognition memory in monkeys: effects of hippocampal, parahippocampal and perirhinal lesions

This study further investigated the specific contributions of the medial temporal lobe structures to contextual recognition memory. Monkeys (Macaca mulatta) with either neurotoxic lesions of the hippocampus, aspiration lesions of the perirhinal cortex and parahippocampal areas TH/TF, or sham operations were tested on five conditions of a visual-paired comparison (VPC) task in which 3-dimensional objects were presented over multicolored backgrounds. In two conditions (Conditions 1 and 2: Context-changes), the sample object was presented on a new background during the retention tests, whereas in the three others (Conditions 3-5: No-context-changes) the sample object was presented over its familiar background. Novelty preference scores of control animals were weaker, but still significantly different from chance, in the Context-changes conditions than on the No-context-changes conditions. Animals in the three experimental groups showed strong preference for novelty on the No-context-change conditions, but weaker novelty preference on the Context-change conditions than controls. Thus, animals in all three lesion types had greater difficulty recognizing an object when its background was different from that used during encoding. The data are consistent with the view that the hippocampal formation, areas TH/TF, and perirhinal cortex contribute interactively to contextual memory processes.

Developmental studies of the hippocampus and hippocampal-dependent behaviors: insights from interdisciplinary studies and tips for new investigators

The hippocampus is not fully developed at birth and, with respect to spatial cognition, only begins to show signs of adult-like function at three postnatal weeks in rodents. Studying the developmental period spanning roughly two to four weeks of age permits an understanding of the neural framework necessary for the emergence of spatial navigation and, quite possibly, human episodic memory. However, due to developmental factors, behavior data collection and interpretation can be severely compromised if inappropriate designs are applied. As such, we propose methodological considerations for the behavioral assessment of hippocampal function in developing rats that take into account animal size, growth rate, and sensory and motor ability. We further summarize recent key interdisciplinary studies that are beginning to unravel the molecular machinery and physiological alterations responsible for hippocampal maturation. In general, hippocampal development is a protracted process during which unique contributions to spatial cognition and complex recognition memory come "on line" at different postnatal ages creating a unique situation for elucidating the neural bases of specific components of higher cognitive abilities.

Isoflurane exposure in newborn rats induces long-term cognitive dysfunction in males but not females

Volatile anesthetics are used widely for achieving a state of unconsciousness, yet these agents are incompletely understood in their mechanisms of action and effects on neural development. There is mounting evidence that children exposed to anesthetic agents sustain lasting effects on learning and memory. The explanation for these behavioral changes remains elusive, although acute neuronal death after anesthesia is commonly believed to be a principal cause. Rodent models have shown that isoflurane exposure in newborns induces acute neuroapoptosis and long-term cognitive impairment. However, the assessment of predisposing factors is lacking. We investigated the role of sex by delivering isoflurane to postnatal day (P)7 male and female Sprague Dawley rats for 4 h. Brain cell death was assessed 12 h later using FluoroJade C staining in the thalamus, CA1-3 regions of hippocampus, and dentate gyrus. Behavior was assessed separately using a series of object recognition tasks and a test of social memory beginning at P38. We found that isoflurane exposure significantly increased neuronal death in each brain region with no difference between sexes. Behavioral outcome was also equivalent in simple novel object recognition. However, only males were impaired in the recognition of objects in different locations and contexts. Males also exhibited deficient social memory while females were intact. The profound behavioral impairment in males relative to females, in spite of comparable cell death, suggests that males are more susceptible to long-term cognitive effects and this outcome may not be exclusively attributed to neuronal death.

Early handling effect on female rat spatial and non-spatial learning and memory

This study aims at providing an insight into early handling procedures on learning and memory performance in adult female rats. Early handling procedures were started on post-natal day 2 until 21, and consisted in 15 min, daily separations of the dams from their litters. Assessment of declarative memory was carried out in the novel-object recognition task; spatial learning, reference- and working memory were evaluated in the Morris water maze (MWM). Our results indicate that early handling induced an enhancement in: (1) declarative memory, in the object recognition task, both at 1h and 24h intervals; (2) reference memory in the probe test and working memory and behavioral flexibility in the "single-trial and four-trial place learning paradigm" of the MWM. Short-term separation by increasing maternal care causes a dampening in HPA axis response in the pups. A modulated activation of the stress response may help to protect brain structures, involved in cognitive function. In conclusion, this study shows the long-term effects of a brief maternal separation in enhancing object recognition-, spatial reference- and working memory in female rats, remarking the impact of early environmental experiences and the consequent maternal care on the behavioral adaptive mechanisms in adulthood.

The rodent hippocampus is essential for nonspatial object memory

Elucidating the role of the rodent hippocampus in object recognition memory is critical for establishing the appropriateness of rodents as models of human memory and for their use in the development of memory disorder treatments. In mammals, spatial memory and nonspatial memory depend upon the hippocampus and associated medial temporal lobe (MTL) structures. Although well established in humans, the role of the rodent hippocampus in object memory remains highly debated due to conflicting findings across temporary and permanent hippocampal lesion studies and evidence that the perirhinal cortex may support object memory. In the current studies, we used intrahippocampal muscimol microinfusions to transiently inactivate the male C57BL/6J mouse hippocampus at distinct stages during the novel object recognition (NOR) task: during object memory encoding and consolidation, just consolidation, and/or retrieval. We also assessed the effect of temporary hippocampal inactivation when objects were presented in different contexts, thus eliminating the spatial or contextual components of the task. Lastly, we assessed extracellular dorsal hippocampal glutamate efflux and firing properties of hippocampal neurons while mice performed the NOR task. Our results reveal a clear and compelling role of the rodent hippocampus in nonspatial object memory.

Lateral entorhinal cortex is necessary for associative but not nonassociative recognition memory

The lateral entorhinal cortex (LEC) provides one of the two major input pathways to the hippocampus and has been suggested to process the nonspatial contextual details of episodic memory. Combined with spatial information from the medial entorhinal cortex it is hypothesised that this contextual information is used to form an integrated spatially selective, context-specific response in the hippocampus that underlies episodic memory. Recently, we reported that the LEC is required for recognition of objects that have been experienced in a specific context (Wilson et al. (2013) Hippocampus 23:352-366). Here, we sought to extend this work to assess the role of the LEC in recognition of all associative combinations of objects, places and contexts within an episode. Unlike controls, rats with excitotoxic lesions of the LEC showed no evidence of recognizing familiar combinations of object in place, place in context, or object in place and context. However, LEC lesioned rats showed normal recognition of objects and places independently from each other (nonassociative recognition). Together with our previous findings, these data suggest that the LEC is critical for associative recognition memory and may bind together information relating to objects, places, and contexts needed for episodic memory formation.

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.

Layer V perirhinal cortical ensemble activity during object exploration: a comparison between young and aged rats

Object recognition memory requires the perirhinal cortex (PRC) and this cognitive function declines during normal aging. Recent electrophysiological recordings from young rats have shown that neurons in Layer V of the PRC are activated by three-dimensional objects. Thus, it is possible that age-related object recognition deficits result from alterations in PRC neuron activity in older animals. To examine this, the present study used cellular compartment analysis of temporal activity by fluorescence in situ hybridization (catFISH) with confocal microscopy to monitor cellular distributions of activity-induced Arc RNA in layer V of the PRC. Activity was monitored during two distinct epochs of object exploration. In one group of rats (6 young/6 aged) animals were placed in a familiar testing arena and allowed to explore five different three-dimensional objects for two 5-min sessions separated by a 20-min rest (AA). The second group of animals (6 young/6 aged) also explored the same objects for two 5-min sessions, but the environment was changed between the first and the second epoch (AB). Behavioral data showed that both age groups spent less time exploring objects during the second epoch, even when the environment changed, indicating successful recognition. Although the proportion of active neurons between epochs did not change in the AA group, in the AB group more neurons were active during epoch 2 of object exploration. This recruitment of neurons into the active neural ensemble could serve to signal that familiar stimuli are being encountered in a new context. When numbers of Arc positive neurons were compared between age groups, the old rats had significantly lower proportions of Arc-positive PRC neurons in both the AA and AB behavioral conditions. These data support the hypothesis that age-associated functional alterations in the PRC contribute to declines in stimulus recognition over the lifespan.

Evidence that the rat hippocampus has contrasting roles in object recognition memory and object recency memory

Adult rats with extensive, bilateral neurotoxic lesions of the hippocampus showed normal forgetting curves for object recognition memory, yet were impaired on closely related tests of object recency memory. The present findings point to specific mechanisms for temporal order information (recency) that are dependent on the hippocampus and do not involve object recognition memory. The object recognition tests measured rats exploring simultaneously presented objects, one novel and the other familiar. Task difficulty was varied by altering the retention delays after presentation of the familiar object, so creating a forgetting curve. Hippocampal lesions had no apparent effect, despite using an apparatus (bow-tie maze) where it was possible to give lists of objects that might be expected to increase stimulus interference. In contrast, the same hippocampal lesions impaired the normal preference for an older (less recent) familiar object over a more recent, familiar object. A correlation was found between the loss of septal hippocampal tissue and this impairment in recency memory. The dissociation in the present study between recognition memory (spared) and recency memory (impaired) was unusually compelling, because it was possible to test the same objects for both forms of memory within the same session and within the same apparatus. The object recency deficit is of additional interest as it provides an example of a nonspatial memory deficit following hippocampal damage.

Crossmodal object recognition in rats with and without multimodal object pre-exposure: no effect of hippocampal lesions

The neural mechanisms and brain circuitry involved in the formation, storage, and utilization of multisensory object representations are poorly understood. We have recently introduced a crossmodal object recognition (CMOR) task that enables the study of such questions in rats. Our previous research has indicated that the perirhinal and posterior parietal cortices functionally interact to mediate spontaneous (tactile-to-visual) CMOR performance in rats; however, it remains to be seen whether other brain regions, particularly those receiving polymodal sensory inputs, contribute to this cognitive function. In the current study, we assessed the potential contribution of one such polymodal region, the hippocampus (HPC), to crossmodal object recognition memory. Rats with bilateral excitotoxic HPC lesions were tested in two versions of crossmodal object recognition: (1) the original CMOR task, which requires rats to compare between a stored tactile object representation and visually-presented objects to discriminate the novel and familiar stimuli; and (2) a novel 'multimodal pre-exposure' version of the CMOR task (PE/CMOR), in which simultaneous exploration of the tactile and visual sensory features of an object 24 h prior to the sample phase enhances CMOR performance across longer retention delays. Hippocampus-lesioned rats performed normally on both crossmodal object recognition tasks, but were impaired on a radial arm maze test of spatial memory, demonstrating the functional effectiveness of the lesions. These results strongly suggest that the HPC, despite its polymodal anatomical connections, is not critically involved in tactile-to-visual crossmodal object recognition memory.

Anterior thalamic nuclei lesions in rats disrupt markers of neural plasticity in distal limbic brain regions

In two related experiments, neurotoxic lesions were placed in the anterior thalamic nuclei of adult rats. The rats were then trained on behavioral tasks, immediately followed by the immunohistochemical measurement of molecules linked to neural plasticity. These measurements were made in limbic sites including the retrosplenial cortex, the hippocampal formation, and parahippocampal areas. In Experiment 1, rats with unilateral anterior thalamic lesions explored either novel or familiar objects prior to analysis of the immediate-early gene zif268. The lesions reduced zif268 activity in the granular retrosplenial cortex and postsubiculum. Exploring novel objects resulted in local changes of hippocampal zif268, but this change was not moderated by anterior thalamic lesions. In Experiment 2, rats that had received either bilateral anterior thalamic lesions or control surgeries were exposed to novel room cues while running in the arms of a radial maze. In addition to zif268, measurements of c-AMP response element binding protein (CREB), phosphorylated CREB (pCREB), and growth associated protein43 (GAP-43) were made. As before, anterior thalamic lesions reduced zif268 in retrosplenial cortex and postsubiculum, but there were also reductions of pCREB in granular retrosplenial cortex. Again, the hippocampus did not show lesion-induced changes in zif268, but there were differential effects on CREB and pCREB consistent with reduced levels of hippocampal CREB phosphorylation following anterior thalamic damage. No changes in GAP-43 were detected. The results not only point to changes in several limbic sites (retrosplenial cortex and hippocampus) following anterior thalamic damage, but also indicate that these changes include decreased levels of pCREB. As pCREB is required for neuronal plasticity, partly because of its regulation of immediate early-gene expression, the present findings reinforce the concept of an 'extended hippocampal system' in which hippocampal function is dependent on distal sites such as the anterior thalamic nuclei.

Object/context specific memory deficits following medial frontal cortex damage in mice

Recent evidence suggests that the medial prefrontal cortex (MFC) is important for processing contextual information. Here we evaluate the performance of mice with MFC damage in a discrimination task that requires an association between an object and the context in which it was experienced (the object/context mismatch task), as well as a version of the novel object preference task that does not require knowledge of contextual information to resolve. Adult C57/BL6 mice received aspiration lesions of the MFC or control surgery. Upon recovery, mice were tested in the object/context mismatch and novel object preference tasks. The object/context mismatch task involved exposing mice to two different contexts, each of which housed a unique pair of identical objects. After a brief delay, mice were re-exposed to one of the contexts, this time with one object that was congruent with that context and one that was not. Novel object preference was performed within a single context, housing an identical pair of objects. After the initial exposure and following a brief delay, mice were re-exposed to the context, this time housing a familiar and a novel object. Control mice were able to successfully resolve the object/context mismatch and novel object preference discriminations, investigating the incongruent/novel object within each task significantly greater than chance. Mice with MFC damage experienced deficits in the object/context mismatch task but not the novel object preference task. These findings add to a growing body of evidence that demonstrate a critical role for the MFC in contextual information processing.

Episodic-like memory: new perspectives from a behavioral test in rats

In order to have a tool to empirically test the ideas derived from a theoretical model, we extended a protocol for evaluation of episodic-like memory in rats, based on the triad "what, where, context" for definition of memories. As with the computational model, our intention was for the animal being tested to store a specific number of object-place-context configurations as different memories, which would then be retrievable from cues. The aim of this work was to evaluate the influence of the number of configurations to be memorized on the performance of the task. Sixty-five Wistar male rats were evaluated. In accordance with previous work, for two configurations, the recognition index was indicative of recognition of the element mismatching the original memory (mean = 0.28; SEM = 0.12). The recognition index for three configurations was lower (mean = 0.15; SEM = 0.10), evidencing less recall with increasing requirements. The results also showed a trend toward recognition of novelty for the first and the last memory when evaluating three configurations (a "U" shape in the exploratory preference's curve), showing the primacy and recency effects typical of memory both in humans and animals. Nonetheless, the data presented a high inter-subject variability which makes the test non-robust for small groups. However, if used before and after a treatment for a same subject, we suggest that the protocol presented in this work can be a useful behavioral test for the evaluation of episodic-like memory in rats in terms of a variable task demand.

The novel object recognition memory: neurobiology, test procedure, and its modifications

Animal models of memory have been considered as the subject of many scientific publications at least since the beginning of the twentieth century. In humans, memory is often accessed through spoken or written language, while in animals, cognitive functions must be accessed through different kind of behaviors in many specific, experimental models of memory and learning. Among them, the novel object recognition test can be evaluated by the differences in the exploration time of novel and familiar objects. Its application is not limited to a field of research and enables that various issues can be studied, such as the memory and learning, the preference for novelty, the influence of different brain regions in the process of recognition, and even the study of different drugs and their effects. This paper describes the novel object recognition paradigms in animals, as a valuable measure of cognition. The purpose of this work was to review the neurobiology and methodological modifications of the test commonly used in behavioral pharmacology.

On the dynamic nature of the engram: evidence for circuit-level reorganization of object memory traces following reactivation

Research has implicated the perirhinal cortex (PRh) in several aspects of object recognition memory. The specific role of the hippocampus (HPC) remains controversial, but its involvement in object recognition may pertain to processing contextual information in relation to objects rather than object representation per se. Here we investigated the roles of the PRh and HPC in object memory reconsolidation using the spontaneous object recognition task for rats. Intra-PRh infusions of the protein synthesis inhibitor anisomycin immediately following memory reactivation prevented object memory reconsolidation. Similar deficits were observed when a novel object or a salient contextual change was introduced during the reactivation phase. Intra-HPC infusions of anisomycin, however, blocked object memory reconsolidation only when a contextual change was introduced during reactivation. Moreover, disrupting functional interaction between the HPC and PRh by infusing anisomycin unilaterally into each structure in opposite hemispheres also impaired reconsolidation when reactivation was done in an altered context. These results show for the first time that the PRh is critical for reconsolidation of object memory traces and provide insight into the dynamic process of object memory storage; the selective requirement for hippocampal involvement following reactivation in an altered context suggests a substantial circuit level object trace reorganization whereby an initially PRh-dependent object memory becomes reliant on both the HPC and PRh and their interaction. Such trace reorganization may play a central role in reconsolidation-mediated memory updating and could represent an important aspect of lingering consolidation processes proposed to underlie long-term memory modulation and stabilization.

Pattern separation deficits may contribute to age-associated recognition impairments

Normal aging is associated with impairments in stimulus recognition. In the current investigation, object recognition was tested in adult and aged rats with the standard spontaneous object recognition (SOR) task or two variants of this task. On the standard SOR task, adult rats showed an exploratory preference for the novel object over delays up to 24 h, whereas the aged rats only showed significant novelty discrimination at the 2-min delay. This age difference appeared to be because of the old rats behaving as if the novel object was familiar. To test this hypothesis directly, rats participated in a variant of the SOR task that allowed the exploration times between the object familiarization and the test phases to be compared, and this experiment confirmed that aged rats falsely "recognize" the novel object. A final control examined whether or not aged rats exhibited reduced motivation to explore objects. In this experiment, when the environmental context changed between familiarization and test, young and old rats failed to show an exploratory preference because both age groups spent more time exploring the familiar object. Together these findings support the view that age-related impairments in object recognition arise from old animals behaving as if novel objects are familiar, which is reminiscent of behavioral impairments in young rats with perirhinal cortical lesions. The current experiments thus suggest that alterations in the perirhinal cortex may be responsible for reducing aged animals' ability to distinguish new stimuli from ones that have been encountered previously.

Impaired conditioned emotional response and object recognition are concomitant to neuronal damage in the amygdala and perirhinal cortex in middle-aged ischemic rats

The current study characterizes fear conditioning responses following global ischemia and evaluates neuronal damage affecting discrete extra-hippocampal areas susceptible to contribute to post ischemic emotional and memory impairments. Conditioned emotional response, Barnes Maze and object recognition tests were used to assess emotional, spatial and recognition memory, respectively. Behavioural testing was initiated in middle-aged animals (10-12 month old) 1 week following sham (n=16) or 4VO occlusion (n=18). Post-mortem cellular assessment was performed in the hippocampal CA1 layer, the perirhinal cortex and basolateral amygdala. Middle-aged ischemic animals showed impaired spatial memory in the initial three testing days in the Barnes Maze and deficit in recognition memory. Of interest, ischemic rats demonstrated a significant reduction of freezing and increased locomotion during the contextual fear testing period, suggesting reduced fear in these animals. Assessment of neuronal density 40 days following global ischemia revealed that CA1 neuronal injury was accompanied by 20-25% neuronal loss in the basolateral nucleus of the amygdala and perirhinal cortex in middle-aged ischemic compared to sham-operated animals. This study represents the first demonstration of altered conditioned fear responses following ischemia. Our findings also indicate a vulnerability of extra-hippocampal neurons to ischemic injury, possibly contributing to discrete emotional and/or memory impairments post ischemia.

New behavioral protocols to extend our knowledge of rodent object recognition memory

Animals often show an innate preference for novelty. This preference facilitates spontaneous exploration tasks of novelty discrimination (recognition memory). In response to limitations with standard spontaneous object recognition procedures for rodents, a new task ("bow-tie maze") was devised. This task combines features of delayed nonmatching-to-sample with spontaneous exploration. The present study explored aspects of object recognition in the bow-tie maze not amenable to standard procedures. Two rat strains (Lister Hooded, Dark Agouti) displayed very reliable object recognition in both the light and dark, with the Lister Hooded strain showing superior performance (Experiment 1). These findings reveal the potential contribution of tactile and odor cues in object recognition. As the bow-tie maze task permits multiple trials within a session, it was possible to derive forgetting curves both within-session and between-sessions (Experiment 1). In Experiment 2, rats with hippocampal or fornix lesions performed at normal levels on the basic version of the recognition task, contrasting with the marked deficits previously seen after perirhinal cortex lesions. Next, the training protocol was adapted (Experiment 3), and this modified version was used successfully with mice (Experiment 4). The overall findings demonstrate the efficacy of this new behavioral task and advance our understanding of object recognition.

Hippocampus is necessary for spatial discrimination using distal cue-configuration

The role of the hippocampus in processing contextual cues has been well recognized. Contextual manipulation often involves transferring animals between different rooms. Because of vague definition of context in such a paradigm, however, it has been difficult to study the role of the hippocampus parametrically in contextual information processing. We designed a novel task in which a different context can be parametrically defined by the spatial configuration of distal cues. In this task, rats were trained to associate two different configurations of distal cue-sets (standard contexts) with different food-well locations at the end of a radial arm. Experiment 1 tested the role of the dorsal hippocampus in retrieving well-learned associations between standard contexts and rewarding food-well locations by comparing rats with neurotoxic lesions in the dorsal hippocampus with controls. We found that the hippocampal-lesioned rats were unable to retrieve the context-place paired associations learned before surgery. To further test the role of the hippocampus in generalizing altered context, in Experiment 2, rats were trained in a task in which modified versions of the standard contexts (ambiguous contexts) were presented, intermixed with the standard contexts. Rats were able to process the ambiguous contexts immediately by using their similarities to the standard contexts, whereas muscimol inactivation of the dorsal hippocampus in the same animals reversibly deprived such capability. The results suggest that rats can effectively associate discrete spatial locations with spatial configuration of distal cues. More important, rats can generalize or orthogonalize modified contextual environments using learned contextual representation of the environment.