Lateral mammillary body neurons in mouse brain are disproportionately vulnerable in Alzheimer's disease.

The neural circuits governing the induction and progression of neurodegeneration and memory impairment in Alzheimer's disease (AD) are incompletely understood. The mammillary body (MB), a subcortical node of the medial limbic circuit, is one of the first brain regions to exhibit amyloid deposition in the 5xFAD mouse model of AD. Amyloid burden in the MB correlates with pathological diagnosis of AD in human postmortem brain tissue. Whether and how MB neuronal circuitry contributes to neurodegeneration and memory deficits in AD are unknown. Using 5xFAD mice and postmortem MB samples from individuals with varying degrees of AD pathology, we identified two neuronal cell types in the MB harboring distinct electrophysiological properties and long-range projections: lateral neurons and medial neurons. lateral MB neurons harbored aberrant hyperactivity and exhibited early neurodegeneration in 5xFAD mice compared with lateral MB neurons in wild-type littermates. Inducing hyperactivity in lateral MB neurons in wild-type mice impaired performance on memory tasks, whereas attenuating aberrant hyperactivity in lateral MB neurons ameliorated memory deficits in 5xFAD mice. Our findings suggest that neurodegeneration may be a result of genetically distinct, projection-specific cellular dysfunction and that dysregulated lateral MB neurons may be causally linked to memory deficits in AD.

High ethanol preference and dissociated memory are co-occurring phenotypes associated with hippocampal GABAAR-δ receptor levels.

Alcohol use disorder (AUD) frequently co-occurs with dissociative disorders and disorders with dissociative symptoms, suggesting a common neurobiological basis. It has been proposed that facilitated information processing under the influence of alcohol, resulting in the formation of dissociated memories, might be an important factor controlling alcohol use. Access to such memories is facilitated under the effect of alcohol, thus further reinforcing alcohol use. To interrogate possible mechanisms associated with these phenotypes, we used a mouse model of dissociative amnesia, combined with a high-alcohol preferring (HAP) model of AUD. Dissociated memory was induced by activation of hippocampal extrasynaptic GABA type A receptor delta subunits (GABAAR-δ), which control tonic inhibition and to which ethanol binds with high affinity. Increased ethanol preference was associated with increased propensity to form dissociated memories dependent on GABAAR-δ in the dorsal hippocampus (DH). Furthermore, the DH level of GABAAR-δ protein, but not mRNA, was increased in HAP mice, and was inversely correlated to the level of miR-365-3p, suggesting an miRNA-mediated post-transcriptional mechanism contributing to elevated GABAAR-δ. The observed changes of DH GABAAR-δ were associated with a severe reduction of excitatory projections stemming from GABAAR-δ-containing pyramidal neurons in the subiculum and terminating in the mammillary body. These results suggest that both molecular and circuit dysfunction involving hippocampal GABAAR-δ receptors might contribute to the co-occurrence of ethanol preference and dissociated information processing.

Acute ethanol induces behavioral changes and alters c-fos expression in specific brain regions, including the mammillary body, in zebrafish.

Ethanol is one of the most commonly abused substances in the world, and ethanol abuse and dependence disorders represent major societal problems. However, appropriate treatment is lacking as we still do not fully understand the molecular bases of these disorders. The zebrafish is one of the model organisms utilized for studying such mechanisms. In this study, we examined the effects of acute ethanol administration on the behavior of zebrafish, and we also analyzed correlated gene expression changes using whole-mount in situ hybridization focusing on a number of genes associated with different neurotransmitter systems, stress response, and neuronal activity. We found ethanol treatment to result in hyperactivity and reduced shoal cohesion compared to control. Analysis of c-fos expression demonstrated altered activity patterns in certain brain regions, including intense activation of the mammillary body in zebrafish with acute ethanol treatment. We also found reduced level of gad1b expression in the cerebellum of ethanol treated fish compared to control. However, we could not detect significant changes in the expression level of other genes, including vglut2b, th, crh, hdc, avp, pomc, and galn in ethanol treated fish compared controls. Our results suggest that zebrafish is a promising animal model for the study of mechanisms underlying alcohol induced behavioral changes and alcohol related human disorders.

Laminar Localization and Projection-Specific Properties of Presubicular Neurons Targeting the Lateral Mammillary Nucleus, Thalamus, or Medial Entorhinal Cortex.

The presubiculum (PrS) is part of an interconnected network of distributed brain regions where individual neurons signal the animals heading direction. PrS sends axons to medial entorhinal cortex (MEC), it is reciprocally connected with anterior thalamic nuclei (ATNs), and it sends feedback projections to the lateral mammillary nucleus (LMN), involved in generating the head direction signal. The intrinsic properties of projecting neurons will influence the pathway-specific transmission of activity. Here, we used projection-specific labeling of presubicular neurons to identify MEC-, LMN-, and ATN-projecting neurons in mice. MEC-projecting neurons located in superficial layers II/III were mostly regular spiking pyramidal neurons, and we also identified a Martinotti-type GABAergic neuron. The cell bodies of LMN-projecting neurons were located in a well-delimited area in the middle portion of the PrS, which corresponds to layer IV. The physiology of LMN projecting, pyramidal neurons stood out with a tendency to fire in bursts of action potentials (APs) with rapid onset. These properties may be uniquely adapted to reliably transmit visual landmark information with short latency to upstream LMN. Neurons projecting to ATN were located in layers V/VI, and they were mostly regular spiking pyramidal neurons. Unsupervised cluster analysis of intrinsic properties suggested distinct physiological features for the different categories of projection neurons, with some similarities between MEC- and ATN-projecting neurons. Projection-specific subpopulations may serve separate functions in the PrS and may be engaged differently in transmitting head direction related information.

Thiazine Red(+) platelet inclusions in Cerebral Blood Vessels are first signs in an Alzheimer's Disease mouse model.

Strong evidence shows an association between cerebral vascular diseases and Alzheimer´s disease (AD). In order to study the interaction of beta-amyloid (Aß) plaques with brain vessels, we crossbred an AD mouse model (overexpressing amyloid precursor protein with the Swedish-Dutch-Iowa mutations, APP_SweDI) with mice expressing green fluorescent protein (GFP) under the flt-1/VEGFR1 promoter in vessels (GFP_FLT1). Our data show, that only very few Aß plaques were seen in 4-months old mice, focused in the mammillary body and in the lateral septal nucleus. The number of plaques markedly increased with age being most prominent in 12-months old mice. Thiazine Red was used to verify the plaques. Several Thiazine Red(+) inclusions were found in GFP(+) vessels, but only in non-perfused 4-months old mice. These inclusions were verified by Resorufin stainings possibly representing cerebral amyloid angiopathy. The inclusions were also seen in non-crossbred APP_SweDI but not in wildtype and GFP_FLT1 mice. In order to characterize these inclusions Flow Cytometry (FACS) analysis demonstrated that platelets were specifically stained by Thiazine Red(+), more pronounced when aggregated. In conclusion, our data show that Thiazine Red(+) inclusions representing aggregated platelets are a first pathological sign in AD before plaque development and may become important therapeutic targets in early AD.

Comparable reduction in Zif268 levels and cytochrome oxidase activity in the retrosplenial cortex following mammillothalamic tract lesions.

Damage to the mammillothalamic tract (MTT) produces memory impairments in both humans and rats, yet it is still not clear why this diencephalic pathway is vital for memory. One suggestion is that it is an important route for midbrain inputs to reach a wider cortical and subcortical network that supports memory. Consistent with this idea, MTT lesions produce widespread hypoactivity in distal brain regions as measured by the immediate-early gene, c-fos. To determine whether these findings were selective to c-fos or reflected more general changes in neuronal function, we assessed the effects of MTT lesions on the expression of the immediate-early gene protein, Zif268 and the metabolic marker, cytochrome oxidase, in the retrosplenial cortex and hippocampus. The lesions decreased levels of both activity markers in the superficial and deep layers of the retrosplenial cortex in both its granular and dysgranular subregions. In contrast, no significant changes were observed in the hippocampus, despite the MTT-lesioned animals showing marked impairments on T-maze alternation. These findings are consistent with MTT lesions providing important, indirect inputs for normal retrosplenial cortex functioning. These distal functional changes may contribute to the memory impairments observed after MTT lesions.

Spatial memory extinction differentially affects dorsal and ventral hippocampal metabolic activity and associated functional brain networks.

Previous studies showed the involvement of brain regions associated with both spatial learning and associative learning in spatial memory extinction, although the specific role of the dorsal and ventral hippocampus and the extended hippocampal system including the mammillary body in the process is still controversial. The present study aimed to identify the involvement of the dorsal and ventral hippocampus, together with cortical regions, the amygdaloid nuclei, and the mammillary bodies in the extinction of a spatial memory task. To address these issues, quantitative cytochrome c oxidase histochemistry was applied as a metabolic brain mapping method. Rats were trained in a reference memory task using the Morris water maze, followed by an extinction procedure of the previously acquired memory task. Results show that rats learned successfully the spatial memory task as shown by the progressive decrease in measured latencies to reach the escape platform and the results obtained in the probe test. Spatial memory was subsequently extinguished as shown by the descending preference for the previously reinforced location. A control naïve group was added to ensure that brain metabolic changes were specifically related with performance in the spatial memory extinction task. Extinction of the original spatial learning task significantly modified the metabolic activity in the dorsal and ventral hippocampus, the amygdala and the mammillary bodies. Moreover, the ventral hippocampus, the lateral mammillary body and the retrosplenial cortex were differentially recruited in the spatial memory extinction task, as shown by group differences in brain metabolic networks. These findings provide new insights on the brain regions and functional brain networks underlying spatial memory, and specifically spatial memory extinction. © 2016 Wiley Periodicals, Inc.

Expression of c-Fos protein in medial septum/diagonal band of Broca and CA3 region, associated with the temporary inactivation of the supramammillary area.

The supramammillary (SuM) area is part of the diencephalic nuclei comprising the mammillary bodies, and is a key structure in the memory and spatial learning processes. It is a critical region in the modulation/generation of hippocampal theta rhythm. In addition, many papers have recently shown a clear involvement of this structure in the processes of spatial learning and memory in animal models, although it is still not known how it modulates spatial navigation and response emotional. The aim of the present research was to study the effect of the temporary inactivation of the SuM area on synaptic plasticity of crucial structures in the formation of spatial memory and emotional response. Sprague-Dawley rats were asigned in three groups: a control group where the animals were not subjected to any treatment, and two groups where the rats received microinjections of tetrodotoxin (TTX) in the SuM area (5ng diluted in 0.5µl of saline) or saline (0.5µl). The microinjections were administered 90min before the perfusion. Later, cellular activity in medial septum/diagonal band of Broca (MS/DBB) and CA3 region of the dorsal hippocampus was assessed, by measuring the immediate early gene c-fos. The results show a clear hiperactivity cellular in medial septum/diagonal band of Broca and a clear hypoactivity cellular in the CA3 region of the hippocampus when there was a functional inactivation of the SuM area. It suggests that the SuM area seems to be part of the connection and information input pathways to CA3 region of the hippocampal formation, key for proper functioning in spatial memory and emotional response.

Population pharmacokinetics of ramosetron.

Ramosetron is a selective serotonergic 5-hydroxy-tryptamine receptor 3 antagonist that is used to prevent and treat postoperative nausea and vomiting. This study aimed to characterize the population pharmacokinetics of ramosetron in patients undergoing surgery with general anesthesia. Patients aged 19-80 years received a single intravenous bolus of ramosetron (0.3, 0.45, or 0.6 mg) 30 min before the end of surgery. Blood samples were collected, and plasma concentrations of ramosetron were measured by high performance liquid chromatography-tandem mass spectrometry. Pooled data from 50 patients and 479 pharmacokinetic samples were used for population pharmacokinetic analysis using the nonlinear mixed effect modeling program (NONMEM(®)). The pharmacokinetics of ramosetron was best described by a three-compartment mammillary model with first-order elimination. Based on allometric principles, body weight was incorporated in the base model, along with fixed allometric exponents. The typical value of clearance was 0.19 L/h in a 60-kg subject, and it decreased approximately 3% for every year of age, starting at age of 57. The bootstrap method and visual predictive check showed that the final pharmacokinetic model was appropriate. A population pharmacokinetic model of ramosetron was constructed in adult surgical patients, providing a foundation for further defining the relationship between ramosetron dose and postoperative nausea and vomiting.

Thalamic amnesia after infarct: The role of the mammillothalamic tract and mediodorsal nucleus.

OBJECTIVE: To improve current understanding of the mechanisms behind thalamic amnesia, as it is unclear whether it is directly related to damage to specific nuclei, in particular to the anterior or mediodorsal nuclei, or indirectly related to lesions of the mammillothalamic tract (MTT). METHODS: We recruited 12 patients with a left thalamic infarction and 25 healthy matched controls. All underwent a comprehensive neuropsychological assessment of verbal and visual memory, executive functions, language, and affect, and a high-resolution structural volumetric MRI scan. Thalamic lesions were manually segmented and automatically localized with a computerized thalamic atlas. As well as comparing patients with controls, we divided patients into subgroups with intact or damaged MTT. RESULTS: Only one patient had a small lesion of the anterior nucleus. Most of the lesions included the mediodorsal (n = 11) and intralaminar nuclei (n = 12). Patients performed worse than controls on the verbal memory tasks, but the 5 patients with intact MTT who showed isolated lesions of the mediodorsal nucleus (MD) only displayed moderate memory impairment. The 7 patients with a damaged MTT performed worse on the verbal memory tasks than those whose MTT was intact. CONCLUSIONS: Lesions in the MTT and in the MD result in memory impairment, severely in the case of MTT and to a lesser extent in the case of MD, thus highlighting the roles played by these 2 structures in memory circuits.

The importance of the context in the hippocampus and brain related areas throughout the performance of a fear conditioning task.

The importance context has been broadly studied in the management of phobias and in the drug addiction literature. The way in which changes to a context influence behavior after the simple acquisition of a passive avoidance task remains unclear. The hippocampus has long been implicated in the contextual and spatial processing required for contextual fear, but its role in encoding the aversive component of a contextual fear memory is still inconclusive. Our work tries to elucidate whether a change in context, represented as differences in the load of the stimuli, is critical for learning about the context-shock association and whether this manipulation of the context could be linked to any change in metabolic brain activity requirements. For this purpose, we used an avoidance conditioning task. Animals were divided into three different experimental conditions. In one group, acquisition was performed in an enriched stimuli environment and retention was performed in a typically lit chamber (the PA-ACQ-CONTX group). In another group, acquisition was performed in the typically lit chamber and retention was undertaken in the highly enriched chamber (the PA-RET-CONTX group). Finally, for the control group, PA-CN-CONTX, acquisition, and retention were performed in the enriched stimuli environment. Our results showed that the PA-ACQ-CONTX group had longer escape latencies and poorer retention than the PA-RET-CONTX and PA-CN-CONTX groups after 24 h of acquisition under contextual changes. To study metabolic brain activity, histochemical labelling of cytochrome c-oxidase (CO) was performed. CO results suggested a neural circuit including the hippocampus, amygdala, thalamus, parahippocampal cortices, and mammillary nuclei that is involved in the learning and memory processes that enable context-dependent behavior. These results highlight how dysfunction in this network may be involved in the contextualization of fear associations that underlie several forms of psychopathology, including post-traumatic stress disorder, schizophrenia, and substance abuse disorders.

A novel TaulacZ allele reveals a requirement for Pitx2 in formation of the mammillothalamic tract.

The hypothalamic mammillary region is critical for spatial memory and vestibular processing. Pitx2 encodes a paired-like transcription factor that is highly expressed in the developing mammillary region and is required for subthalamic nucleus formation. Here we analyzed a loss of function Pitx2-TaulacZ knock-in allele to study the effects of Pitx2 deficiency on neuronal projections in the embryonic mammillary region. Pitx2-expressing neurons contribute axons to principal mammillary, mammillotegmental and mammillotectal tracts. Embryos with Pitx2 deficiency exhibit axonal fibers in the principal mammillary tract that are improperly bundled and disorganized, yet project caudally toward the tectum and tegmentum. Embryos with Nestin-Cre mediated conditional Pitx2 deficiency exhibit truncated mammillothalamic tracts (mtt) that fail to elongate, and reduced Pax6-positive cells at the branching point of the principal mammillary and mtt. These data suggest that Pitx2 mediates cell-autonomous and nonautonomous guidance cues necessary for mammillary collaterals destined to project to the anterior thalamus.

Spatial short-term memory in rats: effects of learning trials on metabolic activity of limbic structures.

The oxidative metabolism was assessed in the hippocampus and related regions in rats that were trained in a spatial short-term memory task in the water maze following distinct training schedules. The cytochrome oxidase (COx) histochemistry was evaluated in groups of rats that received a daily session made up of either two or three learning trials. An untreated group was added to determine baseline levels of COx. We found that the 3-Trials group exhibited a better performance concerning the differences in latency between trials 1 and 2. Trained groups showed higher COx activity than the untreated group in the medial prefrontal cortex, dentate gyrus, CA1, and the mammillary region. However, a decrease in COx activity was found in the dentate gyrus, CA1, and supramammillary region of the 3-Trials group. In addition, COx activity levels found in this group were similar to those of the untreated group in some thalamic nuclei. Most of the regions that presented significant correlations between COx activity and behavioral scores were found in the 3-Trials group. These findings suggest an influence of task difficulty in the oxidative metabolism of brain regions involved in spatial learning.

Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences.

The supramammillary nucleus (SuM) provides substantial projections to the hippocampal formation. This hypothalamic structure is involved in the regulation of hippocampal theta rhythm and therefore the control of hippocampal-dependent cognitive functions as well as emotional behavior. A major goal of this study was to characterize the neurotransmitter identity of the SuM-hippocampal pathways. Our findings demonstrate two distinct neurochemical pathways in rat. The first pathway originates from neurons in the lateral region of the SuM and innervates the supragranular layer of the dorsal dentate gyrus and, to a much lesser extent, the ventral dentate gyrus. This pathway displays a unique dual phenotype for GABAergic and glutamatergic neurotransmission. Axon terminals contain markers of GABAergic neurotransmission, including the synthesizing enzyme of GABA, glutamate decarboxylase 65, and the vesicular GABA transporter and also a marker of glutamatergic neurotransmission, the vesicular glutamate transporter 2. The second pathway originates from neurons in the most posterior and medial part of the SuM and innervates exclusively the inner molecular layer of the ventral dentate gyrus and the CA2/CA3a pyramidal cell layer of the hippocampus. The axon terminals from the medial part of the SuM contain the vesicular glutamate transporter 2 only. These data demonstrate for the first time the heterogeneity of the SuM-hippocampal pathways, not only from an anatomical but also a neurochemical point of view. These pathways, implicated in different neuronal networks, could modulate different hippocampal activities. They are likely to be involved differently in the regulation of hippocampal theta rhythm and associated cognitive functions as well as emotional behavior.

Calcium increase in the mammillary bodies with aging.

To elucidate compositional changes of the mammillary body with aging, the authors investigated age-related changes of elements, relationships among their elements, and gender differences in the mammillary bodies by direct chemical analysis. After ordinary dissection at Nara Medical University was finished, the mammillary bodies were resected from the cerebra of the subjects. The subjects consisted of 19 men and 25 women, ranging in age from 70 to 101 years (average age = 83.8 +/- 7.2 years). After ashing of the mammillary bodies with nitric acid and perchloric acid, element contents were determined by inductively coupled plasma-atomic emission spectrometry. It was found that the Ca content increased slightly and significantly in the mammillary bodies with aging, but the P, S, Mg, Zn, Fe, and Na contents did not change significantly with aging. Regarding the relationships among elements, significant correlations were found among the contents of Ca, P, S, and Mg in the mammillary bodies, with one exception between P and Mg contents. With regard to the seven element contents, no significant gender differences were found in the mammillary bodies.

Early maternal separation and chronic variable stress in adulthood changes the neural activity and the expression of glucocorticoid receptor in limbic structures.

There is increasing evidence that early adverse experience contributes to the development of stress susceptibility, and increases the onset of stress-related psychiatric disorders in stressful environments in adulthood. This study addressed whether or not prolonged maternal separation, a well-established model of early stress, affects adult limbic areas related to the regulation of the hypothalamic-pituitary-adrenal axis in exposure to chronic variable stress in adulthood. Rats were subjected to daily maternal separation for 4.5h during postnatal days 1-21. As adults, the animals were exposed to a variable chronic stress paradigm of 24 days. Persistent changes were assessed in glucocorticoid receptor density and Fos activity in the anterodorsal thalamic nuclei, mammillary nuclei and retrosplenial cortex. Immunohistochemical analysis revealed that adult maternally separated animals had increased levels of c-Fos immunoreactivity in the anterodorsal thalamic nuclei as well as in the mammillary nuclei compared to normal non-maternally separated animals. Chronic variable stress in maternally separated and non-maternally separated animals diminished glucocorticoid receptor density in the anterodorsal thalamic nuclei but not in the rest of the nuclei analyzed. These results indicate that c-Fos immunoreactivity as well as glucocorticoid receptor expression in the anterodorsal thalamic nuclei and mammillary nuclei exhibit long-term alterations in adult rats following repeated maternal separation and subsequent stress exposure. Recognition of these adaptations helps to define the brain regions and neural circuitry associated with persistent alterations induced by early life environment and the development of stress-associated disorders.

Sexually dimorphic c-Fos expression following spatial working memory in young and adult rats.

The sex differences in the functional contribution of brain substrates were explored following acquisition of a spatial working memory task using quantification of c-Fos protein. Rats of both sexes were trained during adolescence and adulthood in Morris water maze using a hidden escape platform with different daily location. Two control groups for each sex and age were added to explore the c-Fos activation not specific to the memory process. These were a free-swimming group (yoked control) and a handled control (CO) group. Behaviorally, no age differences were found in number of days required by males to acquire the task, but females showed a delay in acquisition during adolescence (P30) that improved in adulthood (P90). Both sexes showed a learning-related increase in Fos immunoreactivity in the anterodorsal and anteroventral thalamus and medial and lateral mammillary nuclei during adolescence. Higher levels of learning-related Fos immunoreactivity were found in the infralimbic cortex, CA3 and CA1 only in females. During adulthood the common activated region was the prelimbic cortex with the addition of the infralimbic cortex in the male group and the lateral mammillary nucleus in the female group. These results indicated sex and age differences in brain functioning following working memory task. However, they could not be necessarily linked with differences in performance since similar results were found between males and females during adulthood. The activation of common and interrelated structures suggests that these structures are involved in spatial processing but it also highlights the relevance of developmental changes for understanding the memory process.

Three-dimensional distribution of Fos-positive neurons in the supramammillary nucleus of the rat exposed to novel environment.

The supramammillary nucleus (SuM) in the hypothalamus is proposed to regulate the function of the hippocampus through distinct fiber connection. Several investigations suggest that the SuM is relevant to anxiety and defensive behavior. Function of the SuM, however, is not known exactly. In order to demonstrate the spatial activation of the SuM in physiologically behaving rats, we investigated Fos induction in the SuM by exposure to novel environment. To correct uneven background in microscopic preparations, we applied a convolution filter, resulting in reliable automatic counting of Fos-positive neurons and analyzed the distribution of Fos-positive neurons in the whole region of SuM. A large number of Fos-positive neurons were observed throughout the entire SuM after rats exposed to a novel open field. A three-dimensional density map revealed that density of the Fos-positive neurons was highest in the medial SuM, especially in its core regions. Based on these results we suggest that the medial SuM modulates defensive behavior and that the lateral SuM modulates emotional and memory functions of the hippocampus.

Basal and learning task-related brain oxidative metabolism in cirrhotic rats.

Hepatic encephalopathy is a neurological complication observed in patients with liver disease. Subjects with hepatic encephalopathy can develop memory alterations. In order to investigate brain oxidative metabolism in an animal model of chronic cirrhosis and its modification after spatial working memory task, we determined the neural metabolic activity of several brain limbic system regions by cytochrome oxidase (COx) histochemistry and assessed the spatial working memory in the Morris water maze of rats with cirrhosis by administration of thioacetamide. This COx histochemistry was done in cirrhotic and control rats under basal conditions and after the spatial working memory task. The histochemical results showed differences in basal COx activity between control and cirrhotic rats in hippocampal and thalamic regions. In cirrhotic rats basal COx activity was increased in the CA1 and CA3 areas of the hippocampus and reduced in the anterodorsal and anteroventral thalamic nuclei. We found impaired spatial working memory in animals with cirrhosis. These animals showed absence of metabolic activation of the CA3 hippocampal subfield and the lateral mammillary nucleus and disturbance of COx activity in the medial mammillary nucleus and the anteroventral thalamus. These findings suggest that cirrhotic rats show spatial working memory deficits that could be related to the alteration of metabolic activity of neural regions thought to be involved in the processing of spatial memories.