Parallel but separate inputs from limbic cortices to the mammillary bodies and anterior thalamic nuclei in the rat

Thalamic contributions to psychosis susceptibility: Evidence from co-activation patterns accounting for intra-seed spatial variability (μCAPs)

The temporal variability of the thalamus in functional networks may provide valuable insights into the pathophysiology of schizophrenia. To address the complexity of the role of the thalamic nuclei in psychosis, we introduced micro-co-activation patterns (μCAPs) and employed this method on the human genetic model of schizophrenia 22q11.2 deletion syndrome (22q11.2DS). Participants underwent resting-state functional MRI and a data-driven iterative process resulting in the identification of six whole-brain μCAPs with specific activity patterns within the thalamus. Unlike conventional methods, μCAPs extract dynamic spatial patterns that reveal partially overlapping and non-mutually exclusive functional subparts. Thus, the μCAPs method detects finer foci of activity within the initial seed region, retaining valuable and clinically relevant temporal and spatial information. We found that a μCAP showing co-activation of the mediodorsal thalamus with brain-wide cortical regions was expressed significantly less frequently in patients with 22q11.2DS, and its occurrence negatively correlated with the severity of positive psychotic symptoms. Additionally, activity within the auditory-visual cortex and their respective geniculate nuclei was expressed in two different μCAPs. One of these auditory-visual μCAPs co-activated with salience areas, while the other co-activated with the default mode network (DMN). A significant shift of occurrence from the salience+visuo-auditory-thalamus to the DMN + visuo-auditory-thalamus μCAP was observed in patients with 22q11.2DS. Thus, our findings support existing research on the gatekeeping role of the thalamus for sensory information in the pathophysiology of psychosis and revisit the evidence of geniculate nuclei hyperconnectivity with the audio-visual cortex in 22q11.2DS in the context of dynamic functional connectivity, seen here as the specific hyper-occurrence of these circuits with the task-negative brain networks.

Neuromodulation of the anterior thalamus: Current approaches and opportunities for the future

The role of thalamocortical circuits in memory has driven a recent burst of scholarship, especially in animal models. Investigating this circuitry in humans is more challenging. And yet, the development of new recording and stimulation technologies deployed for clinical indications has created novel opportunities for data collection to elucidate the cognitive roles of thalamic structures. These technologies include stereoelectroencephalography (SEEG), deep brain stimulation (DBS), and responsive neurostimulation (RNS), all of which have been applied to memory-related thalamic regions, specifically for seizure localization and treatment. This review seeks to summarize the existing applications of neuromodulation of the anterior thalamic nuclei (ANT) and highlight several devices and their capabilities that can allow cognitive researchers to design experiments to assay its functionality. Our goal is to introduce to investigators, who may not be familiar with these clinical devices, the capabilities, and limitations of these tools for understanding the neurophysiology of the ANT as it pertains to memory and other behaviors. We also briefly cover the targeting of other thalamic regions including the centromedian (CM) nucleus, dorsomedial (DM) nucleus, and pulvinar, with associated potential avenues of experimentation.

Convergence of location, direction, and theta in the rat anteroventral thalamic nucleus

The thalamus and cortex are anatomically interconnected, with the thalamus providing integral information for cortical functions. The anteroventral thalamic nucleus (AV) is reciprocally connected to retrosplenial cortex (RSC). Two distinct AV subfields, dorsomedial (AVDM) and ventrolateral (AVVL), project differentially to granular vs. dysgranular RSC, respectively. To probe if functional responses of AV neurons differ, we recorded single neurons and local field potentials from AVDM and AVVL in rats during foraging. We observed place cells (neurons modulated by spatial location) in both AVDM and AVVL. Additionally, we characterized neurons modulated by theta oscillations, heading direction, and a conjunction of these. Place cells and conjunctive Theta-by-Head direction cells were more prevalent in AVVL; more non-conjunctive theta and directional neurons were prevalent in AVDM. These findings add further evidence that there are two thalamocortical circuits connecting AV and RSC, and reveal that the signaling involves place information in addition to direction and theta.

Anteromedial thalamus gates the selection and stabilization of long-term memories

Memories initially formed in hippocampus gradually stabilize to cortex over weeks-to-months for long-term storage. The mechanistic details of this brain re-organization remain poorly understood. We recorded bulk neural activity in circuits that link hippocampus and cortex as mice performed a memory-guided virtual-reality task over weeks. We identified a prominent and sustained neural correlate of memory in anterior thalamus, whose inhibition substantially disrupted memory consolidation. More strikingly, gain amplification enhanced consolidation of otherwise unconsolidated memories. To gain mechanistic insights, we developed a technology for simultaneous cellular-resolution imaging of hippocampus, thalamus, and cortex throughout consolidation. We found that whereas hippocampus equally encodes multiple memories, the anteromedial thalamus preferentially encodes salient memories, and gradually increases correlations with cortex to facilitate tuning and synchronization of cortical ensembles. We thus identify a thalamo-cortical circuit that gates memory consolidation and propose a mechanism suitable for the selection and stabilization of hippocampal memories into longer-term cortical storage.

Nonlinear Theta-Gamma Coupling between the Anterior Thalamus and Hippocampus Increases as a Function of Running Speed

The hippocampal theta rhythm strongly correlates to awake behavior leading to theories that it represents a cognitive state of the brain. As theta has been observed in other regions of the Papez circuit, it has been theorized that activity propagates in a reentrant manner. These observations complement the energy cascade hypothesis in which large-amplitude, slow-frequency oscillations reflect activity propagating across a large population of neurons. Higher frequency oscillations, such as gamma, are related to the speed with which inhibitory and excitatory neurons interact and distribute activity on the local level. The energy cascade hypothesis suggests that the larger anatomic loops, maintaining theta, drive the smaller loops. As hippocampal theta increases in power with running speed, so does the power and frequency of the gamma rhythm. If theta is propagated through the circuit, it stands to reason that the local field potential (LFP) recorded in other regions would be coupled to the hippocampal theta, with the coupling increasing with running speed. We explored this hypothesis using open-source simultaneous recorded data from the CA1 region of the hippocampus and the anterior dorsal and anterior ventral thalamus. Cross-regional theta coupling increased with running speed. Although the power of the gamma rhythm was lower in the anterior thalamus, there was an increase in the coupling of hippocampal theta to anterior thalamic gamma. Broadly, the data support models of how activity moves across the nervous system, suggesting that the brain uses large-scale volleys of activity to support higher cognitive processes.

Anteromedial Thalamus Gates the Selection & Stabilization of Long-Term Memories

Memories initially formed in hippocampus gradually stabilize to cortex, over weeks-to-months, for long-term storage. The mechanistic details of this brain re-organization process remain poorly understood. In this study, we developed a virtual-reality based behavioral task and observed neural activity patterns associated with memory reorganization and stabilization over weeks-long timescales. Initial photometry recordings in circuits that link hippocampus and cortex revealed a unique and prominent neural correlate of memory in anterior thalamus that emerged in training and persisted for several weeks. Inhibition of the anteromedial thalamus-to-anterior cingulate cortex projections during training resulted in substantial memory consolidation deficits, and gain amplification more strikingly, was sufficient to enhance consolidation of otherwise unconsolidated memories. To provide mechanistic insights, we developed a new behavioral task where mice form two memories, of which only the more salient memory is consolidated, and also a technology for simultaneous and longitudinal cellular resolution imaging of hippocampus, thalamus, and cortex throughout the consolidation window. We found that whereas hippocampus equally encodes multiple memories, the anteromedial thalamus forms preferential tuning to salient memories, and establishes inter-regional correlations with cortex, that are critical for synchronizing and stabilizing cortical representations at remote time. Indeed, inhibition of this thalamo-cortical circuit while imaging in cortex reveals loss of contextual tuning and ensemble synchrony in anterior cingulate, together with behavioral deficits in remote memory retrieval. We thus identify a thalamo-cortical circuit that gates memory consolidation and propose a mechanism suitable for the selection and stabilization of hippocampal memories into longer term cortical storage.

Running speed and REM sleep control two distinct modes of rapid interhemispheric communication

Rhythmic gamma-band communication within and across cortical hemispheres is critical for optimal perception, navigation, and memory. Here, using multisite recordings in both rats and mice, we show that even faster ~140 Hz rhythms are robustly anti-phase across cortical hemispheres, visually resembling splines, the interlocking teeth on mechanical gears. Splines are strongest in superficial granular retrosplenial cortex, a region important for spatial navigation and memory. Spline-frequency interhemispheric communication becomes more coherent and more precisely anti-phase at faster running speeds. Anti-phase splines also demarcate high-activity frames during REM sleep. While splines and associated neuronal spiking are anti-phase across retrosplenial hemispheres during navigation and REM sleep, gamma-rhythmic interhemispheric communication is precisely in-phase. Gamma and splines occur at distinct points of a theta cycle and thus highlight the ability of interhemispheric cortical communication to rapidly switch between in-phase (gamma) and anti-phase (spline) modes within individual theta cycles during both navigation and REM sleep.

Gamma-rhythmic communication within and across cortical hemispheres is critical for optimal perception, navigation, and memory. Here, Ghosh et al. identify even faster ~140 Hz rhythms, named splines, that reflect anti-phase neuronal synchrony across hemispheres. The balance of anti-phase spline and in-phase gamma communication is dynamically controlled by behavior and sleep.

Anterior thalamic circuits crucial for working memory

Alterations in the structure and functional connectivity of anterior thalamic nuclei (ATN) have been linked to reduced cognition during aging. However, ATN circuits that contribute to higher cognitive functions remain understudied. We found that the anteroventral (AV) subdivision of ATN is necessary specifically during the maintenance phase of a spatial working memory task. This function engages the AV→parasubiculum (PaS)→entorhinal cortex (EC) circuit. Aged mice showed a deficit in spatial working memory, which was associated with a decrease in the excitability of AV neurons. Activation of AV neurons or the AV→PaS circuit in aged mice was sufficient to rescue their working memory performance. Furthermore, rescued aged mice showed improved behavior-induced neuronal activity in prefrontal cortex (PFC), a critical site for working memory processes. Although the direct activation of PFC neurons in aged mice also rescued their working memory performance, we found that these animals exhibited increased levels of anxiety, which was not the case for AV→PaS circuit manipulations in aged mice. These results suggest that targeting AV thalamus in aging may not only be beneficial for cognitive functions but that this approach may have fewer unintended effects compared to direct PFC manipulations.

The limbic memory circuit and the neural basis of contextual memory

The hippocampus, retrosplenial cortex and anterior thalamus are key components of a neural circuit known to be involved in a variety of memory functions, including spatial, contextual and episodic memory. In this review, we focus on the role of this circuit in contextual memory processes. The background environment, or context, is a powerful cue for memory retrieval, and neural representations of the context provide a mechanism for efficiently retrieving relevant memories while avoiding interference from memories that belong to other contexts. Data from experimental lesions and neural manipulation techniques indicate that each of these regions is critical for contextual memory. Neurophysiological evidence from the hippocampus and retrosplenial cortex suggest that contextual information is represented within this circuit by population-level neural firing patterns that reliably differentiate each context a subject encounters. These findings indicate that encoding contextual information to support context-dependent memory retrieval is a key function of this circuit.

Characterization and manipulation of Corticothalamic neurons in associative cortices using Syt6-Cre transgenic mice

Corticothalamic interactions between associative cortices and higher order thalamic nuclei are involved in high-cognitive functions such as decision-making and working memory. Corticothalamic neurons (CTn) in the prefrontal cortex and other associative areas have been much less studied than their counterparts in the primary sensory areas. The availability of characterized transgenic tools to study CTn in associative areas will facilitate their study and contribute to overcome the scarcity of data about their properties, network dynamics, and contribution to cognitive functions. Here, we characterized the Syt6-Cre (KI148Gsat/Mmud) transgenic mouse line, by tracking expression of a Cre-mediated reporter. In this line, Cre-reporter is strongly expressed in the prefrontal, motor, cingulate, and retrosplenial cortices, as well as in other brain areas including the cerebellum and the olfactory tubercle. Cortical expression starts embryonically and reaches the adult expression pattern by postnatal day 15. In the cortex, Cre-reporter is expressed by layer 6-CTn and by layer 5-CTn to a lesser extent. We quantified Syt6-Cre⁺ CTn axon varicosities to estimate the distribution and density of putative corticothalamic driver and modulator inputs to thalamic nuclei in the medial, midline, intralaminar, anterior, and motor groups. Also, we characterized the effect of optogenetic stimulation of Syt6-Cre⁺ neurons in the activity of the prefrontal cortex. CTn stimulation in the prefrontal cortex induces an oscillatory activity in the local field potential that resembles the cortical downstates typically observed during slow-wave sleep or quiet wake.

The nucleus reuniens orchestrates prefrontal-hippocampal synchrony during spatial working memory

Spatial working memory, the ability to temporarily maintain an internal representation of spatial information for use in guiding upcoming decisions, has been shown to be dependent upon a network of brain structures that includes the hippocampus, a region known to be critical for spatial navigation and episodic memory, and the prefrontal cortex (PFC), a region known to be critical for executive function and goal directed behavior. Oscillatory synchronization between the hippocampus and the prefrontal cortex (PFC) is known to increase in situations of high working memory demand. Most of our knowledge about the anatomical connectivity between the PFC and hippocampus comes from the rodent literature. Thus, most of the findings that will be discussed here model human working memory using spatial working memory-dependent maze navigation tasks in rodents. It has been demonstrated that the ventral midline thalamic nucleus reuniens (Re) is reciprocally connected to both the infralimbic and prelimbic subregions of the PFC, collectively referred to as the medial PFC (mPFC), and the hippocampus. Given that the Re serves as a major anatomical route between the mPFC and hippocampus, it is perhaps not surprising that Re has been shown to be critical for spatial working memory. This review will describe the latest findings and ideas on how the Re contributes to prefrontal-hippocampal synchronization and spatial working memory in rodents. The review will conclude with possible future directions that will advance the understanding of the mechanisms that enable the Re to orchestrate long range synchrony in the prefrontal-hippocampal network.

Thalamocortical interactions in cognition and disease: the mediodorsal and anterior thalamic nuclei

The mediodorsal thalamus (MD) and anterior thalamic nuclei (ATN) are two adjacent brain nodes that support our ability to make decisions, learn, update information, form and retrieve memories, and find our way around. The MD and PFC work in partnerships to support cognitive processes linked to successful learning and decision-making, while the ATN and extended hippocampal system together coordinate the encoding and retrieval of memories and successful spatial navigation. Yet, while these distinctions may appear to be segregated, both the MD and ATN together support our higher cognitive functions as they regulate and are influenced by interconnected fronto-temporal neural networks and subcortical inputs. Our review focuses on recent studies in animal models and in humans. This evidence is re-shaping our understanding of the importance of MD and ATN cortico-thalamocortical pathways in influencing complex cognitive functions. Given the evidence from clinical settings and neuroscience research labs, the MD and ATN should be considered targets for effective treatments in neuropsychiatric diseases and disorders and neurodegeneration.

A Thalamic Reticular Circuit for Head Direction Cell Tuning and Spatial Navigation

As we navigate in space, external landmarks and internal information guide our movement. Circuit and synaptic mechanisms that integrate these cues with head-direction (HD) signals remain, however, unclear. We identify an excitatory synaptic projection from the presubiculum (PreS) and the multisensory-associative retrosplenial cortex (RSC) to the anterodorsal thalamic reticular nucleus (TRN), so far classically implied in gating sensory information flow. In vitro, projections to TRN involve AMPA/NMDA-type glutamate receptors that initiate TRN cell burst discharge and feedforward inhibition of anterior thalamic nuclei. In vivo, chemogenetic anterodorsal TRN inhibition modulates PreS/RSC-induced anterior thalamic firing dynamics, broadens the tuning of thalamic HD cells, and leads to preferential use of allo- over egocentric search strategies in the Morris water maze. TRN-dependent thalamic inhibition is thus an integral part of limbic navigational circuits wherein it coordinates external sensory and internal HD signals to regulate the choice of search strategies during spatial navigation.

AMPA Receptor Auxiliary Subunit GSG1L Suppresses Short-Term Facilitation in Corticothalamic Synapses and Determines Seizure Susceptibility

The anterior thalamus (AT) is critical for memory formation, processing navigational information, and seizure initiation. However, the molecular mechanisms that regulate synaptic function of AT neurons remain largely unexplored. We report that AMPA receptor auxiliary subunit GSG1L controls short-term plasticity in AT synapses that receive inputs from the cortex, but not in those receiving inputs from other pathways. A canonical auxiliary subunit stargazin co-exists in these neurons but is functionally absent from corticothalamic synapses. In GSG1L knockout mice, AT neurons exhibit hyperexcitability and the animals have increased susceptibility to seizures, consistent with a negative regulatory role of GSG1L. We hypothesize that negative regulation of synaptic function by GSG1L plays a critical role in maintaining optimal excitation in the AT.

Kamalova et al. report the phenotypes of GSG1L KO mice. The synaptic function of AMPAR auxiliary subunit GSG1L in the anterior thalamus is input specific. GSG1L suppresses short-term facilitation and decreases AMPAR activity specifically in corticothalamic synapses, where stargazin is functionally absent. GSG1L KO mice exhibit hyperexcitability and seizure susceptibility.

The anterior thalamic nuclei and cognition: A role beyond space?

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Anterior thalamic nuclei important for specific classes of temporal discriminations.

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Anterior thalamic nuclei required for hippocampal-dependent contextual processes.

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Critical role for anterior thalamic nuclei in selective attention.

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Significance of anterior thalamic – anterior cingulate interactions.

The anterior thalamic nuclei are a vital node within hippocampal-diencephalic-cingulate circuits that support spatial learning and memory. Reflecting this interconnectivity, the overwhelming focus of research into the cognitive functions of the anterior thalamic nuclei has been spatial processing. However, there is increasing evidence that the functions of the anterior thalamic nuclei extend beyond the spatial realm. This work has highlighted how these nuclei are required for certain classes of temporal discrimination as well as their importance for processing other contextual information; revealing parallels with the non-spatial functions of the hippocampal formation. Yet further work has shown how the anterior thalamic nuclei may be important for other forms of non-spatial learning, including a critical role for these nuclei in attentional mechanisms. This evidence signals the need to reconsider the functions of the anterior thalamic within the framework of their wider connections with sites including the anterior cingulate cortex that subserve non-spatial functions.

Time to put the mammillothalamic pathway into context

The medial diencephalon, in particular the mammillary bodies and anterior thalamic nuclei, has long been linked to memory and amnesia. The mammillary bodies provide a dense input into the anterior thalamic nuclei, via the mammillothalamic tract. In both animal models, and in patients, lesions of the mammillary bodies, mammillothalamic tract and anterior thalamic nuclei all produce severe impairments in temporal and contextual memory, yet it is uncertain why these regions are critical. Mounting evidence from electrophysiological and neural imaging studies suggests that mammillothalamic projections exercise considerable distal influence over thalamo-cortical and hippocampo-cortical interactions. Here, we outline how damage to the mammillary body-anterior thalamic axis, in both patients and animal models, disrupts behavioural performance on tasks that relate to contextual ("where") and temporal ("when") processing. Focusing on the medial mammillary nuclei as a possible 'theta-generator' (through their interconnections with the ventral tegmental nucleus of Gudden) we discuss how the mammillary body-anterior thalamic pathway may contribute to the mechanisms via which the hippocampus and neocortex encode representations of experience.

Flexible and specific contributions of thalamic subdivisions to human cognition

The thalamus participates in multiple functional brain networks supporting different cognitive abilities. How thalamo-cortical connections map onto the architecture of human cognition remains an outstanding question. The aim of this meta-analysis is to map co-activation between thalamic and extra-thalamic brain regions onto separate cognitive domains and to assess thalamic subdivision specificity within each of the cognitive domains considered. We parsed 93 fMRI studies into twelve cognitive domains. Signed Differential Mapping served to obtain co-activation maps. We then projected the contribution of thalamic subdivisions onto a thalamic atlas to assess cognitive domain specificity. A set of brain regions was flexibly involved with thalamus in several cognitive domains. Thalamic subdivisions showed ample cognitive heterogeneity. Our proposed model represents thalamic involvement in cognition as an "ensemble" of functional subdivisions with common cell properties embedded in separate cortical circuits rather than a homogeneous functional unit.

Individual Subnuclei of the Rat Anterior Thalamic Nuclei Differently affect Spatial Memory and Passive Avoidance Tasks

The role of the anterior thalamic nuclei (ATN) has been proven in different learning and memory tasks. The ATN consist of three main subnuclei, the anterodorsal (AD), anteroventral (AV) and anteromedial (AM), which have different biological characteristics such as distinct circuitry, cell population and neurotransmitter content. The role of ATN subnuclei in learning and memory has been shown in several studies. However, their probable role in different phases of memory including acquisition, consolidation and retrieval are not still well-known. For this purpose, the effect of reversible inactivation of each ATN subnucleus on different memory phases in two behavioral tasks including passive avoidance (PA) and Morris water maze (MWM) was studied. Wister male rats were bilaterally implanted with cannulas above the AD, AV or AM subnucleus in separate experimental groups in order to inject lidocaine (4%) for their temporal inactivation or, equal volume of saline. Animals were trained in the behavioral tasks and different phases of memory were investigated. Our findings indicated that the AV inactivation strongly disrupts all memory phases in the MWM, and consolidation and retrieval phases in the PA tasks. The AM inactivation had no effect on acquisition of both tasks while it impaired the PA consolidation and MWM retrieval. However, the AD inactivation could not disrupt memory phases in the PA task but impaired the MWM retrieval. In conclusion, it seems that the ATN distinct subnuclei differently affect different phases of memory in these two tasks.

Deconstructing the Direct Reciprocal Hippocampal-Anterior Thalamic Pathways for Spatial Learning

The hippocampus is essential for normal memory but does not act in isolation. The anterior thalamic nuclei may represent one vital partner. Using DREADDs, the behavioral consequences of transiently disrupting anterior thalamic function were examined, followed by inactivation of the dorsal subiculum. Next, the anterograde transport of an adeno-associated virus expressing DREADDs was paired with localized intracerebral infusions of a ligand to target specific input pathways. In this way, the direct projections from the anterior thalamic nuclei to the dorsal hippocampal formation were inhibited, followed by separate inhibition of the dorsal subiculum projections to the anterior thalamic nuclei. To assay spatial working memory, all animals performed a reinforced T-maze alternation task, then a more challenging version that nullifies intramaze cues. Across all four experiments, deficits emerged on the spatial alternation task that precluded the use of intramaze cues. Inhibiting dorsal subiculum projections to the anterior thalamic nuclei produced the severest spatial working memory deficit. This deficit revealed the key contribution of dorsal subiculum projections to the anteromedial and anteroventral thalamic nuclei for the processing of allocentric information, projections not associated with head-direction information. The overall pattern of results provides consistent causal evidence of the two-way functional significance of direct hippocampal-anterior thalamic interactions for spatial processing. At the same time, these findings are consistent with hypotheses that these same, reciprocal interactions underlie the common core symptoms of temporal lobe and diencephalic anterograde amnesia.

SIGNIFICANCE STATEMENT It has long been conjectured that the anterior thalamic nuclei might be key partners with the hippocampal formation and that, respectively, they are principally responsible for diencephalic and temporal lobe amnesia. However, direct causal evidence for this functional relationship is lacking. Here, we examined the behavioral consequences of transiently silencing the direct reciprocal interconnections between these two brain regions on tests of spatial learning. Disrupting information flow from the hippocampal formation to the anterior thalamic nuclei and vice versa impaired performance on tests of spatial learning. By revealing the conjoint importance of hippocampal-anterior thalamic pathways, these findings help explain why pathology in either the medial diencephalon or the medial temporal lobes can result in profound anterograde amnesic syndromes.

Bilateral postsubiculum lesions impair visual and nonvisual homing performance in rats

Nearly all species rely on visual and nonvisual cues to guide navigation, and which ones they use depend on the environment and task demands. The postsubiculum (PoS) is a crucial brain region for the use of visual cues, but its role in the use of self-movement cues is less clear. We therefore evaluated rats' navigational performance on a food-carrying task in light and in darkness in rats that had bilateral neurotoxic lesions of the PoS. Animals were trained postoperatively to exit a refuge and search for a food pellet, and carry it back to the refuge for consumption. In both light and darkness, control and PoS-lesioned rats made circuitous outward journeys as they searched for food. However, only control rats were able to accurately use visual or self-movement cues to make relatively direct returns to the home refuge. These results suggest the PoS's role in navigation is not limited to the use of visual cues, but also includes the use of self-movement cues. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Separate cortical and hippocampal cell populations target the rat nucleus reuniens and mammillary bodies

Nucleus reuniens receives dense projections from both the hippocampus and the frontal cortices. Reflecting these connections, this nucleus is thought to enable executive functions, including those involving spatial learning. The mammillary bodies, which also support spatial learning, again receive dense hippocampal inputs, as well as lighter projections from medial frontal areas. The present study, therefore, compared the sources of these inputs to nucleus reuniens and the mammillary bodies. Retrograde tracer injections in rats showed how these two diencephalic sites receive projections from separate cell populations, often from adjacent layers in the same cortical areas. In the subiculum, which projects strongly to both sites, the mammillary body inputs originate from a homogenous pyramidal cell population in more superficial levels, while the cells that target nucleus reuniens most often originate from cells positioned at a deeper level. In these deeper levels, a more morphologically diverse set of subiculum cells contributes to the thalamic projection, especially at septal levels. While both diencephalic sites also receive medial frontal inputs, those to nucleus reuniens are especially dense. The densest inputs to the mammillary bodies appear to arise from the dorsal peduncular cortex, where the cells are mostly separate from deeper neurons that project to nucleus reuniens. Again, in those other cortical regions that innervate both nucleus reuniens and the mammillary bodies, there was no evidence of collateral projections. The findings support the notion that these diencephalic nuclei represent components of distinct, but complementary, systems that support different aspects of cognition.

Proximal perimeter encoding in the rat rostral thalamus

Perimeters are an important part of the environment, delimiting its geometry. Here, we investigated how perimeters (vertical walls; vertical drops) affect neuronal responses in the rostral thalamus (the anteromedial and parataenial nuclei in particular). We found neurons whose firing patterns reflected the presence of walls and drops, irrespective of arena shape. Their firing patterns were stable across multiple sleep-wake cycles and were independent of ambient lighting conditions. Thus, rostral thalamic nuclei may participate in spatial representation by encoding the perimeters of environments.

Anterior thalamic nuclei, but not retrosplenial cortex, lesions abolish latent inhibition in rats

The present study examined the effects of excitotoxic lesions in 2 closely related structures, the anterior thalamic nuclei and the retrosplenial cortex, on latent inhibition. Latent inhibition occurs when nonreinforced preexposure to a stimulus retards the subsequent acquisition of conditioned responding to that stimulus. Latent inhibition was assessed in a within-subject procedure with auditory stimuli and food reinforcement. As expected, sham-operated animals were slower to acquire conditioned responding to a stimulus that had previously been experienced without consequence, relative to a non-preexposed stimulus. This latent inhibition effect was absent in rats with excitotoxic lesions in the anterior thalamic nuclei, as these animals conditioned to both stimuli at equivalent rates. The retrosplenial lesions appeared to spare latent inhibition, as these animals displayed a robust stimulus preexposure effect. The demonstration here that anterior thalamic nuclei lesions abolish latent inhibition is consistent with emerging evidence of the importance of these thalamic nuclei for attentional control. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Crybb2 Mutations Consistently Affect Schizophrenia Endophenotypes in Mice

As part of the βγ-superfamily, βB2-crystallin (CRYBB2) is an ocular structural protein in the lens, and mutation of the corresponding gene can cause cataracts. CRYBB2 also is expressed in non-lens tissue such as the adult mouse brain and is associated with neuropsychiatric disorders such as schizophrenia. Nevertheless, the robustness of this association as well as how CRYBB2 may contribute to disease-relevant phenotypes is unknown. To add further clarity to this issue, we performed a comprehensive analysis of behavioral and neurohistological alterations in mice with an allelic series of mutations in the C-terminal end of the Crybb2 gene. Behavioral phenotyping of these three βB2-mutant lines Crybb2^O377, Crybb2^Philly, and Crybb2^Aey2 included assessment of exploratory activity and anxiety-related behavior in the open field, sensorimotor gating measured by prepulse inhibition (PPI) of the acoustic startle reflex, cognitive performance measured by social discrimination, and spontaneous alternation in the Y-maze. In each mutant line, we also quantified the number of parvalbumin-positive (PV+) GABAergic interneurons in selected brain regions that express CRYBB2. While there were allele-specific differences in individual behaviors and affected brain areas, all three mutant lines exhibited consistent alterations in PPI that paralleled alterations in the PV+ cell number in the thalamic reticular nucleus (TRN). The direction of the PPI change mirrored that of the TRN PV+ cell number thereby suggesting a role for TRN PV+ cell number in modulating PPI. Moreover, as both altered PPI and PV+ cell number are schizophrenia-associated endophenotypes, our result implicates mutated Crybb2 in the development of this neuropsychiatric disorder.

Routes to, from and within the subiculum

The subiculum is one of the major output areas of the hippocampus and has extensive projections to extrahippocampal targets. It is likely to play a pivotal role in the distribution of outgoing information from the hippocampus. The hippocampus, including the subiculum, is important for the formation, consolidation and retrieval of memory. These functions require a network that is flexible enough to encode incoming information and also allows for reliable distribution, storage and integration into previously encoded memories. Finally, relevant information has to be retrieved in a context-specific manner to allow for an appropriate behavioral response. The subiculum as a gateway between the hippocampus and cortex might serve to integrate and process information from the hippocampus proper and its other inputs before conveying it to more permanent storage locations. This review summarizes how the subiculum is embedded into upstream and downstream circuits, describes what is known about the local network topology and discusses cellular and functional properties of subicular cells subtypes. Lastly, it describes how these properties might help to separate information into parallel output streams and distribute it to its multiple target areas.

Dissociating Landmark Stability from Orienting Value Using Functional Magnetic Resonance Imaging

Retrosplenial cortex (RSC) plays a role in using environmental landmarks to help orientate oneself in space. It has also been consistently implicated in processing landmarks that remain fixed in a permanent location. However, it is not clear whether the RSC represents the permanent landmarks themselves or instead the orienting relevance of these landmarks. In previous functional magnetic resonance imaging (fMRI) studies, these features have been conflated-stable landmarks were always useful for orienting. Here, we dissociated these two key landmark attributes to investigate which one best reflects the function of the RSC. Before scanning, participants learned the features of novel landmarks about which they had no prior knowledge. During fMRI scanning, we found that the RSC was more engaged when people viewed permanent compared with transient landmarks and was not responsive to the orienting relevance of landmarks. Activity in RSC was also related to the amount of landmark permanence information a person had acquired and, as knowledge increased, the more the RSC drove responses in the anterior thalamus while viewing permanent landmarks. In contrast, the angular gyrus and the hippocampus were engaged by the orienting relevance of landmarks, but not their permanence, with the hippocampus also sensitive to the distance between relevant landmarks and target locations. We conclude that the coding of permanent landmarks in RSC may drive processing in regions like anterior thalamus, with possible implications for the efficacy of functions such as navigation.

Teneurin-3 controls topographic circuit assembly in the hippocampus

Brain functions rely on specific patterns of connectivity. Teneurins are evolutionarily conserved transmembrane proteins that instruct synaptic partner matching in Drosophila and are required for vertebrate visual system development. The roles of vertebrate teneurins in connectivity beyond the visual system remain largely unknown and their mechanisms of action have not been demonstrated. Here we show that mouse teneurin-3 is expressed in multiple topographically interconnected areas of the hippocampal region, including proximal CA1, distal subiculum, and medial entorhinal cortex. Viral-genetic analyses reveal that teneurin-3 is required in both CA1 and subicular neurons for the precise targeting of proximal CA1 axons to distal subiculum. Furthermore, teneurin-3 promotes homophilic adhesion in vitro in a splicing isoform-dependent manner. These findings demonstrate striking genetic heterogeneity across multiple hippocampal areas and suggest that teneurin-3 may orchestrate the assembly of a complex distributed circuit in the mammalian brain via matching expression and homophilic attraction.

Collateral Projections Innervate the Mammillary Bodies and Retrosplenial Cortex: A New Category of Hippocampal Cells

To understand the hippocampus, it is necessary to understand the subiculum. Unlike other hippocampal subfields, the subiculum projects to almost all distal hippocampal targets, highlighting its critical importance for external networks. The present studies, in male rats and mice, reveal a new category of dorsal subiculum neurons that innervate both the mammillary bodies (MBs) and the retrosplenial cortex (RSP). These bifurcating neurons comprise almost half of the hippocampal cells that project to RSP. The termination of these numerous collateral projections was visualized within the medial mammillary nucleus and the granular RSP (area 29). These collateral projections included subiculum efferents that cross to the contralateral MBs. Within the granular RSP, the collateral projections form a particularly dense plexus in deep Layer II and Layer III. This retrosplenial termination site colocalized with markers for VGluT2 and neurotensin. While efferents from the hippocampal CA fields standardly collateralize, subiculum projections often have only one target site. Consequently, the many collateral projections involving the RSP and the MBs present a relatively unusual pattern for the subiculum, which presumably relates to how both targets have complementary roles in spatial processing. Furthermore, along with the anterior thalamic nuclei, the MBs and RSP are key members of a memory circuit, which is usually described as both starting and finishing in the hippocampus. The present findings reveal how the hippocampus simultaneously engages different parts of this circuit, so forcing an important revision of this network.

3D imaging of the brain morphology and connectivity defects in a model of psychiatric disorders: MAP6-KO mice

In the central nervous system, microtubule-associated protein 6 (MAP6) is expressed at high levels and is crucial for cognitive abilities. The large spectrum of social and cognitive impairments observed in MAP6-KO mice are reminiscent of the symptoms observed in psychiatric diseases, such as schizophrenia, and respond positively to long-term treatment with antipsychotics. MAP6-KO mice have therefore been proposed to be a useful animal model for these diseases. Here, we explored the brain anatomy in MAP6-KO mice using high spatial resolution 3D MRI, including a volumetric T_1w method to image brain structures, and Diffusion Tensor Imaging (DTI) for white matter fiber tractography. 3D DTI imaging of neuronal tracts was validated by comparing results to optical images of cleared brains. Changes to brain architecture included reduced volume of the cerebellum and the thalamus and altered size, integrity and spatial orientation of some neuronal tracks such as the anterior commissure, the mammillary tract, the corpus callosum, the corticospinal tract, the fasciculus retroflexus and the fornix. Our results provide information on the neuroanatomical defects behind the neurological phenotype displayed in the MAP6-KO mice model and especially highlight a severe damage of the corticospinal tract with defasciculation at the location of the pontine nuclei.

The effect of pharmacological inactivation of the mammillary body and anterior thalamic nuclei on hippocampal theta rhythm in urethane-anesthetized rats

The mammillary body (MB) and the anterior thalamic nuclei (ATN) are closely related structures, which take part in learning and memory processes. However, the exact role of these structures has remained unclear. In both structures neurons firing according to hippocampal theta rhythm have been found, mainly in the medial mammillary nucleus (MM) and anteroventral thalamic nucleus (AV). These neurons are driven by descending projections from the hippocampal formation and are thought to convey theta rhythm back to the hippocampus (HP). We argue that the MB-ATN axis not only relays theta signal, but may also modulate it. To examine it, we performed a pharmacological inactivation of the MM and AV by local infusion of procaine, and measured changes in theta activity in selected structures of the extended hippocampal system in urethane-anesthetized rats. The inactivation of the MM resulted in decrease in EEG power in the HP and AV, the most evidently in the lower theta frequency bands, i.e. 3-5Hz in the HP (down to 9.2% in 3- to 4-Hz band and 37.6% in 4- to 5-Hz band, in comparison to the power in the control conditions) and 3-4Hz in the AV (down to 24.9%). After the AV inactivation, hippocampal EEG power decreased in theta frequency bands of 3-8Hz (down to 61.6% in 6- to 7-Hz band and 69.4% in 7- to 8-Hz band). Our results suggest that the role of the MB-ATN axis in regulating theta rhythm signaling may be much more important than has been speculated so far.

Hippocampal - diencephalic - cingulate networks for memory and emotion: An anatomical guide

This review brings together current knowledge from tract tracing studies to update and reconsider those limbic connections initially highlighted by Papez (1937) for their presumed role in emotion. These connections link hippocampal and parahippocampal regions with the mammillary bodies, the anterior thalamic nuclei, and the cingulate gyrus, all structures now strongly implicated in memory functions. An additional goal of this review is to describe the routes taken by the various connections within this network. The original descriptions of these limbic connections saw their interconnecting pathways forming a serial circuit that began and finished in the hippocampal formation. It is now clear that, with the exception of the mammillary bodies, these various sites are multiply interconnected with each other, including many reciprocal connections. In addition, these same connections are topographically organised, creating further subsystems. This complex pattern of connectivity helps to explain the difficulty of interpreting the functional outcome of damage to any individual site within the network. For these same reasons, Papez' initial concept of a loop beginning and ending in the hippocampal formation needs to be seen as a much more complex system of hippocampal-diencephalic-cingulate connections. The functions of these multiple interactions might be better viewed as principally providing efferent information from the posterior medial temporal lobe. Both a subcortical diencephalic route (via the fornix) and a cortical cingulate route (via retrosplenial cortex) can be distinguished. These routes provide indirect pathways for hippocampal interactions with prefrontal cortex, with the preponderance of both sets of connections arising from the more posterior hippocampal regions. These multi-stage connections complement the direct hippocampal projections to prefrontal cortex, which principally arise from the anterior hippocampus, thereby creating longitudinal functional differences along the anterior-posterior plane of the hippocampus.

In vivo visualization of connections among revised Papez circuit hubs using full q-space diffusion spectrum imaging tractography

Structural connections among the hubs of the revised Papez circuit remain to be elucidated in the human brain. As the original Papez circuit failed to explain functional imaging findings, a more detailed investigation is needed to delineate connections among the circuit's key hubs. Here we acquired diffusion spectrum imaging (DSI) from eight normal subjects and used data from the Human Connectome Project (HCP) to elucidate connections among hubs in the retrosplenial gyrus, hippocampus, mammillary bodies, and anterior thalamic nuclei. Our results show that the ventral hippocampal commissure (VHC) was visualized in all eight individual DSI datasets, as well as in the DSI and HCP group datasets, but a strictly defined VHC was only visualized in one individual dataset. Thalamic fibers were observed to connect with both the posterior cingulate cortex (PCC) and retrosplenial cortex (RSC). The RSC was mainly responsible for direct hippocampal connections, while the PCC was not. This indicates that the RSC and PCC represent separate functional hubs in humans, as also shown by previous primate axonal tracing studies and functional magnetic resonance imaging observations.

Asymmetric cross-hemispheric connections link the rat anterior thalamic nuclei with the cortex and hippocampal formation

Dense reciprocal connections link the rat anterior thalamic nuclei with the prelimbic, anterior cingulate and retrosplenial cortices, as well as with the subiculum and postsubiculum. The present study compared the ipsilateral thalamic-cortical connections with the corresponding crossed, contralateral connections between these same sets of regions. All efferents from the anteromedial thalamic nucleus to the cortex, as well as those to the subiculum, remained ipsilateral. In contrast, all of these target sites provided reciprocal, bilateral projections to the anteromedial nucleus. While the anteroventral thalamic nucleus often shared this same asymmetric pattern of cortical connections, it received relatively fewer crossed inputs than the anteromedial nucleus. This difference was most marked for the anterior cingulate projections, as those to the anteroventral nucleus remained almost entirely ipsilateral. Unlike the anteromedial nucleus, the anteroventral nucleus also appeared to provide a restricted, crossed projection to the contralateral retrosplenial cortex. Meanwhile, the closely related laterodorsal thalamic nucleus had almost exclusively ipsilateral efferent and afferent cortical connections. Likewise, within the hippocampus, the postsubiculum seemingly had only ipsilateral efferent and afferent connections with the anterior thalamic and laterodorsal nuclei. While the bilateral cortical projections to the anterior thalamic nuclei originated predominantly from layer VI, the accompanying sparse projections from layer V largely gave rise to ipsilateral thalamic inputs. In testing a potentially unifying principle of anterior thalamic - cortical interactions, a slightly more individual pattern emerged that reinforces other evidence of functional differences within the anterior thalamic and also helps to explain the consequences of unilateral interventions involving these nuclei.

Retrieval of high-fidelity memory arises from distributed cortical networks

Medial temporal lobe (MTL) function is well established as necessary for memory of facts and events. It is likely that lateral cortical regions critically guide cognitive control processes to tune in high-fidelity details that are most relevant for memory retrieval. Here, convergent results from functional and structural MRI show that retrieval of detailed episodic memory arises from lateral cortical-MTL networks, including regions of inferior frontal and angular gyrii. Results also suggest that recognition of items based on low-fidelity, generalized information, rather than memory arising from retrieval of relevant episodic details, is not associated with functional connectivity between MTL and lateral cortical regions. Additionally, individual differences in microstructural properties in white matter pathways, associated with distributed MTL-cortical networks, are positively correlated with better performance on a mnemonic discrimination task.

Hippocampal shape alterations are associated with regional Aβ load in cognitively normal elderly individuals

Aβ deposition is a driving force of Alzheimer's disease pathology and can be detected early by amyloid positron emission tomography. Identifying presymptomatic structural brain changes associated with Aβ deposition might lead to a better understanding of its consequences and provide early diagnostic information. In this respect we analyzed measures of cortical thickness and subcortical volumes along with hippocampal, thalamic and striatal shape and surface area by applying novel analysis strategies for structural magnetic resonance imaging. We included 69 cognitively normal elderly subjects after careful clinical and neuropsychological workup. Standardized uptake value ratios (cerebellar reference) for uptake of 11-C-Pittsburgh Compound B (PiB) were calculated from positron emission tomographic data for a cortical measurement and for bilateral hippocampus, thalamus and striatum. Associations to shape, surface area, volume and cortical thickness were tested using regression models that included significant predictors as covariates. Left anterior hippocampal shape was associated with regional PiB uptake (P < 0.05, FDR corrected), whereas volumes of the hippocampi and their subregions were not associated with cortical or regional PiB uptake (all P > 0.05, FDR corrected). Within the entorhinal cortical region of both hemispheres, thickness was negatively associated with cortical PiB uptake (P < 0.05, FDR corrected). Hence, localized shape measures and cortical thickness may be potential biomarkers of presymptomatic Alzheimer's disease.

[Bilateral radiofrequency anterior thalamotomy in intractable epilepsy patients]

BACKGROUND

Identification of the crucial role of the anterior thalamic nuclei (ATN) in the generalization of seizures led to increased interest in surgical interventions in this particular area in intractable epilepsy patients. Simulation of ATN destruction in animals demonstrated its high efficacy for both preventing the seizure development and reducing the seizure rate. However, bilateral radiofrequency destruction of the anterior thalamic nuclei in humans has not yet bee described.

AIM

The study objective was to perform bilateral radiofrequency anterior thalamotomy in intractable epilepsy patients and to evaluate its

RESULTS

MATERIAL AND METHODS

We performed for the first time bilateral stereotactic radiofrequency thermocoagulation of ATN in 13 patients with long-term intractable epilepsy. Before surgery, we assessed the disease duration, age of seizure onset, localization of pathological activity sources, and types of seizures, morphological damages, and ongoing pharmacotherapy. All interventions were performed under local anesthesia and were accompanied by intraoperative microelectrode monitoring of the neuronal activity and by EEG.

RESULTS

Seven males and 6 females, aged 22 to 48 years, were operated on. All patients had epileptogenic foci in the frontal and/or temporal lobes. MRI revealed epileptogenic structural abnormalities in 3 patients. There were no postoperative complications. According to a postoperative examination, 5 patients were seizure-free; a decrease in the seizure rate was 70% in 6 patients and 50% in 1 patient; 1 patient had no response to the surgery. The resulting effect was manifested not only in a reduction in the frequency and severity of seizures but also in a decrease in the dose of administered anticonvulsants. EEG also showed a significant improvement in the majority of patients.

CONCLUSION

Our experience demonstrates that bilateral radiofrequency anterior thalamotomy is a safe and effective technique to control seizures in humans. Further research will clarify, based on the clinical and EEG data, the patient selection criteria for surgical treatment.

Complementary subicular pathways to the anterior thalamic nuclei and mammillary bodies in the rat and macaque monkey brain

The origins of the hippocampal (subicular) projections to the anterior thalamic nuclei and mammillary bodies were compared in rats and macaque monkeys using retrograde tracers. These projections form core components of the Papez circuit, which is vital for normal memory. The study revealed a complex pattern of subicular efferents, consistent with the presence of different, parallel information streams, whose segregation appears more marked in the rat brain. In both species, the cells projecting to the mammillary bodies and anterior thalamic nuclei showed laminar separation but also differed along other hippocampal axes. In the rat, these diencephalic inputs showed complementary topographies in the proximal–distal (columnar) plane, consistent with differential involvement in object‐based (proximal subiculum) and context‐based (distal subiculum) information. The medial mammillary inputs, which arose along the anterior–posterior extent of the rat subiculum, favoured the central subiculum (septal hippocampus) and the more proximal subiculum (temporal hippocampus). In contrast, anterior thalamic inputs were largely confined to the dorsal (i.e. septal and intermediate) subiculum, where projections to the anteromedial nucleus favoured the proximal subiculum while those to the anteroventral nucleus predominantly arose in the distal subiculum. In the macaque, the corresponding diencephalic inputs were again distinguished by anterior–posterior topographies, as subicular inputs to the medial mammillary bodies predominantly arose from the posterior hippocampus while subicular inputs to the anteromedial thalamic nucleus predominantly arose from the anterior hippocampus. Unlike the rat, there was no clear evidence of proximal–distal separation as all of these medial diencephalic projections preferentially arose from the more distal subiculum.

Calcium-binding protein immunoreactivity in Gudden's tegmental nuclei and the hippocampal formation: differential co-localization in neurons projecting to the mammillary bodies

The principal projections to the mammillary bodies arise from just two sites, Gudden’s tegmental nuclei (dorsal and ventral nuclei) and the hippocampal formation (subiculum and pre/postsubiculum). The present study sought to compare the neurochemical properties of these mammillary body inputs in the rat, with a focus on calcium-binding proteins. Neuronal calretinin (CR) immunoreactivity was sparse in Gudden’s tegmental nuclei and showed no co-localization with neurons projecting to the mammillary bodies. In contrast, many of the ventral tegmental nucleus of Gudden cell that project to the mammillary bodies were parvalbumin (PV)-positive whereas a smaller number of mammillary inputs stained for calbindin (CB). Only a few mammillary body projection cells in the dorsal tegmental nucleus of Gudden co-localized with PV and none co-localized with CB. A very different pattern was found in the hippocampal formation. Here, a large proportion of postsubiculum cells that project to the mammillary bodies co-localized with CR, but not CB or PV. While many neurons in the dorsal and ventral subiculum projected to the mammillary bodies, these cells did not co-localize with the immunofluorescence of any of the three tested proteins. These findings highlight marked differences between hippocampal and tegmental inputs to the rat mammillary bodies as well as differences between the medial and lateral mammillary systems. These findings also indicate some conserved neurochemical properties in Gudden’s tegmental nuclei across rodents and primates.

The mammillary bodies and memory: more than a hippocampal relay

Although the mammillary bodies were one of the first neural structures to be implicated in memory, it has long been assumed that their main function was to act primarily as a hippocampal relay, passing information on to the anterior thalamic nuclei and from there to the cingulate cortex. This view not only afforded the mammillary bodies no independent role in memory, it also neglected the potential significance of other, nonhippocampal, inputs to the mammillary bodies. Recent advances have transformed the picture, revealing that projections from the tegmental nuclei of Gudden, and not the hippocampal formation, are critical for sustaining mammillary body function. By uncovering a role for the mammillary bodies that is independent of its subicular inputs, this work signals the need to consider a wider network of structures that form the neural bases of episodic memory.

The subiculum: the heart of the extended hippocampal system

While descriptions of the subiculum often emphasize its role as a recipient of hippocampal inputs, the area also has particular importance as a source of hippocampal projections. The extrinsic projections from the subiculum not only parallel those from hippocampal fields CA1-4 but also terminate in sites that do not receive direct inputs from the rest of the hippocampus. Both electrophysiological and lesion studies reveal how, despite its very dense CA1 inputs, the subiculum has functional properties seemingly independent from the rest of the hippocampus. In understanding the subiculum, it is necessary to appreciate that its connections are topographically organized along all three planes (longitudinal, transverse, and depth). These topographies may enable the subiculum to separate multiple information types and, hence, support multiple functions. The particular significance of the subiculum for learning and memory is underlined by its importance as a source of hippocampal projections to nuclei in the medial diencephalon, which are themselves vital for human memory and rodent spatial learning. Of particular note are its reciprocal connections with the anterior thalamic nuclei, which are not shared by the rest of the hippocampus (CA1-4). These thalamosubiculum connections may be of especial significance for resolving memory problems that suffer high interference and require the flexible use of stimulus representations.

A critical role for the anterior thalamus in directing attention to task-relevant stimuli

The prefrontal cortex mediates adaption to changing environmental contingencies. The anterior thalamic nuclei, which are closely interconnected with the prefrontal cortex, are important for rodent spatial memory, but their potential role in executive function has received scant attention. The current study examined whether the anterior thalamic nuclei are involved in attentional processes akin to those of prefrontal regions. Remarkably, the results repeatedly revealed attentional properties opposite to those of the prefrontal cortex. Two separate cohorts of rats with anterior thalamic lesions were tested on an attentional set-shifting paradigm that measures not only the ability of stimuli dimensions that reliably predict reinforcement to gain attention ("intradimensional shift"), but also their ability to shift attention to another stimulus dimension when contingencies change ("extradimensional shift"). In stark contrast to the effects of prefrontal damage, anterior thalamic lesions impaired intradimensional shifts but facilitated extradimensional shifts. Anterior thalamic lesion animals were slower to acquire discriminations based on the currently relevant stimulus dimension but acquired discriminations involving previously irrelevant stimulus dimensions more rapidly than controls. Subsequent tests revealed that the critical determinant of whether anterior thalamic lesions facilitate extradimensional shifts is the degree to which the stimulus dimension has been established as an unreliable predictor of reinforcement over preceding trials. This pattern of performance reveals that the anterior thalamic nuclei are vital for attending to those stimuli that are the best predictors of reward. In their absence, unreliable predictors of reward usurp attentional control.

The effect of retrosplenial cortex lesions in rats on incidental and active spatial learning

The study examined the importance of the retrosplenial cortex for the incidental learning of the spatial arrangement of distinctive features within a scene. In a modified Morris water-maze, rats spontaneously learnt the location of an escape platform prior to swimming to that location. For this, rats were repeatedly placed on a submerged platform in one corner of either a rectangular (Experiment 1) or square (Experiments 2, 3) pool with walls of different appearance. The rats were then released in the center of the pool for their first test trial. In Experiment 1, the correct corner and its diagonally opposite partner (also correct) were specified by the geometric properties of the pool. Rats with retrosplenial lesions took longer to first reach a correct corner, subsequently showing an attenuated preference for the correct corners. A reduced preference for the correct corner was also found in Experiment 2, when platform location was determined by the juxtaposition of highly salient visual cues (black vs. white walls). In Experiment 3, less salient visual cues (striped vs. white walls) led to a robust lesion impairment, as the retrosplenial lesioned rats showed no preference for the correct corner. When subsequently trained actively to swim to the correct corner over successive trials, retrosplenial lesions spared performance on all three discriminations. The findings not only reveal the importance of the retrosplenial cortex for processing various classes of visuospatial information but also highlight a broader role in the incidental learning of the features of a spatial array, consistent with the translation of scene information.

Visual landmark information gains control of the head direction signal at the lateral mammillary nuclei

The neural representation of directional heading is conveyed by head direction (HD) cells located in an ascending circuit that includes projections from the lateral mammillary nuclei (LMN) to the anterodorsal thalamus (ADN) to the postsubiculum (PoS). The PoS provides return projections to LMN and ADN and is responsible for the landmark control of HD cells in ADN. However, the functional role of the PoS projection to LMN has not been tested. The present study recorded HD cells from LMN after bilateral PoS lesions to determine whether the PoS provides landmark control to LMN HD cells. After the lesion and implantation of electrodes, HD cell activity was recorded while rats navigated within a cylindrical arena containing a single visual landmark or while they navigated between familiar and novel arenas of a dual-chamber apparatus. PoS lesions disrupted the landmark control of HD cells and also disrupted the stability of the preferred firing direction of the cells in darkness. Furthermore, PoS lesions impaired the stable HD cell representation maintained by path integration mechanisms when the rat walked between familiar and novel arenas. These results suggest that visual information first gains control of the HD cell signal in the LMN, presumably via the direct PoS → LMN projection. This visual landmark information then controls HD cells throughout the HD cell circuit.

Looking beyond the hippocampus: old and new neurological targets for understanding memory disorders

Although anterograde amnesia can occur after damage in various brain sites, hippocampal dysfunction is usually seen as the ultimate cause of the failure to learn new episodic information. This assumption is supported by anatomical evidence showing direct hippocampal connections with all other sites implicated in causing anterograde amnesia. Likewise, behavioural and clinical evidence would seem to strengthen the established notion of an episodic memory system emanating from the hippocampus. There is, however, growing evidence that key, interconnected sites may also regulate the hippocampus, reflecting a more balanced, integrated network that enables learning. Recent behavioural evidence strongly suggests that medial diencephalic structures have some mnemonic functions independent of the hippocampus, which can then act upon the hippocampus. Anatomical findings now reveal that nucleus reuniens and the retrosplenial cortex provide parallel, disynaptic routes for prefrontal control of hippocampal activity. There is also growing clinical evidence that retrosplenial cortex dysfunctions contribute to both anterograde amnesia and the earliest stages of Alzheimer's disease, revealing the potential significance of this area for clinical studies. This array of findings underlines the importance of redressing the balance and the value of looking beyond the hippocampus when seeking to explain failures in learning new episodic information.

Episodic memory impairment in systemic lupus erythematosus: involvement of thalamic structures

Episodic memory deficits in systemic lupus erythematosus (SLE) have been frequently reported in the literature; however, little is known about the neural correlates of these deficits. We investigated differences in the volumes of different brain structures of SLE patients with and without episodic memory impairments diagnosed by the Rey Auditory Verbal Learning Test (RAVLT). Groups were paired based on age, education, sex, Mini Mental State Examination score, accumulation of disease burden (SLICC), and focused attention dimension score. Patients underwent magnetic resonance imaging (MRI). Cortical volumetric reconstruction and segmentation of the MR images were performed with the FreeSurfer software program. SLE patients with episodic memory deficits presented shorter time of diagnosis than SLE patients without episodic memory deficits. ANOVA revealed that SLE patients with episodic memory deficits had a larger third ventricle volume than SLE patients without episodic memory deficits and controls. Additionally, covariance analysis indicated group effects on the bilateral thalamus and on the third ventricle. Our findings indicate that episodic memory may be impaired in SLE patients with normal hippocampal volume. In addition, the thalamus may undergo volumetric changes associated with episodic memory loss in SLE.