Processing the head direction cell signal: a review and commentary

The effects of vestibular lesions on hippocampal function in rats

Interest in interaction between the vestibular system and the hippocampus was stimulated by evidence that peripheral vestibular lesions could impair performance in learning and memory tasks requiring spatial information processing. By the 1990s, electrophysiological data were emerging that the brainstem vestibular nucleus complex (VNC) and the hippocampus were connected polysynaptically and that hippocampal place cells could respond to vestibular stimulation. The aim of this review is to summarise and critically evaluate research published in the last 5 years that has seen major progress in understanding the effects of vestibular damage on the hippocampus. In addition to new behavioural studies demonstrating that animals with vestibular lesions exhibit impairments in spatial memory tasks, electrophysiological studies have confirmed long-latency, polysynaptic pathways between the VNC and the hippocampus. Peripheral vestibular lesions have been shown to cause long-term changes in place cell function, hippocampal EEG activity and even CA1 field potentials in brain slices maintained in vitro. During the same period, neurochemical investigations have shown that some hippocampal subregions exhibit long-term changes in the expression of neuronal nitric oxide synthase, arginase I and II, and the NR1 and NR2A N-methyl-D-aspartate (NMDA) receptor subunits following peripheral vestibular damage. Despite the progress, a number of important issues remain to be resolved, such as the possible contribution of auditory damage associated with vestibular lesions, to the hippocampal effects observed. Furthermore, although these studies demonstrate that damage to the vestibular system does have a long-term impact on the electrophysiological and neurochemical function of the hippocampus, they do not indicate precisely how vestibular information might be used in hippocampal functions such as developing spatial representations of the environment. Understanding this will require detailed electrical stimulation and lesion studies to elucidate the way in which different kinds of vestibular information are transmitted to various hippocampal subregions.

Topographic activation of the medial entorhinal cortex by presubicular commissural projections

Previous investigations have shown that presubicular commissural fibers traveling in the caudal part of the dorsal hippocampal commissure (PSD) selectively activated the dorsalmost portion of the entorhinal cortex (EC), where they discharged perforant path neurons to the dorsal dentate gyrus. The dentate activation was followed by that of the dorsal hippocampus. The aim of the present study was to ascertain whether presubiculum commissural projections traveling in the PSD can also activate ventral levels of the EC and, if so, whether this activation is followed by that of the dentate gyrus-hippocampal system in the ventral hippocampus. The experiments were carried out in adult, anesthetized guinea pigs by field potential analysis. The results showed that presubicular fibers traveling at different PSD loci selectively activated specific EC portions, with caudal fibers activating only the dorsal EC and more rostral fibers activating ventral EC points. The region activated by PSD projections corresponded to the medial EC. Current source-density (CSD) analysis revealed that at both dorsal and ventral EC levels excitatory synaptic potentials followed by neuron discharge were generated in layer II, site of origin of the perforant path to the dentate gyrus. Activation of either dorsal or ventral levels of the EC was followed by activation of the dentate gyrus-hippocampal system in corresponding hippocampal segments. The results provide physiological evidence that the commissural presubicular projections activate the EC in a topographic manner. The massive activation of perforant path neurons at all EC levels suggests that presubicular signals may strongly influence the functions played by the EC-dentate-hippocampal system.

Vestibular loss causes hippocampal atrophy and impaired spatial memory in humans

The human hippocampal formation plays a crucial role in various aspects of memory processing. Most literature on the human hippocampus stresses its non-spatial memory functions, but older work in rodents and some other species emphasized the role of the hippocampus in spatial learning and memory as well. A few human studies also point to a direct relation between hippocampal size, navigation and spatial memory. Conversely, the importance of the vestibular system for navigation and spatial memory was until now convincingly demonstrated only in animals. Using magnetic resonance imaging volumetry, we found that patients (n = 10) with acquired chronic bilateral vestibular loss (BVL) develop a significant selective atrophy of the hippocampus (16.9% decrease relative to controls). When tested with a virtual variant (on a PC) of the Morris water task these patients exhibited significant spatial memory and navigation deficits that closely matched the pattern of hippocampal atrophy. These spatial memory deficits were not associated with general memory deficits. The current data on BVL patients and bilateral hippocampal atrophy revive the idea that a major--and probably phylogenetically ancient--function of the archicortical hippocampal tissue is still evident in spatial aspects of memory processing for navigation. Furthermore, these data demonstrate for the first time in humans that spatial navigation critically depends on preserved vestibular function, even when the subjects are stationary, e.g. without any actual vestibular or somatosensory stimulation.

Reconstruction of the postsubiculum head direction signal from neural ensembles

Head direction cells change their firing rates as a function of the orientation of an animal within an environment. Typically, these cells display a unimodal tuning curve with maximal firing at the cell's preferred direction. As different cells have different preferred directions, the population of cells has been hypothesized to represent the orientation of the animal within the environment. Previous research has shown that pairs of simultaneously recorded head direction cells respond similarly to cue manipulations, suggesting that a population of head direction cells acts in concert to represent the animal's orientation within its environment. Ensembles of head direction cells were recorded from the postsubiculum from rats foraging in an open field. Directional responses of each cell were quantified by the nonparametric Watson's U2 statistic, a measure which makes no explicit assumptions of tuning curve shape. Directionally responsive cells were then used to reconstruct each animal's orientation within the open field using population vector, optimal-linear estimator, and Bayesian methods. The results indicated that postsubiculum contained a complete representation of the animal's orientation. The internal consistency of a neural ensemble can be assessed by comparing the ensemble activity to the expected activity given the reconstructed orientation. This has been termed the "coherency" of the neural ensemble. Reconstruction error decreased as the coherency of the orientation representation increased, indicating that coherency could be used to measure a level of confidence in the representation quality. Because coherency is a linear measure dependent only on internal variables, coherency may be a behaviorally relevant measure used to ascertain the animal's confidence in its representation of orientation.

Head motion in humans alternating between straight and curved walking path: combination of stabilizing and anticipatory orienting mechanisms

Anticipatory head orientation relative to walking direction was investigated in humans. Subjects were asked to walk along a 20 m perimeter, figure of eight. The geometry of this path required subjects to steer their body according to both curvature variations (alternate straight with curved walking) and walking direction (clock wise and counter clock wise). In agreement with previous results obtained during different locomotor tasks [R. Grasso, S. Glasauer, Y. Takei, A. Berthoz, The predictive brain: anticipatory control of head direction for the steering of locomotion, NeuroReport 7 (1996) 1170-1174; R. Grasso, P. Prevost, Y.P. Ivanenko, A. Berthoz, Eye-head coordination for the steering of locomotion in humans: an anticipatory synergy, Neurosci. Lett. 253 (2) (1998) 115-118; T. Imai, S.T. Moore, T. Raphan, B. Cohen, Interaction of body, head, and eyes during walking and turning, Exp. Brain Res. 136 (2001) 1-18; P. Prevost, Y. Ivanenko, R. Grasso, A. Berthoz, Spatial invariance in anticipatory orienting behaviour during human navigation, Neurosci. Lett. 339 (2002) 243-247; G. Courtine, M. Schieppati, Human walking along a curved path. I. Body trajectory, segment orientation and the effect of vision, Eur. J. Neurosci. 18 (2003) 177-190], the head turned toward the future walking direction. This anticipatory head behaviour was continuously modulated by the geometrical variations of the path. Two main components were observed in the anticipatory head behaviour. One was related to the geometrical form of the path, the other to the transfer of body mass from one foot to the other during stepping. A clear modulation of the head deviation pattern was observed between walking on curved versus straight parts of the path: head orientation was influenced to a lesser extent by step alternation for curved path where a transient head fixation was observed. We also observed good symmetry in the head deviation profile, i.e. the head tended to anticipate the future walking direction with the same amplitude when turning to the left (29.75 +/- 7.41 degrees of maximum head deviation) or to the right (30.86 +/- 9.92 degrees ). These findings suggest a combination of motor strategies underlying head stabilization in space and more global orienting mechanisms for steering the whole body in the desired direction.

Testing the importance of the retrosplenial guidance system: effects of different sized retrosplenial cortex lesions on heading direction and spatial working memory

The present study: (1) tested the importance of the retrosplenial cortex for learning a specific heading direction and distance and, (2) determined if lesion size could explain apparent inconsistencies in the results of different research groups. Dark agouti rats received either 'complete' cytotoxic retrosplenial cortex lesions or 'standard' lesions, the latter sparing the caudal retrosplenial cortex. Animals were first tested on two versions of a "landmark" task in a water maze. In condition 1 animals could use both heading direction and allocentric position, while in condition 2 only heading direction was effective. In condition 1, animals with complete retrosplenial lesions were impaired by the end of training, their profile of performance being consistent with a failure to use allocentric position information. When the water maze task changed (condition 2) so that allocentric cues became redundant, the animals with complete retrosplenial lesions were able to head in the appropriate direction although they showed longer swim paths. Subsequent testing in the radial-arm maze provided more evidence that retrosplenial lesions can disrupt the use of distal (allocentric) room cues. The impairments seen with retrosplenial lesions were often mild but throughout the study performance of rats with 'complete' lesions was more disrupted than those with 'standard' lesions, who often did not differ from the controls. These findings show that lesion size is a critical factor and may explain why some studies have failed to find comparable deficits after retrosplenial cortex lesions.

Effects of fimbria-fornix, hippocampus, and amygdala lesions on discrimination between proximal locations

The conditioned cue preference (CCP) task was used to study the information required to discriminate between spatial locations defined by adjacent arms of an 8-arm radial maze. Normal rats learned the discrimination after 3 unreinforced preexposure (PE) sessions and 4 food paired-unpaired training trials. Fimbria-fornix lesions made before, but not after, PE, and hippocampus lesions made at either time, blocked the discrimination, suggesting that the 2 structures processed different information. Lateral amygdala lesions made before PE facilitated the discrimination. This amygdala-mediated interference with the discrimination was the result of a conditioned approach response that did not discriminate between the 2 arm locations. A hippocampus/fimbria-fornix system and an amygdala system process different information about the same learning situation simultaneously and in parallel.

Evidence for entorhinal and parietal cortices involvement in path integration in the rat

Rats with lesions of the entorhinal or parietal cortex were tested in a homing task on a circular platform containing food cups and surrounded by curtains. The animals had to leave a refuge, explore the platform to find a hidden piece of food and carry it back to the refuge. Once the rats were proficient at performing the procedural aspects of the task, they were tested in two successive types of trials in which the food pellet was either always located in the central cup (food at center, "FAC" trials) or placed in a randomly chosen cup (food at random, "FAR" trials). Except in the first FAC trials, all groups displayed similar outward paths in FAC and FAR trials, showing that both types of trials involved equivalent path integration demand. Analysis of the homing accuracy showed that rats with entorhinal cortex or parietal cortex lesions exhibited inaccurate returns to the starting hole, suggesting that these two cortical areas are part of a neural network mediating path integration.

Path integration in mammals

It is often assumed that navigation implies the use, by animals, of landmarks indicating the location of the goal. However, many animals (including humans) are able to return to the starting point of a journey, or to other goal sites, by relying on self-motion cues only. This process is known as path integration, and it allows an agent to calculate a route without making use of landmarks. We review the current literature on path integration and its interaction with external, location-based cues. Special importance is given to the correlation between observable behavior and the activity pattern of particular neural cell populations that implement the internal representation of space. In mammals, the latter may well be the first high-level cognitive representation to be understood at the neural level.

Using idiothetic cues to swim a path with a fixed trajectory and distance: necessary involvement of the hippocampus, but not the retrosplenial cortex

Rats rapidly learned to find a submerged platform in a water maze at a constant distance and angle from the start point, which changed on every trial. The rats performed accurately in the light and dark, but prior rotation disrupted the latter condition. The rats were then retested after receiving cytotoxic hippocampal or retrosplenial cortex lesions. Retrosplenial lesions had no apparent effect in either the light or dark. Hippocampal lesions impaired performance in both conditions but spared the ability to locate a platform placed in the center of the pool. A hippocampal deficit emerged when this pool-center task was run in the dark. The spatial effects of hippocampal damage extend beyond allocentric tasks to include aspects of idiothetic guidance.

Effects of Hippocampus and Medial Caudate Nucleus Lesions on Memory for Direction Information in Rats

A delayed matching-to-sample task was designed to assess memory for direction information in rats. During the study phase, rats traversed a maze arm oriented in 1 of 3 directions. After a delay period, a test phase was presented that required a choice between the study phase direction and a foil direction. Once rats reached a learning criterion, probe trials suggested that normal rats favor the use of direction, rather than turning response, information and use vestibular feedback. Rats were then given hippocampus, medial caudate nucleus (MCN), or cortical control lesions. Unlike control rats, those with hippocampus and MCN lesions exhibited marked impairments when retested. However, all rats were able to learn a direction discrimination task. These results suggest that the hippocampus and MCN support processes associated with short-term memory for direction information.

Vestibular influences on CA1 neurons in the rat hippocampus: an electrophysiological study in vivo

Vestibular information is known to be important for accurate spatial orientation and navigation. Hippocampal place cells, which appear to encode an animal's location within the environment, are also thought to play an essential role in spatial orientation. Therefore, it can be hypothesized that vestibular information may influence cornu ammonis region 1 (CA1) hippocampal neuronal activity. To explore this possibility, the effects of electrical stimulation of the medial vestibular nucleus (MVN) on the firing rates of hippocampal CA1 neurons in the urethane-anesthetized rat were investigated using extracellular single unit recordings. The firing rates of CA1 complex spike cells (n=29), which most likely correspond to place cells, consistently increased during electrical stimulation of the MVN in a current intensity dependent manner. Stimulation applied adjacent to the MVN failed to elicit a response. Overall, the firing rates of non-complex spike cells (n=22) did not show a consistent response to vestibular stimulation, although in some cells clear responses to the stimulation were observed. These findings suggest that vestibular inputs may contribute to spatial information processing in the hippocampus.

Unilateral inner ear damage results in lasting changes in hippocampal CA1 field potentials in vitro

We investigated the effects of a surgical lesion of one vestibular inner ear (unilateral vestibular damage [UVD]) on the field potential responses of CA1 neurons in vitro. Hippocampal slices were removed from rats at 4-6 weeks or 5-6 months post-UVD, and the field responses of CA1 neurons to electrical stimulation of the Schaffer collateral commissural pathway were analyzed. Compared with slices from sham and naive control animals, slices from UVD animals at 5-6 months post-UVD exhibited decreases in the population spike amplitude, the somal field excitatory postsynaptic potential (sfEPSP) slope, and the field EPSP (fEPSP) slope. For the population spike amplitude and fEPSP slope, this effect was observed in both CA1 ipsilateral and contralateral to the UVD. On both the ipsilateral and contralateral sides, paired-pulse testing showed increases in paired-pulse inhibition at the shortest interstimulus intervals (ISIs), with increases in paired-pulse facilitation at longer ISIs. This study provides the first evidence that peripheral vestibular damage can produce long-term changes in hippocampal electrophysiological activity in vitro.

Passive transport disrupts directional path integration by rat head direction cells

A subset of neurons in the rat limbic system encodes head direction (HD) by selectively discharging when the rat points its head in a preferred direction in the horizontal plane. The preferred firing direction is sensitive to the location of landmark cues, as well as idiothetic or self-motion cues (i.e., vestibular, motor efference copy, proprioception, and optic flow). Previous studies have shown that the preferred firing direction remains relatively stable (average shift +/- 18 degrees ) after the rat walks from a familiar environment into a novel one, suggesting that without familiar landmarks, the preferred firing direction can be maintained using idiothetic cues, a process called directional path integration. This study repeated this experiment and manipulated the idiothetic cues available to the rat as it moved between the familiar and novel environment. Motor efference copy/proprioceptive cues were disrupted by passively transporting the animal between the familiar and novel environment. Darkening the room as the animal moved to the novel environment eliminated optic flow cues. HD cell preferred firing directions shifted in the novel environment by an average of 30 degrees after locomotion from the familiar environment with the room lights off; by an average of 70 degrees after passive transport from the familiar environment with the room lights on; and by an average of 67 degrees after passive transport with the room lights off. These findings are consistent with the view that motor efference copy/proprioception cues are important for maintaining the preferred firing direction of HD cells under conditions requiring path integration.

Long-term effects of permanent vestibular lesions on hippocampal spatial firing

The hippocampus is thought to be important for spatial representation processes that depend on the integration of both self-movement and allocentric cues. The vestibular system is a particularly important source of self-movement information that may contribute to this spatial representation. To test the hypothesis that the vestibular system provides self-movement information to the hippocampus, rats were given either a bilateral labyrinthectomy (n = 6) or a sham surgery (n = 6), and at least 60 d after surgery hippocampal CA1 neurons were recorded extracellularly while the animals foraged freely in an open arena. Recorded cells were classified as complex spiking (n = 80) or noncomplex spiking (n = 33) neurons, and their spatial firing fields (place fields) were examined. The most striking effect of the lesion was that it appeared to completely abolish location-related firing. The results of this and previous studies provide converging evidence demonstrating that vestibular information is processed by the hippocampus. The disruption of the vestibular input to the hippocampus may interfere with the reconciliation of internal self-movement signals with the changes to the external sensory inputs that occur as a result of that movement. This would disrupt the ability of the animal to integrate allocentric and egocentric information into a coherent representation of space.

Vestibular response kinematics in posterior parietal cortex neurons of macaque monkeys

Perception of extrapersonal space is a fundamental requirement for accurate interaction with the environment and moving in it. Parietal cortical areas are thought to play an important role in this function. A significant sensory input to this area arrives from the vestibular system. We quantified neuronal responses in the ventral intraparietal area and the medial intraparietal area of awake head-fixed macaque monkeys during classical vestibular sinusoidal stimulation protocols and with a newly developed random vestibular testing paradigm. The goal was to study more specifically the signal content of parietal vestibular neurons with respect to head movement kinematics. Traditional sinusoidal stimulation analysis revealed that about one-third of the neurons responded in phase with either head position or head acceleration, besides classical head velocity tuning. Random vestibular stimulation revealed more complex signal profiles in the majority of neurons, although quantification of the kinematic variables that drove the neurons most effectively led to similar results to phase shift analysis. Thus, a majority of cells was principally driven by head velocity, and a minority by either acceleration or position. Nevertheless, random stimulation also revealed the simultaneous presence of all three kinematic response parameters (i.e. velocity, position and acceleration) in a majority of neurons. A minority of cells coded only two kinematic variables, i.e. head velocity coupled with either acceleration or position. Neurons coding only one kinematic variable were not found. We hereby demonstrate for the first time that central vestibular neurons carry several head movement kinematic variables simultaneously.

Spatial invariance in anticipatory orienting behaviour during human navigation

We have recently reported that the head systematically deviates toward the future direction of the trajectory about 500 ms before attaining a turning point of 90 degrees corner trajectories both in light and in darkness. Here, we investigated how this anticipatory strategy is modified whilst varying visual conditions (Experiment 1) and walking speed (Experiment 2). Exp. 1 showed similar anticipatory behaviour when walking with or without vision. Exp. 2 (that varied walking speed; eyes open) showed that the head started to deviate at a constant distance rather than at a constant time to the corner. The results appear inconsistent with optic flow theories of the guidance of walking direction and might highlight the role of landmarks and/or egocentric direction in anticipatory orienting behaviour.

One-trial memory for object-place associations after separate lesions of hippocampus and posterior parahippocampal region in the monkey

In earlier studies of one-trial spatial memory in monkeys (Parkinson et al., 1988; Angeli et al., 1993), severe and chronic memory impairment for both object-place association and place alone was found after ablation of the hippocampal formation. The results appeared to provide the first clear-cut evidence in the monkey of the essential role of the hippocampus in spatial memory, but that interpretation neglected the inclusion in the lesion of the underlying posterior parahippocampal region. To determine the separate contributions of the hippocampus and posterior parahippocampal region to these spatial forms of one-trial memory, we trained 10 rhesus monkeys, as before, to remember the spatial positions of either two different trial-unique objects overlying two of the wells in a three-well test tray (object-place trials) or simply two of the three wells (place trials). Six of the monkeys then received ibotenic acid lesions restricted to the hippocampal formation (group H), and the four others received selective ablations of the posterior parahippocampal region (group P), comprising mainly parahippocampal cortex, parasubiculum, and presubiculum. Group H was found to be completely unaffected postoperatively on both types of trials, whereas group P sustained an impairment on both types equal in magnitude to that observed after the combined lesions in the original studies. Thus, contrary to the previous interpretation, one-trial memory for object-place association and, perhaps more fundamentally, one-trial memory for two different places appear to be critically dependent not on the hippocampal formation but rather on the posterior parahippocampal region.

Nitric oxide synthase and arginase expression in the vestibular nucleus and hippocampus following unilateral vestibular deafferentation in the rat

The aim of this study was to investigate the possible relationship between changes in neuronal and endothelial nitric oxide synthase (nNOS and eNOS) and arginase expression in the vestibular nucleus complex and the hippocampus (CA1, CA2/3 and the dentate gyrus (DG) at 10 h or 2 weeks following a unilateral vestibular deafferentation (UVD) in rats. There were no significant differences in nNOS or arginase II expression in the ipsilateral or contralateral VNC at either 10 h or 2 weeks post-UVD. For eNOS, there was only a significant decrease in expression in the ipsilateral VNC at 2 weeks post-UVD (P<0.01). In the hippocampus, the only significant difference in nNOS expression was a decrease in the ipsilateral DG at 2 weeks post-UVD (P<0.05). There was a significant decrease in eNOS expression in the contralateral CA2/3 region at 10 h post-UVD (P<0.01). The only other significant change in eNOS was an increase in the contralateral DG at 10 h post-UVD (P<0.01). Although arginase II was expressed in all regions of the hippocampus, there were no significant differences in arginase II expression at any time point following UVD. These results suggest that the changes in NOS expression that occur in the VNC and hippocampus following UVD are not correlated with one another or with changes in arginase II.

Does the vestibular system contribute to head direction cell activity in the rat?

Head direction cells (HDC) located in several regions of the brain, including the anterior dorsal nucleus of the thalamus (ADN), postsubiculum (PoS), and lateral mammillary nuclei (LMN), provide the neural substrate for the determination of head direction. Although activity of HDC is influenced by various sensory signals and internally generated cues, lesion studies and some anatomical and physiological evidence suggest that vestibular inputs are critical for the maintenance of directional sensitivity of these cells. However, vestibular inputs must be transformed considerably in order to signal head direction, and the neuronal circuitry that accomplishes this signal processing has not been fully established. Furthermore, it is unclear why the removal of vestibular inputs abolishes the directional sensitivity of HDC, as visual and other sensory inputs and motor feedback signals strongly affect the firing of these neurons and would be expected to maintain their directional-related activity. Further physiological studies will be required to establish the role of vestibular system in producing HDC responses, and anatomical studies are needed to determine the neural circuitry that mediates vestibular influences on determination of head direction.

Changes in Fos expression in the rat brain after unilateral lesions of the anterior thalamic nuclei

Activity of the immediate early gene c-fos was compared across hemispheres in rats with unilateral anterior thalamic lesions. Fos protein was quantified after rats performed a spatial working memory test in the radial-arm maze, a task that is sensitive to bilateral lesions of the anterior thalamic nuclei. Unilateral anterior thalamic lesions produced evidence of a widespread hippocampal hypoactivity, as there were significant reductions in Fos counts in a range of regions within the ipsilateral hippocampal formation (rostral CA1, rostral dentate gyrus, 'dorsal' hippocampus, presubiculum and postsubiculum). A decrease in Fos levels was also found in the rostral and caudal retrosplenial cortex but not in the parahippocampal cortices or anterior cingulate cortices. The Fos changes seem most closely linked to sites that are also required for successful task performance, supporting the notion that the anterior thalamus, retrosplenial cortex and hippocampus form key components of an interdependent neuronal network involved in spatial mnemonic processing.

Double-ring network model of the head-direction system

In the head-direction system, the orientation of an animal's head in space is encoded internally by persistent activities of a pool of cells whose firing rates are tuned to the animal's directional heading. To maintain an accurate representation of the heading information when the animal moves, the system integrates horizontal angular head-velocity signals from the vestibular nuclei and updates the representation of directional heading. The integration is a difficult process, given that head velocities can vary over a large range and the neural system is highly nonlinear. Previous models of integration have relied on biologically unrealistic mechanisms, such as instantaneous changes in synaptic strength, or very fast synaptic dynamics. In this paper, we propose a different integration model with two populations of neurons, which performs integration based on the differential input of the vestibular nuclei to these two populations. We mathematically analyze the dynamics of the model and demonstrate that with carefully tuned synaptic connections it can accurately integrate a large range of the vestibular input, with potentially slow synapses.

Sensory preconditioning in rats with lesions of the anterior thalamic nuclei: evidence for intact nonspatial 'relational' processing

Rats with neurotoxic lesions centered in the anterior thalamic nuclei were trained in two versions of a nonspatial, sensory preconditioning procedure. In both versions, two stimulus compounds (AX and BY) were first presented and then X, but not Y, was paired with an aversive unconditioned stimulus. This procedure resulted in greater conditioned responding to A than B. Anterior thalamic lesions had no apparent effect on these two examples of sensory preconditioning, nor did they affect fear conditioning or conditioned taste aversion. In contrast, the same lesions led to a severe deficit on a test of spatial memory. These results help to refine our understanding of the contribution of the anterior thalamic nuclei to spatial memory.

Fos imaging reveals that lesions of the anterior thalamic nuclei produce widespread limbic hypoactivity in rats

Activity of the immediate early gene c-fos was compared in rats with neurotoxic lesions of the anterior thalamic nuclei and in surgical controls. Fos levels were measured after rats had been placed in a novel room and allowed to run up and down preselected arms of a radial maze. An additional control group showed that in normal rats, this exposure to a novel room leads to a Fos increase in a number of structures, including the anterior thalamic nuclei and hippocampus. In contrast, rats with anterior thalamic lesions were found to have significantly less Fos-positive cells in an array of sites, including the hippocampus (dorsal and ventral), retrosplenial cortex, anterior cingulate cortex, and prelimbic cortex. These results show that anterior thalamic lesions disrupt multiple limbic brain regions, producing hypoactivity in sites associated in rats with spatial memory. Because many of the same sites are implicated in memory processes in humans (e.g., the hippocampus and retrosplenial cortex), this hypoactivity might contribute to diencephalic amnesia.

A view model which accounts for the spatial fields of hippocampal primate spatial view cells and rat place cells

Hippocampal spatial view cells found in primates respond to a region of visual space being looked at, relatively independently of where the monkey is located. Rat place cells have responses which depend on where the rat is located. We investigate the hypothesis that in both types of animal, hippocampal cells respond to a combination of visual cues in the correct spatial relation to each other. In rats, which have a wide visual field, such a combination might define a place. In primates, including humans, which have a much smaller visual field and a fovea which is directed towards a part of the environment, the same mechanism might lead to spatial view cells. A computational model in which the neurons become organized by learning to respond to a combination of a small number of visual cues spread within an angle of a 30 degrees receptive field resulted in cells with visual properties like those of primate spatial view cells. The same model, but operating with a receptive field of 270 degrees, produced cells with visual properties like those of rat place cells. Thus a common hippocampal mechanism operating with different visual receptive field sizes could account for some of the visual properties of both place cells in rodents and spatial view cells in primates.

Directing place representation in the hippocampus

Theoretical models of rodent navigation consider location information and directional heading to be essential. Indeed, the existence of location-selective 'place cells' and orientation-selective 'head direction cells' is well documented. Different models suggest different forms of interaction between information about location and heading direction. However, until recently, there were no clear empirical data that could be used to distinguish the different models in terms of the nature of the integration of location and directional heading information. Recently, Leutgeb et al. provided evidence that head direction and place signals coexist within the CA1 region of hippocampus, and that the head direction signals are likely to be generated by a subpopulation of interneurons. This finding opens up new possibilities for clarifying current models and for developing biologically plausible theories of synaptic interactions between location and head direction codes. In this paper, we first present the issue concerning the nature of the interaction between location and head direction signals, followed by a selective review of place and head direction cell research. The finding of Leutgeb et al. is then summarized, and its implications for current models are discussed. Finally, a view is presented that considers place fields to be a product not only of (external and internal) sensory input, but also of non-spatial variables such as motivation and responses. The finding of Leutgeb et al. and many earlier anatomical studies suggest that hippocampal head direction, motivation and response information may be represented by the interneuron population. Thus, these factors may have strong impact on the location codes of hippocampal pyramidal neurons. Their influence may further define the behavioral context of the current spatial environment.

Mapping the oculomotor system: the power of transneuronal labelling with rabies virus

Neuronal networks underlying and related to horizontal eye movements were visualized by retrograde transneuronal tracing with rabies virus from the left medial rectus muscle in guinea pigs. Time-sequenced labelling revealed distinct circuitries involved in particular oculomotor functions, i.e. vestibulo-ocular reflex and saccade generation (brainstem circuitry), adaptive plasticity (cerebellar modules) and possibly motivation and navigation (limbic, hippocampal and cortical structures). Our results provide a first comprehensive road map of the oculomotor system that is unsurpassed by any previous tracing study. We report a number of unexpected findings that illustrate a much vaster and more complicated network for the control of the relatively simple horizontal eye movements than had been envisioned previously.

Dissociation of the effects of bilateral lesions of the dorsal hippocampus and parietal cortex on path integration in the rat

Rodents are able to rely on self-motion (idiothetic) cues and navigate toward a reference place by path integration. The authors tested the effects of dorsal hippocampal and parietal lesions in a homing task to dissociate the respective roles of the hippocampus and the parietal cortex in path integration. Hippocampal rats exhibited a strong deficit in learning the basic task. Parietal rats displayed a performance impairment as a function of the complexity of their outward paths when the food was placed at varying locations. These results suggest that the parietal cortex plays a specific role in path integration and in the processing of idiothetic information, whereas the hippocampus is involved in the calibration of space used by the path integration system.

Damage to the vestibular inner ear causes long-term changes in neuronal nitric oxide synthase expression in the rat hippocampus

The vestibular inner ear detects head acceleration and initiates compensatory eye movement and postural reflexes that help keep the visual image of the world stable on the retina, and maintain balance, during unexpected head movement. The most primitive vestibular systems are estimated to have evolved more than 500 million years ago and in mammalian and submammalian species the vestibular reflexes are mediated by basic brainstem pathways (see Wilson and Melvill Jones, 1979 for review). Although the contributions of the vestibular system to higher cognitive function have generally received less attention than its reflexive roles, vestibular sensory information is transmitted to higher centres in the brain and humans with vestibular damage are known to experience debilitating perceptual illusions (see Curthoys and Halmagyi, 1995; Berthoz, 1996 for reviews). Increasing behavioural and neurophysiological evidence suggests that the hippocampus uses information from the vestibular inner ear in order to build up maps of space that can be used in the development of spatial memory during learning tasks (McNaughton et al., 1991; Chapuis et al., 1992; Wiener and Berthoz, 1993; O'Mara et al., 1994; Wiener et al., 1995; Gavrilov et al., 1995; Stackman and Taube, 1996; Vitte et al., 1996; Taube et al., 1996; Save et al., 1998; Peruch et al., 1999; Cuthbert et al., 2000; Russell et al., 2000). However, to date, there has been no indication of the long-term neurochemical effects of the loss of vestibular input on hippocampal function. Since nitric oxide has been implicated in the mechanisms of hippocampal synaptic plasticity associated with the development of short-term memory (e.g. Schuman and Madison, 1991; Schuman et al., 1994; Arancio et al., 1996; Wu et al., 1997; Lu et al., 1999), we examined whether changes occur in the activity and expression of the enzymes responsible for nitric oxide production (nitric oxide synthases) in subregions of the rat hippocampus at different times following unilateral peripheral vestibular lesions, using western blotting and radioenzymatic assays. We found a decreased expression of neuronal nitric oxide synthase in the ipsilateral dentate gyrus at 2 weeks following the vestibular damage and not before, that may be related to the long-term effects of the loss of vestibular input on hippocampal function. These results support the hypothesis that head movement and position information derived from the vestibular inner ear may be important for the normal function of the hippocampus.

Active locomotion increases peak firing rates of anterodorsal thalamic head direction cells

Head direction (HD) cells discharge selectively in macaques, rats, and mice when they orient their head in a specific ("preferred") direction. Preferred directions are influenced by visual cues as well as idiothetic self-motion cues derived from vestibular, proprioceptive, motor efferent copy, and command signals. To distinguish the relative importance of active locomotor signals, we compared HD cell response properties in 49 anterodorsal thalamic HD cells of six male Long-Evans rats during active displacements in a foraging task as well as during passive rotations. Since thalamic HD cells typically stop firing if the animals are tightly restrained, the rats were trained to remain immobile while drinking water distributed at intervals from a small reservoir at the center of a rotatable platform. The platform was rotated in a clockwise/counterclockwise oscillation to record directional responses in the stationary animals while the surrounding environmental cues remained stable. The peak rate of directional firing decreased by 27% on average during passive rotations (r(2) = 0.73, P < 0.001). Individual cells recorded in sequential sessions (n = 8) reliably showed comparable reductions in peak firing, but simultaneously recorded cells did not necessarily produce identical responses. All of the HD cells maintained the same preferred directions during passive rotations. These results are consistent with the hypothesis that the level of locomotor activity provides a state-dependent modulation of the response magnitude of AD HD cells. This could result from diffusely projecting neuromodulatory systems associated with motor state.

The anatomical and computational basis of the rat head-direction cell signal

As a rat navigates through space, neurons called head-direction (HD) cells provide a signal of the rat's momentary directional heading. Although partly guided by landmarks, the cells also show a remarkable ability to track directional heading based on angular head movement. Theoretical models suggest that the HD cells are linked together to form an attractor network, and that cells which signal angular velocity update the directional setting of the attractor. Recently, cell types similar to those required theoretically have been discovered in the lateral mammillary and dorsal tegmental nuclei. Lesion and anatomical data suggest these nuclei might constitute the postulated attractor-path integration mechanism, and that they provide the HD cell signal to cortical areas where it has been observed.

Long-term potentiation of single subicular neurons in mice

Subicular neurons receive direct afferent connections from the vast majority of CA1 pyramidal cells and send their axons to the various brain areas. Because of this strategic position, subicular cells can modulate output of the hippocampus and, thus, play a significant part in memory, spatial processing, and seizure amplification and propagation from the hippocampus. Despite its important role as a hippocampal interface with different brain regions, present knowledge of the subiculum and the plastic properties of the synapses on the subicular neurons is rather limited. By using IR-DIC videomicroscopy and whole-cell patch-clamp recordings in mouse hippocampal slices, I demonstrated that long-term potentiation (LTP) in CA1-subicular cell synapses can be readily induced by high-frequency stimulation (HFS) of the afferents, but not by pairing of low-frequency stimulation with depolarization of postsynaptic cells. This tetanus-induced LTP is input specific, insensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist 3-[(R)-2Carboxipiperazin-4-yl]-propyl-1-phosphonic acid (R-CPP), and reduces paired-pulse facilitation in potentiated synapses. Subsequent morphologic analysis of the recorded cells, which were filled either with Lucifer Yellow or Biocytin, revealed pyramidal-shaped neurons localized predominantly in the deep layer of the subiculum, close to the CA1 border. Axons of the majority of these neurons extended to the alveus and on toward the hippocampus, probably exiting it via the fornix. These data indicate that CA1-subicular cell synapses in mice exhibit LTP, which can be expressed presynaptically, and its induction does not require NMDA-receptor activation. The observed activity-dependent plasticity might play an important role in the integrative mechanisms of the subiculum and may influence transfer of information from the hippocampus to subcortical and cortical brain areas.

Modeling place fields in terms of the cortical inputs to the hippocampus

A model of place-cell firing is presented that makes quantitative predictions about specific place cells' spatial receptive fields following changes to the rat's environment. A place cell's firing rate is modeled as a function of the rat's location by the thresholded sum of the firing rates of a number of putative cortical inputs. These inputs are tuned to respond whenever an environmental boundary is at a particular distance and allocentric direction from the rat. The initial behavior of a place cell in any environment is simply determined by its set of inputs and its threshold; learning is not necessary. The model is shown to produce a good fit to the firing of individual place cells, and populations of place cells across environments of differing shape. The cells' behavior can be predicted for novel environments of arbitrary size and shape, or for manipulations such as introducing a barrier. The model can be extended to make behavioral predictions regarding spatial memory.

Dorsal hippocampal function in unreinforced spatial learning

This study examined learning about the spatial environment by rats during a single 10 min period of exploration on an eight-arm radial maze. Because no specific behaviors were learned during this procedure, the existence of learned spatial information was inferred from its retarding effect on subsequent conditioned cue preference (CCP) learning on the same maze. Previous experiments have shown that this form of spatial learning, measured in this way, requires an intact fimbriafornix and functional N-methyl-D-aspartate receptors. However, in the present experiments, large neurotoxic lesions of the dorsal hippocampus that impaired win-shift learning failed to eliminate the retarding effect of exploration on CCP learning. This result was obtained in three independent replications. These findings fail to confirm the hypothesis that the hippocampus is involved in spatial learning when that learning occurs in the absence of reinforcers and does not produce any specific learned behaviors. Previous work showed that this form of "pure" spatial learning requires an intact fimbria-fornix for acquisition but not for expression; the present findings suggest that the hippocampus is not required for either of these processes. The fimbria-fornix may interact with other temporal lobe structures in mediating this form of learning. The function of the hippocampus may be limited in some way to situations that involve reinforcers and/or situations in which specific behaviors are learned.

Hippocampal-parietal cortical interactions in spatial cognition

Growing evidence suggests that the associative parietal cortex (APC) of the rat is involved in the processing of spatial information. This observation raises the issue of the respective functions of the APC and the hippocampus in spatial processing as well as of their possible interactions. In this paper, we review neuroanatomical, electrophysiological, and behavioral data that support the existence of such functional interactions. Our hypothesis is that the APC is involved in the initial combination of visuospatial information and self-motion information necessary for the integration of egocentrically acquired information into allocentrically coded information, the latter step being completed in the hippocampus. The dialogue between the hippocampus and the APC is therefore crucial, particularly when the elaboration and/or updating of an allocentric representation depends on complex combinations of visuospatial and self-motion information.

Inferior parietal lobule projections to the presubiculum and neighboring ventromedial temporal cortical areas

The entorhinal and perirhinal cortices have long been accorded a special role in the communications between neocortical areas and the hippocampal formation. Less attention has been paid to the presubiculum, which, however, is also a component of the parahippocampal gyrus, receives dense inputs from several cortical areas, and itself is a major source of connections to the entorhinal cortex (EC). In part of a closer investigation of corticohippocampal systems, the authors applied single-axon analysis to the connections from the inferior parietal lobule (IPL) to the presubiculum. One major result from this approach was the finding that many of these axons (at least 10 of 14) branch beyond the presubiculum. For 4 axons, branches were followed to area TF and to the border between the perirhinal and entorhinal cortices, raising the suggestion that these areas, which sometimes are viewed as serial stages, are tightly interconnected. In addition, the current data identify several features of presubicular organization that may be relevant to its functional role in visuospatial or memory processes: 1) Terminations from the IPL, as previously reported for prefrontal connections (Goldman-Rakic et al. [1984] Neuroscience 12:719-743), form two to four patches in the superficial layers. These align in stripes, but only for short distances ( approximately 1.5 mm). This pattern suggests a strong compartmentalization in layers I and II that is also indicated by cytochrome oxidase and other markers. 2) Connections tend to be bistratified, terminating in layers I-II and deeper in layer III. 3) Single axons terminate in layer I alone or in different combinations of layers. This may imply some heterogeneity of subtypes. 4) Individual axons, both ipsilateral projecting (n = 14 axons) and contralateral projecting (n = 6 axons), tend to have large arbors (0.3-0.8 mm across). Finally, the authors observe that projections from the IPL, except for its anteriormost portion, converge at the perirhinal-entorhinal border around the posterior tip of the rhinal sulcus. These projections partially overlap with projections from ventromedial areas TE and TF, and this convergence may contribute to the severe deficits in visual recognition memory resulting from ablations of rhinal cortex.

Intact spatial updating during locomotion after right posterior parietal lesions

One function of the posterior parietal cortex (PPC) is to monitor and integrate sensory signals relating to the current pointing direction of the eyes. We investigated the possibility that the human PPC also contributes to spatial updating during larger-scale behaviors. Two groups of patients with brain injuries either including or excluding the right hemisphere PPC and a group of healthy subjects performed a visually-directed walking task, in which the subject views a target and then attempts to walk to it without vision. All groups walked without vision accurately and precisely to remembered targets up to 6 m away; the patient groups also performed similarly to the healthy controls when indicating egocentric distances using non-motoric responses. These results indicate that the right PPC is not critically involved in monitoring and integrating non-visual self-motion signals, at least along linear paths. In addition, visual perception of egocentric distance in multi-cue environments is immune to injury of a variety of brain areas.

Trajectory encoding in the hippocampus and entorhinal cortex

We recorded from single neurons in the hippocampus and entorhinal cortex (EC) of rats to investigate the role of these structures in navigation and memory representation. Our results revealed two novel phenomena: first, many cells in CA1 and the EC fired at significantly different rates when the animal was in the same position depending on where the animal had come from or where it was going. Second, cells in deep layers of the EC, the targets of hippocampal outputs, appeared to represent the similarities between locations on spatially distinct trajectories through the environment. Our findings suggest that the hippocampus represents the animal's position in the context of a trajectory through space and that the EC represents regularities across different trajectories that could allow for generalization across experiences.

Sensory and memory properties of hippocampal place cells

The rat hippocampus contains place cells whose firing is location-specific. These cells fire only when the rat enters a restricted region of the environment called the firing field. In this review, we examine the sensory information that is fundamental to the place cell system for producing spatial firing. While visual information takes precedence in the control of firing fields when it is available, local (olfactory and/or tactile) cues combined with motion-related cues can permit stable spatial firing. Motion-related cues are integrated by hippocampal place cells, but in the absence of external cues do not support stable firing over long periods. While firing fields are based on a variety of sensory cues, they do not strictly depend on such cues. Rather, sensory information is important for activating the representation appropriate to the current environment as reflected by the firing properties of place cell ensembles. Specific sensory channels as well as the memory properties of place cells can support ongoing firing under manipulations of the environment. These memory features raise the question of the role of the place cell system in the acquisition, storage and retrieval of spatial information. Based on the existing literature about the effects of hippocampal lesions and about the metabolic activations in spatial memory tasks, we suggest that a function of the place cell system is to automatically provide the organism with information about its current location so as to allow for the rapid acquisition of novel information.

Understanding hippocampal activity by using purposeful behavior: place navigation induces place cell discharge in both task-relevant and task-irrelevant spatial reference frames

Continuous rotation of an arena in a cue-rich room dissociates the stationary room-bound information from the rotating arena-bound information. This disrupted spatial discharge in the majority of place cells from rats trained to collect randomly scattered food. In contrast, most place cell firing patterns recorded from rats trained to solve a navigation task on the rotating arena were preserved during the rotation. Spatial discharge was preserved in both the task-relevant stationary and the task-irrelevant rotating reference frames, but firing was more organized in the task-relevant frame. It is concluded that, (i) the effects of environmental manipulations can be understood with confidence only when the rat's purposeful behavior is used to formulate interpretations of the data, and (ii) hippocampal place cell activity is organized in multiple overlapping spatial reference frames.

Understanding hippocampal activity by using purposeful behavior: place navigation induces place cell discharge in both task-relevant and task-irrelevant spatial reference frames

Continuous rotation of an arena in a cue-rich room dissociates the stationary room-bound information from the rotating arena-bound information. This disrupted spatial discharge in the majority of place cells from rats trained to collect randomly scattered food. In contrast, most place cell firing patterns recorded from rats trained to solve a navigation task on the rotating arena were preserved during the rotation. Spatial discharge was preserved in both the task-relevant stationary and the task-irrelevant rotating reference frames, but firing was more organized in the task-relevant frame. It is concluded that, (i) the effects of environmental manipulations can be understood with confidence only when the rat's purposeful behavior is used to formulate interpretations of the data, and (ii) hippocampal place cell activity is organized in multiple overlapping spatial reference frames.

Maintenance of rat head direction cell firing during locomotion in the vertical plane

Previous studies have identified a subset of neurons in the rat anterodorsal thalamus (ADN) that encode head direction (HD) in absolute space and may be involved in navigation. These HD cells discharge selectively when the rat points its head in a specific direction (the preferred firing direction) in the horizontal plane. HD cells are typically recorded during free movement about a single horizontal surface. The current experiment examined how HD cell firing was influenced by 1) locomotion in the vertical plane and 2) locomotion on two different horizontal surfaces separated in height. Rats were trained in a cylindrical enclosure containing a single polarizing cue card attached to the cylinder wall, covering approximately 100 degrees of arc. The enclosure contained two horizontal surfaces: the cylinder floor and an annulus around the cylinder top 76 cm above the floor. A 90 degrees vertical mesh ladder that could be affixed at any angular position on the cylinder wall allowed the rats to locomote back and forth between the two horizontal surfaces. Rats were trained to retrieve food pellets on the cylinder floor as well as climb the mesh ladder to retrieve food pellets on the annulus. HD cell activity was monitored as the rat traversed the horizontal and vertical surfaces of the apparatus. When the angular position of the mesh corresponded to the cell's preferred firing direction, the HD cells maintained their peak discharge rate as the rat climbed up the mesh, but did not fire when the rat climbed down the mesh. In contrast, when the mesh was positioned 180 degrees opposite the preferred firing direction, HD cells did not fire when the rat climbed up the mesh, but exhibited maximal firing when the rat climbed down the mesh. When the mesh was placed 90 or 270 degrees from the preferred firing direction, HD cells exhibited background firing rates during climbing up or down the mesh. While preferred firing directions were maintained between the two horizontal surfaces, peak firing rate increased significantly (approximately 30%) on the annulus as compared with the cylinder floor. These data demonstrate that HD cells continue to discharge in the vertical plane if the vertical locomotion began with the rat's orientation corresponding to the preferred firing direction. One model consistent with these data are that HD cells define the horizontal reference frame as the animal's plane of locomotion. Further, we propose that HD cell firing, as viewed within a three-dimensional coordinate system, can be characterized as the surface of a hemitorus.

Head direction cells in rats with hippocampal or overlying neocortical lesions: evidence for impaired angular path integration

Rodents use two distinct navigation strategies that are based on environmental cues (landmark navigation) or internal cues (path integration). Head direction (HD) cells are neurons that discharge when the animal points its head in a particular direction and are responsive to the same cues that support path integration and landmark navigation. Experiment 1 examined whether HD cells in rats with lesions to the hippocampus plus the overlying neocortex or to just the overlying neocortex could maintain a stable preferred firing direction when the rats locomoted from a familiar to a novel environment, a process thought to require path integration. HD cells from both lesion groups were unable to maintain a similar preferred direction between environments, with cells from hippocampal rats showing larger shifts than cells from rats sustaining only cortical damage. When the rats first explored the novel environment, the preferred directions of the cells drifted for up to 4 min before establishing a consistent firing orientation. The preferred direction was usually maintained during subsequent visits to the novel environment but not across longer time periods (days to weeks). Experiment 2 demonstrated that a novel landmark cue was able to establish control over HD cell preferred directions in rats from both lesion groups, showing that the impairment observed in experiment 1 cannot be attributed to an impairment in establishing cue control. Experiment 3 showed that the preferred direction drifted when HD cells in lesioned animals were recorded in the dark. It was also shown that the anticipatory property of anterodorsal thalamic nucleus HD cells was still present in lesioned animals; thus, this property cannot be attributed to an intact hippocampus. These findings suggest that the hippocampus and the overlying neocortex are involved in path integration mechanisms, which enable an animal to maintain an accurate representation of its directional heading when exploring a novel environment.

Noradrenaline and serotonin levels in the guinea pig hippocampus following unilateral vestibular deafferentation

Recent evidence indicates that the hippocampus uses input from the vestibular system in order to accomplish its spatial computational functions. At present, there are few data on the neurochemical basis of the interactions between the vestibular system and the hippocampus. The aim of this study was to determine levels of noradrenaline (NA), serotonin (5-HT) and the 5-HT metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in the CA1, CA2 and dentate gyrus (DG) regions of the ipsilateral and contralateral hippocampi, at 10 h following deafferentation of the peripheral vestibular nerve (UVD) in guinea pig, using high-performance liquid chromatography (HPLC) with electrochemical detection (ECD). There were no significant differences in NA levels in the ipsilateral or contralateral CA1 following UVD. However, there was a significant increase in NA levels in the contralateral CA2 following UVD, compared to both the sham and intact anesthetic control conditions (p<0.05). No such change was seen in the ipsilateral CA2. In the contralateral DG, there was a significant increase in NA levels in both the UVD and sham conditions, compared to the intact anesthetic controls (p<0.05). No significant changes in 5-HT or 5-HIAA levels were seen in the ipsilateral or contralateral CA1, CA2 or DG following UVD. This study provides the first evidence that UVD may cause an increase in NA levels in the CA2 region of the contralateral hippocampus.

Head direction cells in the primate pre-subiculum

The function of the primate hippocampus and related structures was analysed by making recordings from the hippocampus, subiculum, presubiculum, and parahippocampal gyrus in monkeys actively walking in the laboratory. Head direction cells were found in the presubiculum. The firing rate of these cells was a function of the head direction of the monkey, with a response that was typically 10-100 times larger to the best as compared to the opposite direction. The mean half-amplitude width of the tuning of the cells was 76 degrees. The response of head direction cells in the presubiculum was not influenced by the place where the monkey was, there being the same tuning to head direction at different places in a room, and even outside the room. The response of these cells was also independent of the "spatial view" observed by the monkey, and also the position of the eyes in the head. The average information about head direction was 0.64 bits, about place was 0.10 bits, about spatial view was 0.27 bits, and about eye position was 0.04 bits. The cells maintained their tuning for periods of at least several minutes when the view details were obscured or the room was darkened. This representation of head direction could be useful together with the hippocampal spatial view cells and whole body motion cells found in primates in such spatial and memory functions as path integration.

Differential deficits in the Morris water maze following cytotoxic lesions of the anterior thalamus and fornix transection

Rats with complete fornix lesions or cytotoxic lesions placed in the anterior thalamic region were trained on an allocentric spatial memory test (the Morris water maze). While both lesions led to impairments in locating the hidden platform in this test of reference memory, the thalamic lesions led to a significantly greater deficit than that observed after fornix transection as measured by a number of performance indices. The lesions also led to different patterns of swim behaviour in the pool. The severity of the thalamic lesion deficit was associated with anterior thalamic nuclei damage but not with damage to the nucleus medialis dorsalis. Both the fornix and the thalamic lesions also severely impaired T-maze alternation. In contrast, neither set of lesions appeared to affect the recognition of small or large objects. While the study provides further evidence of a close functional relationship between the hippocampus and the anterior thalamic nuclei, it also shows that disconnection of the fornical inputs to the anterior thalamic nuclei does not provide a full explanation of the thalamic deficit.