Anticipation in the Rodent Head Direction System Can Be Explained by an Interaction of Head Movements and Vestibular Firing Properties

Vestibular and active self-motion signals drive visual perception in binocular rivalry

Multisensory integration helps the brain build reliable models of the world and resolve ambiguities. Visual interactions with sound and touch are well established but vestibular influences on vision are less well studied. Here, we test the vestibular influence on vision using horizontally opposed motions presented one to each eye so that visual perception is unstable and alternates irregularly. Passive, whole-body rotations in the yaw plane stabilized visual alternations, with perceived direction oscillating congruently with rotation (leftward motion during leftward rotation, and vice versa). This demonstrates a purely vestibular signal can resolve ambiguous visual motion and determine visual perception. Active self-rotation following the same sinusoidal profile also entrained vision to the rotation cycle – more strongly and with a lesser time lag, likely because of efference copy and predictive internal models. Both experiments show that visual ambiguity provides an effective paradigm to reveal how vestibular and motor inputs can shape visual perception.

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Binocular rivalry between left/right motions is stabilized by congruent head movement

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Left/right head rotations entrain rivalry dynamics so matching direction is perceived

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Active and passive rotations both drive rivalry dominance to match rotation direction

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Resolving ambiguous vision occurs in a broader vestibular and action-based context

A framework to identify structured behavioral patterns within rodent spatial trajectories

Animal behavior is highly structured. Yet, structured behavioral patterns-or "statistical ethograms"-are not immediately apparent from the full spatiotemporal data that behavioral scientists usually collect. Here, we introduce a framework to quantitatively characterize rodent behavior during spatial (e.g., maze) navigation, in terms of movement building blocks or motor primitives. The hypothesis that we pursue is that rodent behavior is characterized by a small number of motor primitives, which are combined over time to produce open-ended movements. We assume motor primitives to be organized in terms of two sparsity principles: each movement is controlled using a limited subset of motor primitives (sparse superposition) and each primitive is active only for time-limited, time-contiguous portions of movements (sparse activity). We formalize this hypothesis using a sparse dictionary learning method, which we use to extract motor primitives from rodent position and velocity data collected during spatial navigation, and successively to reconstruct past trajectories and predict novel ones. Three main results validate our approach. First, rodent behavioral trajectories are robustly reconstructed from incomplete data, performing better than approaches based on standard dimensionality reduction methods, such as principal component analysis, or single sparsity. Second, the motor primitives extracted during one experimental session generalize and afford the accurate reconstruction of rodent behavior across successive experimental sessions in the same or in modified mazes. Third, in our approach the number of motor primitives associated with each maze correlates with independent measures of maze complexity, hence showing that our formalism is sensitive to essential aspects of task structure. The framework introduced here can be used by behavioral scientists and neuroscientists as an aid for behavioral and neural data analysis. Indeed, the extracted motor primitives enable the quantitative characterization of the complexity and similarity between different mazes and behavioral patterns across multiple trials (i.e., habit formation). We provide example uses of this computational framework, showing how it can be used to identify behavioural effects of maze complexity, analyze stereotyped behavior, classify behavioral choices and predict place and grid cell displacement in novel environments.

Baseline of upper teeth: (a) Control organ for spatial navigation? (b) Weak point for misaligned posture and pain?

Our observations question both the current doctrine of spatial orientation as processed by vestibular, visual and proprioceptive impressions as well as the horizontal alignment of the eye axis. Indeed our observations suggest spatial orientation as a physically based, largely mechanically transmitted interaction between individual and environment. It is controlled by an interface defined by the baseline of upper teeth. It simultaneously constitutes both body and environment acting as an integral part of that environment. Consequently, the baseline of upper teeth is part of the aforementioned environment. Instead of the eye axis during spatial orientation it aligns the true horizontal absolutely. This was tested by fixing a cross to upper teeth. While walking, running and jumping it did not deviate by more than 2° from the external axis. Subsequently, we inclined the baseline of upper teeth by inserting an asymmetric wafer so that it angulated the eye axis. Immediately, head, visual and vestibular axes tilted unstably with misaligned body posture. Only the indicative cross remained stably aligned to the external axes. The person felt "upright", not noticing his posture had changed. He was then instructed to straighten his shoulders and trunk until his posture was objectively nearly upright again. The voluntary correction caused the indicative cross to tilt. The person felt uneven while being more upright. We concluded that the automatic posture works by "synchronizing" the baseline of upper teeth to the external axis and that the synchronized position is supported by the vestibular system. Benefit of an interface is that the body's movements in the environment simultaneously happen within the baseline of upper teeth. Therein the vectors of the body and the environment are calculated to remain in balance. This model introduces the transmission of the vector information to postural muscles by the dura mater, controlled by tension between C0-C2. The information is skewed by bony dislocations between C0-C2 caused by an inclination of the interface. The resulting misalignments of posture are foreseeable and specifically correspond to the type of inclination. They occur in a broad section of the population. Diagnosed as muscular weakness, they may cause therapy resistant common diseases like back and joint pain after 5-10 years. Following our observations, the inclination of the baseline of upper teeth originates from inattentive changes in the length of upper teeth in dental treatment. Multiple treatments optimizing teeth length in long term patients improved the patients' situation.

Anticipatory Neural Activity Improves the Decoding Accuracy for Dynamic Head-Direction Signals

Insects and vertebrates harbor specific neurons that encode the animal's head direction (HD) and provide an internal compass for spatial navigation. Each HD cell fires most strongly in one preferred direction. As the animal turns its head, however, HD cells in rat anterodorsal thalamic nucleus (ADN) and other brain areas fire already before their preferred direction is reached, as if the neurons anticipated the future HD. This phenomenon has been explained at a mechanistic level, but a functional interpretation is still missing. To close this gap, we use a computational approach based on the movement statistics of male rats and a simple model for the neural responses within the ADN HD network. Network activity is read out using population vectors in a biologically plausible manner, so that only past spikes are taken into account. We find that anticipatory firing improves the representation of the present HD by reducing the motion-induced temporal bias inherent in causal decoding. The amount of anticipation observed in ADN enhances the precision of the HD compass read-out by up to 40%. More generally, our theoretical framework predicts that neural integration times not only reflect biophysical constraints, but also the statistics of behaviorally relevant stimuli; in particular, anticipatory tuning should be found wherever neurons encode sensory signals that change gradually in time.SIGNIFICANCE STATEMENT Across different brain regions, populations of noisy neurons encode dynamically changing stimuli. Decoding a time-varying stimulus from the population response involves a trade-off: For short read-out times, stimulus estimates are unreliable as the number of stochastic spikes is small; for long read-outs, estimates are biased because they lag behind the true stimulus. We show that optimal decoding of temporally correlated stimuli not only relies on finding the right read-out time window but requires neurons to anticipate future stimulus values. We apply this general framework to the rodent head-direction system and show that the experimentally observed anticipation of future head directions can be explained at a quantitative level from the neuronal tuning properties, network size, and the animal's head-movement statistics.

Retrosplenial and postsubicular head direction cells compared during visual landmark discrimination

BACKGROUND

Visual landmarks are used by head direction (HD) cells to establish and help update the animal's representation of head direction, for use in orientation and navigation. Two cortical regions that are connected to primary visual areas, postsubiculum (PoS) and retrosplenial cortex (RSC), possess HD cells: we investigated whether they differ in how they process visual landmarks.

METHODS

We compared PoS and RSC HD cell activity from tetrode-implanted rats exploring an arena in which correct HD orientation required discrimination of two opposing landmarks having high, moderate or low discriminability.

RESULTS

RSC HD cells had higher firing rates than PoS HD cells and slightly lower modulation by angular head velocity, and anticipated actual head direction by ~48 ms, indicating that RSC spiking leads PoS spiking. Otherwise, we saw no differences in landmark processing, in that HD cells in both regions showed equal responsiveness to and discrimination of the cues, with cells in both regions having unipolar directional tuning curves and showing better discrimination of the highly discriminable cues. There was a small spatial component to the signal in some cells, consistent with their role in interacting with the place cell navigation system, and there was also slight modulation by running speed. Neither region showed theta modulation of HD cell spiking.

CONCLUSIONS

That the cells can immediately respond to subtle differences in spatial landmarks is consistent with rapid processing of visual snapshots or scenes; similarities in PoS and RSC responding may be due either to similar computations being performed on the visual inputs, or to rapid sharing of information between these regions. More generally, this two-cue HD cell paradigm may be a useful method for testing rapid spontaneous visual discrimination capabilities in other experimental settings.

Environmental Anchoring of Head Direction in a Computational Model of Retrosplenial Cortex

Allocentric (world-centered) spatial codes driven by path integration accumulate error unless reset by environmental sensory inputs that are necessarily egocentric (body-centered). Previous models of the head direction system avoided the necessary transformation between egocentric and allocentric reference frames by placing visual cues at infinity. Here we present a model of head direction coding that copes with exclusively proximal cues by making use of a conjunctive representation of head direction and location in retrosplenial cortex. Egocentric landmark bearing of proximal cues, which changes with location, is mapped onto this retrosplenial representation. The model avoids distortions due to parallax, which occur in simple models when a single proximal cue card is used, and can also accommodate multiple cues, suggesting how it can generalize to arbitrary sensory environments. It provides a functional account of the anatomical distribution of head direction cells along Papez' circuit, of place-by-direction coding in retrosplenial cortex, the anatomical connection from the anterior thalamic nuclei to retrosplenial cortex, and the involvement of retrosplenial cortex in navigation. In addition to parallax correction, the same mechanism allows for continuity of head direction coding between connected environments, and shows how a head direction representation can be stabilized by a single within arena cue. We also make predictions for drift during exploration of a new environment, the effects of hippocampal lesions on retrosplenial cells, and on head direction coding in differently shaped environments.

SIGNIFICANCE STATEMENT

The activity of head direction cells signals the direction of an animal's head relative to landmarks in the world. Although driven by internal estimates of head movements, head direction cells must be kept aligned to the external world by sensory inputs, which arrive in the reference frame of the sensory receptors. We present a computational model, which proposes that sensory inputs are correctly associated to head directions by virtue of a conjunctive representation of place and head directions in the retrosplenial cortex. The model allows for a stable head direction signal, even when the sensory input from nearby cues changes dramatically whenever the animal moves to a different location, and enables stable representations of head direction across connected environments.

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

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

Integration of celestial compass cues in the central complex of the locust brain

Many insects rely on celestial compass cues such as the polarization pattern of the sky for spatial orientation. In the desert locust, the central complex (CX) houses multiple sets of neurons, sensitive to the oscillation plane of polarized light and, thus, likely acts as an internal polarization compass. We investigated whether other sky compass cues like direct sunlight or the chromatic gradient of the sky might contribute to this compass. We recorded from polarization-sensitive CX neurons while an unpolarized green or UV light spot was moved around the head of the animal. All types of neuron that were sensitive to the plane of polarization (E-vector) above the animal also responded to the unpolarized light spots in an azimuth-dependent way. The tuning to the unpolarized light spots was independent of wavelength, suggesting that the neurons encode solar azimuth based on direct sunlight and not on the sky chromatic gradient. Two cell types represented the natural 90°-relationship between solar azimuth and zenithal E-vector orientation, providing evidence to suggest that solar azimuth information supports the internal polarization compass. Most neurons showed advances in their tuning to the E-vector and the unpolarized light spots dependent on rotation direction, consistent with anticipatory signaling. The amplitude of responses and its variability were dependent on the level of background firing, possibly indicating different internal states. The integration of polarization and solar azimuth information strongly suggests that besides the polarization pattern of the sky, direct sunlight might be an important cue for sky compass navigation in the locust.

Cellular Basis of Head Direction and Contextual Cues in the Insect Brain

Animals rely upon integrated sensory information for spatial navigation. A question of wide importance in navigation is how sensory cues get transformed into neural codes that represent the animal's orientation within its proximal environment. Here, we investigated the possibility of head-direction coding in the central complex of the cockroach, Blaberus discoidalis. We used extracellular recordings in restrained animals that were rotated on a platform relative to a fixed landmark. The passive rotations allowed us to test for head-direction coding in the absence of self-generated motion cues. Our results indicate that individual cells in the central complex encode the animal's heading relative to a landmark's position in several ways. In some cells, directional tuning was established even in the absence of visual cues, suggesting that the directional code can be maintained solely based on the internal motion cues derived from the passive rotations. Additionally, some cells in the central complex encoded rotation-direction history, a navigational context cue, by increasing or decreasing the firing rate during the stationary periods following clockwise or counterclockwise rotations. Together, these results unveil head-direction cell-like activity in the insect central complex, which highly resemble similarly functioning cells in the mammalian brain that encode head direction. We predict that the observed head-orientation coding and directionally sensitive cells are essential components of the brain circuitry mediating insect navigation.

Septo-hippocampal signal processing: breaking the code

The septo-hippocampal connections appear to be a key element in the neuromodulatory cholinergic control of the hippocampal neurons. The cholinergic neuromodulation is well established in shifting behavioral states of the brain. The pacemaker role of medial septum in the limbic theta rhythm is demonstrated by lesions and pharmacological manipulations of GABAergic neurons, yet the link between the activity of different septal neuronal classes and limbic theta rhythm is not fully understood. We know even less about the information transfer between the medial septum and hippocampus--is there a particular kind of processed information that septo-hippocampal pathways transmit? This review encompasses fundamental findings together with the latest data of septo-hippocampal signal processing to tackle the frontiers of our understanding about the functional significance of medial septum to the hippocampal formation.

Decoding signal processing in thalamo-hippocampal circuitry: implications for theories of memory and spatial processing

A major tool in understanding how information is processed in the brain is the analysis of neuronal output at each hierarchical level through which neurophysiological signals are propagated. Since the experimental brain operation performed on Henry Gustav Molaison (known as patient H.M.) in 1953, the hippocampal formation has gained special attention, resulting in a very large number of studies investigating signals processed by the hippocampal formation. One of the main information streams to the hippocampal formation, vital for episodic memory formation, arises from thalamo-hippocampal projections, as there is extensive connectivity between these structures. This connectivity is sometimes overlooked by theories of memory formation by the brain, in favour of theories with a strong cortico-hippocampal flavour. In this review, we attempt to address some of the complexity of the signals processed within the thalamo-hippocampal circuitry. To understand the signals encoded by the anterior thalamic nuclei in particular, we review key findings from electrophysiological, anatomical, behavioural and computational studies. We include recent findings elucidating the integration of different signal modalities by single thalamic neurons; we focus in particular on the propagation of two prominent signals: head directionality and theta rhythm. We conclude that thalamo-hippocampal processing provides a centrally important, substantive, and dynamic input modulating and moderating hippocampal spatial and mnemonic processing. This article is part of a Special Issue entitled SI: Brain and Memory.

The irregular firing properties of thalamic head direction cells mediate turn-specific modulation of the directional tuning curve

Head direction cells encode an animal's heading in the horizontal plane. However, it is not clear why the directionality of a cell's mean firing rate differs for clockwise, compared with counterclockwise, head turns (this difference is known as the “separation angle”) in anterior thalamus. Here we investigated in freely behaving rats whether intrinsic neuronal firing properties are linked to this phenomenon. We found a positive correlation between the separation angle and the spiking variability of thalamic head direction cells. To test whether this link is driven by hyperpolarization-inducing currents, we investigated the effect of thalamic reticular inhibition during high-voltage spindles on directional spiking. While the selective directional firing of thalamic neurons was preserved, we found no evidence for entrainment of thalamic head direction cells by high-voltage spindle oscillations. We then examined the role of depolarization-inducing currents in the formation of separation angle. Using a single-compartment Hodgkin-Huxley model, we show that modeled neurons fire with higher frequencies during the ascending phase of sinusoidal current injection (mimicking the head direction tuning curve) when simulated with higher high-threshold calcium channel conductance. These findings demonstrate that the turn-specific encoding of directional signal strongly depends on the ability of thalamic neurons to fire irregularly in response to sinusoidal excitatory activation. Another crucial factor for inducing phase lead to sinusoidal current injection was the presence of spike-frequency adaptation current in the modeled neurons. Our data support a model in which intrinsic biophysical properties of thalamic neurons mediate the physiological encoding of directional information.

Resolving the active versus passive conundrum for head direction cells

Head direction (HD) cells have been identified in a number of limbic system structures. These cells encode the animal's perceived directional heading in the horizontal plane and are dependent on an intact vestibular system. Previous studies have reported that the responses of vestibular neurons within the vestibular nuclei are markedly attenuated when an animal makes a volitional head turn compared to passive rotation. This finding presents a conundrum in that if vestibular responses are suppressed during an active head turn how is a vestibular signal propagated forward to drive and update the HD signal? This review identifies and discusses four possible mechanisms that could resolve this problem. These mechanisms are: (1) the ascending vestibular signal is generated by more than just vestibular-only neurons, (2) not all vestibular-only neurons contributing to the HD pathway have firing rates that are attenuated by active head turns, (3) the ascending pathway may be spared from the affects of the attenuation in that the HD system receives information from other vestibular brainstem sites that do not include vestibular-only cells, and (4) the ascending signal is affected by the inhibited vestibular signal during an active head turn, but the HD circuit compensates and uses the altered signal to accurately update the current HD. Future studies will be needed to decipher which of these possibilities is correct.

Multimodal integration of self-motion cues in the vestibular system: active versus passive translations

The ability to keep track of where we are going as we navigate through our environment requires knowledge of our ongoing location and orientation. In response to passively applied motion, the otolith organs of the vestibular system encode changes in the velocity and direction of linear self-motion (i.e., heading). When self-motion is voluntarily generated, proprioceptive and motor efference copy information is also available to contribute to the brain's internal representation of current heading direction and speed. However to date, how the brain integrates these extra-vestibular cues with otolith signals during active linear self-motion remains unknown. Here, to address this question, we compared the responses of macaque vestibular neurons during active and passive translations. Single-unit recordings were made from a subgroup of neurons at the first central stage of sensory processing in the vestibular pathways involved in postural control and the computation of self-motion perception. Neurons responded far less robustly to otolith stimulation during self-generated than passive head translations. Yet, the mechanism underlying the marked cancellation of otolith signals did not affect other characteristics of neuronal responses (i.e., baseline firing rate, tuning ratio, orientation of maximal sensitivity vector). Transiently applied perturbations during active motion further established that an otolith cancellation signal was only gated in conditions where proprioceptive sensory feedback matched the motor-based expectation. Together our results have important implications for understanding the brain's ability to ensure accurate postural and motor control, as well as perceptual stability, during active self-motion.

Head direction cell activity in the anterodorsal thalamus requires intact supragenual nuclei

Neural activity in several limbic areas varies as a function of the animal's head direction (HD) in the horizontal plane. Lesions of the vestibular periphery abolish this HD cell signal, suggesting an essential role for vestibular afference in HD signal generation. The organization of brain stem pathways conveying vestibular information to the HD circuit is poorly understood; however, recent anatomical work has identified the supragenual nucleus (SGN) as a putative relay. To test this hypothesis, we made lesions of the SGN in rats and screened for HD cells in the anterodorsal thalamus. In animals with complete bilateral lesions, the overall number of HD cells was significantly reduced relative to control animals. In animals with unilateral lesions of the SGN, directional activity was present, but the preferred firing directions of these cells were unstable and less influenced by the rotation of an environmental landmark. In addition, we found that preferred directions displayed large directional shifts when animals foraged for food in a darkened environment and when they were navigating from a familiar environment to a novel one, suggesting that the SGN plays a critical role in projecting essential self-motion (idiothetic) information to the HD cell circuit.

Learned integration of visual, vestibular, and motor cues in multiple brain regions computes head direction during visually guided navigation

Effective navigation depends upon reliable estimates of head direction (HD). Visual, vestibular, and outflow motor signals combine for this purpose in a brain system that includes dorsal tegmental nucleus, lateral mammillary nuclei, anterior dorsal thalamic nucleus, and the postsubiculum. Learning is needed to combine such different cues to provide reliable estimates of HD. A neural model is developed to explain how these three types of signals combine adaptively within the above brain regions to generate a consistent and reliable HD estimate, in both light and darkness, which explains the following experimental facts. Each HD cell is tuned to a preferred head direction. The cell's firing rate is maximal at the preferred direction and decreases as the head turns from the preferred direction. The HD estimate is controlled by the vestibular system when visual cues are not available. A well-established visual cue anchors the cell's preferred direction when the cue is in the animal's field of view. Distal visual cues are more effective than proximal cues for anchoring the preferred direction. The introduction of novel cues in either a novel or familiar environment can gain control over a cell's preferred direction within minutes. Turning out the lights or removing all familiar cues does not change the cell's firing activity, but it may accumulate a drift in the cell's preferred direction. The anticipated time interval (ATI) of the HD estimate is greater in early processing stages of the HD system than at later stages. The model contributes to an emerging unified neural model of how multiple processing stages in spatial navigation, including postsubiculum head direction cells, entorhinal grid cells, and hippocampal place cells, are calibrated through learning in response to multiple types of signals as an animal navigates in the world.

Ageing effects on path integration and landmark navigation

Navigation abilities show marked decline in both normal ageing and dementia. Path integration may be particularly affected, as it is supported by the hippocampus and entorhinal cortex, both of which show severe degeneration with ageing. Age differences in path integration based on kinaesthetic and vestibular cues have been clearly demonstrated, but very little research has focused on visual path integration, based only on optic flow. Path integration is complemented by landmark navigation, which may also show age differences, but has not been well studied either. Here we present a study using several simple virtual navigation tasks to explore age differences in path integration both with and without landmark information. We report that, within a virtual environment that provided only optic flow information, older participants exhibited deficits in path integration in terms of distance reproduction, rotation reproduction, and triangle completion. We also report age differences in triangle completion within an environment that provided landmark information. In all tasks, we observed a more restricted range of responses in the older participants, which we discuss in terms of a leaky integrator model, as older participants showed greater leak than younger participants. Our findings begin to explain the mechanisms underlying age differences in path integration, and thus contribute to an understanding of the substantial decline in navigation abilities observed in ageing.

Theta-modulated head direction cells in the rat anterior thalamus

A major tool in understanding how the brain processes information is the analysis of neuronal output at each hierarchical level along the pathway of signal propagation. Theta rhythm and head directionality are the two main signals found across all levels of Papez's circuit, which supports episodic memory formation. Here, we provide evidence that the functional interaction between both signals occurs at a subcortical level. We show that there is population of head direction cells (39%) in rat anteroventral thalamic nucleus that exhibit rhythmic spiking in the theta range. This class of units, termed HD-by-theta (head direction-by-theta) cells, discharged predominantly in spike trains at theta frequency (6-12 Hz). The highest degree of theta rhythmicity was evident when the animal was heading/facing in the preferred direction, expressed by the Gaussian peak of the directional tuning curve. The theta-rhythmic mode of spiking was closely related to the firing activity of local theta-bursting cells. We also found that 32% of anteroventral theta-bursting cells displayed a head-directional modulation of their spiking. This crossover between theta and head-directional signals indicates that anterior thalamus integrates information related to heading and movement, and may therefore actively modulate hippocampo-dencephalic information processing.

Active and passive movement are encoded equally by head direction cells in the anterodorsal thalamus

The head direction (HD) system is composed of cells that represent the direction in which the animal's head is facing. Each HD cell responds optimally when the head is pointing in a particular, or preferred, direction. Although vestibular system input is necessary to generate the directional signal, motor/proprioceptive inputs can also influence HD cell responses. Previous studies comparing active and passive movement have reported significant suppression of the HD signal during passive restraint. However, in each of these studies there was considerable variability across cells, and the animal's head was never completely fixed. To address these issues, we developed a passive restraint system that more fully prevented head and body movement. HD cell responses in the anterodorsal thalamus (ADN) were evaluated during active and passive movement with this new system. Contrary to previous reports, HD cell responses were not affected by passive restraint. Both head-fixed and hand-held restraint failed to produce significant inhibition of the active HD cell response. Furthermore, direction-specific firing was maintained regardless of 1) the animal's previous experience with restraint, 2) whether it was tested in the light or dark, or 3) the position of the animal relative to the axis of rotation. The maintenance of a stable directional signal without appropriate motor, proprioceptive, or visual input indicates that vestibular input is necessary and sufficient for the generation of the HD signal. Motor and proprioceptive influences may therefore be important for the control of the preferred firing direction of HD cells, but not the generation of the signal itself.

Disruption of the head direction cell signal after occlusion of the semicircular canals in the freely moving chinchilla

Head direction (HD) cells in the rat anterodorsal thalamic nucleus (ADN) fire relative to the animal's directional heading. Lesions of the entire vestibular labyrinth have been shown to severely alter VIIIth nerve input and disrupt these HD signals. To assess the specific contributions of the semicircular canals without altering tonic VIIIth nerve input, ADN cells were recorded from chinchillas after bilateral semicircular canal occlusion. Although ADN HD cells (and also hippocampal place cells and theta cells) were identified in intact chinchillas, no direction-specific activity was seen after canal occlusions. Instead, "bursty" cells were observed that exhibited burst-firing patterns similar to normal HD cells but with firing unrelated to the animal's actual head direction. Importantly, when pairs of bursty cells were recorded, the temporal order of their firing was dependent on the animal's turning direction, as is the case for pairs of normal HD cells. These results suggest that bursty cells are actually disrupted HD cells. The present findings further suggest that the HD cell network is still able to generate spiking activity after canal occlusions, but the semicircular canal input is critical for updating the network activity in register with changes in the animal's HD.

Recurrent networks with short term synaptic depression

Cortical circuitry shows an abundance of recurrent connections. A widely used model that relies on recurrence is the ring attractor network, which has been used to describe phenomena as diverse as working memory, visual processing and head direction cells. Commonly, the synapses in these models are static. Here, we examine the behaviour of ring attractor networks when the recurrent connections are subject to short term synaptic depression, as observed in many brain regions. We find that in the presence of a uniform background current, the network activity can be in either of three states: a stationary attractor state, a uniform state, or a rotating attractor state. The rotation speed can be adjusted over a large range by changing the background current, opening the possibility to use the network as a variable frequency oscillator or pattern generator. Finally, using simulations we extend the network to two-dimensional fields and find a rich range of possible behaviours.

Head direction cell instability in the anterior dorsal thalamus after lesions of the interpeduncular nucleus

Previous research has identified a population of cells throughout the limbic system that discharge as a function of the animal's head direction (HD). Altering normal motor cues can alter the HD cell responses and disrupt the updating of their preferred firing directions, thus suggesting that motor cues contribute to processing the HD signal. A pathway that conveys motor information may stem from the interpeduncular nucleus (IPN), a brain region that has reciprocal connections with HD cell circuitry. To test this hypothesis, we produced electrolytic or neurotoxic lesions of the IPN and recorded HD cells in the anterior dorsal thalamus (ADN) of rats. Direction-specific firing remained present in the ADN after lesions of the IPN, but measures of HD cell properties showed that cells had reduced peak firing rates, large directional firing ranges, and firing that predicted the animal's future heading more than in intact controls. Furthermore, preferred firing directions were moderately less influenced by rotation of a salient visual landmark. Finally, the preferred directions of cells in lesioned rats exhibited large shifts when the animals foraged for scattered food pellets in a darkened environment and when locomoting from a familiar environment to a novel one. We propose that the IPN contributes motor information about the animal's movements to the HD cell circuitry. Furthermore, these results suggest that the IPN plays a broad role in the discharge properties and stability of direction-specific activity in the HD cell circuit.

Compensating time delays with neural predictions: are predictions sensory or motor?

Neural delays are a general property of computations carried out by neural circuits. Delays are a natural consequence of temporal summation and coding used by the nervous system to integrate information from multiple resources. For adaptive behaviour, however, these delays must be compensated. In order to sense and interact with moving objects, for example, the visual system must predict the future position of the object to compensate for delays. In this paper, we address two critical questions concerning the implementation of the compensation mechanisms in the brain, namely, where does compensation occur and how is it realized. We present evidence showing that compensation can happen in both the motor and sensory systems, and that compensation using 'diagonal neural pathways' is a suitable strategy for implementing compensation in the visual system. In this strategy, neural signals in the early stage of information processing are sent to the future cortical positions that correspond to the distance the object will travel in the period of transmission delay. We propose a computational model to elucidate this using the retinal visual information pathway.