Passive movements of the head do not abolish anticipatory firing properties of head direction cells

Short-term postsynaptic plasticity facilitates predictive tracking in continuous attractors

Introduction

The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic transmission and is associated with various neurological and psychiatric disorders. Recently, a novel form of postsynaptic plasticity known as NMDAR-based short-term postsynaptic plasticity (STPP) has been identified. It has been suggested that long-lasting glutamate binding to NMDAR allows for the retention of input information in brain slices up to 500 ms, leading to response facilitation. However, the impact of STPP on the dynamics of neuronal populations remains unexplored.

Methods

In this study, we incorporated STPP into a continuous attractor neural network (CANN) model to investigate its effects on neural information encoding in populations of neurons. Unlike short-term facilitation, a form of presynaptic plasticity, the temporally enhanced synaptic efficacy resulting from STPP destabilizes the network state of the CANN by increasing its mobility.

Results

Our findings demonstrate that the inclusion of STPP in the CANN model enables the network state to predictively respond to a moving stimulus. This nontrivial dynamical effect facilitates the tracking of the anticipated stimulus, as the enhanced synaptic efficacy induced by STPP enhances the system's mobility.

Discussion

The discovered STPP-based mechanism for sensory prediction provides valuable insights into the potential development of brain-inspired computational algorithms for prediction. By elucidating the role of STPP in neural population dynamics, this study expands our understanding of the functional implications of NMDAR-related plasticity in information processing within the brain.

Conclusion

The incorporation of STPP into a CANN model highlights its influence on the mobility and predictive capabilities of neural networks. These findings contribute to our knowledge of STPP-based mechanisms and their potential applications in developing computational algorithms for sensory prediction.

Coregistration of heading to visual cues in retrosplenial cortex

Spatial cognition depends on an accurate representation of orientation within an environment. Head direction cells in distributed brain regions receive a range of sensory inputs, but visual input is particularly important for aligning their responses to environmental landmarks. To investigate how population-level heading responses are aligned to visual input, we recorded from retrosplenial cortex (RSC) of head-fixed mice in a moving environment using two-photon calcium imaging. We show that RSC neurons are tuned to the animal's relative orientation in the environment, even in the absence of head movement. Next, we found that RSC receives functionally distinct projections from visual and thalamic areas and contains several functional classes of neurons. While some functional classes mirror RSC inputs, a newly discovered class coregisters visual and thalamic signals. Finally, decoding analyses reveal unique contributions to heading from each class. Our results suggest an RSC circuit for anchoring heading representations to environmental visual landmarks.

Anterior thalamic NMDA-induced damage impairs extrapolation relying on serial stimulus patterns, in rats

Extrapolation of serial stimulus patterns seems to depend upon identification and application of patterns relating sequences of stimuli stored in memory, thus allowing prediction of pending events never experienced before. There have been proposals that such a "generator of predictions system" would include the subiculum, mammillary bodies, anteroventral thalamus and cingulate cortex (e.g., Gray, 1982). The anteroventral thalamus (AVT) seems to be in a strategic position, both hodologically and experimentally, to allow testing of this hypothesis. This study investigated the effect of NMDA-induced damage to the anteroventral thalamus [part of the anterodorsal (AD) thalamus was also damaged in some animals], following stereotaxic minute topic microinjections, on the ability of male Wistar rats to extrapolate relying on serial stimulus patterns. Corresponding sham-operated controls received phosphate-saline buffer microinjections at the same stereotaxic coordinates. The subjects were trained to run through a straight alleyway along 31 sessions, one session per day, to get rewarded. Each session included four successive trials. Subjects exposed to the monotonic serial pattern received 14, 7, 3, 1 sunflower seeds along trials. Subjects exposed to the non-monotonic serial pattern received 14, 3, 7, 1 sunflower seeds. On the 32nd testing session, a fifth trial, never experienced before, was included immediately after the fourth trial. Sham-operated control subjects exposed to the monotonic serial pattern were expected to exhibit longer running times, since the content of their prediction in the fifth trial should be "less than 1 sunflower seeds". In contrast, control subjects exposed to the non-monotonic serial pattern were expected to exhibit shorter running times, since the content of their prediction would be "more than 1 sunflower seeds". Confirming these predictions, control subjects exposed to the monotonic serial pattern exhibited longer running times as compared to both, their own running times in previous trials within the same session and control subjects exposed to the non-monotonic schedule, thus indicating the occurrence of extrapolation. In contrast, AVT/AD lesioned subjects exposed to the monotonic schedule did not exhibit this increase in running times on the fifth trial, indicating lack of extrapolation. These results indicate that extrapolation relying on serial stimulus patterns is disrupted following extensive NMDA-induced damage to AVT and part of the AD. This represents the first consistent demonstration that the anterior thalamic nuclei are required for extrapolation of serial stimulus patterns and generation of predictions.

Human brain dynamics in active spatial navigation

Spatial navigation is a complex cognitive process based on multiple senses that are integrated and processed by a wide network of brain areas. Previous studies have revealed the retrosplenial complex (RSC) to be modulated in a task-related manner during navigation. However, these studies restricted participants' movement to stationary setups, which might have impacted heading computations due to the absence of vestibular and proprioceptive inputs. Here, we present evidence of human RSC theta oscillation (4-8 Hz) in an active spatial navigation task where participants actively ambulated from one location to several other points while the position of a landmark and the starting location were updated. The results revealed theta power in the RSC to be pronounced during heading changes but not during translational movements, indicating that physical rotations induce human RSC theta activity. This finding provides a potential evidence of head-direction computation in RSC in healthy humans during active spatial navigation.

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.

Neurología de la anticipación y sus implicaciones en el deporte

<p>El movimiento es una acción que involucra interconexiones complejas, por lo cual se requiere profundizar en los procesos de adaptación, predicción y anticipación que permiten entender la importancia de estos aspectos desde sus bases filogenéticas y ontogenéticas hasta su implicación en movimientos complejos. Parte de la optimización de los procesos descritos se haya en la calidad de información aferente, la cual permite la relación con el entorno —especialmente la entrada visual— que reconoce un flujo de imágenes y una proyección al contexto en el que se está inmerso. Las estructuras e interconexiones implicadas en la anticipación y predicción de movimientos son descritas de modo que se evidencia la congruencia y continuidad del flujo de información que caracteriza esta especialidad neuromecánica de movimiento. Por otro lado, se aborda la integración de centros puntuales del sistema nervioso central y redes neuronales que permiten el entramado de procesos de aprendizaje por observación, además de proveer equilibrio y eficiencia al sistema en la recepción de estímulos y su relación con la generación de eferencias motoras que cumplan con objetivos específicos. En el ámbito deportivo estos procesos favorecen la eficiencia del gesto optimizando el movimiento.</p>

Passive Transport Disrupts Grid Signals in the Parahippocampal Cortex

Navigation is usually thought of relative to landmarks, but neural signals representing space also use information generated by an animal's movements. These signals include grid cells, which fire at multiple locations, forming a repeating grid pattern. Grid cell generation depends upon theta rhythm, a 6-10 Hz electroencephalogram (EEG) oscillation that is modulated by the animals' movement velocity. We passively moved rats in a clear cart to eliminate motor related self-movement cues that drive moment-to-moment changes in theta rhythmicity. We found that passive movement maintained theta power and frequency at levels equivalent to low active movement velocity, spared overall head-direction (HD) cell characteristics, but abolished both velocity modulation of theta rhythmicity and grid cell firing patterns. These results indicate that self-movement motor cues are necessary for generating grid-specific firing patterns, possibly by driving velocity modulation of theta rhythmicity, which may be used as a speed signal to generate the repeating pattern of grid cells.

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.

Impairment of the anterior thalamic head direction cell network following administration of the NMDA antagonist MK-801

Head direction (HD) cells, found in the rodent Papez circuit, are thought to form the neural circuitry responsible for directional orientation. Because NMDA transmission has been implicated in spatial tasks requiring directional orientation, we sought to determine if the NMDA antagonist dizocilpine (MK-801) would disrupt the directional signal carried by the HD network. Anterior thalamic HD cells were isolated in female Long-Evans rats and initially monitored for baseline directional activity while the animals foraged in a familiar enclosure. The animals were then administered MK-801 at a dose of .05 mg/kg or 0.1 mg/kg, or isotonic saline, and cells were re-examined for changes in directional specificity and landmark control. While the cells showed no changes in directional specificity and landmark control following administration of saline or the lower dose of MK-801, the higher dose of MK-801 caused a dramatic attenuation of the directional signal, characterized by decreases in peak firing rates, signal to noise, and directional information content. While the greatly attenuated directional specificity of cells in the high dose condition usually remained stable relative to the landmarks within the recording enclosure, a few cells in this condition exhibited unstable preferred directions within and between recording sessions. Our results are discussed relative to the possibility that the findings explain the effects of MK-801 on the acquisition and performance of spatial tasks.

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.

Nucleus reuniens of the thalamus contains head direction cells

Discrete populations of brain cells signal heading direction, rather like a compass. These 'head direction' cells are largely confined to a closely-connected network of sites. We describe, for the first time, a population of head direction cells in nucleus reuniens of the thalamus in the freely-moving rat. This novel subcortical head direction signal potentially modulates the hippocampal CA fields directly and, thus, informs spatial processing and memory.

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.

The anterior thalamus provides a subcortical circuit supporting memory and spatial navigation

The anterior thalamic nuclei (ATN), a central component of Papez' circuit, are generally assumed to be key constituents of the neural circuits responsible for certain categories of learning and memory. Supporting evidence for this contention is that damage to either of two brain regions, the medial temporal lobe and the medial diencephalon, is most consistently associated with anterograde amnesia. Within these respective regions, the hippocampal formation and the ATN (anteromedial, anteroventral, and anterodorsal) are the particular structures of interest. The extensive direct and indirect hippocampal-anterior thalamic interconnections and the presence of theta-modulated cells in both sites further support the hypothesis that these structures constitute a neuronal network crucial for memory and cognition. The 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 coupled with neuroanatomical studies. Here, we discuss the electrophysiological properties of cells in the ATN with an emphasis on their role in spatial navigation. In addition, we describe neuroanatomical and functional relationships between the ATN and hippocampal formation.

Is navigation in virtual reality with FMRI really navigation?

Identifying the neural mechanisms underlying spatial orientation and navigation has long posed a challenge for researchers. Multiple approaches incorporating a variety of techniques and animal models have been used to address this issue. More recently, virtual navigation has become a popular tool for understanding navigational processes. Although combining this technique with functional imaging can provide important information on many aspects of spatial navigation, it is important to recognize some of the limitations these techniques have for gaining a complete understanding of the neural mechanisms of navigation. Foremost among these is that, when participants perform a virtual navigation task in a scanner, they are lying motionless in a supine position while viewing a video monitor. Here, we provide evidence that spatial orientation and navigation rely to a large extent on locomotion and its accompanying activation of motor, vestibular, and proprioceptive systems. Researchers should therefore consider the impact on the absence of these motion-based systems when interpreting virtual navigation/functional imaging experiments to achieve a more accurate understanding of the mechanisms underlying navigation.

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.

Tracking and characterizing the head motion of unanaesthetized rats in positron emission tomography

Positron emission tomography (PET) is an important in vivo molecular imaging technique for translational research. Imaging unanaesthetized rats using motion-compensated PET avoids the confounding impact of anaesthetic drugs and enables animals to be imaged during normal or evoked behaviour. However, there is little published data on the nature of rat head motion to inform the design of suitable marker-based motion-tracking set-ups for brain imaging-specifically, set-ups that afford close to uninterrupted tracking. We performed a systematic study of rat head motion parameters for unanaesthetized tube-bound and freely moving rats with a view to designing suitable motion-tracking set-ups in each case. For tube-bound rats, using a single appropriately placed binocular tracker, uninterrupted tracking was possible greater than 95 per cent of the time. For freely moving rats, simulations and measurements of a live subject indicated that two opposed binocular trackers are sufficient (less than 10% interruption to tracking) for a wide variety of behaviour types. We conclude that reliable tracking of head pose can be achieved with marker-based optical-motion-tracking systems for both tube-bound and freely moving rats undergoing PET studies without sedation.

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.

The vestibular system: multimodal integration and encoding of self-motion for motor control

Understanding how sensory pathways transmit information under natural conditions remains a major goal in neuroscience. The vestibular system plays a vital role in everyday life, contributing to a wide range of functions from reflexes to the highest levels of voluntary behavior. Recent experiments establishing that vestibular (self-motion) processing is inherently multimodal also provide insight into a set of interrelated questions. What neural code is used to represent sensory information in vestibular pathways? How do the interactions between the organism and the environment shape encoding? How is self-motion information processing adjusted to meet the needs of specific tasks? This review highlights progress that has recently been made towards understanding how the brain encodes and processes self-motion to ensure accurate motor control.

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.

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.

Head direction cell activity in mice: robust directional signal depends on intact otolith organs

The head direction (HD) cell signal is a representation of an animal's perceived directional heading with respect to its environment. This signal appears to originate in the vestibular system, which includes the semicircular canals and otolith organs. Preliminary studies indicate the semicircular canals provide a necessary component of the HD signal, but involvement of otolithic information in the HD signal has not been tested. The present study was designed to determine the otolithic contribution to the HD signal, as well as to compare HD cell activity of mice with that of rats. HD cell activity in the anterodorsal thalamus was assessed in wild-type C57BL/6J and otoconia-deficient tilted mice during locomotion within a cylinder containing a prominent visual landmark. HD cell firing properties in C57BL/6J mice were generally similar to those in rats. However, in C57BL/6J mice, landmark rotation failed to demonstrate dominant control of the HD signal in 36% of the sessions. In darkness, directional firing became unstable during 42% of the sessions, but landmark control was not associated with HD signal stability in darkness. HD cells were identified in tilted mice, but directional firing properties were not as robust as those of C57BL/6J mice. Most HD cells in tilted mice were controlled by landmark rotation but showed substantial signal degradation across trials. These results support current models that suggest otolithic information is involved in the perception of directional heading. Furthermore, compared with rats, the HD signal in mice appears to be less reliably anchored to prominent environmental cues.

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.

Vestibular system: the many facets of a multimodal sense

Elegant sensory structures in the inner ear have evolved to measure head motion. These vestibular receptors consist of highly conserved semicircular canals and otolith organs. Unlike other senses, vestibular information in the central nervous system becomes immediately multisensory and multimodal. There is no overt, readily recognizable conscious sensation from these organs, yet vestibular signals contribute to a surprising range of brain functions, from the most automatic reflexes to spatial perception and motor coordination. Critical to these diverse, multimodal functions are multiple computationally intriguing levels of processing. For example, the need for multisensory integration necessitates vestibular representations in multiple reference frames. Proprioceptive-vestibular interactions, coupled with corollary discharge of a motor plan, allow the brain to distinguish actively generated from passive head movements. Finally, nonlinear interactions between otolith and canal signals allow the vestibular system to function as an inertial sensor and contribute critically to both navigation and spatial orientation.

Landmark control and updating of self-movement cues are largely maintained in head direction cells after lesions of the posterior parietal cortex

Head direction (HD) cells discharge as a function of the rat's directional orientation with respect to its environment. Because animals with posterior parietal cortex (PPC) lesions exhibit spatial and navigational deficits, and the PPC is indirectly connected to areas containing HD cells, we determined the effects of bilateral PPC lesions on HD cells recorded in the anterodorsal thalamus. HD cells from lesioned animals had similar firing properties compared to controls and their preferred firing directions shifted a corresponding amount following rotation of the major visual landmark. Because animals were not exposed to the visual landmark until after surgical recovery, these results provide evidence that the PPC is not necessary for visual landmark control or the establishment of landmark stability. Further, cells from lesioned animals maintained a stable preferred firing direction when they foraged in the dark and were only slightly less stable than controls when they self-locomoted into a novel enclosure. These findings suggest that PPC does not play a major role in the use of landmark and self-movement cues in updating the HD cell signal, or in its generation.

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

The rodent head-direction (HD) system, which codes for the animal's head direction in the horizontal plane, is thought to be critically involved in spatial navigation. Electrophysiological recording studies have shown that HD cells can anticipate the animal's HD by up to 75–80 ms. The origin of this anticipation is poorly understood. In this modeling study, we provide a novel explanation for HD anticipation that relies on the firing properties of neurons afferent to the HD system. By incorporating spike rate adaptation and postinhibitory rebound as observed in medial vestibular nucleus neurons, our model produces realistic anticipation on a large corpus of rat movement data. In addition, HD anticipation varies between recording sessions of the same cell, between active and passive movement, and between different studies. Such differences do not appear to be correlated with behavioral variables and cannot be accounted for using earlier models. In the present model, anticipation depends on the power spectrum of the head movements. By direct comparison with recording data, we show that the model explains 60–80% of the observed anticipation variability. We conclude that HD afferent dynamics and the statistics of rat head movements are important in generating HD anticipation. This result contributes to understanding the functional circuitry of the HD system and has methodological implications for studies of HD anticipation.

The head direction signal: origins and sensory-motor integration

Navigation first requires accurate perception of one's spatial orientation within the environment, which consists of knowledge about location and directional heading. Cells within several limbic system areas of the mammalian brain discharge allocentrically as a function of the animal's directional heading, independent of the animal's location and ongoing behavior. These cells are referred to as head direction (HD) cells and are believed to encode the animal's perceived directional heading with respect to its environment. Although HD cells are found in several areas, the principal circuit for generating this signal originates in the dorsal tegmental nucleus and projects serially, with some reciprocal connections, to the lateral mammillary nucleus --> anterodorsal thalamus --> PoS, and terminates in the entorhinal cortex. HD cells receive multimodal information about landmarks and self-generated movements. Vestibular information appears critical for generating the directional signal, but motor/proprioceptive and landmark information are important for updating it.

Differential regulation of action potential firing in adult murine thalamocortical neurons by Kv3.2, Kv1, and SK potassium and N-type calcium channels

Sensory signals of widely differing dynamic range and intensity are transformed into a common firing rate code by thalamocortical neurons. While a great deal is known about the ionic currents, far less is known about the specific channel subtypes regulating thalamic firing rates. We hypothesized that different K(+) and Ca(2+) channel subtypes control different stimulus-response curve properties. To define the channels, we measured firing rate while pharmacologically or genetically modulating specific channel subtypes. Inhibiting Kv3.2 K(+) channels strongly suppressed maximum firing rate by impairing membrane potential repolarization, while playing no role in the firing response to threshold stimuli. By contrast, inhibiting Kv1 channels with alpha-dendrotoxin or maurotoxin strongly increased firing rates to threshold stimuli by reducing the membrane potential where action potentials fire (V(th)). Inhibiting SK Ca(2+)-activated K(+) channels with apamin robustly increased gain (slope of the stimulus-response curve) and maximum firing rate, with minimum effects on threshold responses. Inhibiting N-type Ca(2+) channels with omega-conotoxin GVIA or omega-conotoxin MVIIC partially mimicked apamin, while inhibiting L-type and P/Q-type Ca(2+) channels had small or no effects. EPSC-like current injections closely mimicked the results from tonic currents. Our results show that Kv3.2, Kv1, SK potassium and N-type calcium channels strongly regulate thalamic relay neuron sensory transmission and that each channel subtype controls a different stimulus-response curve property. Differential regulation of threshold, gain and maximum firing rate may help vary the stimulus-response properties across and within thalamic nuclei, normalize responses to diverse sensory inputs, and underlie sensory perception disorders.

Discrimination between active and passive head movements by macaque ventral and medial intraparietal cortex neurons

An important prerequisite for effective motor action is the discrimination between active and passive body movements. Passive movements often require immediate reflexes, whereas active movements may demand suppression of the latter. The vestibular system maintains correct body and head posture in space through reflexes. Since vestibular inputs have been reported to be largely suppressed in the vestibular nuclei during active head movements, we investigated whether head movement-related signals in the primate parietal cortex, a brain region involved in self-motion perception, could support both reflex functions and self-movement behaviour. We employed a paradigm that made available direct comparison of neuronal discharge under active and passive movement conditions. In this study, we demonstrate that a population of intraparietal (VIP (ventral) and MIP (medial)) cortex neurons change their preferred directions during horizontal head rotations depending on whether animals have performed active movements, or if they were moved passively. In other neurons no such change occurred. A combination of these signals would provide differential information about the active or passive nature of an ongoing movement. Moreover, some neurons' responses clearly anticipated the upcoming active head movement, providing a possible basis for vestibular-related reflex suppression. Intraparietal vestibular neurons thus distinguish between active and passive head movements, and their responses differ substantially from those reported in brainstem vestibular neurons, regarding strength, timing, and direction selectivity. We suggest that the contextual firing characteristics of these neurons have far-reaching implications for the suppression of reflex movements during active movement, and for the representation of space during self-movement.