Lost in time: Relocating the perception of duration outside the brain.

It is well-accepted in neuroscience that animals process time internally to estimate the duration of intervals lasting between one and several seconds. More than 100 years ago, Henri Bergson nevertheless remarked that, because animals have memory, their inner experience of time is ever-changing, making duration impossible to measure internally and time a source of change. Bergson proposed that quantifying the inner experience of time requires its externalization in movements (observed or self-generated), as their unfolding leaves measurable traces in space. Here, studies across species are reviewed and collectively suggest that, in line with Bergson's ideas, animals spontaneously solve time estimation tasks through a movement-based spatialization of time. Moreover, the well-known scalable anticipatory responses of animals to regularly spaced rewards can be explained by the variable pressure of time on reward-oriented actions. Finally, the brain regions linked with time perception overlap with those implicated in motor control, spatial navigation and motivation. Thus, instead of considering time as static information processed by the brain, it might be fruitful to conceptualize it as a kind of force to which animals are more or less sensitive depending on their internal state and environment.

The Dorsal Striatum Energizes Motor Routines.

The dorsal striatum (dS) has been implicated in storing procedural memories and controlling movement kinematics. Since procedural memories are expressed through movements, the exact nature of the dS function has proven difficult to delineate. Here, we challenged rats in complementary locomotion-based tasks designed to alleviate this confound. Surprisingly, dS lesions did not impair the rats' ability to remember the procedure for the successful completion of motor routines. However, the speed and initiation of the reward-oriented phase of the routines were irreversibly altered by the dS lesion. Further behavioral analyses, combined with modeling in the optimal control framework, indicated that these kinematic alterations were well explained by an increased sensitivity to effort. Our work provides evidence supporting a primary role of the dS in modulating the kinematics of reward-oriented actions, a function that may be related to the optimization of the energetic costs of moving.

Author Correction: Response outcomes gate the impact of expectations on perceptual decisions.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Turning the body into a clock: Accurate timing is facilitated by simple stereotyped interactions with the environment.

How animals adapt their behavior according to regular time intervals between events is not well understood, especially when intervals last several seconds. One possibility is that animals use disembodied internal neuronal representations of time to decide when to initiate a given action at the end of an interval. However, animals rarely remain immobile during time intervals but tend to perform stereotyped behaviors, raising the possibility that motor routines improve timing accuracy. To test this possibility, we used a task in which rats, freely moving on a motorized treadmill, could obtain a reward if they approached it after a fixed interval. Most animals took advantage of the treadmill length and its moving direction to develop, by trial-and-error, the same motor routine whose execution resulted in the precise timing of their reward approaches. Noticeably, when proficient animals did not follow this routine, their temporal accuracy decreased. Then, naïve animals were trained in modified versions of the task designed to prevent the development of this routine. Compared to rats trained in the first protocol, these animals didn't reach a comparable level of timing accuracy. Altogether, our results indicate that timing accuracy in rats is improved when the environment affords cues that animals can incorporate into motor routines.

Response outcomes gate the impact of expectations on perceptual decisions.

Perceptual decisions are based on sensory information but can also be influenced by expectations built from recent experiences. Can the impact of expectations be flexibly modulated based on the outcome of previous decisions? Here, rats perform an auditory task where the probability to repeat the previous stimulus category is varied in trial-blocks. All rats capitalize on these sequence correlations by exploiting a transition bias: a tendency to repeat or alternate their previous response using an internal estimate of the sequence repeating probability. Surprisingly, this bias is null after error trials. The internal estimate however is not reset and it becomes effective again after the next correct response. This behavior is captured by a generative model, whereby a reward-driven modulatory signal gates the impact of the latent model of the environment on the current decision. These results demonstrate that, based on previous outcomes, rats flexibly modulate how expectations influence their decisions.

Dynamic control of hippocampal spatial coding resolution by local visual cues.

The ability to flexibly navigate an environment relies on a hippocampal-dependent cognitive map. External space can be internally mapped at different spatial resolutions. However, whether hippocampal spatial coding resolution can rapidly adapt to local features of an environment remains unclear. To explore this possibility, we recorded the firing of hippocampal neurons in mice navigating virtual reality environments, embedding or not local visual cues (virtual 3D objects) in specific locations. Virtual objects enhanced spatial coding resolution in their vicinity with a higher proportion of place cells, smaller place fields, increased spatial selectivity and stability. This effect was highly dynamic upon objects manipulations. Objects also improved temporal coding resolution through improved theta phase precession and theta timescale spike coordination. We propose that the fast adaptation of hippocampal spatial coding resolution to local features of an environment could be relevant for large-scale navigation.

The Timing of Sensory-Guided Behavioral Response is Represented in the Mouse Primary Somatosensory Cortex.

Whisker-guided decision making in mice is thought to critically depend on information processing occurring in the primary somatosensory cortex. However, it is not clear if neuronal activity in this "early" sensory region contains information about the timing and speed of motor response. To address this question we designed a new task in which freely moving mice learned to associate a whisker stimulus to reward delivery. The task was tailored in such a way that a wide range of delays between whisker stimulation and reward collection were observed due to differences of motivation and perception. After training, mice were anesthetized and neuronal responses evoked by stimulating trained and untrained whiskers were recorded across several cortical columns of barrel cortex. We found a strong correlation between the delay of the mouse behavioral response and the timing of multiunit activity evoked by the trained whisker, outside its principal cortical column, in layers 4 and 5A but not in layer 2/3. Circuit mapping ex vivo revealed this effect was associated with a weakening of layer 4 to layer 2/3 projection. We conclude that the processes controlling the propagation of key sensory inputs to naive cortical columns and the timing of sensory-guided action are linked.

No Discrete Start/Stop Signals in the Dorsal Striatum of Mice Performing a Learned Action.

A popular hypothesis is that the dorsal striatum generates discrete "traffic light" signals that initiate, maintain, and terminate the execution of learned actions. Alternatively, the striatum may continuously monitor the dynamics of movements associated with action execution by processing inputs from somatosensory and motor cortices. Here, we recorded the activity of striatal neurons in mice performing a run-and-stop task and characterized the diversity of firing rate modulations relative to run performance (tuning curves) across neurons. We found that the tuning curves could not be statistically clustered in discrete functional groups (start or stop neurons). Rather, their shape varied continuously according to the movement dynamics of the task. Moreover, striatal spiking activity correlated with running speed on a run-by-run basis and was modulated by task-related non-locomotor movements, such as licking. We hypothesize that such moment-to-moment movement monitoring by the dorsal striatum contributes to the learning of adaptive actions and/or updating their kinematics.

To move or to sense? Incorporating somatosensory representation into striatal functions.

A long-standing hypothesis postulates that the striatum is essential for the concurrent selection of adaptive actions and repression of inappropriate alternatives. Here, classical and recent anatomical and physiological studies are reviewed to show that, in mammals, the striatum can detect discrete task-relevant sensory stimuli and continuously track somatosensory information associated with the generation of simple movements and more complex actions. Rather than contributing to the immediate selection of actions, the striatum may monitor the sensorimotor state of animals by integrating somatosensory information and motor-related signals on a moment-by-moment basis. Such function could be critical for the progressive acquisition or updating of adaptive actions and the emergence of an embodied sense of time.

Local or Not Local: Investigating the Nature of Striatal Theta Oscillations in Behaving Rats.

In the cortex and hippocampus, neuronal oscillations of different frequencies can be observed in local field potentials (LFPs). LFPs oscillations in the theta band (6-10 Hz) have also been observed in the dorsolateral striatum (DLS) of rodents, mostly during locomotion, and have been proposed to mediate behaviorally-relevant interactions between striatum and cortex (or between striatum and hippocampus). However, it is unclear if these theta oscillations are generated in the striatum. To address this issue, we recorded LFPs and spiking activity in the DLS of rats performing a running sequence on a motorized treadmill. We observed an increase in rhythmical activity of the LFP in the theta-band during run compared to rest periods. However, several observations suggest that these oscillations are mainly generated outside of the striatum. First, theta oscillations disappeared when LFPs were rereferenced against a striatal recording electrode and the imaginary coherence between LFPs recorded at different locations within the striatum was null. Second, 8% of the recorded neurons had their spiking activity phase-locked to the theta rhythm. Third, Granger causality analyses between LFPs simultaneously recorded in the cortex and the striatum revealed that the interdependence between these two signals in the theta range was mostly accounted for by a common external source. The most parsimonious interpretation of these results is that theta oscillations observed in striatal LFPs are largely contaminated by volume-conducted signals. We propose that striatal LFPs are not optimal proxies of network dynamics in the striatum and should be interpreted with caution.

Abnormal UP/DOWN Membrane Potential Dynamics Coupled with the Neocortical Slow Oscillation in Dentate Granule Cells during the Latent Phase of Temporal Lobe Epilepsy.

The dentate gyrus, a major entry point to the hippocampus, gates (or filters) incoming information from the cortex. During sleep or anesthesia, the slow-wave oscillation (SWO) orchestrates hippocampus-neocortex communication, which is important for memory formation. The dentate gate is altered in temporal lobe epilepsy (TLE) early during epileptogenesis, which favors the propagation of pathological activities. Yet, whether the gating of physiological SWO by dentate granule cells (DGCs) is altered in TLE has remained unexplored. We combined intracellular recordings of membrane potential (V m) of DGCs and local field potential recordings of the SWO in parietal cortex in anesthetized rats early during epileptogenesis [post-status epilepticus (SE) rats]. As expected, in control rats, the V m of DGCs weakly and rarely oscillated in the SWO frequency range. In contrast, in post-SE rats, the V m of DGCs displayed strong and long-lasting SWO. In these cells, clear UP and DOWN states, in phase with the neocortical SWO, led to a bimodal V m distribution. In post-SE rats, the firing of DGCs was increased and more temporally modulated by the neocortical SWO. We conclude that UP/DOWN state dynamics dominate the V m of DGCs and firing early during epileptogenesis. This abnormally strong neocortical influence on the dynamics of DGCs may profoundly modify the hippocampus-neocortex dialogue during sleep and associated cognitive functions.

The striatum multiplexes contextual and kinematic information to constrain motor habits execution.

The striatum is required for the acquisition of procedural memories, but its contribution to motor control once learning has occurred is unclear. We created a task in which rats learned a difficult motor sequence characterized by fine-tuned changes in running speed adjusted to spatial and temporal constraints. After training and extensive practice, we found that the behavior was habitual, yet tetrode recordings in the dorsolateral striatum (DLS) revealed continuous integrative representations of running speed, position and time. These representations were weak in naive rats that were hand-guided to perform the same sequence and developed slowly after learning. Finally, DLS inactivation in well-trained animals preserved the structure of the sequence while increasing its trial-by-trial variability. We conclude that, after learning, the DLS continuously integrates task-relevant information to constrain the execution of motor habits. Our results provide a straightforward mechanism by which the basal ganglia may contribute to habit formation and motor control.

Striatal GABAergic and cortical glutamatergic neurons mediate contrasting effects of cannabinoids on cortical network synchrony.

Activation of type 1 cannabinoid receptors (CB1R) decreases GABA and glutamate release in cortical and subcortical regions, with complex outcomes on cortical network activity. To date there have been few attempts to disentangle the region- and cell-specific mechanisms underlying the effects of cannabinoids on cortical network activity in vivo. Here we addressed this issue by combining in vivo electrophysiological recordings with local and systemic pharmacological manipulations in conditional mutant mice lacking CB1R expression in different neuronal populations. First we report that cannabinoids induce hypersynchronous thalamocortical oscillations while decreasing the amplitude of faster cortical oscillations. Then we demonstrate that CB1R at striatonigral synapses (basal ganglia direct pathway) mediate the thalamocortical hypersynchrony, whereas activation of CB1R expressed in cortical glutamatergic neurons decreases cortical synchrony. Finally we show that activation of CB1 expressed in cortical glutamatergic neurons limits the cannabinoid-induced thalamocortical hypersynchrony. By reporting that CB1R activations in cortical and subcortical regions have contrasting effects on cortical synchrony, our study bridges the gap between cellular and in vivo network effects of cannabinoids. Incidentally, the thalamocortical hypersynchrony we report suggests a potential mechanism to explain the sensory "high" experienced during recreational consumption of marijuana.

Locomotion-related oscillatory body movements at 6-12 Hz modulate the hippocampal theta rhythm.

The hippocampal theta rhythm is required for accurate navigation and spatial memory but its relation to the dynamics of locomotion is poorly understood. We used miniature accelerometers to quantify with high temporal and spatial resolution the oscillatory movements associated with running in rats. Simultaneously, we recorded local field potentials in the CA1 area of the hippocampus. We report that when rats run their heads display prominent vertical oscillations with frequencies in the same range as the hippocampal theta rhythm (i.e., 6-12 Hz). In our behavioral set-up, rats run mainly with speeds between 50 and 100 cm/s. In this range of speeds, both the amplitude and frequency of the "theta" head oscillations were increasing functions of running speed, demonstrating that the head oscillations are part of the locomotion dynamics. We found evidence that these rhythmical locomotor dynamics interact with the neuronal activity in the hippocampus. The amplitude of the hippocampal theta rhythm depended on the relative phase shift with the head oscillations, being maximal when the two signals were in phase. Despite similarity in frequency, the head movements and LFP oscillations only displayed weak phase and frequency locking. Our results are consistent with that neurons in the CA1 region receive inputs that are phase locked to the head acceleration signal and that these inputs are integrated with the ongoing theta rhythm.

Alteration of theta timescale dynamics of hippocampal place cells by a cannabinoid is associated with memory impairment.

The integrity of the hippocampus is critical for both spatial navigation and episodic memory, but how its neuronal firing patterns underlie those functions is not well understood. In particular, the modality by which hippocampal place cells contribute to spatial memory is debated. We found that administration of the cannabinoid receptor agonist CP55940 (2-[(1S,2R,5S)-5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]-5-(2-methyloctan-2-yl)phenol) induced a profound and reversible behavioral deficit in the hippocampus-dependent delayed spatial alternation task. On the one hand, despite severe memory impairment, the location-dependent firing of CA1 hippocampal place cells remained mostly intact. On the other hand, both spike-timing coordination between place cells at the theta timescale and theta phase precession of spikes were reversibly reduced. These results raise the possibility that cannabinoids impair memory primarily by altering short-term temporal dynamics of hippocampal neurons. We hypothesize that precise temporal coordination of hippocampal neurons is necessary for guiding behavior in spatial memory tasks.

Role of the cyclic-AMP/PKA cascade and of P/Q-type Ca++ channels in endocannabinoid-mediated long-term depression in the nucleus accumbens.

Glutamate transmission between prefrontal cortex (PFC) and accumbens (NAc) plays a crucial role in the establishment and expression of addictive behaviors. At these synapses exogenous cannabinoid receptor 1 (CB1R) agonists reversibly inhibit excitatory transmission, and the sustained release of endogenous cannabinoids (eCB) following prolonged cortical stimulation leads to long-term depression (LTD). Activation of presynaptic K(+) channels mediates the effects of exocannabinoids, but the transduction pathway underlying the protracted phase of eCB-LTD is unknown. Here we report that the maintenance of eCB-LTD does not involve presynaptic K(+) channels: eCB-LTD was not affected by blockade of K(+) channels with 4-AP (100 microM) and BaCl(2) (300 microM) (fEPSP=78.9+/-5.4% of baseline 58-60 min after tetanus, compared to 78.9+/-5.9% in control slices). In contrast, eCB-LTD was blocked by treatment of the slices with the adenylyl cyclase (AC) activator forskolin (10 microM), and with the protein kinase A (PKA) inhibitor KT5720 (1 microM) (fEPSP=108.9+/-5.7% in forskolin and 110.5+/-7.7% in KT5720, compared to 80.6+/-3.9% in control conditions). Additionally, selective blockade of P/Q-type Ca(2+) channels with omega-agatoxin-IVA (200 nM) occluded the expression of eCB-LTD (fEPSP=113.4+/-15.9% compared to 78.6+/-4.4% in control slices), while blockade of N- with omega-conotoxin-GVIA (1 microM) or L-type Ca(2+) channels with nimodipine (1 microM), was without effect (fEPSP was 83.7+/-5.3% and 87+/-8.9% respectively). These data show that protracted inhibition of AC/PKA activity and P/Q-type Ca(2+) channels are necessary for expression of eCB-LTD at NAc synapses.

Hippocampal place cell assemblies are speed-controlled oscillators.

The phase of spikes of hippocampal pyramidal cells relative to the local field theta oscillation shifts forward ("phase precession") over a full theta cycle as the animal crosses the cell's receptive field ("place field"). The linear relationship between the phase of the spikes and the travel distance within the place field is independent of the animal's running speed. This invariance of the phase-distance relationship is likely to be important for coordinated activity of hippocampal cells and space coding, yet the mechanism responsible for it is not known. Here we show that at faster running speeds place cells are active for fewer theta cycles but oscillate at a higher frequency and emit more spikes per cycle. As a result, the phase shift of spikes from cycle to cycle (i.e., temporal precession slope) is faster, yet spatial-phase precession stays unchanged. Interneurons can also show transient-phase precession and contribute to the formation of coherently precessing assemblies. We hypothesize that the speed-correlated acceleration of place cell assembly oscillation is responsible for the phase-distance invariance of hippocampal place cells.

Cannabinoids reveal importance of spike timing coordination in hippocampal function.

Cannabinoids impair hippocampus-dependent memory in both humans and animals, but the network mechanisms responsible for this effect are unknown. Here we show that the cannabinoids Delta(9)-tetrahydrocannabinol and CP55940 decreased the power of theta, gamma and ripple oscillations in the hippocampus of head-restrained and freely moving rats. These effects were blocked by a CB1 antagonist. The decrease in theta power correlated with memory impairment in a hippocampus-dependent task. By simultaneously recording from large populations of single units, we found that CP55940 severely disrupted the temporal coordination of cell assemblies in short time windows (<100 ms) yet only marginally affected population firing rates of pyramidal cells and interneurons. The decreased power of local field potential oscillations correlated with reduced temporal synchrony but not with firing rate changes. We hypothesize that reduced spike timing coordination and the associated impairment of physiological oscillations are responsible for cannabinoid-induced memory deficits.

Presynaptic homeostatic plasticity rescues long-term depression after chronic Delta 9-tetrahydrocannabinol exposure.

Alterations of long-term synaptic plasticity have been proposed to participate in the development of addiction. To preserve synaptic functions, homeostatic processes must be engaged after exposure to abused drugs. At the mouse cortico-accumbens synapses, a single in vivo injection of Delta9-tetrahydrocannabinol (THC) suppresses endocannabinoid-mediated long-term depression. Using biochemical and electrophysiological approaches, we now report that 1 week of repeated in vivo THC treatment reduces the coupling efficiency of cannabinoid CB1 receptors (CB1Rs) to G(i/o) transduction proteins, as well as CB1R-mediated inhibition of excitatory synaptic transmission at the excitatory synapses between the prefrontal cortex and the nucleus accumbens (NAc). Nonetheless, we found that cortico-accumbens synapses unexpectedly express normal long-term depression because of a reversible switch in its underlying mechanisms. The present data show that, in THC-treated mice, long-term depression is expressed because a presynaptic mGluR2/3 (metabotropic glutamate receptor 2/3)-dependent mechanism replaces the impaired endocannabinoid system. Thus, in the NAc, a novel form of presynaptic homeostasis rescues synaptic plasticity from THC-induced deficits.