Reinforcing operandum: rapid and reliable learning of skilled forelimb movements by head-fixed rodents

Translational validity of neuropsychological tasks of sustained attention between rodents and humans: A systematic review of three rodent tasks

Atypical sustained attention is a symptom in a number of neurological and psychological conditions. Investigations into its neural underpinnings are required for improved management and treatment. Rodents are useful in investigating the neurobiology underlying atypical sustained attention and several rodent tasks have been developed for use in touchscreen testing platforms that mimic methodology used in human clinical attention assessment. This systematic review was conducted to assess how translatable these rodent tasks are to equivalent clinical human tasks. Studies using the rodent Continuous Performance Task (rCPT), Sustained Attention Task (SAT), and 5-choice CPT (5C-CPT) were sought and screened. Included in the review were 138 studies, using the rCPT (n = 21), SAT (n = 90), and 5C-CPT (n = 27). Translatability between rodent and human studies was assessed based on (1) methodological similarity, (2) performance similarity, and (3) replication of results. The 5C-CPT was found to be the most translatable cross-species paradigm with good utility, while the rCPT and SAT require adaptation and further development to meet these translatability benchmarks. With greater replication and more consistent results, greater confidence in the translation of sustained attention results between species will be engendered.

A novel micro-ECoG recording method for recording multisensory neural activity from the parietal to temporal cortices in mice

Characterization of inter-regional interactions in brain is essential for understanding the mechanism relevant to normal brain function and neurological disease. The recently developed flexible micro (μ)-electrocorticography (μECoG) device is one prominent method used to examine large-scale cortical activity across multiple regions. The sheet-shaped μECoG electrodes arrays can be placed on a relatively wide area of cortical surface beneath the skull by inserting the device into the space between skull and brain. Although rats and mice are useful tools for neuroscience, current μECoG recording methods in these animals are limited to the parietal region of cerebral cortex. Recording cortical activity from the temporal region of cortex in mice has proven difficult because of surgical barriers created by the skull and surrounding temporalis muscle anatomy. Here, we developed a sheet-shaped 64-channel μECoG device that allows access to the mouse temporal cortex, and we determined the factor determining the appropriate bending stiffness for the μECoG electrode array. We also established a surgical technique to implant the electrode arrays into the epidural space over a wide area of cerebral cortex covering from the barrel field to olfactory (piriform) cortex, which is the deepest region of the cerebral cortex. Using histology and computed tomography (CT) images, we confirmed that the tip of the μECoG device reached to the most ventral part of cerebral cortex without causing noticeable damage to the brain surface. Moreover, the device simultaneously recorded somatosensory and odor stimulus-evoked neural activity from dorsal and ventral parts of cerebral cortex in awake and anesthetized mice. These data indicate that our μECoG device and surgical techniques enable the recording of large-scale cortical activity from the parietal to temporal cortex in mice, including somatosensory and olfactory cortices. This system will provide more opportunities for the investigation of physiological functions from wider areas of the mouse cerebral cortex than those currently available with existing ECoG techniques.

A cranial implant for stabilizing whole-cell patch-clamp recordings in behaving rodents

BACKGROUND

In vivo patch-clamp recording techniques provide access to the sub- and suprathreshold membrane potential dynamics of individual neurons during behavior. However, maintaining recording stability throughout behavior is a significant challenge, and while methods for head restraint are commonly used to enhance stability, behaviorally related brain movement relative to the skull can severely impact the success rate and duration of whole-cell patch-clamp recordings.

NEW METHOD

We developed a low-cost, biocompatible, and 3D-printable cranial implant capable of locally stabilizing brain movement, while permitting equivalent access to the brain when compared to a conventional craniotomy.

RESULTS

Experiments in head-restrained behaving mice demonstrate that the cranial implant can reliably reduce the amplitude and speed of brain displacements, significantly improving the success rate of recordings across repeated bouts of motor behavior.

COMPARISON WITH EXISTING METHOD(S)

Our solution offers an improvement on currently available strategies for brain stabilization. Due to its small size, the implant can be retrofitted to most in vivo electrophysiology recording setups, providing a low cost, easily implementable solution for increasing intracellular recording stability in vivo.

CONCLUSIONS

By facilitating stable whole-cell patch-clamp recordings in vivo, biocompatible 3D printed implants should accelerate the investigation of single neuron computations underlying behavior.

Dopamine error signal to actively cope with lack of expected reward

To obtain more of a particular uncertain reward, animals must learn to actively overcome the lack of reward and adjust behavior to obtain it again. The neural mechanisms underlying such coping with reward omission remain unclear. Here, we developed a task in rats to monitor active behavioral switch toward the next reward after no reward. We found that some dopamine neurons in the ventral tegmental area exhibited increased responses to unexpected reward omission and decreased responses to unexpected reward, following the opposite responses of the well-known dopamine neurons that signal reward prediction error (RPE). The dopamine increase reflected in the nucleus accumbens correlated with behavioral adjustment to actively overcome unexpected no reward. We propose that these responses signal error to actively cope with lack of expected reward. The dopamine error signal thus cooperates with the RPE signal, enabling adaptive and robust pursuit of uncertain reward to ultimately obtain more reward.

Conditional Regulation of Blood Pressure in Response to Emotional Stimuli by the Central Nucleus of the Amygdala in Rats

Humans and animals can determine whether a situation is favorable to them and act accordingly. For this, the autonomic tuning of the cardiovascular system to supply energy to active skeletal muscles through the circulatory system is as important as motor control. However, how the autonomic cardiovascular responses are regulated in dynamically changing environments and the neuronal mechanisms underlying these responses remain unclear. To resolve these issues, we recorded the blood pressure and heart rate of head-restrained rats during dynamically changing appetitive and aversive classical conditioning tasks. The rats displayed various associations between conditioned stimuli and unconditioned stimuli in appetitive (sucrose water), neutral (no outcome), and aversive (air puff) blocks. The blood pressure and heart rate in the appetitive block gradually increased in response to the reward-predicting cue and the response to the actual reward vigorously increased. The reward-predictive response was significantly higher than the responses obtained in the neutral and aversive condition blocks. To investigate whether the reward-predictive pressor response was caused by orofacial movements such as anticipatory licking behavior, we separately analyzed high- and low-licking trials. The conditioned pressor response was observed even in trials with low-licking behaviors. Blood pressure and heart rate responses to the air puff-predicting cue in the aversive block were not significantly different from the responses in the neutral block. The conditioned blood pressure response rapidly changed with condition block switching. Furthermore, to examine the contribution of the amygdala as an emotion center to these conditioned responses, we bilaterally microinjected a GABA_A receptor agonist, muscimol, into the central nucleus of the amygdala. Pharmacological inactivation of the central nucleus of the amygdala significantly decreased the reward-predictive pressor responses. These results suggest that the blood pressure is adaptively and rapidly regulated by emotional conditioned stimuli and that the central nucleus of the amygdala participates in regulating the pressor response in dynamically changing situations.

The primary motor cortex: the hub of motor learning in rodents

The primary motor cortex, a dynamic center for overall motion control and decision making, undergoes significant alterations upon neural stimulation. Over the last few decades, data from numerous studies using rodent models have improved our understanding of the morphological and functional plasticity of the primary motor cortex. In particular, spatially specific formation of dendritic spines and their maintenance during distinct behaviors is considered crucial for motor learning. However, whether the modifications of specific synapses are associated with motor learning should be studied further. In this review, we summarized the findings of prior studies on the features and dynamics of the primary motor cortex in rodents.

The contribution of low contrast-preferring neurons to information representation in the primary visual cortex after learning

Animals exhibit improved perception of lower-contrast visual objects after training. We explored this neuronal mechanism using multiple single-unit recordings from deep layers of the primary visual cortex (V1) of trained rats during orientation discrimination. We found that the firing rates of a subset of neurons increased by reducing luminance contrast, being at least above basal activities at low contrast. These low contrast–preferring neurons were rare during passive viewing without training or anesthesia after training. They fired more frequently in correct-choice than incorrect-choice trials. At single-neuron and population levels, they efficiently represented low-contrast orientations. Following training, in addition to generally enhanced excitation, the phase synchronization of spikes to beta oscillations at high contrast was stronger in putative inhibitory than excitatory neurons. The change in excitation-inhibition balance might contribute to low-contrast preference. Thus, low-contrast preference in V1 activity is strengthened in an experience-dependent manner, which may contribute to low-contrast visual discrimination.

Prepulse inhibition predicts subjective hearing in rats

Auditory studies in animals benefit from quick and accurate audiometry. The auditory brainstem response (ABR) and prepulse inhibition (PPI) have been widely used for hearing assessment in animals, but how well these assessments predict subjective audiometry still remains unclear. Human studies suggest that subjective audiometry is consistent with the ABR-based audiogram, not with the PPI-based audiogram, likely due to top-down processing in the cortex that inhibits PPI. Here, we challenged this view in Wistar rats, as rodents exhibit less complexity of cortical activities and thereby less influence of the cerebral cortex on PPI compared to humans. To test our hypothesis, we investigated whether subjective audiometry correlates with ABR- or PPI-based audiograms across the range of audible frequencies in Wistar rats. The subjective audiogram was obtained through pure-tone audiometry based on operant conditioning. Our results demonstrated that both the ABR-based and PPI-based audiograms significantly correlated to the subjective audiogram. We also found that ASR strength was information-rich, and adequate interpolation of this data offered accurate audiometry. Thus, unlike in humans, PPI could be used to predict subjective audibility in rats.

Noradrenaline modulates neuronal and perceptual visual detectability via β-adrenergic receptor

RATIONALE

Noradrenaline (NA) is a neuromodulator secreted from noradrenergic neurons in the locus coeruleus to the whole brain depending on the physiological state and behavioral context. It regulates various brain functions including vision via three major adrenergic receptor (AR) subtypes. Previous studies investigating the noradrenergic modulations on vision reported different effects, including improvement and impairment of perceptual visual sensitivity in rodents via β-AR, an AR subtype. Therefore, it remains unknown how NA affects perceptual visual sensitivity via β-AR and what neuronal mechanisms underlie it.

OBJECTIVES

The current study investigated the noradrenergic modulation of perceptual and neuronal visual sensitivity via β-AR in the primary visual cortex (V1).

METHODS

We performed extracellular multi-point recordings from V1 of rats performing a go/no-go visual detection task under the head-fixed condition. A β-AR blocker, propranolol (10 mM), was topically administered onto the V1 surface, and the drug effect on behavioral and neuronal activities was quantified by comparing pre-and post-drug administration.

RESULTS

The topical administration of propranolol onto the V1 surface significantly improved the task performance. An analysis of the multi-unit activity in V1 showed that propranolol significantly suppressed spontaneous activity and facilitated the visual response of the recording sites in V1. We further calculated the signal-to-noise ratio (SNR), finding that the SNR was significantly improved after propranolol administration.

CONCLUSIONS

Pharmacological blockade of β-AR in V1 improves perceptual visual detectability by modifying the SNR of neuronal activity.

A spike analysis method for characterizing neurons based on phase locking and scaling to the interval between two behavioral events

Standard analysis of neuronal functions assesses the temporal correlation between animal behaviors and neuronal activity by aligning spike trains with the timing of a specific behavioral event, e.g., visual cue. However, spike activity is often involved in information processing dependent on a relative phase between two consecutive events rather than a single event. Nevertheless, less attention has so far been paid to such temporal features of spike activity in relation to two behavioral events. Here, we propose "Phase-Scaling analysis" to simultaneously evaluate the phase locking and scaling to the interval between two events in task-related spike activity of individual neurons. This analysis method can discriminate conceptual "scaled"-type neurons from "nonscaled"-type neurons using an activity variation map that combines phase locking with scaling to the interval. Its robustness was validated by spike simulation using different spike properties. Furthermore, we applied it to analyzing actual spike data from task-related neurons in the primary visual cortex (V1), posterior parietal cortex (PPC), primary motor cortex (M1), and secondary motor cortex (M2) of behaving rats. After hierarchical clustering of all neurons using their activity variation maps, we divided them objectively into four clusters corresponding to nonscaled-type sensory and motor neurons and scaled-type neurons including sustained and ramping activities, etc. Cluster/subcluster compositions for V1 differed from those of PPC, M1, and M2. The V1 neurons showed the fastest functional activities among those areas. Our method was also applicable to determine temporal "forms" and the latency of spike activity changes. These findings demonstrate its utility for characterizing neurons.NEW & NOTEWORTHY Phase-Scaling analysis is a novel technique to unbiasedly characterize the temporal dependency of functional neuron activity on two behavioral events and objectively determine the latency and form of the activity change. This powerful analysis can uncover several classes of latently functioning neurons that have thus far been overlooked, which may participate differently in intermediate processes of a brain function. The Phase-Scaling analysis will yield profound insights into neural mechanisms for processing internal information.

Operant conditioning paradigm for juxtacellular recordings in functionally identified cortical neurons during motor execution in head-fixed rats

BACKGROUND

Understanding the configuration of neural circuits and the specific role of distinct cortical neuron types involved in behavior, requires the study of structure-function and connectivity relationships with single cell resolution in awake behaving animals. Despite head-fixed behaving rats have been used for in vivo measuring of neuronal activity, it is a concern that head fixation could change the performance of behavioral task.

NEW METHOD

We describe the procedures for efficiently training Wistar rats to develop a behavioral task, involving planning and execution of a qualified movement in response to a visual cue under head-fixed conditions. The behavioral and movement performance in freely moving vs head-fixed conditions was analyzed.

RESULTS

The best behavioral performance was obtained in the rats that were trained first in freely moving conditions and then placed in a head-restrained condition compared with the animals which first were habituated to head-restriction and then learned the task. Moreover, head restriction did not alter the movement performance. Stable juxtacellular recordings from sensorimotor cortex neurons were obtained while the rats were performing forelimb movements. Biocytin electroporation and retrograde tracer injections, permits identify the hodology of individual long-range projecting neurons.

COMPARISON WITH EXISTING METHODS

Our method shows no difference in the behavioral performance of head fixed and freely moving conditions. Also includes a computer aided design of a discrete and ergonomic head-post allowing enough stability to perform juxtacellular recording and labeling of cortical neurons.

CONCLUSIONS

Our method is suitable for the in vivo characterization of neuronal circuits and their long-range connectivity.

Differential Changes in the Lateralized Activity of Identified Projection Neurons of Motor Cortex in Hemiparkinsonian Rats

In the parkinsonian state, the motor cortex and basal ganglia (BG) undergo dynamic remodeling of movement representation. One such change is the loss of the normal contralateral lateralized activity pattern. The increase in the number of movement-related neurons responding to ipsilateral or bilateral limb movements may cause motor problems, including impaired balance, reduced bimanual coordination, and abnormal mirror movements. However, it remains unknown how individual types of motor cortical neurons organize this reconstruction. To explore the effect of dopamine depletion on lateralized activity in the parkinsonian state, we used a partial hemiparkinsonian model [6-hydroxydopamine (6-OHDA) lesion] in Long-Evans rats performing unilateral movements in a right-left pedal task, while recording from primary (M1) and secondary motor cortex (M2). The lesion decreased contralateral preferred activity in both M1 and M2. In addition, this change differed among identified intratelencephalic (IT) and pyramidal tract (PT) cortical projection neurons, depending on the cortical area. We detected a decrease in lateralized activity only in PT neurons in M1, whereas in M2, this change was observed in IT neurons, with no change in the PT population. Our results suggest a differential effect of dopamine depletion in the lateralized activity of the motor cortex, and suggest possible compensatory changes in the contralateral hemisphere.

In Vivo Spiking Dynamics of Intra- and Extratelencephalic Projection Neurons in Rat Motor Cortex

In motor cortex, 2 types of deep layer pyramidal cells send their axons to other areas: intratelencephalic (IT)-type neurons specifically project bilaterally to the cerebral cortex and striatum, whereas neurons of the extratelencephalic (ET)-type, termed conventionally pyramidal tract-type, project ipsilaterally to the thalamus and other areas. Although they have totally different synaptic and membrane potential properties in vitro, little is known about the differences between them in ongoing spiking dynamics in vivo. We identified IT-type and ET-type neurons, as well as fast-spiking-type interneurons, using novel multineuronal analysis based on optogenetically evoked spike collision along their axons in behaving/resting rats expressing channelrhodopsin-2 (Multi-Linc method). We found "postspike suppression" (~100 ms) as a characteristic of ET-type neurons in spike auto-correlograms, and it remained constant independent of behavioral conditions in functionally different ET-type neurons. Postspike suppression followed even solitary spikes, and spike bursts significantly extended its duration. We also observed relatively strong spike synchrony in pairs containing IT-type neurons. Thus, spiking dynamics in IT-type and ET-type neurons may be optimized differently for precise and coordinated motor control.

Ventral striatum links motivational and motor networks during operant-conditioned movement in rats

Voluntary actions require motives. It is already known that the medial prefrontal cortex (MPFC) assess the motivational values. However, it remains unclear how the motivational process gains access to the motor execution system in the brain. Here we present evidence that the ventral striatum (VS) plays a hub-like role in mediating motivational and motor processing in operant behavior. We used positron emission tomography (PET) to detect the neural activation areas associated with motivational action. Using obtained regions, partial correlation analysis was performed to examine how the motivational signals propagate to the motor system. The results revealed that VS activity propagated to both MPFC and primary motor cortex through the thalamus. Moreover, muscimol injection into the VS suppressed the motivational behavior, supporting the idea of representations of motivational signals in VS that trigger motivational behavior. These results suggest that the VS-thalamic pathway plays a pivotal role for both motivational processing through interactions with the MPFC and for motor processing through interactions with the motor BG circuits.

Automated home cage training of mice in a hold-still center-out reach task

An obstacle to understanding neural mechanisms of movement is the complex, distributed nature of the mammalian motor system. Here we present a novel behavioral paradigm for high-throughput dissection of neural circuits underlying mouse forelimb control. Custom touch-sensing joysticks were used to quantify mouse forelimb trajectories with micron-millisecond spatiotemporal resolution. Joysticks were integrated into computer-controlled, rack-mountable home cages, enabling batches of mice to be trained in parallel. Closed loop behavioral analysis enabled online control of reward delivery for automated training. We used this system to show that mice can learn, with no human handling, a direction-specific hold-still center-out reach task in which a mouse first held its right forepaw still before reaching out to learned spatial targets. Stabilogram diffusion analysis of submillimeter-scale micromovements produced during the hold demonstrate that an active control process, akin to upright balance, was implemented to maintain forepaw stability. Trajectory decomposition methods, previously used in primates, were used to segment hundreds of thousands of forelimb trajectories into millions of constituent kinematic primitives. This system enables rapid dissection of neural circuits for controlling motion primitives from which forelimb sequences are built. NEW & NOTEWORTHY A novel joystick design resolves mouse forelimb kinematics with micron-millisecond precision. Home cage training is used to train mice in a hold-still center-out reach task. Analytical methods, previously used in primates, are used to decompose mouse forelimb trajectories into kinematic primitives.

Monitoring and Updating of Action Selection for Goal-Directed Behavior through the Striatal Direct and Indirect Pathways

The basal ganglia play key roles in adaptive behaviors guided by reward and punishment. However, despite accumulating knowledge, few studies have tested how heterogeneous signals in the basal ganglia are organized and coordinated for goal-directed behavior. In this study, we investigated neuronal signals of the direct and indirect pathways of the basal ganglia as rats performed a lever push/pull task for a probabilistic reward. In the dorsomedial striatum, we found that optogenetically and electrophysiologically identified direct pathway neurons encoded reward outcomes, whereas indirect pathway neurons encoded no-reward outcome and next-action selection. Outcome coding occurred in association with the chosen action. In support of pathway-specific neuronal coding, light activation induced a bias on repeat selection of the same action in the direct pathway, but on switch selection in the indirect pathway. Our data reveal the mechanisms underlying monitoring and updating of action selection for goal-directed behavior through basal ganglia circuits.

Area-specific Modulation of Functional Cortical Activity During Block-based and Trial-based Proactive Inhibition

Animals can suppress their behavioral response in advance according to changes in environmental context (proactive inhibition: delaying the start of response), a process in which several cortical areas may participate. However, it remains unclear how this process is adaptively regulated according to contextual changes on different timescales. To address the issue, we used an improved stop-signal task paradigm to behaviorally and electrophysiologically characterize the temporal aspect of proactive inhibition in head-fixed rats. In the task, they must respond to a go cue as quickly as possible (go trial), but did not have to respond if a stop cue followed the go cue (stop trial). The task alternated between a block of only go trials (G-block) and a block of go-and-stop trials (GS-block). We observed block-based and trial-based proactive inhibition (emerging in GS-block and after stop trial, respectively) by behaviorally evaluating the delay in reaction time in correct go trials depending on contextual changes on different timescales. We electrophysiologically analyzed task-related neuronal activity in the primary and secondary motor, posterior parietal, and orbitofrontal cortices (M1, M2, PPC, and OFC, respectively). Under block-based proactive inhibition, spike activity of cue-preferring OFC neurons was attenuated continuously, while M1 and M2 activity was enhanced during motor preparation. Subsequently, M1 activity was attenuated during motor decision/execution. Under trial-based proactive inhibition, the OFC activity was continuously enhanced, and PPC and M1 activity was also enhanced shortly during motor decision/execution. These results suggest that different cortical mechanisms underlie the two types of proactive inhibition in rodents.

Subjective decision threshold for accurate visual detection performance in rats

The dissociation between a subjective-criterion performance and forced performance in a sensory detection can provide critical insights into the neural correlates of sensory awareness. Here, we established a behavioral task for rats to test their spatial-visual cue detection ability, using a two alternative choice task with and without a third choice option where animals get rewards only in the objective absence of a visual cue. In the trials without the third option, spatial choice accuracy decreased from near perfect to near chance levels as the visual cue brightness decreased. In contrast, with the third option, the rats exhibited >90% spatial choice accuracy regardless of the cue brightness. The rats chose the third choice option less frequently when the cue was brighter, suggesting that rats have a generalized strategy to make spatial choices only when their internal detection criterion is met. Interestingly, even when the animals chose the third option, they could still significantly and correctly choose the direction of the visual stimuli if they were forced. Our data suggest that the rats' variable detection performance with identical set of stimuli is derived from stochastic processing of visual signals with a certain internal detection threshold rather than general motivational threshold.

Two-photon imaging of neuronal activity in motor cortex of marmosets during upper-limb movement tasks

Two-photon imaging in behaving animals has revealed neuronal activities related to behavioral and cognitive function at single-cell resolution. However, marmosets have posed a challenge due to limited success in training on motor tasks. Here we report the development of protocols to train head-fixed common marmosets to perform upper-limb movement tasks and simultaneously perform two-photon imaging. After 2–5 months of training sessions, head-fixed marmosets can control a manipulandum to move a cursor to a target on a screen. We conduct two-photon calcium imaging of layer 2/3 neurons in the motor cortex during this motor task performance, and detect task-relevant activity from multiple neurons at cellular and subcellular resolutions. In a two-target reaching task, some neurons show direction-selective activity over the training days. In a short-term force-field adaptation task, some neurons change their activity when the force field is on. Two-photon calcium imaging in behaving marmosets may become a fundamental technique for determining the spatial organization of the cortical dynamics underlying action and cognition.

Marmosets are an important model organism in neuroscience but there has only been limited success in training them on behavioral tasks. Here the authors report their ability to train marmosets in various motor tasks and simultaneously image neural dynamics in motor cortex with 2-photon imaging.

Development of stereotaxic recording system for awake marmosets (Callithrix jacchus)

The common marmoset has been proposed as a potential alternative to macaque monkey as a primate model for neuroscience and medical research. Here, we have newly developed a stereotaxic neuronal recording system for awake marmosets under the head-fixed condition by modifying that for macaque monkeys. Using this system, we recorded neuronal activity in the cerebral cortex of awake marmosets and successfully identified the primary motor cortex by intracortical microstimulation. Neuronal activities of deep brain structures, such as the basal ganglia, thalamus, and cerebellum, in awake marmosets were also successfully recorded referring to magnetic resonance images. Our system is suitable for functional mapping of the brain, since the large recording chamber allows access to arbitrary regions over almost the entire brain, and the recording electrode can be easily moved stereotaxically from one site to another. In addition, our system is desirable for neuronal recording during task performance to assess motor skills and cognitive function, as the marmoset sits in the marmoset chair and can freely use its hands. Moreover, our system can be used in combination with cutting-edge techniques, such as two-photon imaging and optogenetic manipulation. This recording system will contribute to boosting neuroscience and medical research using marmosets.

Distinct Laterality in Forelimb-Movement Representations of Rat Primary and Secondary Motor Cortical Neurons with Intratelencephalic and Pyramidal Tract Projections

Two distinct motor areas, the primary and secondary motor cortices (M1 and M2), play crucial roles in voluntary movement in rodents. The aim of this study was to characterize the laterality in motor cortical representations of right and left forelimb movements. To achieve this goal, we developed a novel behavioral task, the Right-Left Pedal task, in which a head-restrained male rat manipulates a right or left pedal with the corresponding forelimb. This task enabled us to monitor independent movements of both forelimbs with high spatiotemporal resolution. We observed phasic movement-related neuronal activity (Go-type) and tonic hold-related activity (Hold-type) in isolated unilateral movements. In both M1 and M2, Go-type neurons exhibited bias toward contralateral preference, whereas Hold-type neurons exhibited no bias. The contralateral bias was weaker in M2 than M1. Moreover, we differentiated between intratelencephalic (IT) and pyramidal tract (PT) neurons using optogenetically evoked spike collision in rats expressing channelrhodopsin-2. Even in identified PT and IT neurons, Hold-type neurons exhibited no lateral bias. Go-type PT neurons exhibited bias toward contralateral preference, whereas IT neurons exhibited no bias. Our findings suggest a different laterality of movement representations of M1 and M2, in each of which IT neurons are involved in cooperation of bilateral movements, whereas PT neurons control contralateral movements.SIGNIFICANCE STATEMENT In rodents, the primary and secondary motor cortices (M1 and M2) are involved in voluntary movements via distinct projection neurons: intratelencephalic (IT) neurons and pyramidal tract (PT) neurons. However, it remains unclear whether the two motor cortices (M1 vs M2) and the two classes of projection neurons (IT vs PT) have different laterality of movement representations. We optogenetically identified these neurons and analyzed their functional activity using a novel behavioral task to monitor movements of the right and left forelimbs separately. We found that contralateral bias was reduced in M2 relative to M1, and in IT relative to PT neurons. Our findings suggest that the motor information processing that controls forelimb movement is coordinated by a distinct cell population.

A mouse's spontaneous eating repertoire aids performance on laboratory skilled reaching tasks: A motoric example of instinctual drift with an ethological description of the withdraw movements in freely-moving and head-fixed mice

Rodents display a spontaneous "order-common" pattern of food eating: they pick up food using the mouth, sit on their haunches, and transfer the food to the hands for handling/chewing. The present study examines how this pattern of behaviour influences performance on "skilled-reaching" tasks, in which mice purchase food with a single hand. Here five types of withdraw movement, the retraction of the hand, in three reaching tasks: freely-moving single-pellet, head-fixed single-pellet, and head-fixed pasta-eating is described. The withdraw movement varied depending upon whether a reach was anticipatory, no food present, or was unsuccessful or successful with food present. Ease of withdraw is dependent upon the extent to which animals used order-common movements. For freely-moving mice, a hand-to-mouth movement was assisted by a mouth-to-hand movement and food transfer to the mouth depended upon a sitting posture and using the other hand to assist food holding, both order-common movements. In the head-fixed single-pellet task, with postural and head movements prevented, withdraw was made with difficulty and tongue protrude movements assisted food transfer to the mouth once the hand reached the mouth. Only when a head-fixed mouse made a bilateral hand-to-mouth movement, a component of order-common eating, was the withdraw movement made with ease. The results are discussed with respect to the use of order-common movements in skilled-reaching tasks and with respect to the optimal design of tasks used to assess rodent skilled hand movement.

Temporal and Rate Coding for Discrete Event Sequences in the Hippocampus

Although the hippocampus is critical to episodic memory, neuronal representations supporting this role, especially relating to nonspatial information, remain elusive. Here, we investigated rate and temporal coding of hippocampal CA1 neurons in rats performing a cue-combination task that requires the integration of sequentially provided sound and odor cues. The majority of CA1 neurons displayed sensory cue-, combination-, or choice-specific (simply, "event"-specific) elevated discharge activities, which were sustained throughout the event period. These event cells underwent transient theta phase precession at event onset, followed by sustained phase locking to the early theta phases. As a result of this unique single neuron behavior, the theta sequences of CA1 cell assemblies of the event sequences had discrete representations. These results help to update the conceptual framework for space encoding toward a more general model of episodic event representations in the hippocampus.

Spike Timing Rigidity Is Maintained in Bursting Neurons under Pentobarbital-Induced Anesthetic Conditions

Pentobarbital potentiates γ-aminobutyric acid (GABA)-mediated inhibitory synaptic transmission by prolonging the open time of GABA_A receptors. However, it is unknown how pentobarbital regulates cortical neuronal activities via local circuits in vivo. To examine this question, we performed extracellular unit recording in rat insular cortex under awake and anesthetic conditions. Not a few studies apply time-rescaling theorem to detect the features of repetitive spike firing. Similar to these methods, we define an average spike interval locally in time using random matrix theory (RMT), which enables us to compare different activity states on a universal scale. Neurons with high spontaneous firing frequency (>5 Hz) and bursting were classified as HFB neurons (n = 10), and those with low spontaneous firing frequency (<10 Hz) and without bursting were classified as non-HFB neurons (n = 48). Pentobarbital injection (30 mg/kg) reduced firing frequency in all HFB neurons and in 78% of non-HFB neurons. RMT analysis demonstrated that pentobarbital increased in the number of neurons with repulsion in both HFB and non-HFB neurons, suggesting that there is a correlation between spikes within a short interspike interval (ISI). Under awake conditions, in 50% of HFB and 40% of non-HFB neurons, the decay phase of normalized histograms of spontaneous firing were fitted to an exponential function, which indicated that the first spike had no correlation with subsequent spikes. In contrast, under pentobarbital-induced anesthesia conditions, the number of non-HFB neurons that were fitted to an exponential function increased to 80%, but almost no change in HFB neurons was observed. These results suggest that under both awake and pentobarbital-induced anesthetized conditions, spike firing in HFB neurons is more robustly regulated by preceding spikes than by non-HFB neurons, which may reflect the GABA_A receptor-mediated regulation of cortical activities. Whole-cell patch-clamp recording in the IC slice preparation was performed to compare the regularity of spike timing between pyramidal and fast-spiking (FS) neurons, which presumably correspond to non-HFB and HFB neurons, respectively. Repetitive spike firing of FS neurons exhibited a lower variance of ISI than pyramidal neurons both in control and under application of pentobarbital, supporting the above hypothesis.

Large-scale analysis reveals populational contributions of cortical spike rate and synchrony to behavioural functions

KEY POINTS

There have been few systematic population-wide analyses of relationships between spike synchrony within a period of several milliseconds and behavioural functions. In this study, we obtained a large amount of spike data from > 23,000 neuron pairs by multiple single-unit recording from deep layer neurons in motor cortical areas in rats performing a forelimb movement task. The temporal changes of spike synchrony in the whole neuron pairs were statistically independent of behavioural changes during the task performance, although some neuron pairs exhibited correlated changes in spike synchrony. Mutual information analyses revealed that spike synchrony made a smaller contribution than spike rate to behavioural functions. The strength of spike synchrony between two neurons was statistically independent of the spike rate-based preferences of the pair for behavioural functions.

ABSTRACT

Spike synchrony within a period of several milliseconds in presynaptic neurons enables effective integration of functional information in the postsynaptic neuron. However, few studies have systematically analysed the population-wide relationships between spike synchrony and behavioural functions. Here we obtained a sufficiently large amount of spike data among regular-spiking (putatively excitatory) and fast-spiking (putatively inhibitory) neuron subtypes (> 23,000 pairs) by multiple single-unit recording from deep layers in motor cortical areas (caudal forelimb area, rostral forelimb area) in rats performing a forelimb movement task. After holding a lever, rats pulled the lever either in response to a cue tone (external-trigger trials) or spontaneously without any cue (internal-trigger trials). Many neurons exhibited functional spike activity in association with forelimb movements, and the preference of regular-spiking neurons in the rostral forelimb area was more biased toward externally triggered movement than that in the caudal forelimb area. We found that a population of neuron pairs with spike synchrony does exist, and that some neuron pairs exhibit a dependence on movement phase during task performance. However, the population-wide analysis revealed that spike synchrony was statistically independent of the movement phase and the spike rate-based preferences of the pair for behavioural functions, whereas spike rates were clearly dependent on the movement phase. In fact, mutual information analyses revealed that the contribution of spike synchrony to the behavioural functions was small relative to the contribution of spike rate. Our large-scale analysis revealed that cortical spike rate, rather than spike synchrony, contributes to population coding for movement.

Development of a removable head fixation device for longitudinal behavioral and imaging studies in rats

BACKGROUND

In some behavioral neuroscience studies, an attachment is surgically fixed onto the head of an awake animal to allow the animal to perform learning tasks repeatedly in the same position in a task-training system. A recently developed task-training system enables operant conditioning of head-fixed rats within only a few days, and this system has been rigorously applied to record learning-associated neural activity using electrophysiological techniques. However, the head attachment of this device is made of metal and thus is not suitable for simultaneous brain imaging studies with X-ray computed tomography (CT), magnetic resonance imaging (MRI) or positron emission tomography (PET).

NEW METHOD

We developed a novel head fixation device with a removable attachment to position the rat head precisely in both imaging and training devices across different sessions. The device consisted of a removable attachment, a clamp and a stage, all of which were made of PET/MRI compatible acrylic resin. We tested the usefulness of the device with (18)F-fluorodeoxyglucose (FDG) PET and CT.

RESULTS

The new device did not substantially affect (18)F-FDG PET images. Repositioning of the rat's head across sessions and experimenters was at a level of submillimeter accuracy.

COMPARISON WITH EXISTING METHOD

The errors of radioactivity concentration of (18)F-FDG in the PET image were lower with the present attachment than with the conventional metal attachment. Repositioning accuracy was considerably improved compared with a visual inspection method.

CONCLUSIONS

The developed fixation device is useful for longitudinal behavioral and brain imaging studies in rats.

Brief hind paw stimulation is sufficient to induce delayed somatosensory discrimination learning in C57BL/6 mice

Somatosensory learning and memory studies in rodents have primarily focused on the role of whiskers and the barrel structure of the sensory cortex, characteristics unique to rodents. In contrast, whether associative learning can occur in animals (and humans) via foot stimulation remains unclear. The sensory cortex corresponding to the plantar foot surface is localized in the centroparietal area, providing relatively easy access for studying somatosensory learning and memory. To assess the contribution of sole stimulation to somatosensory learning and memory, we developed a novel operant-lever-pressing task. In Experiment 1, head-fixed mice were trained to press a lever to receive a water reward upon presentation of an associated stimulus (S+). Following training, they were administered a reversal-learning protocol, in which "S+ " and "S-" (a stimulus not associated with reward) were switched. Mice were then submitted to training with a progressively extended delay period between stimulation and lever presentation. In Experiment 2, the delayed discrimination training was replicated with longer delay periods and restricted training days, to further explore the results of Experiment 1. When the stimuli were presented to a single left hind paw, we found that male C57BL/6J mice were capable of learning to discriminate between different foot stimuli (electrical or mechanical), and of retaining this information for 10s. This novel task has potential applications for electrophysiological and optogenetic studies to clarify the neural circuits underlying somatosensory learning and behavior.

Acrylic Resin Molding Based Head Fixation Technique in Rodents

Head fixation is a technique of immobilizing animal's head by attaching a head-post on the skull for rigid clamping. Traditional head fixation requires surgical attachment of metallic frames on the skull. The attached frames are then clamped to a stationary platform resulting in immobilization of the head. However, metallic frames for head fixation have been technically difficult to design and implement in general laboratory environment. In this study, we provide a novel head fixation method. Using a custom-made head fixation bar, head mounter is constructed during implantation surgery. After the application of acrylic resin for affixing implants such as electrodes and cannula on the skull, additional resins applied on top of that to build a mold matching to the port of the fixation bar. The molded head mounter serves as a guide rails, investigators conveniently fixate the animal's head by inserting the head mounter into the port of the fixation bar. This method could be easily applicable if implantation surgery using dental acrylics is necessary and might be useful for laboratories that cannot easily fabricate CNC machined metal head-posts.

Efficient training protocol for rapid learning of the two-alternative forced-choice visual stimulus detection task

The potential of genetically engineered rodent models has accelerated demand for training procedures of behavioral tasks. Such training is generally time consuming and often shows large variability in learning speed between animals. To overcome these problems, we developed an efficient and stable training system for the two-alternative forced-choice (2AFC) visual stimulus detection task for freely behaving rodents. To facilitate the task learning, we introduced a spout-lever as the operandum and a three-step training program with four ingenuities: (1) a salient stimulus to draw passive attention, (2) a reward-guaranteed trial to keep motivation, (3) a behavior-corrective trial, and (4) switching from a reward-guaranteed trial to a nonguaranteed one to correct behavioral patterns. Our new training system realizes 1-week completion of the whole learning process, during which all rats were able to learn effortlessly the association between (1) lever-manipulation and reward and (2) visual stimulus and reward in a step-by-step manner. Thus, our new system provides an effective and stable training method for the 2AFC visual stimulus detection task. This method should help accelerate the move toward research bridging the visual functions measured in behavioral tasks and the contributing specific neurons/networks that are genetically manipulated or optically controlled.

Flat-floored air-lifted platform: a new method for combining behavior with microscopy or electrophysiology on awake freely moving rodents

It is widely acknowledged that the use of general anesthetics can undermine the relevance of electrophysiological or microscopical data obtained from a living animal’s brain. Moreover, the lengthy recovery from anesthesia limits the frequency of repeated recording/imaging episodes in longitudinal studies. Hence, new methods that would allow stable recordings from non-anesthetized behaving mice are expected to advance the fields of cellular and cognitive neurosciences. Existing solutions range from mere physical restraint to more sophisticated approaches, such as linear and spherical treadmills used in combination with computer-generated virtual reality. Here, a novel method is described where a head-fixed mouse can move around an air-lifted mobile homecage and explore its environment under stress-free conditions. This method allows researchers to perform behavioral tests (e.g., learning, habituation or novel object recognition) simultaneously with two-photon microscopic imaging and/or patch-clamp recordings, all combined in a single experiment. This video-article describes the use of the awake animal head fixation device (mobile homecage), demonstrates the procedures of animal habituation, and exemplifies a number of possible applications of the method.

Different modulation of common motor information in rat primary and secondary motor cortices

Rodents have primary and secondary motor cortices that are involved in the execution of voluntary movements via their direct and parallel projections to the spinal cord. However, it is unclear whether the rodent secondary motor cortex has any motor function distinct from the primary motor cortex to properly control voluntary movements. In the present study, we quantitatively examined neuronal activity in the caudal forelimb area (CFA) of the primary motor cortex and rostral forelimb area (RFA) of the secondary motor cortex in head-fixed rats performing forelimb movements (pushing, holding, and pulling a lever). We found virtually no major differences between CFA and RFA neurons, regardless of neuron subtypes, not only in their basal spiking properties but also in the time-course, amplitude, and direction preference of their functional activation for simple forelimb movements. However, the RFA neurons, as compared with the CFA neurons, showed obviously a greater susceptibility of their functional activation to an alteration in a behavioral situation, a 'rewarding' response that leads to reward or a 'consummatory' response that follows reward water, which might be accompanied by some internal adaptations without affecting the motor outputs. Our results suggest that, although the CFA and RFA neurons commonly process fundamental motor information to properly control forelimb movements, the RFA neurons may be functionally differentiated to integrate motor information with internal state information for an adaptation to goal-directed behaviors.

Two distinct layer-specific dynamics of cortical ensembles during learning of a motor task

The primary motor cortex (M1) possesses two intermediate layers upstream of the motor-output layer: layer 2/3 (L2/3) and layer 5a (L5a). Although repetitive training often improves motor performance and movement coding by M1 neuronal ensembles, it is unclear how neuronal activities in L2/3 and L5a are reorganized during motor task learning. We conducted two-photon calcium imaging in mouse M1 during 14 training sessions of a self-initiated lever-pull task. In L2/3, the accuracy of neuronal ensemble prediction of lever trajectory remained unchanged globally, with a subset of individual neurons retaining high prediction accuracy throughout the training period. However, in L5a, the ensemble prediction accuracy steadily improved, and one-third of neurons, including subcortical projection neurons, evolved to contribute substantially to ensemble prediction in the late stage of learning. The L2/3 network may represent coordination of signals from other areas throughout learning, whereas L5a may participate in the evolving network representing well-learned movements.

Blockade of muscarinic receptors impairs the retrieval of well-trained memory

Acetylcholine (ACh) is known to play an important role in memory functions, and its deficit has been proposed to cause the cognitive decline associated with advanced age and Alzheimer's disease (the cholinergic hypothesis). Although many studies have tested the cholinergic hypothesis for recently acquired memory, only a few have investigated the role of ACh in the retrieval process of well-trained cognitive memory, which describes the memory established from repetition and daily routine. To examine this point, we trained rats to perform a two-alternative forced-choice visual detection task. Each trial was started by having the rats pull upward a central-lever, which triggered the presentation of a visual stimulus to the right or left side of the display monitor, and then pulling upward a stimulus-relevant choice-lever located on both sides. Rats learned the task within 10 days, and the task training was continued for a month. Task performance was measured with or without systemic administration of a muscarinic ACh receptor (mAChR) antagonist, scopolamine (SCOP), prior to the test. After 30 min of SCOP administration, rats stopped manipulating any lever even though they explored the lever and surrounding environment, suggesting a loss of the task-related associative memory. Three hours later, rats were recovered to complete the trial, but the rats selected the levers irrespective of the visual stimulus, suggesting they remembered a series of lever-manipulations in association with a reward, but not association between the reward and visual stimulation. Furthermore, an m1-AChR, but not nicotinic AChR antagonist caused a similar deficit in the task execution. SCOP neither interfered with locomotor activity nor drinking behavior, while it influenced anxiety. These results suggest that the activation of mAChRs at basal ACh levels is essential for the recall of well-trained cognitive memory.

Reward-modulated motor information in identified striatum neurons

It is widely accepted that dorsal striatum neurons participate in either the direct pathway (expressing dopamine D1 receptors) or the indirect pathway (expressing D2 receptors), controlling voluntary movements in an antagonistically balancing manner. The D1- and D2-expressing neurons are activated and inactivated, respectively, by dopamine released from substantia nigra neurons encoding reward expectation. However, little is known about the functional representation of motor information and its reward modulation in individual striatal neurons constituting the two pathways. In this study, we juxtacellularly recorded the spike activity of single neurons in the dorsolateral striatum of rats performing voluntary forelimb movement in a reward-predictable condition. Some of these neurons were identified morphologically by a combination of juxtacellular visualization and in situ hybridization for D1 mRNA. We found that the striatal neurons exhibited distinct functional activations before and during the forelimb movement, regardless of the expression of D1 mRNA. They were often positively, but rarely negatively, modulated by expecting a reward for the correct motor response. The positive reward modulation was independent of behavioral differences in motor performance. In contrast, regular-spiking and fast-spiking neurons in any layers of the motor cortex displayed only minor and unbiased reward modulation of their functional activation in relation to the execution of forelimb movement. Our results suggest that the direct and indirect pathway neurons cooperatively rather than antagonistically contribute to spatiotemporal control of voluntary movements, and that motor information is subcortically integrated with reward information through dopaminergic and other signals in the skeletomotor loop of the basal ganglia.

A novel variable delay Go/No-Go task to study attention, motivation and working memory in the head-fixed rodent

In order to parse the causal elements underlying complex behaviors and decision-making processes, appropriate behavioral methods must be developed and used in concurrence with molecular, pharmacological, and electrophysiological approaches. Presented is a protocol for a novel Go/No-Go behavioral paradigm to study the brain attention and motivation/reward circuitry in awake, head-restrained rodents. This experimental setup allows: (1) Pharmacological and viral manipulation of various brain regions via targeted guide cannula; (2) Optogenetic cell-type specific activation and silencing with simultaneous electrophysiological recording and; (3) Repeated electrophysiological single and multiple unit recordings during ongoing behavior. The task consists of three components. The subject first makes an observing response by initiating a trial by lever pressing in response to distinctive Go or No-Go tones.  Then, after a variable delay period, the subject is presented with a challenge period cued by white noise during which they must respond with a lever press for the Go condition or withhold from lever pressing for the duration of the cue in the No-Go condition. After correctly responding during the challenge period (Challenge) and a brief delay, a final reward tone of the same frequency as the initiation tone is presented and sucrose reward delivery is available and contingent upon lever pressing. Here, we provide a novel procedure and validating data set that allows researchers to study and manipulate components of behavior such as attention, motivation, impulsivity, and reward-related working memory during an ongoing operant behavioral task while limiting interference from non task-related behaviors.

Neural dynamics and information representation in microcircuits of motor cortex

The brain has to analyze and respond to external events that can change rapidly from time to time, suggesting that information processing by the brain may be essentially dynamic rather than static. The dynamical features of neural computation are of significant importance in motor cortex that governs the process of movement generation and learning. In this paper, we discuss these features based primarily on our recent findings on neural dynamics and information coding in the microcircuit of rat motor cortex. In fact, cortical neurons show a variety of dynamical behavior from rhythmic activity in various frequency bands to highly irregular spike firing. Of particular interest are the similarity and dissimilarity of the neuronal response properties in different layers of motor cortex. By conducting electrophysiological recordings in slice preparation, we report the phase response curves (PRCs) of neurons in different cortical layers to demonstrate their layer-dependent synchronization properties. We then study how motor cortex recruits task-related neurons in different layers for voluntary arm movements by simultaneous juxtacellular and multiunit recordings from behaving rats. The results suggest an interesting difference in the spectrum of functional activity between the superficial and deep layers. Furthermore, the task-related activities recorded from various layers exhibited power law distributions of inter-spike intervals (ISIs), in contrast to a general belief that ISIs obey Poisson or Gamma distributions in cortical neurons. We present a theoretical argument that this power law of in vivo neurons may represent the maximization of the entropy of firing rate with limited energy consumption of spike generation. Though further studies are required to fully clarify the functional implications of this coding principle, it may shed new light on information representations by neurons and circuits in motor cortex.

Spatiotemporal dynamics of functional clusters of neurons in the mouse motor cortex during a voluntary movement

Functional clustering of neurons is frequently observed in the motor cortex. However, it is unknown if, when, and how fine-scale (<100 μm) functional clusters form relative to voluntary forelimb movements. In addition, the implications of clustering remain unclear. To address these issues, we conducted two-photon calcium imaging of mouse layer 2/3 motor cortex during a self-initiated lever-pull task. In the imaging session after 8-9 days of training, head-restrained mice had to pull a lever for ∼600 ms to receive a water drop, and then had to wait for >3 s to pull it again. We found two types of task-related cells in the mice: cells whose peak activities occurred during lever pulls (pull cells) and cells whose peak activities occurred after the end of lever pulls. The activity of pull cells was strongly associated with lever-pull duration. In ∼40% of imaged fields, functional clusterings were temporally detected during the lever pulls. Spatially, there were ∼70-μm-scale clusters that consisted of more than four pull cells in ∼50% of the fields. Ensemble and individual activities of pull cells within the cluster more accurately predicted lever movement trajectories than activities of pull cells outside the cluster. This was likely because clustered pull cells were more often active in the individual trials than pull cells outside the cluster. This higher fidelity of activity was related to higher trial-to-trial correlations of activities of pairs within the cluster. We propose that strong recurrent network clusters may represent the execution of voluntary movements.