Neuronal activity in rat barrel cortex underlying texture discrimination

An effective textured Novel Object Recognition Test (tNORT) for repeated measure of whisker sensitivity of rodents

Rodents use their whisker system to discriminate surface texture. Whisker-based texture discrimination tasks are often used to investigate the mechanisms encoding tactile sensation. One such task is the textured Novel Object Recognition Test (tNORT). It takes advantage of a tendency of rodents to explore novel objects more than familiar ones and assesses the sensitivity of whiskers in discriminating different textures of objects. It requires little training of the animals and the equipment involved is a simple arena with typically two objects placed inside. The success of the test relies on rodents spending sufficient time exploring these objects. Animals may lose interests in such tasks when performed repetitively within a limited time frame. However, such repeated tests may be crucial when establishing a sensitivity threshold of the whisker system. Here we present an adapted rodent tNORT protocol designed to maintain sustained interest in the objects even with repeated testing. We constructed complex objects from three simple-shaped objects. Different textures were provided by sandpapers of varying grit sizes. To minimise olfactory clues, we used the sandy and the laminar side of the same sandpaper as the familiar and novel textures assigned at random. We subsequently conducted repeated tNORTs on eight rats in order to identify a critical threshold of the sandpaper grit size below which rats would be unable to discriminate the sandy from the laminar side. With an inter-test-interval of seven days and after five tNORTs, the protocol enabled us to successfully identify the threshold. We suggest that the proposed tNORT is a useful tool for investigating the sensitivity threshold of the whisker system of rodent, and for testing the effectiveness of an intervention by comparing sensitivity threshold pre- and post-intervention.

Reliability and stability of tactile perception in the whisker somatosensory system

Rodents rely on their whiskers as vital sensory tools for tactile perception, enabling them to distinguish textures and shapes. Ensuring the reliability and constancy of tactile perception under varying stimulus conditions remains a fascinating and fundamental inquiry. This study explores the impact of stimulus configurations, including whisker movement velocity and object spatial proximity, on texture discrimination and stability in rats. To address this issue, we employed three distinct approaches for our investigation. Stimulus configurations notably affected tactile inputs, altering whisker vibration's kinetic and kinematic aspects with consistent effects across various textures. Through a texture discrimination task, rats exhibited consistent discrimination performance irrespective of changes in stimulus configuration. However, alterations in stimulus configuration significantly affected the rats' ability to maintain stability in texture perception. Additionally, we investigated the influence of stimulus configurations on cortical neuronal responses by manipulating them experimentally. Notably, cortical neurons demonstrated substantial and intricate changes in firing rates without compromising the ability to discriminate between textures. Nevertheless, these changes resulted in a reduction in texture neuronal response stability. Stimulating multiple whiskers led to improved neuronal texture discrimination and maintained coding stability. These findings emphasize the importance of considering numerous factors and their interactions when studying the impact of stimulus configuration on neuronal responses and behavior.

Tactile processing in mouse cortex depends on action context

The brain receives constant tactile input, but only a subset guides ongoing behavior. Actions associated with tactile stimuli thus endow them with behavioral relevance. It remains unclear how the relevance of tactile stimuli affects processing in the somatosensory (S1) cortex. We developed a cross-modal selection task in which head-fixed mice switched between responding to tactile stimuli in the presence of visual distractors or to visual stimuli in the presence of tactile distractors using licking movements to the left or right side in different blocks of trials. S1 spiking encoded tactile stimuli, licking actions, and direction of licking in response to tactile but not visual stimuli. Bidirectional optogenetic manipulations showed that sensory-motor activity in S1 guided behavior when touch but not vision was relevant. Our results show that S1 activity and its impact on behavior depend on the actions associated with a tactile stimulus.

Global and local neuronal coding of tactile information in the barrel cortex

During tactile sensation in rodents, the whisker movements across surfaces give rise to intricate whisker motions that encompass discrete and transient stick-slip events, effectively conveying valuable information regarding surface properties. These surface characteristics are transformed into cortical neuronal responses. This study examined the coding strategies underlying these transformations in rat whiskers. We found that changes in surface coarseness modified the number and magnitude of stick-slip events, which in turn both modulated properties of neuronal responses. Global changes in the number of stick-slip events primarily affected neuronal discharge rates and the degree of neuronal synchronization. In contrast, local changes in the magnitude of stick-slip events affected the transformation of these kinematic and kinetic characteristics into neuronal discharges. Most cortical neurons exhibited surface coarseness selectivity through global and local stick-slip event properties. However, this selectivity varied across coding strategies in the same neurons, given that each coding strategy reflected different aspects of changes in whisker-surface interactions. The degree of spatial similarity in surface coarseness preference in adjacently recorded neurons differed among these coding strategies. Adjacently recorded neurons exhibited the same surface coarseness preference in their firing rates but not through other coding strategies. Through these results, we were able to show that local stick-slip event properties contribute to texture discrimination, complementing and surpassing global coding in this context. These findings suggest that the representation of surface coarseness in the cortex may rely on concurrent coding strategies that integrate tactile information across different spatiotemporal scales.

Autogenous cerebral processes: an invitation to look at the brain from inside out

While external stimulation can reliably trigger neuronal activity, cerebral processes can operate independently from the environment. In this study, we conceptualize autogenous cerebral processes (ACPs) as intrinsic operations of the brain that exist on multiple scales and can influence or shape stimulus responses, behavior, homeostasis, and the physiological state of an organism. We further propose that the field should consider exploring to what extent perception, arousal, behavior, or movement, as well as other cognitive functions previously investigated mainly regarding their stimulus-response dynamics, are ACP-driven.

Long and short whiskers differently guide snout/pellet interaction in rat oral grasping

We studied the role of rat whisker/snout tactile sense during oral grasping, comparing control data with those obtained, respectively, 1-3 and 5-7 days after bilateral long or short whisker trimming and 3-5 and 8-10 days after bilateral infraorbital nerve (ION) severing. Two behavioural phases were identified: whisker-snout contact by nose-N or lip-L and snout-tongue contact. The second phase involved either: snout passing over stationary pellet (Still pellet); pellet rolling as the snout passed over it (Rolling pellet); pellet being pushed forward by the snout (Pushed pellet); or pellet being hit and pushed away (Hit/Lost pellet). In controls, success was 100%, with N-contact prevailing over L-contact in the first phase and Still pellet in the second. In long whisker-trimmed versus controls, success was still 100%, but L-contact increased in frequency, Pushed pellet prevailed and the second phase duration increased. In short whisker-trimmed versus controls, success remained 100%, with increased L-contact frequency; the first phase duration did not change, but the second phase increased since in pushed trials, the pellet rolled around the snout. In ION-severed versus controls, both phases changed drastically: L-contact frequency increased, Pushed pellet prevailed and contact was persistently maintained; Hit/Lost pellet emerged, Still and Rolling pellets disappeared and the oral-grasping sequence was not triggered. These results suggest that long and short whiskers, respectively, optimize the first and second phases of snout-pellet interaction and that whisker/snout sense is necessary to trigger oral grasping. Kinematic trajectory analysis supports the conclusion that movement from whisker to snout contact is an orientation response.

Layer- and cell-type-specific differences in neural activity in mouse barrel cortex during a whisker detection task

To address the question which neocortical layers and cell types are important for the perception of a sensory stimulus, we performed multielectrode recordings in the barrel cortex of head-fixed mice performing a single-whisker go/no-go detection task with vibrotactile stimuli of differing intensities. We found that behavioral detection probability decreased gradually over the course of each session, which was well explained by a signal detection theory-based model that posits stable psychometric sensitivity and a variable decision criterion updated after each reinforcement, reflecting decreasing motivation. Analysis of multiunit activity demonstrated highest neurometric sensitivity in layer 4, which was achieved within only 30 ms after stimulus onset. At the level of single neurons, we observed substantial heterogeneity of neurometric sensitivity within and across layers, ranging from nonresponsiveness to approaching or even exceeding psychometric sensitivity. In all cortical layers, putative inhibitory interneurons on average proffered higher neurometric sensitivity than putative excitatory neurons. In infragranular layers, neurons increasing firing rate in response to stimulation featured higher sensitivities than neurons decreasing firing rate. Offline machine-learning-based analysis of videos of behavioral sessions showed that mice performed better when not moving, which at the neuronal level, was reflected by increased stimulus-evoked firing rates.

Coexisting neuronal coding strategies in the barrel cortex

During tactile sensation by rodents, whisker movements across surfaces generate complex whisker motions, including discrete, transient stick-slip events, which carry information about surface properties. The characteristics of these events and how the brain encodes this tactile information remain enigmatic. We found that cortical neurons show a mixture of synchronized and nontemporally correlated spikes in their tactile responses. Synchronous spikes convey the magnitude of stick-slip events by numerous aspects of temporal coding. These spikes show preferential selectivity for kinetic and kinematic whisker motion. By contrast, asynchronous spikes in each neuron convey the magnitude of stick-slip events by their discharge rates, response probability, and interspike intervals. We further show that the differentiation between these two types of activity is highly dependent on the magnitude of stick-slip events and stimulus and response history. These results suggest that cortical neurons transmit multiple components of tactile information through numerous coding strategies.

Response Facilitation Induced by Insulin-Like Growth Factor-I in the Primary Somatosensory Cortex of Mice Was Reduced in Aging

Aging is accompanied by a decline in cognition that can be due to a lower IGF-I level. We studied response facilitation induced in primary somatosensory (S1) cortical neurons by repetitive stimulation of whiskers in young and old mice. Layer 2/3 and 5/6 neurons were extracellularly recorded in young (≤ 6 months of age) and old (≥ 20 month of age) anesthetized mice. IGF-I injection in S1 cortex (10 nM; 0.2 μL) increased whisker responses in young and old animals. A stimulation train at 8 Hz induced a long-lasting response facilitation in only layer 2/3 neurons of young animals. However, all cortical neurons from young and old animals showed long-lasting response facilitation when IGF-I was applied in the S1 cortex. The reduction in response facilitation in old animals can be due to a reduction in the IGF-I receptors as was indicated by the immunohistochemistry study. Furthermore, a reduction in the performance of a whisker discrimination task was observed in old animals. In conclusion, our findings indicate that there is a reduction in the synaptic plasticity of S1 neurons during aging that can be recovered by IGF-I. Therefore, it opens the possibility of use IGF-I as a therapeutic tool to ameliorate the effects of heathy aging.

Adaptive Whisking in Mice

Rodents generate rhythmic whisking movements to explore their environment. Whisking trajectories, for one, appear as a fixed pattern of whisk cycles at 5–10 Hz driven by a brain stem central pattern generator. In contrast, whisking behavior is thought to be versatile and adaptable to behavioral goals. To begin to systematically investigate such behavioral adaptation, we established a whisking task, in which mice altered the trajectories of whisking in a goal-oriented fashion to gain rewards. Mice were trained to set the whisker to a defined starting position and generate a protraction movement across a virtual target (no touch-related tactile feedback). By ramping up target distance based on reward history, we observed that mice are able to generate highly specific whisking patterns suited to keep reward probability constant. On a sensorimotor level, the behavioral adaptation was realized by adjusting whisker kinematics: more distant locations were targeted using higher velocities (i.e., pointing to longer force generation), rather than by generating higher acceleration (i.e., pointing to stronger forces). We tested the suitability of the paradigm of tracking subtle alteration in whisking motor commands using small lesions in the rhythmic whisking subfield (RW) of the whisking-related primary motor cortex. Small contralateral RW lesions generated the deterioration of whisking kinematics with a latency of 12 days post-lesion, a change that was readily discriminated from changes in the behavioral adaptation by the paradigm.

Sensory Adaptation in the Whisker-Mediated Tactile System: Physiology, Theory, and Function

In the natural environment, organisms are constantly exposed to a continuous stream of sensory input. The dynamics of sensory input changes with organism's behaviour and environmental context. The contextual variations may induce >100-fold change in the parameters of the stimulation that an animal experiences. Thus, it is vital for the organism to adapt to the new diet of stimulation. The response properties of neurons, in turn, dynamically adjust to the prevailing properties of sensory stimulation, a process known as "neuronal adaptation." Neuronal adaptation is a ubiquitous phenomenon across all sensory modalities and occurs at different stages of processing from periphery to cortex. In spite of the wealth of research on contextual modulation and neuronal adaptation in visual and auditory systems, the neuronal and computational basis of sensory adaptation in somatosensory system is less understood. Here, we summarise the recent finding and views about the neuronal adaptation in the rodent whisker-mediated tactile system and further summarise the functional effect of neuronal adaptation on the response dynamics and encoding efficiency of neurons at single cell and population levels along the whisker-mediated touch system in rodents. Based on direct and indirect pieces of evidence presented here, we suggest sensory adaptation provides context-dependent functional mechanisms for noise reduction in sensory processing, salience processing and deviant stimulus detection, shift between integration and coincidence detection, band-pass frequency filtering, adjusting neuronal receptive fields, enhancing neural coding and improving discriminability around adapting stimuli, energy conservation, and disambiguating encoding of principal features of tactile stimuli.

Detection of Barely Visible Impact Damage in Polymeric Laminated Composites Using a Biomimetic Tactile Whisker

This is a novel investigation on the possibility of detecting barely visible impact damage (BVID) in composite materials by whisking across the surface via tactile whisker sensors that resemble rats’ whiskers. A series of drop tower low-velocity impact tests were performed on quasi-isotropic composite plates. The plates were made from unidirectional T800 carbon/MTM49-3 epoxy prepregs with the stacking sequence of [45/0/90/−45]_4S. Investigating the specimens’ surface by the naked eye does not reveal any significant damage, rather than a small dent on the surface, with no tangible difference in the different impact energy levels. Ultrasonic C-scan observations showed the existence of BVID in all the impact energy levels, with an increasing trend in the damage size by increasing the impact energy level. The collected data from whisker sensors were analyzed using the support vector machine classifier, based on their vibrational properties, to identify the impacted region and classify the impact severity. It was observed that after training for 13 whisker contacts, the BVID severity can be classified with an accuracy of 100%. This is offering a new BVID detection technique, with a high potential for automation and high reliability that can be used as an alternative or combined with available inspection systems.

State-Dependent Changes in Perception and Coding in the Mouse Somatosensory Cortex

An animal's behavioral state is reflected in the dynamics of cortical population activity and its capacity to process sensory information. To better understand the relationship between behavioral states and information processing, mice are trained to detect varying amplitudes of whisker-deflection under two-photon calcium imaging. Layer 2/3 neurons in the vibrissal primary somatosensory cortex are imaged across different behavioral states, defined based on detection performance (low to high-state) and pupil diameter. The neurometric curve in each behavioral state mirrors the corresponding psychometric performance, with calcium signals predictive of the animal's choice. High behavioral states are associated with lower network synchrony, extending over shorter cortical distances. The decrease in correlation across neurons in high state results in enhanced information transmission capacity at the population level. The observed state-dependent changes suggest that the coding regime within the first stage of cortical processing may underlie adaptive routing of relevant information through the sensorimotor system.

Of mice and monkeys: Somatosensory processing in two prominent animal models

Our understanding of the neural basis of somatosensation is based largely on studies of the whisker system of mice and rats and the hands of macaque monkeys. Results across these animal models are often interpreted as providing direct insight into human somatosensation. Work on these systems has proceeded in parallel, capitalizing on the strengths of each model, but has rarely been considered as a whole. This lack of integration promotes a piecemeal understanding of somatosensation. Here, we examine the functions and morphologies of whiskers of mice and rats, the hands of macaque monkeys, and the somatosensory neuraxes of these three species. We then discuss how somatosensory information is encoded in their respective nervous systems, highlighting similarities and differences. We reflect on the limitations of these models of human somatosensation and consider key gaps in our understanding of the neural basis of somatosensation.

Developmental plasticity of texture discrimination following early vision loss in the marsupial Monodelphis domestica

Behavioral strategies that depend on sensory information are not immutable; rather they can be shaped by the specific sensory context in which animals develop. This behavioral plasticity depends on the remarkable capacity of the brain to reorganize in response to alterations in the sensory environment, particularly when changes in sensory input occur at an early age. To study this phenomenon, we utilize the short-tailed opossum, a marsupial that has been a valuable animal model to study developmental plasticity due to the extremely immature state of its nervous system at birth. Previous studies in opossums have demonstrated that removal of retinal inputs early in development results in profound alterations to cortical connectivity and functional organization of visual and somatosensory cortex; however, behavioral consequences of this plasticity are not well understood. We trained early blind and sighted control opossums to perform a two-alternative forced choice texture discrimination task. Whisker trimming caused an acute deficit in discrimination accuracy for both groups, indicating the use of a primarily whisker-based strategy to guide choices based on tactile cues. Mystacial whiskers were important for performance in both groups; however, genal whiskers only contributed to behavioral performance in early blind animals. Early blind opossums significantly outperformed their sighted counterparts in discrimination accuracy, with discrimination thresholds that were lower by ∼75 μm. Our results support behavioral compensation following early blindness using tactile inputs, especially the whisker system.

Constraints on the deformation of the vibrissa within the follicle

Nearly all mammals have a vibrissal system specialized for tactile sensation, composed of whiskers growing from sensor-rich follicles in the skin. When a whisker deflects against an object, it deforms within the follicle and exerts forces on the mechanoreceptors inside. In addition, during active whisking behavior, muscle contractions around the follicle and increases in blood pressure in the ring sinus will affect the whisker deformation profile. To date, however, it is not yet possible to experimentally measure how the whisker deforms in an intact follicle or its effects on different groups of mechanoreceptors. The present study develops a novel model to predict vibrissal deformation within the follicle sinus complex. The model is based on experimental results from a previous ex vivo study on whisker deformation within the follicle, and on a new histological analysis of follicle tissue. It is then used to simulate whisker deformation within the follicle during passive touch and active whisking. Results suggest that the most likely whisker deformation profile is “S-shaped,” crossing the midline of the follicle right below the ring sinus. Simulations of active whisking indicate that an increase in overall muscle stiffness, an increase in the ratio between deep and superficial intrinsic muscle stiffness, and an increase in sinus blood pressure will all enhance tactile sensitivity. Finally, we discuss how the deformation profiles might map to the responses of primary afferents of each mechanoreceptor type. The mechanical model presented in this study is an important first step in simulating mechanical interactions within whisker follicles.

Many mammals rely on whiskers as a mode of tactile sensation, especially when exploring in darkness. Active, rhythmic protraction and retraction of the whiskers, commonly referred to as “whisking,” is observed among many whisker specialist animals. During whisker-based sensing, forces and moments generated by external stimuli are transmitted to the base of the whisker shaft inside the follicle. Within the follicle, the interaction between the whisker’s deformation and the surrounding tissue determines how different groups of mechanoreceptors will deform, thereby transducing the mechanical signals into electrical signals. However, it is not yet possible to experimentally measure this interaction in vivo. We therefore created a mechanical model of the follicle sinus complex to simulate whisker deformation within the follicle resulting from external whisker deflection. Our results provide the first estimate of whisker shape as it deforms in the follicle, during both passive touch and active whisking. In turn, these shape estimates allow us to predict how the whisker will deform against different types of mechanoreceptors at different locations within the follicle. In addition, we find that both intrinsic muscle contraction and an increase in blood pressure will improve the tactile sensitivity of the whisker system.

An analog of psychedelics restores functional neural circuits disrupted by unpredictable stress

Psychological stress affects a wide spectrum of brain functions and poses risks for many mental disorders. However, effective therapeutics to alleviate or revert its deleterious effects are lacking. A recently synthesized psychedelic analog tabernanthalog (TBG) has demonstrated anti-addictive and antidepressant potential. Whether TBG can rescue stress-induced affective, sensory, and cognitive deficits, and how it may achieve such effects by modulating neural circuits, remain unknown. Here we show that in mice exposed to unpredictable mild stress (UMS), administration of a single dose of TBG decreases their anxiety level and rescues deficits in sensory processing as well as in cognitive flexibility. Post-stress TBG treatment promotes the regrowth of excitatory neuron dendritic spines lost during UMS, decreases the baseline neuronal activity, and enhances whisking-modulation of neuronal activity in the somatosensory cortex. Moreover, calcium imaging in head-fixed mice performing a whisker-dependent texture discrimination task shows that novel textures elicit responses from a greater proportion of neurons in the somatosensory cortex than do familiar textures. Such differential response is diminished by UMS and is restored by TBG. Together, our study reveals the effects of UMS on cortical neuronal circuit activity patterns and demonstrate that TBG combats the detrimental effects of stress by modulating basal and stimulus-dependent neural activity in cortical networks.

Of city and village mice: behavioural adjustments of striped field mice to urban environments

A fundamental question of current ecological research concerns the drives and limits of species responses to human-induced rapid environmental change (HIREC). Behavioural responses to HIREC are a key component because behaviour links individual responses to population and community changes. Ongoing fast urbanization provides an ideal setting to test the functional role of behaviour for responses to HIREC. Consistent behavioural differences between conspecifics (animal personality) may be important determinants or constraints of animals' adaptation to urban habitats. We tested whether urban and rural populations of small mammals differ in mean trait expression, flexibility and repeatability of behaviours associated to risk-taking and exploratory tendencies. Using a standardized behavioural test in the field, we quantified spatial exploration and boldness of striped field mice (Apodemus agrarius, n = 96) from nine sub-populations, presenting different levels of urbanisation and anthropogenic disturbance. The level of urbanisation positively correlated with boldness, spatial exploration and behavioural flexibility, with urban dwellers being bolder, more explorative and more flexible in some traits than rural conspecifics. Thus, individuals seem to distribute in a non-random way in response to human disturbance based on their behavioural characteristics. Animal personality might therefore play a key role in successful coping with the challenges of HIREC.

Temporal Sharpening of Sensory Responses by Layer V in the Mouse Primary Somatosensory Cortex

The timing of stimulus-evoked spikes encodes information about sensory stimuli. Here we studied the neural circuits controlling this process in the mouse primary somatosensory cortex. We found that brief optogenetic activation of layer V pyramidal cells just after whisker deflection modulated the membrane potential of neurons and interrupted their long-latency whisker responses, increasing their accuracy in encoding whisker deflection time. In contrast, optogenetic inhibition of layer V during either passive whisker deflection or active whisking decreased accuracy in encoding stimulus or touch time, respectively. Suppression of layer V pyramidal cells increased reaction times in a texture discrimination task. Moreover, two-color optogenetic experiments revealed that cortical inhibition was efficiently recruited by layer V stimulation and that it mainly involved activation of parvalbumin-positive rather than somatostatin-positive interneurons. Layer V thus performs behaviorally relevant temporal sharpening of sensory responses through circuit-specific recruitment of cortical inhibition.

White matter structure and myelin-related gene expression alterations with experience in adult rats

White matter (WM) plasticity during adulthood is a recently described phenomenon by which experience can shape brain structure. It has been observed in humans using diffusion tensor imaging (DTI) and myelination has been suggested as a possible mechanism. Here, we set out to identify molecular and cellular changes associated with WM plasticity measured by DTI. We combined DTI, immunohistochemistry and mRNA expression analysis and examined the effects of somatosensory experience in adult rats. First, we observed experience-induced DTI differences in WM and in grey matter structure. C-Fos mRNA expression, a marker of cortical activity, in the barrel cortex correlated with the MRI WM metrics, indicating that molecular correlates of cortical activity relate to macroscale measures of WM structure. Analysis of myelin-related genes revealed higher myelin basic protein (MBP) mRNA expression. Higher MBP protein expression was also found via immunohistochemistry in WM. Finally, unbiased RNA sequencing analysis identified 134 differentially expressed genes encoding proteins involved in functions related to cell proliferation and differentiation, regulation of myelination and neuronal activity modulation. In conclusion, macroscale measures of WM plasticity are supported by both molecular and cellular evidence and confirm that myelination is one of the underlying mechanisms.

The effect of Wi-Fi electromagnetic waves on neuronal response properties in rat barrel cortex

There is a growing number of studies on the possible biological effects of Wi-Fi radiations on nervous system. In this study we investigated the effect of Wi-Fi exposure on single neuron responses to natural stimuli by using whisker to barrel pathway. This study was done on 29 male Wistar rats. Neuronal spontaneous activity and ON and OFF responses to displacement of principal whisker (PW), adjacent whisker (AW) and combination of PW-AW stimulation (as natural stimuli) were recorded in barrel cortex of anaesthetised rats. A D-link Wi-Fi device was used for 1 h exposure to 2.4 GHz microwaves in data mode (18.2 dBm and 44% for power and duty cycle). A condition test ratio (CTR) was calculated for assessing neuronal integrative properties. Wi-Fi radiations decreased CTR for ON responses. However, neuronal spontaneous activity and ON and OFF responses were not significantly changed following exposure to Wi-Fi signals. The results of this study demonstrated that exposure to Wi-Fi radiation could modulate integrative responses to natural stimuli in barrel cortex.

Sensorimotor integration in the whisker somatosensory brain stem trigeminal loop

The rodent's vibrissal system is a useful model system for studying sensorimotor integration in perception. This integration determines the way in which sensory information is acquired by sensory organs and the motor commands that control them. The initial instance of sensorimotor integration in the whisker somatosensory system is implemented in the brain stem loop and may be essential to the way rodents explore and sense their environment. To examine the nature of these sensorimotor interactions, we recorded from lightly anesthetized rats in vivo and brain stem slices in vitro and isolated specific parts of this loop. We found that motor feedback to the vibrissal pad serves as a dynamic gain controller that controls the response of first-order sensory neurons by increasing and decreasing sensitivity to lower and higher tactile stimulus magnitudes, respectively. This delicate mechanism is mediated through tactile stimulus magnitude-dependent motor feedback. Conversely, tactile inputs affect the motor whisking output through influences on the rhythmic whisking circuitry, thus changing whisking kinetics. Similarly, tactile influences also modify the whisking amplitude through synaptic and intrinsic neuronal interaction in the facial nucleus, resulting in facilitation or suppression of whisking amplitude. These results point to the vast range of mechanisms underlying sensorimotor integration in the brain stem loop.NEW & NOTEWORTHY Sensorimotor integration is a process in which sensory and motor information is combined to control the flow of sensory information, as well as to adjust the motor system output. We found in the rodent's whisker somatosensory system mutual influences between tactile inputs and motor output, in which motor neurons control the flow of sensory information depending on their magnitude. Conversely, sensory information can control the magnitude and kinetics of whisker movement.

Whisker-Mediated Touch System in Rodents: From Neuron to Behavior

A key question in systems neuroscience is to identify how sensory stimuli are represented in neuronal activity, and how the activity of sensory neurons in turn is “read out” by downstream neurons and give rise to behavior. The choice of a proper model system to address these questions, is therefore a crucial step. Over the past decade, the increasingly powerful array of experimental approaches that has become available in non-primate models (e.g., optogenetics and two-photon imaging) has spurred a renewed interest for the use of rodent models in systems neuroscience research. Here, I introduce the rodent whisker-mediated touch system as a structurally well-established and well-organized model system which, despite its simplicity, gives rise to complex behaviors. This system serves as a behaviorally efficient model system; known as nocturnal animals, along with their olfaction, rodents rely on their whisker-mediated touch system to collect information about their surrounding environment. Moreover, this system represents a well-studied circuitry with a somatotopic organization. At every stage of processing, one can identify anatomical and functional topographic maps of whiskers; “barrelettes” in the brainstem nuclei, “barreloids” in the sensory thalamus, and “barrels” in the cortex. This article provides a brief review on the basic anatomy and function of the whisker system in rodents.

Timing Is of the Essence: Improvement in Perception During Active Sensing

Active sensing refers to the concept of animals perceiving their environment while involving self-initiated motor acts. As a consequence of these motor acts, this activity produces direct and timely changes in the sensory surface. Is the brain able to take advantage of the precise time-locking that occurs during active sensing? Is the intrinsic predictability present during active sensing, impacting the sensory processes? We conjecture that if stimuli presentation is evoked by a self-initiated motor act, sensory discrimination and timing accuracy would improve. We studied this phenomenon when rats had to locate the position of a brief light stimulus, either when it was elicited by a warning light [passive condition (PC)] or when it was generated by a lever press [active condition (AC)]. We found that during the PC, rats had 66% of correct responses, vs. a significantly higher 77% of correct responses in AC. Furthermore, reaction times reduced from 1,181 ms during AC to 816 ms during PC For the latter condition, the probability of detecting the side of the light stimulus was negatively correlated with the time lag between the motor act and the evoked light and with a 38% reduction on performance per second of delay. These experiment shows that the mechanism that underlies sensory improvement during active behaviors have a constrained time dynamic, where the peak performances occur during the motor act, decreasing proportionally to the lag between the motor act and the stimulus presentation. This result is consistent with the evidence already found in humans, of a precise time dynamic of the improvement of sensory acuity after a motor act and reveals an equivalent process in rodents. Our results support the idea that perception and action are precisely coordinated in the brain.

Diverse tuning underlies sparse activity in layer 2/3 vibrissal cortex of awake mice

KEY POINTS

Sparse population activity is a common feature observed across cortical areas, yet the implications for sensory coding are not clear. We recorded single neuron activity in the vibrissal somatosensory cortex of awake head-fixed mice using the cell-attached technique. Unlike the anaesthetised condition, in awake mice a high-velocity, piezo-controlled whisker deflection excited only a small fraction of neurons. Manual probing of whiskers revealed that the majority of these silent neurons could be activated by specific forms of whisker-object contact. Our results suggest that sparse coding in vibrissal cortex may be due to high dimensionality of the stimulus space and narrow tuning of individual neurons.

ABSTRACT

It is widely reported that superficial layers of the somatosensory cortex exhibit sparse firing. This sparseness could reflect weak feedforward sensory inputs that are not sufficient to generate action potentials in these layers. Alternatively, sparseness might reflect tuning to unknown or higher-level complex features that are not fully explored in the stimulus space. Here, we examined these hypotheses by applying a range of vibrotactile and manual vibrissal stimuli in awake, head-fixed mice while performing loose-seal cell-attached recordings from the vibrissal primary somatosensory (vS1) cortex. A high-velocity stimulus delivered by a piezo-electric actuator evoked activity in a small fraction of regular spiking supragranular neurons (23%) in the awake condition. However, a majority of the supragranular regular spiking neurons (84%) were driven by manual stimulation of whiskers. Our results suggest that most neurons in the superficial layers of vS1 cortex contribute to coding in the awake condition when neurons may encounter their preferred feature(s) during whisker-object interactions.

Response Adaptation in Barrel Cortical Neurons Facilitates Stimulus Detection during Rhythmic Whisker Stimulation in Anesthetized Mice

Rodents use rhythmic whisker movements at frequencies between 4 and 12 Hz to sense the environment that will be disturbed when the animal touches an object. The aim of this work is to study the response adaptation to rhythmic whisker stimulation trains at 4 Hz in the barrel cortex and the sensitivity of cortical neurons to changes in the timing of the stimulation pattern. Longitudinal arrays of four iridium oxide electrodes were used to obtain single-unit recordings in supragranular, granular, and infragranular neurons in urethane anesthetized mice. The stimulation protocol consisted in a stimulation train of three air puffs (20 ms duration each) in which the time interval between the first and the third stimuli was fixed (500 ms) and the time interval between the first and the second stimuli changed (regular: 250 ms; “accelerando”: 375 ms; or “decelerando” stimulation train: 125 ms interval). Cortical neurons adapted strongly their response to regular stimulation trains. Response adaptation was reduced when accelerando or decelerando stimulation trains were applied. This facilitation of the shifted stimulus was mediated by activation of NMDA receptors because the effect was blocked by AP5. The facilitation was not observed in thalamic nuclei. Facilitation increased during periods of EEG activation induced by systemic application of IGF-I, probably by activation of NMDA receptors, as well. We suggest that response adaptation is the outcome of an intrinsic cortical information processing aimed at contributing to improve the detection of “unexpected” stimuli that disturbed the rhythmic behavior of exploration.

Temporal cueing enhances neuronal and behavioral discrimination performance in rat whisker system

Since sensory systems operate with a finite quantity of processing resources, an animal would benefit from prioritizing processing of sensory stimuli within a time window that is expected to provide key information. This behavioral manifestation of such prioritization is known as attention. Here, we investigate attention with temporal cueing and its neuronal correlates in the rat primary vibrissal somatosensory (vS1) cortex. Rats were trained in a simple whisker vibration detection task. A vibration was presented at one of two spatial locations (left or right), sometimes after an unknown time interval and sometimes after receiving an auditory cue. The auditory cue provided temporal but not spatial information about the vibration. We found that for all rats ( n = 6), the auditory cue consistently enhanced detection of the vibration stimulus. Neuronal activity in vS1 cortex reflected the observed behavioral enhancement from temporal cueing with single units responded differentially to the whisker vibration stimulus when it was temporally predicted by the auditory cue, exhibiting an enhanced signal-to-noise ratio. Our findings indicate that rats are capable of prioritizing processing within a specified time window and provide evidence that the primary sensory cortex may participate in the temporal allocation of resources.

NEW & NOTEWORTHY We demonstrate a novel paradigm of temporal cueing in rats. In a two-alternative whisker detection task, an auditory cue provided information about the timing of the stimulus but not the correct choice. In the presence of cue, detection was faster and more accurate, and neuronal activity from the primary somatosensory cortex revealed enhanced representation of vibrations. These results thus establish the rat as an alternative model organism to primates for studying temporal attention.

Deficits in Behavioral Functions of Intact Barrel Cortex Following Lesions of Homotopic Contralateral Cortex

Focal unilateral injuries to the somatosensory whisker barrel cortex have been shown cause long-lasting deficits in the activity and experience-dependent plasticity of neurons in the intact contralateral barrel cortex. However, the long-term effect of these deficits on behavioral functions of the intact contralesional cortex is not clear. In this study, we used the “Gap-crossing task” a barrel cortex-dependent, whisker-sensitive, tactile behavior to test the hypothesis that unilateral lesions of the somatosensory cortex would affect behavioral functions of the intact somatosensory cortex and degrade the execution of a bilaterally learnt behavior. Adult rats were trained to perform the Gap-crossing task using whiskers on both sides of the face. The barrel cortex was then lesioned unilaterally by subpial aspiration. As observed in other studies, when rats used whiskers that directly projected to the lesioned hemisphere the performance of Gap-crossing was drastically compromised, perhaps due to direct effect of lesion. Significant and persistent deficits were present when the lesioned rats performed Gap-crossing task using whiskers that projected to the intact cortex. The deficits were specific to performance of the task at the highest levels of sensitivity. Comparable deficits were seen when normal, bilaterally trained, rats performed the Gap-crossing task with only the whiskers on one side of the face or when they used only two rows of whiskers (D row and E row) intact on both side of the face. These findings indicate that the prolonged impairment in execution of the learnt task by rats with unilateral lesions of somatosensory cortex could be because sensory inputs from one set of whiskers to the intact cortex is insufficient to provide adequate sensory information at higher thresholds of detection. Our data suggest that optimal performance of somatosensory behavior requires dynamic activity-driven interhemispheric interactions from the entire somatosensory inputs between homotopic areas of the cerebral cortex. These results imply that focal unilateral cortical injuries, including those in humans, are likely to have widespread bilateral effects on information processing including in intact areas of the cortex.

Pupil-linked arousal modulates behavior in rats performing a whisker deflection direction discrimination task

Non-luminance-mediated changes in pupil size have been widely used to index arousal state. Recent animal studies have demonstrated correlations between behavioral state-related pupil dynamics and sensory processing. However, the relationship between pupil-linked arousal and behavior in animals performing perceptual tasks has not been fully elucidated. In the present study, we trained head-fixed rats to discriminate between directions of whisker movements using a Go/No-Go discrimination paradigm while imaging their pupils. Reaction times in this discrimination task were significantly slower than in previously reported detection tasks with similar setup, suggesting that discrimination required an increased cognitive load. We found the pupils dilated for all trials following stimulus presentation. Interestingly, in correct rejection trials, where pupil dilations solely resulted from cognitive processing, dilations were larger for more difficult stimuli. Baseline pupil size before stimulus presentation strongly correlated with behavior, as perceptual sensitivity peaked at intermediate pupil baselines and reaction time was fastest at large baselines. We further explored these relationships by investigating to what extent pupil baseline was predictive of upcoming behavior and found that a Bayesian decoder had significantly greater-than-chance probability in correctly predicting behavioral outcomes. Moreover, the outcome of the previous trial showed a strong correlation with behavior on present trials. Animals were more liberal and faster in responding following hit trials, whereas perceptual sensitivity was greatest following correct rejection trials. Taken together, these results suggest a tight correlation between pupil dynamics, perceptual performance, and reaction time in behaving rats, all of which are modulated by fluctuating arousal state. NEW & NOTEWORTHY In this study, we for the first time demonstrated that head-fixed rats were able to discriminate different directions of whisker movement. Interestingly, we found that the pupil dilated more when discriminating more difficult stimuli, a phenomenon reported in human subjects but not in animals. Baseline pupil size before stimulus presentation was found to strongly correlate with behavior, and a Bayesian decoder had significantly greater-than-chance probability in correctly predicting behavioral outcomes based on the baseline pupil size.

Biomechanical Texture Coding in Rat Whiskers

Classically, texture discrimination has been thought to be based on ‘global’ codes, i.e. frequency (signal analysis based on Fourier analysis) or intensity (signal analysis based on averaging), which both rely on integration of the vibrotactile signal across time and/or space. Recently, a novel ‘local’ coding scheme based on the waveform of frictional movements, discrete short lasting kinematic events (i.e. stick-slip movements called slips) has been formulated. We performed biomechanical measurements of relative movements of a rat vibrissa across sandpapers of different roughness. We find that the classic global codes convey some information about texture identity, but are consistently outperformed by the slip-based local code. Moreover, the slip code also surpasses the global ones in coding for active scanning parameters. This is remarkable as it suggests that the slip code would explicitly allow the whisking rat to optimize perception by selecting goal-specific scanning strategies.

Whisker touch sensing guides locomotion in small, quadrupedal mammals

All small mammals have prominent facial whiskers that they employ as tactile sensors to guide navigation and foraging in complex habitats. Nocturnal, arboreal mammals tend to have the longest and most densely packed whiskers, and semi-aquatic mammals have the most sensitive. Here we present evidence to indicate that many small mammals use their whiskers to tactually guide safe foot positioning. Specifically, in 11, small, non-flying mammal species, we demonstrate that forepaw placement always falls within the ground contact zone of the whisker field and that forepaw width is always smaller than whisker span. We also demonstrate commonalities of whisker scanning movements (whisking) and elements of active control, associated with increasing contact with objects of interest, across multiple small mammal species that have previously only been shown in common laboratory animals. Overall, we propose that guiding locomotion, alongside environment exploration, is a common function of whisker touch sensing in small, quadrupedal mammals.

Interspersed Distribution of Selectivity to Kinematic Stimulus Features in Supragranular Layers of Mouse Barrel Cortex

Neurons in the primary sensory regions of neocortex have heterogeneous response properties. The spatial arrangement of neurons with particular response properties is a key aspect of population representations and can shed light on how local circuits are wired. Here, we investigated how neurons with sensitivity to different kinematic features of whisker stimuli are distributed across local circuits in supragranular layers of the barrel cortex. Using 2-photon calcium population imaging in anesthetized mice, we found that nearby neurons represent diverse kinematic features, providing a rich population representation at the local scale. Neurons interspersed in space therefore responded differently to a common stimulus kinematic feature. Conversely, neurons with similar feature selectivity were located no closer to each other than predicted by a random distribution null hypothesis. This finding relied on defining a null hypothesis that was specific for testing the spatial distribution of tuning across neurons. We also measured how neurons sensitive to specific features were distributed relative to barrel boundaries, and found no systematic organization. Our results are compatible with randomly distributed selectivity to kinematic features, with no systematic ordering superimposed upon the whisker map.

Task-specific sensory coding strategies are matched to detection and discrimination performance

The acquisition of sensory information is limited by the neural encoding method used, constraining perceptual abilities. The most relevant aspects of stimuli may change as behavioral context changes, making efficient encoding of information more challenging. Sensory systems must balance rapid detection of a stimulus with perception of fine details that enable discrimination between similar stimuli. Here, we show that in a species of weakly electric fish, Apteronotus leptorhynchus, two coding strategies are employed for these separate behavioral tasks. Using communication signals, we demonstrate a strong correlation between neural coding strategies and behavioral performance on a discrimination task. Extracellular recordings of pyramidal cells within the electrosensory lateral line lobe of alert fish show two distinct response patterns, either burst discharges with little variation between different signals of the same category, or a graded, heterogeneous response that contains sufficient information to discriminate between signals with slight variations. When faced with a discrimination-based task, the behavioral performance of the fish closely matches predictions based on coding strategy. Comparisons of these results with neural and behavioral responses observed in other model systems suggest that our study highlights a general principle in the way sensory systems utilize different neural codes.

The TrkB agonist 7,8-dihydroxyflavone changes the structural dynamics of neocortical pyramidal neurons and improves object recognition in mice

BDNF and its receptor TrkB have important roles in neurodevelopment, neural plasticity, learning, and memory. Alterations in TrkB expression have been described in different CNS disorders. Therefore, drugs interacting with TrkB, specially agonists, are promising therapeutic tools. Among them, the recently described 7,8-dihydroxyflavone (DHF), an orally bioactive compound, has been successfully tested in animal models of these diseases. Recent studies have shown the influence of this drug on the structure of pyramidal neurons, specifically on dendritic spine density. However, there is no information yet on how DHF may alter the structural dynamics of these neurons (i.e., real-time study of the addition/elimination of dendritic spines and axonal boutons). To gain knowledge on these effects of DHF, we have performed a real-time analysis of spine and axonal dynamics in pyramidal neurons of barrel cortex, using cranial windows and 2-photon microscopy during a chronic oral treatment with this drug. After confirming TrkB expression in these neurons, we found that DHF increased the gain rates of spines and axonal boutons, as well as improved object recognition memory. These results help to understand how the activation of the BDNF-TrkB system can improve basic behavioral tasks through changes in the structural dynamics of pyramidal neurons. Moreover, they highlight DHF as a promising therapeutic vector for certain brain disorders in which this system is altered.

Slip-Based Coding of Local Shape and Texture in Mouse S1

Tactile objects have both local geometry (shape) and broader macroscopic texture, but how these different spatial scales are simultaneously encoded during active touch is unknown. In the whisker system, we tested for a shared code based on localized whisker micromotions (stick-slips) and slip-evoked spikes. We trained mice to discriminate smooth from rough surfaces, including ridged gratings and sandpaper. Whisker slips locked to ridges and evoked temporally precise spikes (<10 ms jitter) in somatosensory cortex (S1) that could resolve ridges with ∼1 mm accuracy. Slip-sensitive neurons also encoded touch and texture. On rough surfaces, both slip-evoked spikes and an additional non-slip signal elevated mean firing rate, allowing accurate rough-smooth texture decoding from population firing rate. Eighteen percent of neurons were selective among rough surfaces. Thus, slips elicit spatially and temporally precise spiking in S1 that simultaneously encodes local shape (ridges) and is integrated into a macroscopic firing rate code for roughness.

Long-range cortical dynamics: a perspective from the mouse sensorimotor whisker system

In the mammalian neocortex, the capacity to dynamically route and coordinate the exchange of information between areas is a critical feature of cognitive function, enabling processes such as higher-level sensory processing and sensorimotor integration. Despite the importance attributed to long-range connections between cortical areas, their exact operations and role in cortical function remain an open question. In recent years, progress has been made in understanding long-range cortical circuits through work focused on the mouse sensorimotor whisker system. In this review, we examine recent studies dissecting long-range circuits involved in whisker sensorimotor processing as an entry point for understanding the rules that govern long-range cortical circuit function.

Neocortical dynamics during whisker-based sensory discrimination in head-restrained mice

A fundamental task frequently encountered by brains is to rapidly and reliably discriminate between sensory stimuli of the same modality, be it distinct auditory sounds, odors, visual patterns, or tactile textures. A key mammalian brain structure involved in discrimination behavior is the neocortex. Sensory processing not only involves the respective primary sensory area, which is crucial for perceptual detection, but additionally relies on cortico-cortical communication among several regions including higher-order sensory areas as well as frontal cortical areas. It remains elusive how these regions exchange information to process neural representations of distinct stimuli to bring about a decision and initiate appropriate behavioral responses. Likewise, it is poorly understood how these neural computations are conjured during task learning. In this review, we discuss recent studies investigating cortical dynamics during discrimination behaviors that utilize head-fixed behavioral tasks in combination with in vivo electrophysiology, two-photon calcium imaging, and cell-type-specific targeting. We particularly focus on information flow in distinct cortico-cortical pathways when mice use their whiskers to discriminate between different objects or different locations. Within the primary and secondary somatosensory cortices (S1 and S2, respectively) as well as vibrissae motor cortex (M1), intermingled functional representations of touch, whisking, and licking were found, which partially re-organized during discrimination learning. These findings provide first glimpses of cortico-cortical communication but emphasize that for understanding the complete process of discrimination it will be crucial to elucidate the details of how neural processing is coordinated across brain-wide neuronal networks including the S1-S2-M1 triangle and cortical areas beyond.

Organization of sensory feature selectivity in the whisker system

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Neurons in the whisker system are selective to spatial and dynamical properties – features – of sensory stimuli.

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At each stage of the pathway, different neurons encode distinct features, generating a rich population representation.

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Whisker touch is robustly represented; neurons respond to touch-driven fast fluctuations in forces at the whisker base.

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Cortical neurons have more complex and context-dependent selectivity than subcortical, e.g., to collective whisker motion.

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Understanding how these signals are integrated to construct whisker-mediated percepts requires further research.

Our sensory receptors are faced with an onslaught of different environmental inputs. Each sensory event or encounter with an object involves a distinct combination of physical energy sources impinging upon receptors. In the rodent whisker system, each primary afferent neuron located in the trigeminal ganglion innervates and responds to a single whisker and encodes a distinct set of physical stimulus properties – features – corresponding to changes in whisker angle and shape and the consequent forces acting on the whisker follicle. Here we review the nature of the features encoded by successive stages of processing along the whisker pathway. At each stage different neurons respond to distinct features, such that the population as a whole represents diverse properties. Different neuronal types also have distinct feature selectivity. Thus, neurons at the same stage of processing and responding to the same whisker nevertheless play different roles in representing objects contacted by the whisker. This diversity, combined with the precise timing and high reliability of responses, enables populations at each stage to represent a wide range of stimuli. Cortical neurons respond to more complex stimulus properties – such as correlated motion across whiskers – than those at early subcortical stages. Temporal integration along the pathway is comparatively weak: neurons up to barrel cortex (BC) are sensitive mainly to fast (tens of milliseconds) fluctuations in whisker motion. The topographic organization of whisker sensitivity is paralleled by systematic organization of neuronal selectivity to certain other physical features, but selectivity to touch and to dynamic stimulus properties is distributed in “salt-and-pepper” fashion.

The brain during free movement - What can we learn from the animal model

Animals, just like humans, can freely move. They do so for various important reasons, such as finding food and escaping predators. Observing these behaviors can inform us about the underlying cognitive processes. In addition, while humans can convey complicated information easily through speaking, animals need to move their bodies to communicate. This has prompted many creative solutions by animal neuroscientists to enable studying the brain during movement. In this review, we first summarize how animal researchers record from the brain while an animal is moving, by describing the most common neural recording techniques in animals and how they were adapted to record during movement. We further discuss the challenge of controlling or monitoring sensory input during free movement. However, not only is free movement a necessity to reflect the outcome of certain internal cognitive processes in animals, it is also a fascinating field of research since certain crucial behavioral patterns can only be observed and studied during free movement. Therefore, in a second part of the review, we focus on some key findings in animal research that specifically address the interaction between free movement and brain activity. First, focusing on walking as a fundamental form of free movement, we discuss how important such intentional movements are for understanding processes as diverse as spatial navigation, active sensing, and complex motor planning. Second, we propose the idea of regarding free movement as the expression of a behavioral state. This view can help to understand the general influence of movement on brain function. Together, the technological advancements towards recording from the brain during movement, and the scientific questions asked about the brain engaged in movement, make animal research highly valuable to research into the human "moving brain".

Radial Distance Estimation with Tapered Whisker Sensors

Rats use their whiskers as tactile sensors to sense their environment. Active whisking, moving whiskers back and forth continuously, is one of prominent features observed in rodents. They can discriminate different textures or extract features of a nearby object such as size, shape and distance through active whisking. There have been studies to localize objects with artificial whiskers inspired by rat whiskers. The linear whisker model based on beam theory has been used to estimate the radial distance, that is, the distance between the base of the whisker and a target object. In this paper, we investigate deflection angle measurements instead of forces or moments, based on a linear tapered whisker model to see the role of tapered whiskers found in real animals. We analyze how accurately this model estimates the radial distance, and quantify the estimation errors and noise sensitivity. We also compare the linear model simulation and nonlinear numerical solutions. It is shown that the radial distance can be estimated using deflection angles at two different positions on the tapered whisker. We argue that the tapered whisker has an advantage of estimating the radial distance better, as compared to an untapered whisker, and active sensing allows that estimation without the whisker’s material property and thickness or the moment at base. In addition, we investigate the potential of passive sensing for tactile localization.

Bilateral Discrimination of Tactile Patterns without Whisking in Freely Running Rats

A majority of whisker discrimination tasks in rodents are performed on head-fixed animals to facilitate tracking or control of the sensory inputs. However, head fixation critically restrains the behavior and thus the incoming stimuli compared with those occurring in natural conditions. In this study, we investigated whether freely behaving rats can discriminate fine tactile patterns while running, in particular when stimuli are presented simultaneously on both sides of the snout. We developed a two-alternative forced-choice task in an automated modified T-maze. Stimuli were either a surface with no bars (smooth) or with vertical bars spaced irregularly or regularly. While running at full speed, rats encountered simultaneously the two discriminanda placed on the two sides of the central aisle. Rats learned to recognize regular bars versus a smooth surface in 8 weeks. They solved the task while running at an average speed of 1 m/s, so that the contact with the stimulus lasted <1 typical whisking cycle, precluding the use of active whisking. Whisker-tracking analysis revealed an asymmetry in the position of the whiskers: they oriented toward the rewarded stimulus during successful trials as early as 60 ms after the first possible contact. We showed that the whiskers and activity in the primary somatosensory cortex are involved during the discrimination process. Finally, we identified irregular patterns of bars that the rats can discriminate from the regular one. This novel task shows that freely moving rodents can make simultaneous bilateral tactile discrimination without whisking.SIGNIFICANCE STATEMENT The whisker system of rodents is a widely used model to study tactile processing. Rats show remarkable abilities in discriminating surfaces by actively moving their whiskers (whisking) against stimuli, typically sampling them several times. This motor strategy affects considerably the way that tactile information is acquired and thus the way that neuronal networks process the information. However, when rats run at high speed, they protract their whiskers in front of the snout without large movements. Here, we investigated whether rats are able to discriminate regular and irregular patterns of vertical bars while running without whisking. We found that the animals can perform a bilateral simultaneous discrimination without whisking and that this involves both whiskers and barrel cortex activity.

Channel noise effects on first spike latency of a stochastic Hodgkin-Huxley neuron

While it is widely accepted that information is encoded in neurons via action potentials or spikes, it is far less understood what specific features of spiking contain encoded information. Experimental evidence has suggested that the timing of the first spike may be an energy-efficient coding mechanism that contains more neural information than subsequent spikes. Therefore, the biophysical features of neurons that underlie response latency are of considerable interest. Here we examine the effects of channel noise on the first spike latency of a Hodgkin-Huxley neuron receiving random input from many other neurons. Because the principal feature of a Hodgkin-Huxley neuron is the stochastic opening and closing of channels, the fluctuations in the number of open channels lead to fluctuations in the membrane voltage and modify the timing of the first spike. Our results show that when a neuron has a larger number of channels, (i) the occurrence of the first spike is delayed and (ii) the variation in the first spike timing is greater. We also show that the mean, median, and interquartile range of first spike latency can be accurately predicted from a simple linear regression by knowing only the number of channels in the neuron and the rate at which presynaptic neurons fire, but the standard deviation (i.e., neuronal jitter) cannot be predicted using only this information. We then compare our results to another commonly used stochastic Hodgkin-Huxley model and show that the more commonly used model overstates the first spike latency but can predict the standard deviation of first spike latencies accurately. We end by suggesting a more suitable definition for the neuronal jitter based upon our simulations and comparison of the two models.

Temporally precise control of single-neuron spiking by juxtacellular nanostimulation

Temporal patterns of action potentials influence a variety of activity-dependent intra- and intercellular processes and play an important role in theories of neural coding. Elucidating the mechanisms underlying these phenomena requires imposing spike trains with precisely defined patterns, but this has been challenging due to the limitations of existing stimulation techniques. Here we present a new nanostimulation method providing control over the action potential output of individual cortical neurons. Spikes are elicited through the juxtacellular application of short-duration fluctuating currents ("kurzpulses"), allowing for the sub-millisecond precise and reproducible induction of arbitrary patterns of action potentials at all physiologically relevant firing frequencies (<120 Hz), including minute-long spike trains recorded in freely moving animals. We systematically compared our method to whole cell current injection, as well as optogenetic stimulation, and show that nanostimulation performance compares favorably with these techniques. This new nanostimulation approach is easily applied, can be readily performed in awake behaving animals, and thus promises to be a powerful tool for systematic investigations into the temporal elements of neural codes, as well as the mechanisms underlying a wide variety of activity-dependent cellular processes.NEW & NOTEWORTHY Assessing the impact of temporal features of neuronal spike trains requires imposing arbitrary patterns of spiking on individual neurons during behavior, but this has been difficult to achieve due to limitations of existing stimulation methods. We present a technique that overcomes these limitations by using carefully designed short-duration fluctuating juxtacellular current injections, which allow for the precise and reliable evocation of arbitrary patterns of neuronal spikes in single neurons in vivo.

Short Time-Scale Sensory Coding in S1 during Discrimination of Whisker Vibrotactile Sequences

Rodent whisker input consists of dense microvibration sequences that are often temporally integrated for perceptual discrimination. Whether primary somatosensory cortex (S1) participates in temporal integration is unknown. We trained rats to discriminate whisker impulse sequences that varied in single-impulse kinematics (5–20-ms time scale) and mean speed (150-ms time scale). Rats appeared to use the integrated feature, mean speed, to guide discrimination in this task, consistent with similar prior studies. Despite this, 52% of S1 units, including 73% of units in L4 and L2/3, encoded sequences at fast time scales (≤20 ms, mostly 5–10 ms), accurately reflecting single impulse kinematics. 17% of units, mostly in L5, showed weaker impulse responses and a slow firing rate increase during sequences. However, these units did not effectively integrate whisker impulses, but instead combined weak impulse responses with a distinct, slow signal correlated to behavioral choice. A neural decoder could identify sequences from fast unit spike trains and behavioral choice from slow units. Thus, S1 encoded fast time scale whisker input without substantial temporal integration across whisker impulses.

Recordings in whisker somatosensory cortex of rats during discrimination of rapid whisker deflection sequences show that whisker input is encoded at very short time scales (less than 20 ms).

Sensory input is rich in temporal patterns, but how the brain processes this temporal information is not well understood. This process is important in the whisker tactile system of rodents, in which active whisking on objects generates dense streams of stick-slip and contact events. Rats can discriminate vibrotactile sequences applied to the whiskers, and prior studies show that this often involves behavioral integration over time to calculate mean whisker-speed. How the brain represents and integrates vibrotactile input is not known. We recorded neural activity in primary somatosensory cortex as rats discriminated rapid vibrotactile sequences. We found that neurons in the primary somatosensory cortex encoded whisker sensory information at very fast time scales (<20 ms), without evidence for substantial temporal integration. A subset of neurons encoded relatively little stimulus information but strongly encoded the rat’s behavioral choice on each trial. Thus, primary sensory cortex represents immediate sensory input, suggesting that temporal integration occurs in downstream brain areas.

Air-Track: a real-world floating environment for active sensing in head-fixed mice

Natural behavior occurs in multiple sensory and motor modalities and in particular is dependent on sensory feedback that constantly adjusts behavior. To investigate the underlying neuronal correlates of natural behavior, it is useful to have access to state-of-the-art recording equipment (e.g., 2-photon imaging, patch recordings, etc.) that frequently requires head fixation. This limitation has been addressed with various approaches such as virtual reality/air ball or treadmill systems. However, achieving multimodal realistic behavior in these systems can be challenging. These systems are often also complex and expensive to implement. Here we present "Air-Track," an easy-to-build head-fixed behavioral environment that requires only minimal computational processing. The Air-Track is a lightweight physical maze floating on an air table that has all the properties of the "real" world, including multiple sensory modalities tightly coupled to motor actions. To test this system, we trained mice in Go/No-Go and two-alternative forced choice tasks in a plus maze. Mice chose lanes and discriminated apertures or textures by moving the Air-Track back and forth and rotating it around themselves. Mice rapidly adapted to moving the track and used visual, auditory, and tactile cues to guide them in performing the tasks. A custom-controlled camera system monitored animal location and generated data that could be used to calculate reaction times in the visual and somatosensory discrimination tasks. We conclude that the Air-Track system is ideal for eliciting natural behavior in concert with virtually any system for monitoring or manipulating brain activity.

Correlation between Cortical State and Locus Coeruleus Activity: Implications for Sensory Coding in Rat Barrel Cortex

Cortical state modulates the background activity of cortical neurons, and their evoked response to sensory stimulation. Multiple mechanisms are involved in switching between cortical states including various neuromodulatory systems. Locus Coeruleus (LC) is one of the major neuromodulatory nuclei in the brainstem with widespread projections throughout the brain and modulates the activity of cells and networks. Here, we quantified the link between the LC spontaneous activity, cortical state and sensory processing in the rat vibrissal somatosensory "barrel" cortex (BC). We simultaneously recorded unit activity from LC and BC along with prefrontal electroencephalogram (EEG) while presenting brief whisker deflections under urethane anesthesia. The ratio of low to high frequency components of EEG (referred to as the L/H ratio) was employed to identify cortical state. We found that the spontaneous activity of LC units exhibited a negative correlation with the L/H ratio. Cross-correlation analysis revealed that changes in LC firing preceded changes in the cortical state: the correlation of the LC firing profile with the L/H ratio was maximal at an average lag of -1.2 s. We further quantified BC neuronal responses to whisker stimulation during the synchronized and desynchronized states. In the desynchronized state, BC neurons showed lower stimulus detection threshold, higher response fidelity, and shorter response latency. The most prominent change was observed in the late phase of BC evoked activity (100-400 ms post stimulus onset): almost every BC unit exhibited a greater late response during the desynchronized state. Categorization of the BC evoked responses based on LC activity (into high and low LC discharge rates) resulted in highly similar response profiles compared to categorization based on the cortical state (low and high L/H ratios). These findings provide evidence for the involvement of the LC neuromodulatory system in desynchronization of cortical state and the consequent enhancement of sensory coding efficiency.