Publications

Sensory information is coded in space and in time. The organization of neuronal activity in space maintains straightforward relationships with the spatial organization of the perceived environment. In contrast, the temporal organization of neuronal activity is not trivially related to external features due to sensor motion. Still, the temporal organization shares similar principles across sensory modalities. Likewise, thalamocortical circuits exhibit common features across senses. Focusing on touch, vision, and audition, we review their shared coding principles and suggest that thalamocortical systems include circuits that allow analogous recoding mechanisms in all three senses. These thalamocortical circuits constitute oscillations-based phase-locked loops, that translate temporally-coded sensory information to rate-coded cortical signals, signals that can integrate information across sensory and motor modalities. The loop also allows predictive locking to the onset of future modulations of the sensory signal. The paper thus suggests a theoretical framework in which a common thalamocortical mechanism implements temporal demodulation across senses.

Natural vision is a dynamic and continuous process. Under natural conditions, visual object recognition typically involves continuous interactions between ocular motion and visual contrasts, resulting in dynamic retinal activations. In order to identify the dynamic variables that participate in this process and are relevant for image recognition, we used a set of images that are just above and below the human recognition threshold and whose recognition typically requires >2 s of viewing. We recorded eye movements of participants while attempting to recognize these images within trials lasting 3 s. We then assessed the activation dynamics of retinal ganglion cells resulting from ocular dynamics using a computational model. We found that while the saccadic rate was similar between recognized and unrecognized trials, the fixational ocular speed was significantly larger for unrecognized trials. Interestingly, however, retinal activation level was significantly lower during these unrecognized trials. We used retinal activation patterns and oculomotor parameters of each fixation to train a binary classifier, classifying recognized from unrecognized trials. Only retinal activation patterns could predict recognition, reaching 80% correct classifications on the fourth fixation (on average, ∼2.5 s from trial onset). We thus conclude that the information that is relevant for visual perception is embedded in the dynamic interactions between the oculomotor sequence and the image. Hence, our results suggest that ocular dynamics play an important role in recognition and that understanding the dynamics of retinal activation is crucial for understanding natural vision.

In whisking rodents, mystacial pad is supplied with vibrissae and contains a collagenous skeleton that is a part of the snout fascia. Collagenous skeleton is built of fibrillary, spongy and tough connective tissue that splits into three interconnected layers: superficial, deep spongy mesh and subcapsular fibrous mat. The integrity of the collagenous skeleton is maintained by tough follicular capsules of which ends are embedded into superficial layer and subcapsular fibrous mat. To move vibrissae, the forces of intrinsic muscles are applied directly to the capsules of the vibrissa follicles, whereas the forces of extrinsic muscles, to other parts of the collagenous skeleton which transmit the forces to the capsules. According to the spatial distribution and anchoring sites of the muscles and fascia, extrinsic muscles provide vibrissa protraction or retraction by pulling superficial layer of the collagenous skeleton rostral or caudal, respectively. Vibrissae can be also retracted when the efforts of extrinsic muscles are applied to the subcapsular fibrous mat that has a matrix composed of oriented rostrocaudal wavy fibrils. When the muscles relax, vibrissae return to their resting position. Deep spongy layer encompasses vibrissal follicles providing a uniform distribution of stresses and strains during whisking. In the mystacial pad, fascia is a dominant type of tissue that maintains integrity of the vibrissa motor plant, translates muscular momentum to the vibrissae and plays a role in vibrissae movements.

Physics and neuroscience share overlapping objectives, the major of which is probably the attempt to reduce the observed universe to a set of rules. The approaches are complementary, attempting to find a reduced description of the universe or of the observer, respectively. We propose here that combining the two approaches within an observer-inclusive physical scheme, bears significant advantages. In such a scheme, the same set of rules applies to the universe and its observers, and the two descriptions are entangled. We show here that analyzing special relativity in an observer-inclusive framework can resolve its contradiction with the observed non-locality of physical interactions. The contradiction is resolved by reducing the universe (including the observer) to a dynamic distribution of closed strings (\u201cceons\u201d) whose vibration waves travel at c. This ceons model is consistent with special and general relativity, non-locality and the holographic principle; it also eliminates Zenos motion paradoxes. Yet, the model entails several new empirical predictions. Finally, the ceons model suggests a fundamental physical implementation of active biological perception. Paraphrasing Torricelli, this paper suggests that we live submerged in a c of light.

Whisking mammals move their whiskers in the rostrocaudal and dorsoventral directions with simultaneous rolling about their long axes (torsion). Whereas muscular control of the first two types of whisker movement was already established, the anatomic muscular substrate of the whisker torsion remains unclear. Specifically, it was not clear whether torsion is induced by asymmetrical operation of known muscles or by other largely unknown muscles. Here, we report that mystacial pads of newborn and adult rats and mice contain oblique intrinsic muscles (OMs) that connect diagonally adjacent vibrissa follicles. Each of the OMs is supplied by a cluster of motor end plates. In rows A and B, OMs connect the ventral part of the rostral follicle with the dorsal part of the caudal follicle. In rows C-E, in contrast, OMs connect the dorsal part of the rostral follicle to the ventral part of the caudal follicle. This inverse architecture is consistent with previous behavioral observations [Knutsen et al.: Neuron 59 (2008) 35-42]. In newborn mice, torsion occurred in irregular single twitches. In adult anesthetized rats, microelectrode mediated electrical stimulation of an individual OM that is coupled with two adjacent whiskers was sufficient to induce a unidirectional torsion of both whiskers. Torsional movement was associated with protracting movement, indicating that in the vibrissal system, like in the ocular system, torsional movement is mechanically coupled to horizontal and vertical movements. This study shows that torsional whisker rotation is mediated by specific OMs whose morphology and attachment sites determine rotation direction and mechanical coupling, and motor innervation determines rotation dynamics.

Perception of external objects involves sensory acquisition via the relevant sensory organs. A widely-accepted assumption is that the sensory organ is the first station in a serial chain of processing circuits leading to an internal circuit in which a percept emerges. This open-loop scheme, in which the interaction between the sensory organ and the environment is not affected by its concurrent downstream neuronal processing, is strongly challenged by behavioral and anatomical data. We present here a hypothesis in which the perception of external objects is a closed-loop dynamical process encompassing loops that integrate the organism and its environment and converging towards organism-environment steady-states. We discuss the consistency of closed-loop perception (CLP) with empirical data and show that it can be synthesized in a robotic setup. Testable predictions are proposed for empirical distinction between open and closed loop schemes of perception.

In this paper, the phrases should be corrected as follows:(page 575, ref. 20)For \u201cparent\u201dRead \u201ctrunk\u201d(page 576, ref. 39)For \u201cperoxidise\u201dRead \u201cperoxidase\u201d

It is commonly assumed that object perception is the combination of sensory features into unified perceptual entities. Tactile object perception may therefore be defined as the perception of objects whose feature information is acquired via touch. Consequently, research relevant to the topic of tactile objects has focused on exploring the primitives of the tactile system, their interrelation, and how they may be bound together. The current discussion does not explicitly rule out, nor does it address, kinesthetic sensation. As such, tactile perception is used here interchangeably with haptic perception (Lederman and Klatzky 2009).

In the article \u201cMotion Makes Sense: An Adaptive Motor-Sensory Strategy Underlies the Perception of Object Location in Rats\u201d by Inbar Saraf-Sinik, Eldad Assa, and Ehud Ahissar, which appeared on pages 87778789 of the June 10, 2015 issue, several errors were discovered and are corrected.

Coordinated action of facial muscles during whisking, sniffing, and touching objects is an important component of active sensing in rodents. Accumulating evidence suggests that the anatomical schemes that underlie active sensing are similar across the majority of whisking rodents. Intriguingly, however, muscle architecture in the mystacial pad of the mouse was reported to be different, possessing only one extrinsic vibrissa protracting muscle (M. nasalis) in the rostral part of the snout. In this study, the organization of the muscles that move the nose and the mystacial vibrissae in mice was re-examined and compared with that reported previously in other rodents. We found that muscle distribution within the mystacial pad and around the tip of the nose in mice is isomorphic with that found in other whisking rodents. In particular, in the rostral part of the mouse snout, we describe both protractors and retractors of the vibrissae. Nose movements are controlled by the M. dilator nasi and five subunits of the M. nasolabialis profundus, with involvement of the nasal cartilaginous skeleton as a mediator in the muscular effort translation.

What are the functions implemented by neurons in the sensory nuclei of the thalamus? It seems that this question has accompanied cortical and thalamic studies since their onset some 6 decades ago. Over the years, the simplistic, traditional view of thalamic neurons as mere relays of sensory information has given way to more sophisticated views, of which several alternative hypotheses have been proposed. This commentary briefly reviews the 2 current major hypotheses and shows how a new, pioneering experiment, published in Cerebral Cortex by Groh, Acsady and colleagues, discriminates between them. The commentary further elaborates on the thalamo-cortical processing suggested by the new findings, the general sensory-motor scheme to which these findings may be relevant, and the possible roles such thalamo-cortical processing may have in sensory-motor control.

In a number of mammals muscle dilator nasi (naris) has been described as a muscle that reduces nasal airflow resistance by dilating the nostrils. Here we show that in rats the tendon of this muscle inserts into the aponeurosis above the nasal cartilage. Electrical stimulation of this muscle raises the nose and deflects it laterally towards the side of stimulation, but does not change the size of the nares. In alert head-restrained rats, electromyographic recordings of muscle dilator nasi reveal that it is active during nose motion rather than nares dilation. Together these results suggest an alternative role for the muscle dilator nasi in directing the nares for active odor sampling rather than dilating the nares. We suggest that dilation of the nares results from contraction of muscles of the maxillary division of muscle nasolabialis profundus. This muscle group attaches to the outer wall of the nasal cartilage and to the plate of the mystacial pad. Contraction of these muscles exerts a dual action: it pulls the lateral nasal cartilage outward, thus dilating the naris, and drags the plate of the mystacial pad rostrally to produce a slight retraction of the vibrissae. On the basis of these results, we propose that muscle dilator nasi of the rat should be re-named muscle deflector nasi, and that the maxillary parts of muscle nasolabialis profundus should be referred to as muscle dilator nasi.

Tactile perception is obtained by coordinated motor-sensory processes. We studied the processes underlying the perception of object location in freely moving rats. We trained rats to identify the relative location of two vertical poles placed in front of them and measured at high resolution the motor and sensory variables (19 and 2 variables, respectively) associated with this whiskers-based perceptual process. We found that the rats developed stereotypic head and whisker movements to solve this task, in a manner that can be described by several distinct behavioral phases. During two of these phases, the rats' whiskers coded object position by first temporal and then angular coding schemes. We then introduced wind (in two opposite directions) and remeasured their perceptual performance and motor-sensory variables. Our rats continued to perceive object location in a consistent manner under wind perturbations while maintaining all behavioral phases and relatively constant sensory coding. Constant sensory coding was achieved by keeping one group of motor variables (the \u201ccontrolled variables\u201d) constant, despite the perturbing wind, at the cost of strongly modulating another group of motor variables (the \u201cmodulated variables\u201d). The controlled variables included coding-relevant variables, such as head azimuth and whisker velocity. These results indicate that consistent perception of location in the rat is obtained actively, via a selective control of perception-relevant motor variables.

Rats repeatedly sweep their facial whiskers back and forth in order to explore their environment. Such explorative whisking appears to be driven by central pattern generators (CPGs) that operate independently of direct sensory feedback. Nevertheless, whisking can be modulated by sensory feedback, and it has been hypothesized that some of this modulation already occurs within the brainstem. However, the interaction between sensory feedback and CPG activity is poorly understood. Using the visual language of statecharts, a dynamic, bottom-up computerized model of the brainstem loop of the whisking system was built in order to investigate the interaction between sensory feedback and CPG activity during whisking behavior. As a benchmark, we used a previously quantified closed-loop phenomenon of the whisking system, touched-induced pump (TIP), which is thought to be mediated by the brainstem loop. First, we showed that TIPs depend on sensory feedback, by comparing TIP occurrence in intact rats with that in rats whose sensory nerve was experimentally cut. We then inspected several possible feedback mechanisms of TIPs using our model. The model ruled out all hypothesized mechanisms but one, which adequately simulated the corresponding motion observed in the rat. Results of the simulations suggest that TIPs are generated via sensory feedback that activates extrinsic retractor muscles in the mystacial pad. The model further predicted that in addition to the touching whisker, all whiskers found on the same side of the snout should exhibit a TIP. We present experimental results that confirm the predicted movements in behaving rats, establishing the validity of the hypothesized interaction between sensory feedback and CPG activity we suggest here for the generation of TIPs in the whisking system. Copyright:

In the vibrissal system, touch information is conveyed by a receptorless whisker hair to follicle mechanoreceptors, which then provide input to the brain. We examined whether any processing, that is, meaningful transformation, occurs in the whisker itself. Using high-speed videography and tracking the movements of whiskers in anesthetized and behaving rats, we found that whisker-related morphological phase planes, based on angular and curvature variables, can represent the coordinates of object position after contact in a reliable manner, consistent with theoretical predictions. By tracking exposed follicles, we found that the follicle-whisker junction is rigid, which enables direct readout of whisker morphological coding by mechanoreceptors. Finally, we found that our behaving rats pushed their whiskers against objects during localization in a way that induced meaningful morphological coding and, in parallel, improved their localization performance, which suggests a role for pre-neuronal morphological computation in active vibrissal touch.

Perception involves motor control of sensory organs. However, the dynamics underlying emergence of perception from motorsensory interactions are not yet known. Two extreme possibilities are as follows: (1) motor and sensory signals interact within an open-loop scheme in which motor signals determine sensory sampling but are not affected by sensory processing and (2) motor and sensory signals are affected by each other within a closed-loop scheme. We studied the scheme of motor-sensory interactions in humans using a novel object localization task that enabled monitoring the relevant overt motor and sensory variables. We found that motor variables were dynamically controlled within each perceptual trial, such that they gradually converged to steady values. Training on this task resulted in improvement in perceptual acuity, which was achieved solely by changes in motor variables, without any change in the acuity of sensory readout. The within-trial dynamics is captured by a hierarchical closed-loop model in which lower loops actively maintain constant sensory coding, and higher loops maintain constant sensory update flow. These findings demonstrate interchangeability of motor and sensory variables in perception, motor convergence during perception, and a consistent hierarchical closed-loop perceptual model.

A curious agent acts so as to optimize its learning about itself and its environment, without external supervision. We present a model of hierarchical curiosity loops for such an autonomous active learning agent, whereby each loop selects the optimal action that maximizes the agent's learning of sensory-motor correlations. The model is based on rewarding the learner's prediction errors in an actor-critic reinforcement learning (RL) paradigm. Hierarchy is achieved by utilizing previously learned motor-sensory mapping, which enables the learning of other mappings, thus increasing the extent and diversity of knowledge and skills. We demonstrate the relevance of this architecture to active sensing using the well-studied vibrissae (whiskers) system, where rodents acquire sensory information by virtue of repeated whisker movements. We show that hierarchical curiosity loops starting from optimally learning the internal models of whisker motion and then extending to object localization result in free-air whisking and object palpation, respectively.

A curious agent, be it a robot, animal or human, acts so as to learn as much as possible about itself and its environment. Such an agent can also learn without external supervision, but rather actively probe its surrounding and autonomously induce the relations between its action's effects on the environment and the resulting sensory input. We present a model of hierarchical motor-sensory loops for such an autonomous active learning agent, meaning a model that selects the appropriate action in order to optimize the agent's learning. Furthermore, learning one motor-sensory mapping enables the learning of other mappings, thus increasing the extent and diversity of knowledge and skills, usually in hierarchical manner. Each such loop attempts to optimally learn a specific correlation between the agent's available internal information, e. g. sensory signals and motor efference copies, by finding the action that optimizes that learning. We demonstrate this architecture on the well-studied vibrissae system, and show how sensory-motor loops are actively learnt from the bottom-up, starting with the forward and inverse models of whisker motion and then extending them to object localization. The model predicts transition from free-air whisking that optimally learns the self-generated motor-sensory mapping to touch-induced palpation that optimizes object localization, both observed in naturally behaving rats.

The vibrissal system of the rat is an example of active tactile sensing, and has recently been used as a prototype in construction of touch-oriented robots. Active vibrissal exploration and touch are enabled and controlled by musculature of the mystacial pad. So far, knowledge about motor control of the rat vibrissal system has been extracted from what is known about the vibrissal systems of other species, mainly mice and hamsters, since a detailed description of the musculature of the rat mystacial pad was lacking. In the present work, the musculature of the rat mystacial pad was revealed by slicing the mystacial pad in four different planes, staining of mystacial pad slices for cytochrome oxidase, and tracking spatial organization of mystacial pad muscles in consecutive slices. We found that the rat mystacial pad contains four superficial extrinsic muscles and five parts of the M. nasolabialis profundus. The connection scheme of the three parts of the M. nasolabialis profundus is described here for the first time. These muscles are inserted into the plate of the mystacial pad, and thus, their contraction causes whisker retraction. All the muscles of the rat mystacial pad contained three types of skeletal striated fibers (red, white, and intermediate). Although the entire rat mystacial pad usually functions as unity, our data revealed its structural segmentation into nasal and maxillary subdivisions. The mechanisms of whisking in the rat, and hypotheses concerning biomechanical interactions during whisking, are discussed with respect to the muscle architecture of the rat mystacial pad.

Mammals acquire much of their sensory information by actively moving their sensory organs. Rats, in particular, scan their surrounding environment with their whiskers. This form of active sensing induces specific patterns of temporal encoding of sensory information, which are based on a conversion of space into time via sensor movement. We investigate the ways in which object location is encoded by the whiskers and decoded by the brain. We recorded from first-order neurons located in the trigeminal ganglion (TG) of anaesthetized rats during epochs of artificial whisking induced by electrical stimulation of the facial motor nerve. We found that TG neurons encode the three positional coordinates with different codes. The horizontal coordinate (along the backward-forward axis) is encoded by two encoding schemes, both relying on the firing times of one type of TG neuron, the 'contact cell'. The radial coordinate (from face outward) is encoded primarily by the firing magnitude of another type of TG neurons, the 'pressure cell'. The vertical coordinate (from ground up) is encoded by the identity of activated neurons. The decoding schemes of at least some of these sensory cues, our data suggest, are also active: cortical representations are generated by a thalamic comparison of cortical expectations with incoming sensory data.

Correction to the acknowledgements.

to the editor: In a recent Epub issue of the Journal of Neurophysiology, a paper (Masri et al. 2008) appeared in which the authors claim to replicate experiments, but not results, previously obtained in our laboratory (Ahissar et al. 2000; Sosnik et al. 2001). We maintain that Masri et al. 1) did not replicate our experiments, 2) probed the vibrissal system in a different parametric regime, and 3) their results are not inconsistent with ours.

Traditionally, sensory processing and motor control have been studied separately, reflecting the belief that sensory and motor streams remain independent until linked via cortical \u201cassociative\u201d areas. Although this belief no longer dominates neuroscience, the traditional tendency to continue to study sensory processing and motor control separately is not easily overcome. Only after closely examining operation of sensory organs does one realize how important motor control is for sensation. The recent elegant study of Herfst and Brecht reveals how accurate sensation-targeted motor control should be in one such systemthe vibrissal system.

Using their large mystacial vibrissas, rats perform a variety of tasks, including localization and identification of objects. We report on the discriminatory thresholds and behavior of rats trained in a horizontal object localization task. Using an adaptive training procedure, rats learned to discriminate offsets in horizontal (anteroposterior) location with all, one row, or one arc of whiskers intact, but not when only a single whisker (C2) was intact on each cheek. However, rats initially trained with multiple whiskers typically improved when retested later with a single whisker intact. Individual rats reached localization thresholds as low as 0.24 mm (∼1°). Among the tested groups, localization acuity was finest (

In active sensation, sensory information is acquired via movements of sensory organs; rats move their whiskers repetitively to scan the environment, thus detecting, localizing, and identifying objects. Sensory information, in turn, affects future motor movements. How this motor-sensory-motor functional loop is implemented across anatomical loops of the whisker system is not yet known. While inducing artificial whisking in anesthetized rats, we recorded the activity of individual neurons from three thalamic nuclei of the whisker system, each belonging to a different major afferent pathway: paralemniscal, extralemniscal (a recently discovered pathway), or lemniscal. We found that different sensory signals related to active touch are conveyed separately via the thalamus by these three parallel afferent pathways. The paralemniscal pathway conveys sensor motion (whisking) signals, the extralemniscal conveys contact (touch) signals, and the lemniscal pathway conveys combined whisking-touch signals. This functional segregation of anatomical pathways raises the possibility that different sensory-motor processes, such as those related to motion control, object localization, and object identification, are implemented along different motor-sensory-motor loops.

A temporal sensory code occurs in posterior medial (POm) thalamus of the rat vibrissa system, where the latency for the spike rate to peak is observed to increase with increasing frequency of stimulation between 2 and 11 Hz. In contrast, the latency of the spike rate in the ventroposterior medial (VPm) thalamus is constant in this frequency range. We consider the hypothesis that two factors are essential for latency coding in the POm. The first is GABA B-mediated feedback inhibition from the reticular thalamic (Rt) nucleus, which provides delayed and prolonged input to thalamic structures. The second is sensory input that leads to an accelerating spike rate in brain stem nuclei. Essential aspects of the experimental observations are replicated by the analytical solution of a rate-based model with a minimal architecture that includes only the POm and Rt nuclei, i.e., an increase in stimulus frequency will increase the level of inhibitory output from Rt thalamus and lead to a longer latency in the activation of POm thalamus. This architecture, however, admits period-doubling at high levels of GABA B-mediated conductance. A full architecture that incorporates the VPm nucleus suppresses period-doubling. A clear match between the experimentally measured spike rates and the numerically calculated rates for the full model occurs when VPm thalamus receives stronger brain stem input and weaker GABA B-mediated inhibition than POm thalamus. Our analysis leads to the prediction that the latency code will disappear if GABA B-mediated transmission is blocked in POm thalamus or if the onset of sensory input is too abrupt. We suggest that GABA B-mediated inhibition is a substrate of temporal coding in normal brain function.

The response properties of neurons of the postero-medial barrel sub-field of the somatosensory cortex (the cortical structure receiving information from the mystacial vibrissae can be modified as a consequence of peripheral manipulations of the afferent activity. This plasticity depends on the integrity of the cortical cholinergic innervation, which originates at the nucleus basalis magnocellularis (NBM). The activity of the NBM is related to the behavioral state of the animal and the putative cholinergic neurons are activated by specific events, such as reward-related signals, during behavioral learning. Experimental studies on acetylcholine (ACh)-dependent cortical plasticity have shown that ACh is needed for both the induction and the expression of plastic modifications induced by sensory-cholinergic pairings. Here we review and discuss ACh-dependent plasticity and activity-dependent plasticity and ask whether these two mechanisms are linked. To address this question, we analyzed our data and tested whether changes mediated by ACh were activity-dependent. We show that ACh-dependent potentiation of response in the barrel cortex of rats observed after sensory-cholinergic pairing was not correlated to the changes in activity induced during pairing. Since these results suggest that the effect of ACh during pairing is not exerted through a direct control of the post-synaptic activity, we propose that ACh might induce its effect either pre- or post-synaptically through activation of second messenger cascades.

Sensory information is encoded both in space and in time. Spatial encoding is based on the identity of activated receptors, while temporal encoding is based on the timing of activation. In order to generate accurate internal representations of the external world, the brain must decode both types of encoded information, even when processing stationary stimuli. We review here evidence in support of a parallel processing scheme for spatially and temporally encoded information in the tactile system and discuss the advantages and limitations of sensory-derived temporal coding in both the tactile and visual systems. Based on a large body of data, we propose a dynamic theory for vision, which avoids the impediments of previous dynamic theories.

While scanning a textured surface with fingers, tactile information is encoded both spatially, by differential activation of adjacent receptors, and temporally, by changes in receptor activation during movements of the fingers across the surface. We used a tactile discrimination task to examine the dependence of human tactile perception on the availability of spatial and temporal cues. Subjects discriminated between spatial frequencies of metal gratings presented simultaneously to both hands. Tactile temporal cues were eliminated by preventing lateral hand movements; tactile spatial cues were eliminated by using gloves with an attached rubber pin. Analysis revealed separation of the subjects into two groups: "spatiotemporal" (ST) and "latent-temporal" (LT). Under normal conditions, the performance of ST subjects was significantly better than that of the LT subjects. Prevention of lateral movements impaired performance of both ST and LT subjects. However, when only temporal cues were available, the performance of ST subjects was significantly impaired, whereas that of the LT subjects either improved or did not change. Under the latter condition, LT subjects changed strategy to scanning with alternating hands, at velocities similar to the velocities normally used by ST subjects. These velocities generated temporal frequencies between 15 and 30 Hz. The LT subjects were unaware of their improved performance. Nine of ten LT subjects significantly improved their performance under normal conditions when trained to scan gratings using alternating hands and velocities similar to those used by ST subjects. We conclude that (1) temporal cues are essential for spatial-frequency discrimination, (2) human subjects vary in the tactile strategies they use for texture exploration, and (3) poor tactile performers can significantly improve by using strategies that emphasize temporal cues.

This chapter discusses the temporal and spatial coding in the rat vibrissal system. Spatial encoding of the world is accomplished by arrays of receptors, which are spatially organized across the sensory organs. Each receptor is sensitive to a specific and limited range within the sensation spectrum. Temporal encoding is attained by the temporal pattern of receptor firing, which is not limited to the dynamic aspects of the stimulus, and can also be used for encoding stationary aspects. Primates explore the texture of objects by moving their fingers across it, and rodents scan their environment by moving their whiskers. Tactile acquisition by the whiskers is a motor sensory active process. Whiskers are moved by the motor system to acquire information, which is then analyzed by the sensory system. Changes in pressure, caused by the presence of an object, or by the ridges and grooves across its surface, are detected by the mechanoreceptors. The rat vibrissal system provides a clear example of dissociated coding schemes.

Part of the information obtained by rodent whiskers is carried by the frequency of their movement. In the thalamus of anesthetized rats, the whisker frequency is represented by two different coding schemes: by amplitude and spike count (i.e., response amplitudes and spike counts decrease as a function of frequency) in the lemniscal thalamus and by latency and spike count (latencies increase and spike counts decrease as a function of frequency) in the paralemniscal thalamus (see accompanying paper). Here we investigated neuronal representations of the whisker frequency in the primary somatosensory ("barrel") cortex of the anesthetized rat, which receives its input from both the lemniscal and paralemniscal thalamic nuclei. Single and multi-units were recorded from layers 2/3, 4 (barrels only), 5a, and 5b during vibrissal stimulation. Typically, the input frequency was represented by amplitude and spike count in the barrels of layer 4 and in layer 5b (the "lemniscal layers") and by latency and spike count in layer 5a (the "paralemniscal layer"). Neurons of layer 2/3 displayed a mixture of the two coding schemes. When the pulse width of the stimulus was reduced from 50 to 20 ms, the latency coding in layers 5a and 2/3 was dramatically reduced, while the spike-count coding was not affected; in contrast, in layers 4 and 5b, the latencies remained constant, but the spike counts were reduced with 20-ms stimuli. The same effects were found in the paralemniscal and lemniscal thalamic nuclei, respectively (see accompanying paper). These results are consistent with the idea that thalamocortical loops of different pathways, although terminating within the same cortical columns, perform different computations in parallel. Furthermore, the mixture of coding schemes in layer 2/3 might reflect an integration of lemniscal and paralemniscal outputs.

The arrangements of vibrissae in guinea pigs and golden hamsters were previously reported to be different from those in mice and rats. Whereas the mystacial pads in mice and rats include four straddlers and five rows of vibrissae, guinea pigs were described to possess six rows of irregularly aligned mystacial vibrissae and no straddlers, and golden hamsters to include seven vibrissal rows and also no straddlers. We found that all of these four species possess similar vibrissal arrangements within the mystacial pad. To demonstrate this similarity, we developed a new method of sinus hair visualization in flattened and cleared preparations of the mystacial pad. Intrinsic muscles of the mystacial pad that were revealed in thick histological preparations showed clearly the structural and functional relationships between straddlers and vibrissal rows. To verify this finding, and to extend the knowledge of vibrissal cortical representations in guinea pigs and golden hamsters, we have investigated the spatial organization and the functional vibrissal representations of barrels in the posteromedial barre1 subfield (PMBSF) of these rodents. The barrel morphology was clearly preserved in Nissl-stained sections and sections processed for cytochrome oxidase of flattened cerebral cortices. We demonstrate that the vibrissal arrangement in the mystacial pad is replicated in the PMBSF of guinea pigs and golden hamsters and that this arrangement is similar to that found in mice and rats. To facilitate comparative studies, these findings strongly recommend the use, in guinea pigs and golden hamsters, of the same classifications and nomenclatures that are used in mice and rats to describe mystacial vibrissae and cortical barrels.