Specialized subregions in the cat motor cortex: anatomical demonstration of differential projections to rostral and caudal sectors

Signals from posterior parietal area 5 to motor cortex during locomotion

Area 5 of the parietal cortex is part of the "dorsal stream" cortical pathway which processes visual information for action. The signals that area 5 ultimately conveys to motor cortex, the main area providing output to the spinal cord, are unknown. We analyzed area 5 neuronal activity during vision-independent locomotion on a flat surface and vision-dependent locomotion on a horizontal ladder in cats focusing on corticocortical neurons (CCs) projecting to motor cortex from the upper and deeper cortical layers and compared it to that of neighboring unidentified neurons (noIDs). We found that upon transition from vision-independent to vision-dependent locomotion, the low discharge of CCs in layer V doubled and the proportion of cells with 2 bursts per stride tended to increase. In layer V, the group of 2-bursters developed 2 activity peaks that coincided with peaks of gaze shifts along the surface away from the animal, described previously. One-bursters and either subpopulation in supragranular layers did not transmit any clear unified stride-related signal to the motor cortex. Most CC group activities did not mirror those of their noID counterparts. CCs with receptive fields on the shoulder, elbow, or wrist/paw discharged in opposite phases with the respective groups of pyramidal tract neurons of motor cortex, the cortico-spinal cells.

Chronic recordings from the marmoset motor cortex reveals modulation of neural firing and local field potentials overlap with macaques

Objective.The common marmoset has been increasingly used in neural interfacing studies due to its smaller size, easier handling, and faster breeding compared to Old World non-human primate (NHP) species. While assessment of cortical anatomy in marmosets has shown strikingly similar layout to macaques, comprehensive assessment of electrophysiological properties underlying forelimb reaching movements in this bridge species does not exist. The objective of this study is to characterize electrophysiological properties of signals recorded from the marmoset primary motor cortex (M1) during a reach task and compare with larger NHP models such that this smaller NHP model can be used in behavioral neural interfacing studies.Approach and main results.Neuronal firing rates and local field potentials (LFPs) were chronically recorded from M1 in three adult, male marmosets. Firing rates, mu + beta and high gamma frequency bands of LFPs were evaluated for modulation with respect to movement. Firing rate and regularity of neurons of the marmoset M1 were similar to that reported in macaques with a subset of neurons showing selectivity to movement direction. Movement phases (rest vs move) was classified from both neural spiking and LFPs. Microelectrode arrays provide the ability to sample small regions of the motor cortex to drive brain-machine interfaces (BMIs). The results demonstrate that marmosets are a robust bridge species for behavioral neuroscience studies with motor cortical electrophysiological signals recorded from microelectrode arrays that are similar to Old World NHPs.Significance. As marmosets represent an interesting step between rodent and macaque models, successful demonstration that neuron modulation in marmoset motor cortex is analogous to reports in macaques illustrates the utility of marmosets as a viable species for BMI studies.

Microstimulation of the Premotor Cortex of the Cat Produces Phase-Dependent Changes in Locomotor Activity

To determine the functional organization of premotor areas in the cat pericruciate cortex we applied intracortical microstimulation (ICMS) within multiple cytoarchitectonically identified subregions of areas 4 and 6 in the awake cat, both at rest and during treadmill walking. ICMS in most premotor areas evoked clear twitch responses in the limbs and/or head at rest. During locomotion, these same areas produced phase-dependent modifications of muscle activity. ICMS in the primary motor cortex (area 4γ) produced large phase-dependent responses, mostly restricted to the contralateral forelimb or hindlimb. Stimulation in premotor areas also produced phase-dependent responses that, in some cases, were as large as those evoked from area 4γ. However, responses from premotor areas had more widespread effects on multiple limbs, including the ipsilateral limbs, than did stimulation in 4γ. During locomotion, responses in both forelimb and hindlimb muscles were evoked from cytoarchitectonic areas 4γ, 4δ, 6aα, and 6aγ. However, the prevalence of effects in a given limb varied from one area to another. The results suggest that premotor areas may contribute to the production, modification, and coordination of activity in the limbs during locomotion and may be particularly pertinent during modifications of gait.

Induction of BDNF Expression in Layer II/III and Layer V Neurons of the Motor Cortex Is Essential for Motor Learning

Motor learning depends on synaptic plasticity between corticostriatal projections and striatal medium spiny neurons. Retrograde tracing from the dorsolateral striatum reveals that both layer II/III and V neurons in the motor cortex express BDNF as a potential regulator of plasticity in corticostriatal projections in male and female mice. The number of these BDNF-expressing cortical neurons and levels of BDNF protein are highest in juvenile mice when adult motor patterns are shaped, while BDNF levels in the adult are low. When mice are trained by physical exercise in the adult, BDNF expression in motor cortex is reinduced, especially in layer II/III projection neurons. Reduced expression of cortical BDNF in 3-month-old mice results in impaired motor learning while space memory is preserved. These findings suggest that activity regulates BDNF expression differentially in layers II/III and V striatal afferents from motor cortex and that cortical BDNF is essential for motor learning.

SIGNIFICANCE STATEMENT Motor learning in mice depends on corticostriatal BDNF supply, and regulation of BDNF expression during motor learning is highest in corticostriatal projection neurons in cortical layer II/III.

Neuromuscular electrical stimulation-promoted plasticity of the human brain

The application of neuromuscular electrical stimulation (NMES) to paretic limbs has demonstrated utility for motor rehabilitation following brain injury. When NMES is delivered to a mixed peripheral nerve, typically both efferent and afferent fibres are recruited. Muscle contractions brought about by the excitation of motor neurons are often used to compensate for disability by assisting actions such as the formation of hand aperture, or by preventing others including foot drop. In this context, exogenous stimulation provides a direct substitute for endogenous neural drive. The goal of the present narrative review is to describe the means through which NMES may also promote sustained adaptations within central motor pathways, leading ultimately to increases in (intrinsic) functional capacity. There is an obvious practical motivation, in that detailed knowledge concerning the mechanisms of adaptation has the potential to inform neurorehabilitation practice. In addition, responses to NMES provide a means of studying CNS plasticity at a systems level in humans. We summarize the fundamental aspects of NMES, focusing on the forms that are employed most commonly in clinical and experimental practice. Specific attention is devoted to adjuvant techniques that further promote adaptive responses to NMES thereby offering the prospect of increased therapeutic potential. The emergent theme is that an association with centrally initiated neural activity, whether this is generated in the context of NMES triggered by efferent drive or via indirect methods such as mental imagery, may in some circumstances promote the physiological changes that can be induced through peripheral electrical stimulation.

Memory-Guided Stumbling Correction in the Hindlimb of Quadrupeds Relies on Parietal Area 5

In complex environments, tripping over an unexpected obstacle evokes the stumbling corrective reaction, eliciting rapid limb hyperflexion to lift the leg over the obstruction. While stumbling correction has been characterized within a single limb in the cat, this response must extend to both forelegs and hindlegs for successful avoidance in naturalistic settings. Furthermore, the ability to remember an obstacle over which the forelegs have tripped is necessary for hindleg clearance if locomotion is delayed. Therefore, memory-guided stumbling correction was studied in walking cats after the forelegs tripped over an unexpected obstacle. Tactile input to only one foreleg was often sufficient in modulating stepping of all four legs when locomotion was continuous, or when hindleg clearance was delayed. When obstacle height was varied, animals appropriately scaled step height to obstacle height. As tactile input without foreleg clearance was insufficient in reliably modulating stepping, efference, or proprioceptive information about modulated foreleg stepping may be important for producing a robust, long-lasting memory. Finally, cooling-induced deactivation of parietal area 5 altered hindleg stepping in a manner indicating that animals no longer recalled the obstacle over which they had tripped. Altogether, these results demonstrate the integral role area 5 plays in memory-guided stumbling correction.

Stable Delay Period Representations in the Posterior Parietal Cortex Facilitate Working-Memory-Guided Obstacle Negotiation

In complex environments, information about surrounding obstacles is stored in working memory (WM) and used to coordinate appropriate movements for avoidance. In quadrupeds, this WM system is particularly important for guiding hindleg stepping, as an animal can no longer see the obstacle underneath the body following foreleg clearance. Such obstacle WM involves the posterior parietal cortex (PPC), as deactivation of area 5 incurs WM deficits, precluding successful avoidance. However, the neural underpinnings of this involvement remain undefined. To reveal the neural substrates of this behavior, microelectrode arrays were implanted to record neuronal activity in area 5 during an obstacle WM task in cats. Early in the WM delay, neurons were modulated generally by obstacle presence or more specifically in relation to foreleg step height. Thus, information about the obstacle or about foreleg clearance can be retained in WM. In a separate set of neurons, this information was recalled later in the delay in order to plan subsequent hindleg stepping. Such early and late delay period signals were temporally bridged by neurons exhibiting obstacle-modulated activity sustained throughout the delay. These neurons represented a specialized subset of all recorded neurons, which maintained stable information coding across the WM delay. Ultimately, these various patterns of task-related modulation enable stable representations of obstacle-related information within the PPC to support successful WM-guided obstacle negotiation in the cat.

Enhancing Generalization of Visuomotor Adaptation by Inducing Use-dependent Learning

Learning a motor task in one condition typically generalizes to another, although it is unclear why it generalizes substantially in certain situations, but only partially in other situations (e.g., across movement directions and motor effectors). Here, we demonstrate that generalization of motor learning across directions and effectors can be enhanced substantially by inducing use-dependent learning, that is, by having subjects experience motor actions associated with a desired trajectory repeatedly during reaching movements. In Experiments 1 and 2, healthy human adults adapted to a visuomotor rotation while concurrently experiencing repetitive passive movements guided by a robot. This manipulation increased the extent of generalization across movement directions (Expt. 1) and across the arms (Expt. 2) by up to 50% and 42%, respectively, indicating crucial contribution of use-dependent learning to motor generalization. In Experiment 3, we applied repetitive transcranial magnetic stimulation (rTMS) to the left primary motor cortex (M1) of the human subjects prior to passive training with the right arm to increase cortical excitability. This intervention resulted in increased motor-evoked potentials (MEPs) and decreased short-interval intracortical inhibition (SICI) in the rTMS group, but not in the sham group. These changes observed in the rTMS group were accompanied by enhanced generalization of visuomotor adaptation across the arms, which was not the case in the sham group. Collectively, these findings confirm the involvement of M1 in use-dependent learning, and suggest that use-dependent learning can contribute not only to motor learning, but also to motor generalization.

Cortical contributions to sensory gating in the ipsilateral somatosensory cortex during voluntary activity

KEY POINTS

It has long been known that the somatosensory cortex gates sensory inputs from the contralateral side of the body. Here, we examined the contribution of the ipsilateral somatosensory cortex (iS1) to sensory gating during index finger voluntary activity. The amplitude of the P25/N33, but not other somatosensory evoked potential (SSEP) components, was reduced during voluntary activity compared with rest. Interhemispheric inhibition between S1s and intracortical inhibition in the S1 modulated the amplitude of the P25/N33. Note that changes in interhemispheric inhibition between S1s correlated with changes in cortical circuits in the ipsilateral motor cortex. Our findings suggest that cortical circuits, probably from somatosensory and motor cortex, contribute to sensory gating in the iS1 during voluntary activity in humans.

ABSTRACT

An important principle in the organization of the somatosensory cortex is that it processes afferent information from the contralateral side of the body. The role of the ipsilateral somatosensory cortex (iS1) in sensory gating in humans remains largely unknown. Using electroencephalographic (EEG) recordings over the iS1 and electrical stimulation of the ulnar nerve at the wrist, we examined somatosensory evoked potentials (SSEPs; P14/N20, N20/P25 and P25/N33 components) and paired-pulse SSEPs between S1s (interhemispheric inhibition) and within (intracortical inhibition) the iS1 at rest and during tonic index finger voluntary activity. We found that the amplitude of the P25/N33, but not other SSEP components, was reduced during voluntary activity compared with rest. Interhemispheric inhibition increased the amplitude of the P25/N33 and intracortical inhibition reduced the amplitude of the P25/N33, suggesting a cortical origin for this effect. The P25/N33 receives inputs from the motor cortex, so we also examined the contribution of distinct sets of cortical interneurons by testing the effect of ulnar nerve stimulation on motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the ipsilateral motor cortex with the coil in the posterior-anterior (PA) and anterior-posterior (AP) orientation. Afferent input attenuated PA, but not AP, MEPs during voluntary activity compared with rest. Notably, changes in interhemispheric inhibition correlated with changes in PA MEPs. Our novel findings suggest that interhemispheric projections between S1s and intracortical circuits, probably from somatosensory and motor cortex, contribute to sensory gating in the iS1 during voluntary activity in humans.

Afferent input and sensory function after human spinal cord injury

Spinal cord injury (SCI) often disrupts the integrity of afferent (sensory) axons projecting through the spinal cord dorsal columns to the brain. Examinations of ascending sensory tracts, therefore, are critical for monitoring the extent of SCI and recovery processes. In this review, we discuss the most common electrophysiological techniques used to assess transmission of afferent inputs to the primary motor cortex (i.e., afferent input-induced facilitation and inhibition) and the somatosensory cortex [i.e., somatosensory evoked potentials (SSEPs), dermatomal SSEPs, and electrical perceptual thresholds] following human SCI. We discuss how afferent input modulates corticospinal excitability by involving cortical and spinal mechanisms depending on the timing of the effects, which need to be considered separately for upper and lower limb muscles. We argue that the time of arrival of afferent input onto the sensory and motor cortex is critical to consider in plasticity-induced protocols in humans with SCI. We also discuss how current sensory exams have been used to detect differences between control and SCI participants but might be less optimal to characterize the level and severity of injury. There is a need to conduct some of these electrophysiological examinations during functionally relevant behaviors to understand the contribution of impaired afferent inputs to the control, or lack of control, of movement. Thus the effects of transmission of afferent inputs to the brain need to be considered on multiple functions following human SCI.

Transcranial Direct Current Stimulation Targeting Primary Motor Versus Dorsolateral Prefrontal Cortices: Proof-of-Concept Study Investigating Functional Connectivity of Thalamocortical Networks Specific to Sensory-Affective Information Processing

The pain matrix is comprised of an extensive network of brain structures involved in sensory and/or affective information processing. The thalamus is a key structure constituting the pain matrix. The thalamus serves as a relay center receiving information from multiple ascending pathways and relating information to and from multiple cortical areas. However, it is unknown how thalamocortical networks specific to sensory-affective information processing are functionally integrated. Here, in a proof-of-concept study in healthy humans, we aimed to understand this connectivity using transcranial direct current stimulation (tDCS) targeting primary motor (M1) or dorsolateral prefrontal cortices (DLPFC). We compared changes in functional connectivity (FC) with DLPFC tDCS to changes in FC with M1 tDCS. FC changes were also compared to further investigate its relation with individual's baseline experience of pain. We hypothesized that resting-state FC would change based on tDCS location and would represent known thalamocortical networks. Ten right-handed individuals received a single application of anodal tDCS (1 mA, 20 min) to right M1 and DLPFC in a single-blind, sham-controlled crossover study. FC changes were studied between ventroposterolateral (VPL), the sensory nucleus of thalamus, and cortical areas involved in sensory information processing and between medial dorsal (MD), the affective nucleus, and cortical areas involved in affective information processing. Individual's perception of pain at baseline was assessed using cutaneous heat pain stimuli. We found that anodal M1 tDCS and anodal DLPFC tDCS both increased FC between VPL and sensorimotor cortices, although FC effects were greater with M1 tDCS. Similarly, anodal M1 tDCS and anodal DLPFC tDCS both increased FC between MD and motor cortices, but only DLPFC tDCS modulated FC between MD and affective cortices, like DLPFC. Our findings suggest that M1 stimulation primarily modulates FC of sensory networks, whereas DLPFC stimulation modulates FC of both sensory and affective networks. Our findings when replicated in a larger group of individuals could provide useful evidence that may inform future studies on pain to differentiate between effects of M1 and DLPFC stimulation. Notably, our finding that individuals with high baseline pain thresholds experience greater FC changes with DLPFC tDCS implies the role of DLPFC in pain modulation, particularly pain tolerance.

REVIEW ■ : Mechanisms of Cortical Control of Hand Function

The direct cortico-motoneuronal (CM) projection from motor cortex to spinal motoneurons is a distinctive primate feature, and appears to be of particular importance for the control of fine hand and finger move ments. This review considers recent neurophysiological evidence showing that the degree of dexterity in different nonhuman primates is reflected in the strength of the CM connections to hand motoneurons. It also shows that phylogenetically older, indirect pathways may have been superseded by the direct CM system. The unique contribution of the CM system to hand function may reside in the focused and powerful facilitation it exerts on selective groups of muscles, as shown by recordings from identified CM neurons during performance of precision grip by conscious, trained monkeys. In man, damage by stroke to the corticospinal system often results in weakness and poverty of hand movement. New evidence from the use of transcranial magnetic stimulation (TMS) in stroke patients demonstrates that any recovery in the voluntary activation of hand muscles is always accompanied by recovery of short-latency responses to TMS, which are probably mediated by the fast corticospinal system, suggesting that the integrity of this system is es sential for recovery of hand function. NEUROSCIENTIST 3:389-398, 1997

Modulation of human corticospinal excitability by paired associative stimulation

Paired Associative Stimulation (PAS) has come to prominence as a potential therapeutic intervention for the treatment of brain injury/disease, and as an experimental method with which to investigate Hebbian principles of neural plasticity in humans. Prototypically, a single electrical stimulus is directed to a peripheral nerve in advance of transcranial magnetic stimulation (TMS) delivered to the contralateral primary motor cortex (M1). Repeated pairing of the stimuli (i.e., association) over an extended period may increase or decrease the excitability of corticospinal projections from M1, in manner that depends on the interstimulus interval (ISI). It has been suggested that these effects represent a form of associative long-term potentiation (LTP) and depression (LTD) that bears resemblance to spike-timing dependent plasticity (STDP) as it has been elaborated in animal models. With a large body of empirical evidence having emerged since the cardinal features of PAS were first described, and in light of the variations from the original protocols that have been implemented, it is opportune to consider whether the phenomenology of PAS remains consistent with the characteristic features that were initially disclosed. This assessment necessarily has bearing upon interpretation of the effects of PAS in relation to the specific cellular pathways that are putatively engaged, including those that adhere to the rules of STDP. The balance of evidence suggests that the mechanisms that contribute to the LTP- and LTD-type responses to PAS differ depending on the precise nature of the induction protocol that is used. In addition to emphasizing the requirement for additional explanatory models, in the present analysis we highlight the key features of the PAS phenomenology that require interpretation.

Signals from the ventrolateral thalamus to the motor cortex during locomotion

The activity of the motor cortex during locomotion is profoundly modulated in the rhythm of strides. The source of modulation is not known. In this study we examined the activity of one of the major sources of afferent input to the motor cortex, the ventrolateral thalamus (VL). Experiments were conducted in chronically implanted cats with an extracellular single-neuron recording technique. VL neurons projecting to the motor cortex were identified by antidromic responses. During locomotion, the activity of 92% of neurons was modulated in the rhythm of strides; 67% of cells discharged one activity burst per stride, a pattern typical for the motor cortex. The characteristics of these discharges in most VL neurons appeared to be well suited to contribute to the locomotion-related activity of the motor cortex. In addition to simple locomotion, we examined VL activity during walking on a horizontal ladder, a task that requires vision for correct foot placement. Upon transition from simple to ladder locomotion, the activity of most VL neurons exhibited the same changes that have been reported for the motor cortex, i.e., an increase in the strength of stride-related modulation and shortening of the discharge duration. Five modes of integration of simple and ladder locomotion-related information were recognized in the VL. We suggest that, in addition to contributing to the locomotion-related activity in the motor cortex during simple locomotion, the VL integrates and transmits signals needed for correct foot placement on a complex terrain to the motor cortex.

Motor cortical networks for skilled movements have dynamic properties that are related to accurate reaching

Neurons in the Primary Motor Cortex (MI) are known to form functional ensembles with one another in order to produce voluntary movement. Neural network changes during skill learning are thought to be involved in improved fluency and accuracy of motor tasks. Unforced errors during skilled tasks provide an avenue to study network connections related to motor learning. In order to investigate network activity in MI, microwires were implanted in the MI of cats trained to perform a reaching task. Spike trains from eight groups of simultaneously recorded cells (95 neurons in total) were acquired. A point process generalized linear model (GLM) was developed to assess simultaneously recorded cells for functional connectivity during reaching attempts where unforced errors or no errors were made. Whilst the same groups of neurons were often functionally connected regardless of trial success, functional connectivity between neurons was significantly different at fine time scales when the outcome of task performance changed. Furthermore, connections were shown to be significantly more robust across multiple latencies during successful trials of task performance. The results of this study indicate that reach-related neurons in MI form dynamic spiking dependencies whose temporal features are highly sensitive to unforced movement errors.

A Contribution of Area 5 of the Posterior Parietal Cortex to the Planning of Visually Guided Locomotion: Limb-Specific and Limb-Independent Effects

We tested the hypothesis that area 5 of the posterior parietal cortex (PPC) contributes to the planning of visually guided gait modifications. We recorded 121 neurons from the PPC of two cats during a task in which cats needed to process visual input to step over obstacles attached to a moving treadmill belt. During unobstructed locomotion, 64/121 (53%) of cells showed rhythmic activity. During steps over the obstacles, 102/121 (84%) of cells showed a significant change of their activity. Of these, 46/102 were unmodulated during the control task. We divided the 102 task-related cells into two groups on the basis of their discharge when the limb contralateral to the recording site was the first to pass over the obstacle. One group (41/102) was characterized by a brief, phasic discharge as the lead forelimb passed over the obstacle (Step-related cells). These cells were recorded primarily from area 5a. The other group (61/102) showed a progressive increase in activity prior to the onset of the swing phase in the modified limb and frequently diverged from control at least one step cycle before the gait modification (Step-advanced cells). Most of these cells were recorded in area 5b. In both groups, some cells maintained a fixed relationship to the activity of the contralateral forelimb regardless of which limb was the first to pass over the obstacle (limb-specific cells), whereas others changed their phase of activity so that they were always related to activity of the first limb to pass over the obstacle, either contralateral or ipsilateral (limb-independent cells). Limb-independent cells were more common among the Step-advanced cell population. We suggest that both populations of cells contribute to the gait modification and that the discharge characteristics of the Step-advanced cells are compatible with a contribution to the planning of the gait modification.

Patterns of spatio-temporal correlations in the neural activity of the cat motor cortex during trained forelimb movements

In order to study how neurons in the primary motor cortex (MI) are dynamically linked together during skilled movement, we recorded simultaneously from many cortical neurons in cats trained to perform a reaching and retrieval task using their forelimbs. Analysis of task-related spike activity in the MI of the hemisphere contralateral to the reaching forelimb (in identified forelimb or hindlimb representations) recorded through chronically implanted microwires, was followed by pairwise evaluation of temporally correlated activity in these neurons during task performance using shuffle corrected cross-correlograms. Over many months of recording, a variety of task-related modulations of neural activities were observed in individual efferent zones. Positively correlated activity (mainly narrow peaks at zero or short latencies) was seen during task performance frequently between neurons recorded within the forelimb representation of MI, rarely within the hindlimb area of MI, and never between forelimb and hindlimb areas. Correlated activity was frequently observed between neurons with different patterns of task-related activity or preferential activity during different task elements (reaching, feeding, etc.), and located in efferent zones with dissimilar representation as defined by intracortical microstimulation. The observed synchronization of action potentials among selected but functionally varied groups of MI neurons possibly reflects dynamic recruitment of network connections between efferent zones during skilled movement.

Changes in electrical thresholds for evoking movements from the cat cerebral cortex following lesions of the sensori-motor area

We evaluated motor maps in the cerebral cortex and motor performance in cats before and after lesions of the forelimb representation in the primary motor area. After the lesion there was a reduction in the use of the affected forelimb and loss of accuracy in prehension tasks using the forelimb; some recovery occurred during the mapping study. Electrode tracts and lesion sites were located in cytoarchitectonically identified cortical areas 4gamma, 4delta, 6aalpha, 6agamma, 3a. The lesions were mainly in area 4gamma. In the lesioned hemisphere there were many points around the lesion site (in areas 4gamma and 3a) from which movements could not be evoked. In some areas distant from the lesion site (e.g. area 6agamma) the mean thresholds for evoking forelimb movements were significantly elevated. Mean thresholds for evoking hindlimb and facial movements were not different from before. In the contralateral hemisphere mean thresholds for evoking forelimb, but not hindlimb or facial movements, were significantly elevated in several sensorimotor areas (area 4gamma, 6agamma and 3a). Mean thresholds for evoking forelimb movements appeared to progressively increase during the time of study. Minimal currents required to evoke forelimb movements from the cerebral cortex increase (possibly progressively) following a lesion of the forelimb representation in the primary motor area, affecting many interconnected motor areas in the hemispheres ipsilateral and contralateral to the lesioned site. This increase in thresholds may play a role in the changes in cortical control of the affected and contralateral limbs following brain lesions and explain the increased sense of effort required to produce movements.

The function of sensory information from the first somatosensory cortex for facial movements during ingestion in cats

The aim of the present study was to investigate the relationship between the facial region of the first somatosensory cortex (facial SI) and facial region of the motor cortex (facial MI), as the basis of orofacial behaviors during ingestion of fish paste. Area M in the ventral cortex of the cruciate sulcus that was defined as part of the facial MI by and, showed various facial twitches evoked by intracortical microstimulation (ICMS) and recorded many mastication-related neurons (MRNs). Many MRNs in area M had receptive fields (RFs) in lingual, perioral and mandibular regions. The 60% value of activity patterns of MRNs (n = 124) recorded in area M of normal cats, were the pre-SB type (the sustained and pre-movement type) that showed increased firing prior to the start of mastication and then tonic activity during the masticatory period. MRNs recorded in area M of cats with the facial SI lesion, showed a noticeable decrease in MRNs with RFs in the perioral and mandibular regions and with activity of the pre-SB type. These results strongly suggest that blocking facial SI sensory inputs evoked by mastication interferes with the relay of important facial sensory information to area M required for the appropriate manipulation of food during mastication.

Subdivisions of primary motor cortex based on cortico-motoneuronal cells

We used retrograde transneuronal transport of rabies virus from single muscles of rhesus monkeys to identify cortico-motoneuronal (CM) cells in the primary motor cortex (M1) that make monosynaptic connections with motoneurons innervating shoulder, elbow, and finger muscles. We found that M1 has 2 subdivisions. A rostral region lacks CM cells and represents an "old" M1 that is the standard for many mammals. The descending commands mediated by corticospinal efferents from old M1 must use the integrative mechanisms of the spinal cord to generate motoneuron activity and motor output. In contrast, a caudal region of M1 contains shoulder, elbow, and finger CM cells. This region represents a "new" M1 that is present only in some higher primates and humans. The direct access to motoneurons afforded by CM cells enables the newly recognized M1 to bypass spinal cord mechanisms and sculpt novel patterns of motor output that are essential for highly skilled movements.

The sense of touch: embodied simulation in a visuotactile mirroring mechanism for observed animate or inanimate touch

Previous studies have shown a shared neural circuitry in the somatosensory cortices for the experience of one's own body being touched and the sight of intentional touch. Using functional magnetic resonance imaging (fMRI), the present study aimed to elucidate whether the activation of a visuotactile mirroring mechanism during touch observation applies to the sight of any touch, that is, whether it is independent of the intentionality of observed touching agent. During fMRI scanning, healthy participants viewed video clips depicting a touch that was intentional or accidental, and occurring between animate or inanimate objects. Analyses showed equal overlapping activation for all the touch observation conditions and the experience of one's own body being touched in the bilateral secondary somatosensory cortex (SII), left inferior parietal lobule (IPL)/supramarginal gyrus, bilateral temporal-occipital junction, and left precentral gyrus. A significant difference between the sight of an intentional touch, compared to an accidental touch, was found in the left primary somatosensory cortex (SI/Brodmann's area [BA] 2). Interestingly, activation in SI/BA 2 significantly correlated with the degree of intentionality of the observed touch stimuli as rated by participants. Our findings show that activation of a visuotactile mirroring mechanism for touch observation might underpin an abstract notion of touch, whereas activation in SI might reflect a human tendency to "resonate" more with a present or assumed intentional touching agent.

Differential activity-dependent development of corticospinal control of movement and final limb position during visually guided locomotion

Although we understand that activity- and use-dependent processes are important in determining corticospinal axon terminal development in the spinal cord, little is known about the role of these processes in development of skilled control of limb movements. In the present study we determined the effects of unilateral motor cortex activity blockade produced by muscimol infusion during the corticospinal axon terminal refinement period, between postnatal weeks 5-7, on visually guided locomotion. We examined stepping and forepaw placement on the rungs of a horizontal ladder and gait modifications as animals stepped over obstacles during treadmill walking. When cats traversed the horizontal ladder, the limb contralateral to inactivation was placed significantly farther forward on the rungs than the ipsilateral limb, indicating defective endpoint control. Similarly, when animals stepped over obstacles on a treadmill, the contralateral limb was placed farther in front of the obstacle, but only when it was the first (i.e., leading) limb to step over the obstacle, not when it was the second (i.e., trailing) limb. This is also indicative of an endpoint control deficit. In contrast, neither during ladder walking, nor when stepping over obstacles on the treadmill, was there any consistent evidence for a major impairment in limb trajectory. These results point to distinct and possibility independent corticospinal mechanisms for movement trajectory control and endpoint control. Although corticospinal activity during early postnatal development is needed to refine circuits for accurate endpoint control, this activity-dependent refinement is not needed for movement trajectory control.

Lesions of Area 5 of the Posterior Parietal Cortex in the Cat Produce Errors in the Accuracy of Paw Placement During Visually Guided Locomotion

We developed a novel locomotor task in which cats step over obstacles that move at a different speed from that of the treadmill on which the cat is walking: we refer to this as a visual dissociation locomotion task. Slowing the speed of the obstacle with respect to that of the treadmill sometimes led to a major change in strategy so that cats made two steps with the hindlimbs before stepping over the obstacle (double step strategy) instead of the single step (standard strategy) observed when the obstacle was at the same speed as the treadmill. In addition, in the step preceding the step over the obstacle, the paws were placed significantly closer to the obstacle in the visual dissociation task than when the treadmill and the obstacle were at the same speed. After unilateral lesion of area 5 of the posterior parietal cortex (PPC), the cats frequently hit the obstacle as they stepped over it, especially in the visual dissociation task. This locomotor deficit was linked to significant differences in the location in which the forelimbs were placed in the step preceding that over the obstacle compared with the prelesion control. Cats also frequently hit the obstacle with their hindlimbs even when the forelimbs negotiated the obstacle successfully; this suggests an important role for the posterior parietal cortex in the coordination of the forelimbs and hindlimbs. Together, these results suggest an important contribution of the PPC to the planning of visually guided gait modifications.

Organization of the projections from the posterior parietal cortex to the rostral and caudal regions of the motor cortex of the cat

The posterior parietal cortex (PPC) is an important source of input to the motor cortex in both the primate and the cat. However, the available evidence from the cat suggests that the projection from the PPC to those rostral areas of the motor cortex that project to the intermediate and ventral parts of the spinal gray matter is relatively small. This leaves in question the importance of the contribution of the PPC to the initiation and modulation of voluntary movements in the cat. As this anatomical evidence is not entirely compatible with the physiological data, we reinvestigated the PPC projection to the motor cortex by injecting dextran amine tracers either into the proximal or distal representations of the forelimb in the rostral motor cortex, into the representation of the forelimb in the caudal motor cortex, or into the hindlimb representation. The results show strong projections from the PPC to each of these regions. However, projections to the rostral motor cortex were observed primarily from the caudal bank of the ansate sulcus and the adjacent gyrus, whereas those to the caudal motor cortex were generally located more rostrally. There was also evidence of some topographic organization with the distal limb being located progressively more laterally and rostrally in the PPC than the areas projecting to more proximal regions. In contrast to previous anatomical investigations, these results suggest that the PPC can potentially modulate motor activity via its strong projection to the more rostral regions of the motor cortex.

Transient suppression of ipsilateral primary somatosensory cortex during tactile finger stimulation

The whole human primary somatosensory (SI) cortex is activated by contralateral tactile stimuli, whereas its subarea 2 displays neuronal responses also to ipsilateral stimuli. Here we report on a transient deactivation of area 3b of the ipsilateral SI during long-lasting tactile stimulation. We collected functional magnetic resonance imaging data with a 3 T scanner from 10 healthy adult subjects while tactile pulses were delivered at 1, 4, or 10 Hz in 25 s blocks to three right-hand fingers. In the contralateral SI cortex, activation [positive blood oxygenation level-dependent (BOLD) response] outlasted the stimulus blocks by 20 s, with an average duration of 45 s. In contrast, a transient deactivation (negative BOLD response) occurred in the ipsilateral rolandic cortex with an average duration of 18 s. Additional recordings on 10 subjects confirmed that the deactivation was not limited to the right SI but occurred in the SI cortex ipsilateral to the stimulated hand. Moreover, the primary motor cortex (MI) contained voxels that were phasically deactivated in response to both ipsilateral and contralateral touch. These data indicate that unilateral touch of fingers is associated, in addition to the well known activation of the contralateral SI cortex, with deactivation of the ipsilateral SI cortex and of the MI cortex of both hemispheres. The ipsilateral SI deactivation could result from transcallosal inhibition, whereas intracortical SI-MI connections could be responsible for the MI deactivation. The shorter time course of deactivation than activation would agree with stronger decay of inhibitory than EPSP at the applied stimulus repetition rates.

Quantitative characteristics of the associative projections of field 4y to subfields of the sensorimotor and parietal cortex of the cat

The Nauta-Gygax method was used to study the ipsilateral associative connections of motor cortex field 4y after local electrolytic lesioning of this zone. The relative quantitative distribution of associative fibers running from field 4y to somatosensory areas I and II, the motor cortex, and the parietal cortex was determined. The greatest projections of field 4y were found to be directed to field 2pri (the secondary somatosensory zone) and field 5ab. Occasional degenerative fibers passed to fields 1, 2, 3a, and 3b of the primary somatosensory zone of the cortex. Efferent fibers from field 4y were not directed to fields 4fu, 4sfu, 6aa, 6ab, or 6ifu. It is suggested that the morphological basis of motor reactions mediated by field 4y is not provided by the fundal (4fu, 4sfu, 6ifu) or premotor (6aa, 6ab) fields but by field 2pri and 5ab, with which it has more extensive connections.

Dissociation of sensorimotor deficits after rostral versus caudal lesions in the primary motor cortex hand representation

Primary motor cortex (M1) has traditionally been considered a motor structure. Although neurophysiologic studies have demonstrated that M1 is also influenced by somatosensory inputs (cutaneous and proprioceptive), the behavioral significance of these inputs has yet to be fully defined in primates. The present study describes differential sensory-related deficits after small ischemic lesions in either the rostral or caudal subregion of the M1 hand area in a nonhuman primate. Squirrel monkeys retrieved food pellets out of different sized wells drilled into a Plexiglas board. Before the lesion, monkeys retrieved pellets by directing the hand to the well, inserting fingers directly into it, and extracting the pellet. After a lesion to the rostral portion of M1, monkeys frequently failed to direct the hand accurately to the well. Instead, fingers contacted the surface of the board outside the well before entering the well. These aiming errors are consistent with both the large amount of proximal motor outputs and the predominant proprioceptive inputs of rostral M1. Overall, these aiming errors are suggestive of dysfunctional processing of proprioceptive information or the failure to integrate proprioceptive information with motor commands. In contrast, after a lesion to the caudal portion of M1, monkeys frequently examined their palm visually for the presence of the pellet after an attempted retrieval. These errors are consistent with both the large amount of distal motor outputs and the predominant cutaneous inputs of caudal M1. Thus these errors are suggestive of a deficit in processing of cutaneous information or the failure to integrate cutaneous information with motor commands. Rostral and caudal M1 lesions result in different deficits in sensory-dependent motor control that appear to correlate with broad segregation of motor outputs and previously described sensory inputs of M1.

Comparison of electrical thresholds for evoking movements from sensori-motor areas of the cat cerebral cortex and its relation to motor training

Motor maps and electrical thresholds for evoking movements from motor areas of the cerebral cortex were evaluated in normal cats by using intracortical microstimulation techniques. Stainless steel chambers were implanted over craniotomies in adult cats trained to perform reaching and retrieval movements with their forelimbs. Prehensile motor training was continued and movement performance monitored for about 6-10 weeks during which the cortex was progressively explored with sharp tungsten electrodes inserted into cortical gyri (anterior and posterior sigmoid, and coronal) and the banks of sulci (cruciate, presylvian and coronal). Twice weekly, under light general anaesthesia, 3-4 tracks were made in either hemisphere till about 50 tracks were made in each hemisphere. Mean thresholds for evoking forelimb movements from different cytoarchitectonic areas (4gamma, 4delta, 6agamma and 3a) were compared and no consistent or significant differences were observed between the different areas. In the animals (4/6) which used either forelimb to perform the tasks, there were no consistent differences in the mean thresholds for evoking forelimb movements from the two hemispheres. However, in 2 animals, which used their right forelimbs predominantly or exclusively to perform all the tasks, mean thresholds for evoking forelimb movements was significantly higher in areas 4gamma and 6agamma of the left hemisphere (compared to the right); no consistent differences in the mean thresholds for evoking hindlimb or facial movements were observed between the two hemispheres. These findings suggest that ICMS thresholds for evoking forelimb movements may be similar in different sensorimotor areas of the cat cerebral cortex, and these thresholds could be influenced by motor training.

Integration of motor and visual information in the parietal area 5 during locomotion

The parietal cortex receives both visual- and motor-related information and is believed to be one of the sites of visuo-motor coordination. This study for the first time characterizes integration of visual and motor information in activity of neurons of parietal area 5 during locomotion under conditions that require visuo-motor coordination. The activity of neurons was recorded in cats during walking on a flat surface-a task with no visuo-motor coordination required (flat locomotion), walking along a horizontal ladder or a series of barriers-a task requiring visuo-motor coordination for an accurate foot placement on surface that is heterogeneous along the direction of progression (ladder and barriers locomotion), and walking along a narrow pathway-a task requiring visuo-motor coordination on surface homogeneous along the direction of progression (narrow locomotion). During flat locomotion, activity of 66% of the neurons was modulated in rhythm of stepping, usually with one peak per cycle. During ladder and barrier locomotion, the proportion of rhythmically active neurons significantly increased, their modulation became stronger, and the majority of neurons had two peaks of activity per cycle. During narrow locomotion, however, the activity of neurons was similar to that during flat locomotion. We concluded that, during locomotion, parietal area 5 integrates two types of information: signals about the activity of basic locomotion mechanisms and signals about heterogeneity of the surface along the direction of progression. We describe here the modes of integration of these two types of information during locomotion.

Origin of corticospinal neurones evoking monosynaptic excitation in C3--C4 propriospinal neurones in the cat

Intracellular recording was made from propriospinal neurones (PNs) in the C3-C4 spinal cord segments in the cat (alpha-chloralose anaesthesia). The effect of electrical stimulation of corticospinal neurones (CSNs) in the cortex was investigated. Short C3-C4 PNs were identified by antidromic activation of their axons in the ventral horn in C6/C7 and in the lateral reticular nucleus. Long PNs were antidromically identified from Th12-13. In short PNs, monosynaptic excitory postsynoptic potentials (EPSPs) were elicited from the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6. Two subtypes of short PNs were identified. PNs of type I received monosynaptic EPSPs from the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma, which is from the same region as disynaptic cortical EPSPs were evoked in forelimb motoneurones. PNs of type II received monosynaptic EPSPs from regions slightly more rostrally in the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6, which is outside the region from which disynaptic EPSPs could be evoked in forelimb motoneurones. Long PNs received monosynaptic EPSPs, like the short PNs, by stimulation in the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6. In contrast, the long PNs also received monosynaptic EPSPs from area 3b near the border of area 1. The present results show segregation of the cortical control to functionally different premotoneuronal systems and suggest that this control could in part be separated for subtypes of short C3-C4 PNs.

Topographical organization of projections to cat motor cortex from nucleus interpositus anterior and forelimb skin

1. The activation of the motor cortex from focal electrical stimulation of sites in the forelimb area of cerebellar nucleus interpositus anterior (NIA) was investigated in barbiturate-anaesthetized cats. Using a microelectrode, nuclear sites were identified by the cutaneous climbing fibre receptive fields of their afferent Purkinje cells. These cutaneous receptive fields can be identified by positive field potentials reflecting inhibition from Purkinje cells activated on natural stimulation of the skin. Thereafter, the sites were microstimulated and the evoked responses were systematically recorded over the cortical surface with a ball-tipped electrode. The topographical organization in the motor cortex of responses evoked by electrical stimulation of the forelimb skin was also analysed. 2. Generally, sites in the forelimb area of NIA projected to the lateral part of the anterior sigmoid gyrus (ASG). Sites in the hindlimb area of NIA also projected to lateral ASG and in addition to a more medial region. Sites in the face area of NIA, however, projected mainly to the middle part of the posterior sigmoid gyrus (PSG). 3. For sites in the forelimb area of NIA, the topographical organization and strength of the projections varied specifically with the cutaneous climbing fibre receptive field of the site. The largest cortical responses were evoked from sites with receptive fields on the distal or ventral skin of the forelimb. 4. Microelectrode recordings in the depth of the motor cortex revealed that responses evoked by cerebellar nuclear stimulation were due to an excitatory process in layer III. 5. Short latency surface responses evoked from the forelimb skin were found in the caudolateral part of the motor cortex. At gradually longer latencies, responses appeared in sequentially more rostromedial parts of the motor cortex. Since the responses displayed several temporal peaks that appeared in specific cortical regions for different areas of the forelimb skin, several somatotopic maps were seen. 6. The cerebellar and cutaneous projections activated mainly different cortical regions and had topographical organizations that apparently were constant between animals. Their patterns of activation may constitute a frame of reference for investigations of the functional organization of the motor cortex.

Effects of electrical stimulation of the caudate nucleus on functionally identified neurons of the sensorimotor cortex in the cat brain

Paired stimulation was used to study the effects of the caudate nucleus on the specific and nonspecific responses of projection neurons of the sensorimotor cortex in the cat brain. Caudal influences on the neurons being studied had insignificant effects on specific peripheral evoked responses. Nonspecific peripherally evoked activity was in most cases inhibited by caudate spike activity, and the pattern of evoked activity underwent significant modulation in conditions of a constant type of peripherally evoked response. It is suggested that the caudate nucleus acts as a filter of proprioceptive information in the cortex or in pathways to the cortex: specific corticopetal information is passed unchanged, while nonspecific signals are predominantly inhibited or significantly modulated.

Comparison of the cortical connections of areas 4 gamma and 4 delta in the cat cerebral cortex

Area 4 of the cat cerebral cortex has been subdivided into several regions based on cytoarchitectonic studies: areas 4 gamma, 4 delta, 4sfu, and 4fu (Hassler and Muhs-Clement [1964] J. Hirnforsch. 6:377-420). In a previous study, we found separate representations of contralateral limb movements in areas 4 gamma and 4 delta (Ghosh [1997] J. Comp. Neurol. 380:191-214). To investigate the relationship between these representations, the ipsilateral cortical connections of area 4 gamma and 4 delta were compared by the use of the retrograde neural tracers. After intracortical microstimulation of area 4, tracer was injected into one or two of the following regions: the forelimb regions of the rostral and caudal subdivisions of areas 4 gamma and 4 delta (r4 gamma, r4 delta, c4 gamma, c4 delta, separated by the cruciate sulcus) and the hindlimb regions of c4 gamma and c4 delta. Retrogradely labeled neural profiles were counted in every fourth section of the coronal series and located in cytoarchitectonic areas of the ipsilateral cortex. We found topographically organized reciprocal connections between areas 4 gamma and 4 delta; these connections were part of a rich network of interconnections between the cytoarchitectonic subdivisions of area 4. The forelimb regions of c4 gamma and c4 delta, of r4 gamma and c4 delta, and of r4 gamma and r4 delta were interconnected. These findings support the location of a secondary motor area in 4 delta. No interconnections between the forelimb regions of r4 gamma and c4 gamma, and of r4 delta and c4 gamma, could be found. Area 6 (particularly area 6a gamma) was found to project strongly to the forelimb regions of r4 gamma and r4 delta and relatively weakly to the forelimb region of c4 delta. Retrogradely labeled neurons were also detected in areas 3a, 3b, 1, 2pri, 5, 7, and insula after tracer injections in area 4.

Cytoarchitecture of sensorimotor areas in the cat cerebral cortex

The organization of multiple motor areas in the cerebral cortex has been investigated frequently in primates but rarely in nonprimate species. To compare sensorimotor areas in cats and primates, the cytoarchitecture of frontal and parietal areas of the cat cerebral cortex was described and mapped from coronal sections stained with cresyl violet or thionine. Multiple subdivisions of areas 4 and 6 were recognized; of these, the cytoarchitecture of area 4 gamma is similar to that of area 4 described in other carnivores and in primates and is characterized by giant pyramidal cells in multiple rows or clusters in lamina V. In other subdivisions of area 4 (4 delta, 4sfu, and 4fu), giant pyramidal cells are few or absent in lamina V, and these subdivisions resemble area 6 of primates. Area 6 of the cat cortex is heterogeneous, and differences in laminar appearance and size of pyramidal cells in lamina V distinguish its four subdivisions (6a alpha, 6a beta, 6a gamma, and 6iffu). The adjoining prefrontal areas are distinguishable from area 6 by the presence of a thin internal granular lamina (lamina IV) and the reduced size of pyramidal cells in lamina V. Laminae are poorly differentiated in the cingulate areas, where a rostral and caudal subdivision can be distinguished on the basis of the absence or presence of lamina IV. Area 3a is characterized by a thin lamina IV and is located between frontal agranular and parietal granular (well-defined lamina IV) fields (3b, 1, 2, 2pri, 5, and 7). Insular cortex can be subdivided into granular and agranular fields.

Ipsilateral cortical connections of area 6 in the cat cerebral cortex

Many motor areas have been identified within cytoarchitectonic area 6 of the cerebral cortex in primates. To provide a better understanding of the motor functions of area 6 in the cat, the ipsilateral cortical connections of the different cytoarchitectonic subdivisions of area 6 (areas 6a alpha, 6a beta, 6a gamma, and 6iffu) were studied by the use of fluorescent retrograde tracers. Tracer injections, made in the forelimb and face regions of areas 6a alpha and 6a gamma, were guided by data from intracortical microstimulation (ICMS). ICMS did not evoke movements from areas 6a beta and 6iffu. Retrogradely labeled neurons were enumerated in cytoarchitectonically identified areas in the frontal and parietal lobes to show that the subdivisions of area 6 are strongly interconnected except for areas 6a gamma and 6a beta. There are considerable differences in the pattern of connections of the area 6 subdivisions with area 4 and with prefrontal, cingulate, and parietal cortices. Area 4 gamma projects strongly to area 6a gamma but not to the other subdivisions. Areas 4 delta, 4fu, and 4sfu project strongly to 6a alpha and 6iffu but relatively weakly to area 6a beta. Prefrontal areas project strongly to area 6a beta and 6iffu, moderately to 6a alpha, but weakly to area 6a gamma. Cingulate areas project strongly to area 6iffu and moderately to areas 6a alpha and 6a beta but do not project to area 6a gamma. Parietal projections from primary and second somatosensory cortex were directed to area 6a gamma, whereas areas 5, 7, and insula were found to project to all the subdivisions of area 6. These findings support earlier suggestions that secondary motor areas may be located in areas 6a alpha and 6a gamma (Ghosh [1997] J. Comp. Neurol. 380:191-214). Features of the pattern of cortical connections of area 6 common to the cat and primates suggest that their motor areas in the frontal lobe are organized in a similar manner.

Functional organization of the projection from area 2 to area 4gamma in the cat

It has been shown that tetanic stimulation of area 2 of the somatosensory cortex produces long-term potentiation in neurons in area 4gamma, and this has been suggested as the basis of learning new motor skills. The purpose of this study was to further elucidate the characteristics of this projection by the use of evoked potentials in area 4gamma elicited by intracortical microstimulation of area 2. The experiments were carried out in cats and the following results were obtained. 1) In six animals, stimulation of a given site in area 2 elicited evoked potentials in a restricted region of area 4gamma, the size of which ranged from 1 to 1.5 mm2. These responses were labeled "localized responses." The evoked potentials were recorded from the superficial layers of the cortex, and were positive monophasic in shape. 2) In 16 animals, stimulation of a given site in area 2 elicited a focal response that was surrounded by smaller positive and monophasic potentials of <50% amplitude that spread broadly over area 4gamma. These responses were labeled "graded responses." 3) The sites that produced focal evoked potentials in area 4gamma extended along the direction of the radial fibers in area 2. These sites were defined as most effective segments (MESs). 4) The receptive fields of neurons along the MES in area 2 were similar to those of neurons recorded at the foci of the evoked potentials in area 4gamma. The results demonstrate that some of the projections from area 2 to area 4gamma are highly specific and that the somatosensory and motor areas that are connected by these specific projections receive functionally related peripheral input. These results are discussed in relation to possible mechanisms underlying motor learning.

Identification of motor areas of the cat cerebral cortex based on studies of cortical stimulation and corticospinal connections

The location and topography of motor areas in the cat cerebral cortex were studied by electrical stimulation of the cortex in five animals, and by the injection of retrograde tracers into the spinal cord of four animals. Movements evoked by intracortical microstimulation (ICMS) of the anterior, posterior and lateral sigmoid gyri, both banks of the cruciate sulcus and the dorsal bank of the presylvian sulcus were observed in anaesthetized cats. Fluorescent tracers (Fast Blue and/or Diamadino Yellow) were injected into the lateral funiculus in the second cervical segment, into the gray matter of cervical segments C3-T1 and/or into the gray matter of lumbar segments L2-S1. Contraction of the contralateral forelimb, hindlimb or facial muscles was observed following electrical stimulation of several cytoarchitectonic areas: 4 gamma, 4 delta, 6a alpha, 6a gamma, and 3a. These findings suggested representations of contralateral forelimb and hindlimb movements in areas 4 gamma and 4 delta, and of the contralateral forelimb muscles in areas 6a alpha and 6a gamma. Corticospinal neurons were located in all the above cytoarchitectonic areas as well as in areas 3b, 1, 2, 2pri, and 5. Large numbers of neurons were labeled in areas 4 gamma and 4 delta, and moderate labeling was observed in areas 6a gamma and 6a alpha. Corticospinal neurons projecting to cervical and lumbar segments were located in areas 4 gamma and 4 delta, while those projecting only to cervical segments were detected in areas 6a alpha and 6a gamma. Based on these findings it is proposed that within the motor cortex of the cat there are representations of limb movements in several cytoarchitectonic subdivisions. Many of these representations may be candidate secondary motor areas.

Alterations in intralaminar and motor thalamic physiology following nigrostriatal dopamine depletion

The response of central median/central lateral (CM/CL) and ventral anterior/ventral lateral (VA/VL) thalamic neurons to tactile sensory stimulation of the face and electrical stimulation of the striatum was assessed in awake cats before and after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) exposure. When cats exhibited Parkinson-like motor deficits, there was a significant decrease in the number of CM/CL and VA/VL neurons responsive to tactile stimulation of the face. Mean spontaneous firing rates decreased by 58% in the CM/CL nuclei, 65% in the VA, and 49% in the VL. The number of thalamic neurons responding to electrical stimulation of the striatum was also significantly decreased in parkinsonian animals. Approximately 6 weeks after MPTP exposure, when cats had spontaneously recovered gross motor function, thalamic responses to peripheral sensory stimulation, electrical stimulation of the CD, and spontaneous activity rates, returned to approximately normal levels in all thalamic areas studied. These findings support the concept that abnormalities in the transmission of information through the thalamus, and in particular, a decrease in sensory responsiveness in intralaminar and motor thalamic regions subsequent to nigrostriatal dopamine depletion, may contribute to the generation of Parkinson-like motor and sensorimotor deficits.

Differential spinal projections from the forelimb areas of the rostral and caudal subregions of primary motor cortex in the cat

We used anterograde transport of WGA-HRP to examine the topography of corticospinal projections from the forelimb areas within the rostral and caudal motor cortex subregions in the cat. We compared the pattern of these projections with those from the somatic sensory cortex. The principal finding of this study was that the laminar distribution of projections to the contralateral gray matter from the two motor cortex subregions was different. The rostral motor cortex projected preferentially to laminae VI-VIII, whereas caudal motor cortex projected primarily to laminae IV-VI. Confirming earlier findings, somatic sensory cortex projected predominantly to laminae I-VI inclusive. We found that only rostral motor cortex projected to territories in the rostral cervical cord containing propriospinal neurons of cervical spinal segments C3-4 and, in the cervical enlargement, to portions presumed to contain Ia inhibitory interneurons. We generated contour maps of labeling probability on averaged segmental distributions of anterograde labeling for all analyzed sections using the same algorithm. For rostral motor cortex, heaviest label in the dorsal part of lamina VII in the contralateral cord was consistently located in separate medial and lateral zones. In contrast, no consistent differences in the mediolateral location of label was noted for caudal motor cortex. To summarize, laminae I-III received input only from the somatic sensory cortex, while laminae IV-V received input from both somatic sensory and caudal motor cortex. Lamina VI received input from all cortical fields examined. Laminae VII-IX received input selectively from the rostral motor cortex. For motor cortex, our findings suggest that projections from the two subregions comprise separate descending pathways that could play distinct functional roles in movement control and sensorimotor integration.

Cortico-cortical mediation of short-latency (lemniscal) sensory input to the motor cortex in deeply pentobarbitone anaesthetized cats

In pentobarbitone-anaesthetized cats, responses were recorded as surface positive potentials in the motor cortex on forelimb and brachium conjunctivum stimulation. In such a preparation, the forelimb nerve responses are mediated via the spino-cervical tract and the dorsal column-lemniscal pathway. Lesions of the sensory cortex (sparing only the depth of the coronary sulcus) abolished or reduced short-latency peripheral responses, in the motor cortex, on both skin and muscle nerve stimulation to less than 10% of control, while brachium conjunctivum responses were unchanged. Lesions of the second somatosensory area alone reduced the motor cortex responses on peripheral nerve stimulation by 10-20%. When the sensory cortex was inactivated by spreading depression, peripheral responses in the motor cortex were abolished before the spreading depression reached the recording point, as judged from the brachium conjunctivum response. The depth distribution of positive and negative field potentials, constituting the early components of a peripheral response in the motor cortex, closely resembled that of a cortico-cortical response evoked on stimulation in area 3. It differed from that of thalamo-cortical response evoked on brachium conjunctivum stimulation. These data suggest that most, if not all, sensory input through the dorsal column and spino-cervical tract to the motor cortex is mediated via the sensory cortex.

A comparison of the ipsilateral cortical projections to the dorsal and ventral subdivisions of the macaque premotor cortex

The cortical connections of the dorsal (PMd) and ventral (PMv) subdivisions of the premotor area (PM, lateral area 6) were studied in four monkeys (Macaca fascicularis) through the use of retrograde tracers. In two animals, tracer was injected ventral to the arcuate sulcus (PMv), in a region from which forelimb movements could be elicited by intracortical microstimulation (ICMS). Tracer injections dorsal to the arcuate sulcus (PMd) were made in two locations. In one animal, tracer was injected caudal to the genu of the arcuate sulcus (in caudal PMd [cPMd], where ICMS was effective in eliciting forelimb movements); in another animal, it was injected rostral to the genu of the arcuate sulcus (in rostral PMd [rPMd], where ICMS was ineffective in eliciting movements). Retrogradely labeled neurons were counted in the ipsilateral hemisphere and located in cytoarchitectonically identified areas of the frontal and parietal lobes. Although both PMv and PMd were found to receive inputs from other motor areas, the prefrontal cortex, and the parietal cortex, there were differences in the topography and the relative strength of projections from these areas. There were few common inputs to PMv and PMd; only the supplementary eye fields projected to all three areas studied. Interconnections within PMd or PMv appeared to link hindlimb and forelimb representations, and forelimb and face representations; however, connections between PMd and PMv were sparse. Areas cPMd and PMv were found to receive inputs from other motor areas--the primary motor area, the supplementary motor area, and the cingulate motor area--but the topography and strength of projections from these areas varied. Area rPMd was found to receive sparse inputs, if any, from these motor areas. The frontal eye field (area 8a) was found to project to PMv and rPMd, and area 46 was labeled substantially only from rPMd. Parietal projections to PMv were found to originate from a variety of somatosensory and visual areas, including the second somatosensory cortex and related areas in the parietal operculum of the lateral sulcus, as well as areas 5, 7a, and 7b, and the anterior intraparietal area. By contrast, projections to cPMd arose only from area 5. Visual areas 7m and the medial intraparietal area were labeled from rPMd. Relatively more parietal neurons were labeled after tracer injections in PMv than in PMd. Thus, PMv and PMd appear to be parts of separate, parallel networks for movement control.

Information processing within the motor cortex. II. Intracortical connections between neurons receiving somatosensory cortical input and motor output neurons of the cortex

Connections between motor cortical neurons receiving somatosensory inputs from area 2 and large pyramidal cells in layer V were examined in the cat via intracellular injection of biocytin and immunohistochemistry of nonphosphorylated neurofilament proteins (npNFP). Biocytin was injected into pyramidal cells in layers II/III of the motor cortex that responded monosynaptically and polysynaptically to microstimulation of the somatosensory cortex and subsequently stained black by the avidin-biotinylated peroxidase complex method with diaminobenzidine (DAB) and nickel. By using a monoclonal antibody SMI-32 and a modified peroxidase-antiperoxidase method with Tris-aminophenyl-methane (TAPM) and p-cresol as a chromogen, pyramidal cells in layers III and V of the motor cortex were stained red for npNFP. In particular, all the large pyramidal cells in layer V, Betz cells, displayed intense npNFP immunoreactivity not only in the perikarya but also in the dendrites. Double staining with DAB/nickel and TAPM/p-cresol showed that biocytin-filled axon varicosities of the pyramidal cells, which were thought to receive monosynaptic inputs from area 2, made contacts with npNFP-positive dendrites in layers I-III around the biocytin-injected cell and in layers V-VI beneath the cell. The present results suggest that the corticocortical input from area 2 to pyramidal cells in layers II/III of the motor cortex is transferred to layer V pyramidal cells, including Betz cells, as well as to neighboring layer II/III pyramidal cells. Since tetanic stimulation of the somatosensory cortex reportedly produces long-term potentiation in layer II/III cells of the motor cortex, it seems reasonable to assume that a given area of the somatosensory cortex can produce a long-lasting change in the activity of a given group of output cells in the motor cortex.

Information processing within the motor cortex. I. Responses of morphologically identified motor cortical cells to stimulation of the somatosensory cortex

Inputs from the somatosensory cortex to the motor cortex have been proposed to function in learning of motor skills. In an attempt to analyze how these somatosensory inputs were processed in the motor cortex, neurons in the superficial layer of the cat motor cortex were classified into three groups on the basis of synaptic responses elicited by intracortical microstimulation (ICMS) of area 2. ICMS was delivered through seven electrodes implanted in area 2. When ICMS through one of the seven sites produced a response that was greater than 50% of the response produced by stimulating the seven sites at a time, the site was called a "dominant" site. Type I cells were those that had a dominant stimulation site and showed a constant response latency when examined by a double shock test. Type II cells were those that had a dominant site but displayed a variable latency. Type III cells had no dominant site and showed a variable latency. Latency of type I responses was 1.2-2.6 milliseconds, which was much shorter than that of type II and type III responses. Seventy-nine neurons in layers II/III of the motor cortex, which responded to ICMS in area 2, were stained by intracellular injection of biocytin. From the presence of an apical dendrite and rich spines on the dendrites, 23 type I, 21 type II, and 15 type III cells were classified as pyramidal cells. Type II pyramidal cells were located more superficially than type I and type III pyramidal cells. On the basis of the absence or sparseness of dendritic spines, three type I and four type II cells in layers II/III were classified as nonpyramidal cells. These cells consisted of five small multipolar cells in layer II and a large multipolar cell and a small bitufted cell in layer III. The remaining 11 cells were not classified because of insufficient staining. Since type I and type II cells are considered to represent monosynaptic and polysynaptic responses to stimulation of area 2, respectively, information flow from type I cells to more superficially located type II cells is presumed in layers II/III of the motor cortex. Type III responses suggest the presence of a convergent flow of impulses inside of and/or between areas 2 and 4.

Regional distribution of GABAA receptor binding sites in cat somatosensory and motor cortex

Inhibition in primary sensory cortex plays a role in neuronal responses to peripheral stimuli. For many neurons in cat primary somatosensory cortex, blockade of GABAA receptors by bicuculline results in receptive field enlargement. The magnitude of this effect varies with the neuron's adaptation characteristics and its location in particular laminae and submodality regions. To test whether these variations are correlated with the distribution of GABAA receptors, we analyzed [3H]muscimol binding in cat primary somatosensory and motor cortical areas. The highest levels of binding were in layers I-III, and the lowest levels were in layers V-VI. In somatosensory cortical areas, layer IV was distinguished by higher levels of binding than in adjacent layers. Within layer IV, levels of binding were significantly higher in posterior area 3b than in anterior area 3b. These differences may correspond to the rapidly adapting and slowly adapting submodality regions which have been described in this area. The laminar distribution of [3H]muscimol binding differed from that of [3H]flunitrazepam, and neither resembled the distribution of the magnitude of bicuculline's effects on receptive field size. The laminar distribution of [3H]muscimol binding was highly correlated with the areal density of GABA-immunoreactive neurons described in a companion study.

Thalamic afferents of area 4 and 6 in the dog: a multiple retrograde fluorescent dye study

In the present study, we compared the distribution of thalamocortical afferents of cortical area 4 to that of cortical area 6 in the dog, using fluorescent tracers. Multiple injections of combinations of two dyes (diamidino yellow dihydrochloride, Evans blue, fast blue, granular blue) were made into either the anterior and posterior sigmoid gyri or into the medial and lateral regions of the anterior sigmoid gyrus in the anesthetized dog. We found that the thalamic afferents of areas 4 and 6 arise from topographically organized bands of cells that traverse several thalamic nuclei and extend throughout the rostrocaudal extent of the thalamus. The most medial band included area 6-projecting neurons in the anterior nuclei, the rhomboid nucleus, the ventral anterior nucleus (VA), ventromedial nucleus (VM) and mediodorsal nucleus (MD). Within this band, cells projecting to medial area 6 a alpha tended to be more numerous in the anterior nuclei, anterior parts of VA and VM and anterior and caudal parts of MD. Fewer cells in MD but more cells in caudal parts of VA and VM projected to lateral area 6 a beta. Lateral bands of cells in central through lateral parts of VA and VL projected topographically to lateral area 4 on the anterior sigmoid gyrus and lateral through medial parts of postcruciate area 4. The most lateral band of cells in VL continued ventrally into the zona incerta. Area 4 also received input from VM and the central lateral (CL) and centrum medianum (CM) nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the discharge patterns of cat motor cortex neurones during unexpected perturbations of on-going locomotion

1. The impulse activity of single neurones in the forelimb part of the motor cortex was recorded extracellularly in unrestrained cats during self-paced locomotion on a horizontal circular ladder. 2. Fifty-one cells (forty-nine of which discharged rhythmically in time with the step cycle) were recorded during encounters with a number of rungs that could be locked firmly in position or, alternatively, held in position by weak springs so that when stepped on they unexpectedly descended (under the weight of the animal) from 1 to 5 cm before contacting a mechanical stop. 3. In eleven cells (22%) including four fast-axon pyramidal tract neurones (PTNs), an increase in discharge occurred when the contralateral forelimb descended unexpectedly. Onset latency relative to the start of rung movement ranged from ca 20 to ca 100 ms. In eight cells latency was such that most of the response preceded contact of the rung with the stop; averaged over a number of trials the altered discharge in five of these cells (including two PTNs) represented an accurate profile of the averaged velocity of rung (and foot) descent. The three remaining cells appeared to be responding largely to the cessation of rung movement. 4. Thirty-six of the cells were also studied during unexpected descent of the ipsilateral forelimb and six (17%) displayed an increase in discharge (onset latency ca 35 to ca 80 ms); three of these were among those that also responded to contralateral descents. 5. These findings for skilled locomotion requiring a high degree of visuomotor coordination are discussed and it is concluded that the motor cortex is rapidly informed regarding unexpected perturbations delivered to the contralateral forelimb at the onset of stance and that changes are evoked in the pattern of impulse traffic descending via the pyramidal tract.

Low-threshold motor effects produced by stimulation of the facial area of the fifth somatosensory cortex in the cat

The motor effective sites of the fifth somatosensory cortex (SV) in the cat were mapped in detail by using unit recording and intracortical microstimulation (ICMS) techniques. The motor effective sites for facial muscle contraction were identified using stimulus currents of less than 30 microA. Of the 257 effective sites detected, 49% were activated by stimulus currents of less than 20 microA and of these, 51% responded to stimulus currents of less than 10 microA. ICMS within the facial area of the SV neuron produced contralateral eye-blinking, the lowest threshold current for which was 2 microA and ICMS within the SV neurons produced whisker movements, the minimum threshold current for which was 4 microA. Furthermore, stimulation of some SV neurons at a threshold current as low as 4 microA produced whisker movements and some responded to both visual and cutaneous stimuli. Ablation of areas 6a beta, 3a, SII, SIII and the motor cortex did not eliminate or reduce the low-threshold responses elicited by this weak stimulation of the SV. These motor effective areas receive input from the contralateral cutaneous of the surrounding muscle motor effective region. Our results suggest that the described effect is independent of motor effective areas.