Changes in the discharge patterns of motor cortical neurones associated with volitional changes in stepping in the cat

Cortical contribution to visuomotor coordination in locomotion and reaching

One of the hallmarks of mammals is their ability to make precise visually guided limb movements to attain objects. This is best exemplified by the reach and grasp movements of primates, although it is not unique to this mammalian order. Precise, coordinated, visually guided movements are equally as important during locomotion in many mammalian species, especially in predators. In this context, vision is used to guide paw trajectory and placement. In this review we examine the contribution of the fronto-parietal network in the control of such movements. We suggest that this network is responsible for visuomotor coordination across behaviours and species. We further argue for analogies between cytoarchitectonically similar cortical areas in primates and cats.

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.

Treadmill-belt width, but not feedback from the lower visual field, influences the noise characteristics of step width time series

Step kinematic variability, which has been associated with gait-related fall risk, is thought to be attributed to neuromotor noise. Altered neuromotor control of step kinematics would be expected to manifest as changes in the noise-related characteristics of the step kinematic time series. This study determined the effects of eliminating feedback from the lower visual field and reducing treadmill-belt width on the noise characteristics of step width time series and statistical measures of step width variability during treadmill walking. We hypothesized that eliminating feedback from the lower visual field and reducing treadmill-belt width would both alter the noise characteristics of step width time series, reflected by decreased fractal scaling, and increase statistical measures of step width variability. Eighteen young adults performed four randomly ordered walking trials during which we manipulated visual feedback from the lower visual field (normal and obstructed) and treadmill-belt width (wide and narrow). Reducing treadmill-belt width, but not eliminating feedback from the lower visual field, significantly reduced the fractal scaling of step width time series, indicating a shift towards white, uncorrelated noise. These results suggest that accounting for the influence of treadmill-belt width on step width time series may be an important consideration in both laboratory and clinical settings. Further work is needed to clarify the effects of vision on measures of step width, identify the mechanism(s) underlying the observed shift towards white, uncorrelated noise associated with reduced treadmill-belt width, and to assess the potential relationship between the noise characteristics of step width time series and fall risk.

Contribution of the Entopeduncular Nucleus and the Globus Pallidus to the Control of Locomotion and Visually Guided Gait Modifications in the Cat

We tested the hypothesis that the entopeduncular (EP) nucleus (feline equivalent of the primate GPi) and the globus pallidus (GPe) contribute to both the planning and execution of locomotion and voluntary gait modifications in the cat. We recorded from 414 cells distributed throughout these two nuclei (referred to together as the pallidum) while cats walked on a treadmill and stepped over an obstacle that advanced towards them. Neuronal activity in many cells in both structures was modulated on a step-by-step basis during unobstructed locomotion and was modified in the step over the obstacle. On a population basis, the most frequently observed change, in both the EP and the GPe, was an increase in activity prior to and/or during the swing phase of the step over the obstacle by the contralateral forelimb, when it was the first limb to pass over the obstacle. Our results support a contribution of the pallidum, in concert with cortical structures, to the control of both the planning and the execution of the gait modifications. We discuss the results in the context of current models of pallidal action on thalamic activity, including the possibility that cells in the EP with increased activity may sculpt thalamo-cortical activity.

Context-dependent limb movement encoding in neuronal populations of motor cortex

Neuronal networks of the mammalian motor cortex (M1) are important for dexterous control of limb joints. Yet it remains unclear how encoding of joint movement in M1 depends on varying environmental contexts. Using calcium imaging we measured neuronal activity in layer 2/3 of the M1 forelimb region while mice grasped regularly or irregularly spaced ladder rungs during locomotion. We found that population coding of forelimb joint movements is sparse and varies according to the flexibility demanded from individual joints in the regular and irregular context, even for equivalent grasping actions across conditions. This context-dependence of M1 encoding emerged during task learning, fostering higher precision of grasping actions, but broke apart upon silencing of projections from secondary motor cortex (M2). These findings suggest that M1 exploits information from M2 to adapt encoding of joint movements to the flexibility demands of distinct familiar contexts, thereby increasing the accuracy of motor output.

Network activity in primary motor cortex (M1) controls dexterous limb movements. Here, the authors show that the M1 population code varies according to contextual motor demands that are conveyed via the secondary motor cortex (M2).

Increased central common drive to ankle plantar flexor and dorsiflexor muscles during visually guided gait

When we walk in a challenging environment, we use visual information to modify our gait and place our feet carefully on the ground. Here, we explored how central common drive to ankle muscles changes in relation to visually guided foot placement. Sixteen healthy adults aged 23 ± 5 years participated in the study. Electromyography (EMG) from the Soleus (Sol), medial Gastrocnemius (MG), and the distal and proximal ends of the Tibialis anterior (TA) muscles and electroencephalography (EEG) from Cz were recorded while subjects walked on a motorized treadmill. A visually guided walking task, where subjects received visual feedback of their foot placement on a screen in real-time and were required to place their feet within narrow preset target areas, was compared to normal walking. There was a significant increase in the central common drive estimated by TA-TA and Sol-MG EMG-EMG coherence in beta and gamma frequencies during the visually guided walking compared to normal walking. EEG-TA EMG coherence also increased, but the group average did not reach statistical significance. The results indicate that the corticospinal tract is involved in modifying gait when visually guided placement of the foot is required. These findings are important for our basic understanding of the central control of human bipedal gait and for the design of rehabilitation interventions for gait function following central motor lesions.

EMG synchrony to assess impaired corticomotor control of locomotion after stroke

Adapting one's gait pattern requires a contribution from cortical motor commands. Evidence suggests that frequency-based analysis of electromyography (EMG) can be used to detect this cortical contribution. Specifically, increased EMG synchrony between synergistic muscles in the Piper frequency band has been linked to heightened corticomotor contribution to EMG. Stroke-related damage to cerebral motor pathways would be expected to diminish EMG Piper synchrony. The objective of this study is therefore to test the hypothesis that EMG Piper synchrony is diminished in the paretic leg relative to nonparetic and control legs, particularly during a long-step task of walking adaptability. Twenty adults with post-stroke hemiparesis and seventeen healthy controls participated in this study. EMG Piper synchrony increased more for the control legs compare to the paretic legs when taking a non-paretic long step (5.02±3.22% versus 0.86±2.62%), p<0.01) and when taking a paretic long step (2.04±1.98% versus 0.70±2.34%, p<0.05). A similar but non-significant trend was evident when comparing non-paretic and paretic legs. No statistically significant differences in EMG Piper synchrony were found between legs for typical walking. EMG Piper synchrony was positively associated with walking speed and step length within the stroke group. These findings support the assertion that EMG Piper synchrony indicates corticomotor contribution to walking.

Similar Motor Cortical Control Mechanisms for Precise Limb Control during Reaching and Locomotion

Throughout the course of evolution there has been a parallel development of the complexity and flexibility of the nervous system and the skeletomuscular system that it controls. This development is particularly evident for the cerebral cortical areas and the transformation of the use of the upper limbs from a purely locomotor function to one including, or restricted to, reaching and grasping. This study addresses the issue of whether the control of reaching has involved the development of new cortical circuits or whether the same neurons are used to control both locomotion and reaching. We recorded the activity of pyramidal tract neurons in the motor cortex of the cat both during voluntary gait modifications and during reaching. All cells showed generally similar patterns of activity in both tasks. More specifically, we showed that, in many cases, cells maintained a constant temporal relationship to the activity of synergistic muscle groups in each task. In addition, in some cells the relationship between the intensity of the cell discharge activity and the magnitude of the EMG activity was equally constant during gait modifications and reaching. As such, the results are compatible with the hypothesis that the corticospinal circuits used to control reaching evolved from those used to precisely modify gait.

Known and unexpected constraints evoke different kinematic, muscle, and motor cortical neuron responses during locomotion

During navigation through complex natural environments, people and animals must adapt their movements when the environment changes. The neural mechanisms of such adaptations are poorly understood, especially with respect to constraints that are unexpected and must be adapted to quickly. In this study, we recorded forelimb-related kinematics, muscle activity, and the activity of motor cortical neurons in cats walking along a raised horizontal ladder, a complex locomotion task requiring accurate limb placement. One of the crosspieces was motorized, and displaced before the cat stepped on the ladder or at different points along the cat's progression over the ladder, either towards or away from the cat. We found that, when the crosspiece was displaced before the cat stepped onto the ladder, the kinematic modifications were complex and involved all forelimb joints. When the crosspiece displaced unexpectedly while the cat was on the ladder, the kinematic modifications were minimalistic and primarily involved distal joints. The activity of M. triceps and M. extensor digitorum communis differed based on the direction of displacement. Out of 151 neurons tested, 69% responded to at least one condition; however, neurons were significantly more likely to respond when crosspiece displacement was unexpected. Most often they responded during the swing phase. These results suggest that different neural mechanisms and motor control strategies are used to overcome constraints for locomotor movements depending on whether they are known or emerge unexpectedly.

Walking adaptability after a stroke and its assessment in clinical settings

Control of walking has been described by a tripartite model consisting of stepping, equilibrium, and adaptability. This review focuses on walking adaptability, which is defined as the ability to modify walking to meet task goals and environmental demands. Walking adaptability is crucial to safe ambulation in the home and community environments and is often severely compromised after a stroke. Yet quantification of walking adaptability after stroke has received relatively little attention in the clinical setting. The objectives of this review were to examine the conceptual challenges for clinical measurement of walking adaptability and summarize the current state of clinical assessment for walking adaptability. We created nine domains of walking adaptability from dimensions of community mobility to address the conceptual challenges in measurement and reviewed performance-based clinical assessments of walking to determine if the assessments measure walking adaptability in these domains. Our literature review suggests the lack of a comprehensive well-tested clinical assessment tool for measuring walking adaptability. Accordingly, recommendations for the development of a comprehensive clinical assessment of walking adaptability after stroke have been presented. Such a clinical assessment will be essential for gauging recovery of walking adaptability with rehabilitation and for motivating novel strategies to enhance recovery of walking adaptability after stroke.

Modification of cutaneous reflexes during visually guided walking

Although it has become apparent that cutaneous reflexes can be adjusted based on the phase and context of the locomotor task, it is not clear to what extent these reflexes are regulated when locomotion is modified under visual guidance. To address this, we compared the amplitude of cutaneous reflexes while subjects performed walking tasks that required precise foot placement. In one experiment, subjects walked overground and across a horizontal ladder with narrow raised rungs. In another experiment, subjects walked and stepped onto a series of flat targets, which required different levels of precision (large vs. narrow targets). The superficial peroneal or tibial nerve was electrically stimulated in multiple phases of the gait cycle in each condition and experiment. Reflexes between 50 and 120 ms poststimulation were sorted into 10 equal phase bins, and the amplitudes were then averaged. In each experiment, differences in cutaneous reflexes between conditions occurred predominantly during swing phase when preparation for precise foot placement was necessary. For instance, large excitatory cutaneous reflexes in ipsilateral tibialis anterior were present in the ladder condition and when stepping on narrow targets compared with inhibitory responses in the other conditions, regardless of the nerve stimulated. In the ladder experiments, additional effects of walking condition were evident during stance phase when subjects had to balance on the narrow ladder rungs and may be related to threat and/or the unstable foot-surface interaction. Taken together, these results suggest that cutaneous reflexes are modified when visual feedback regarding the terrain is critical for successful walking.

Synchronous EMG activity in the piper frequency band reveals the corticospinal demand of walking tasks

Evidence indicates that the frequency-domain characteristics of surface electromyogram (EMG) signals are modulated according to the contributing sources of neural drive. Modulation of inter-muscular EMG synchrony within the Piper frequency band (30-60 Hz) during movement tasks has been linked to drive from the corticospinal tract. However, it is not known whether EMG synchrony is sufficiently sensitive to detect task-dependent differences in the corticospinal contribution to leg muscle activation during walking. We investigated this question in seventeen healthy older men and women. It was hypothesized that, relative to typical steady state walking, Piper band EMG synchrony of the triceps surae muscle group would be reduced for dual-task walking (because of competition for cortical resources), similar for fast walking (because walking speed is directed by an indirect locomotor pathway rather than by the corticospinal tract), and increased when taking a long step (because voluntary gait pattern modifications are directed by the corticospinal tract). Each of these hypotheses was confirmed. These findings support the use of frequency-domain analysis of EMG in future investigations into the corticospinal contribution to control of healthy and disordered human walking.

Differential responses of fast- and slow-conducting pyramidal tract neurons to changes in accuracy demands during locomotion

Most movements need to be accurate. The neuronal mechanisms controlling accuracy during movements are poorly understood. In this study we compare the activity of fast- and slow-conducting pyramidal tract neurons (PTNs) of the motor cortex in cats as they walk over both a flat surface, a task that does not require accurate stepping and can be accomplished without the motor cortex, as well as along a horizontal ladder, a task that requires accuracy and the activity of the motor cortex to be successful. Fast- and slow-conducting PTNs are known to have distinct biophysical properties as well as different afferent and efferent connections. We found that while the activity of all PTNs changes substantially upon transition from simple locomotion to accurate stepping on the ladder, slow-conducting PTNs respond in a much more concerted manner than fast-conducting ones. As a group, slow-conducting PTNs increase discharge rate, especially during the late stance and early swing phases, decrease discharge variability, have a tendency to shift their preferred phase of the discharge into the swing phase, and almost always produce a single peak of activity per stride during ladder locomotion. In contrast, the fast-conducting PTNs do not display such concerted changes to their activity. In addition, upon transfer from simple locomotion to accurate stepping on the ladder slow-conducting PTNs more profoundly increase the magnitude of their stride-related frequency modulation compared with fast-conducting PTNs. We suggest that slow-conducting PTNs are involved in control of accuracy of locomotor movements to a greater degree than fast-conducting PTNs.

Chapter 6--motor planning of locomotor adaptations on the basis of vision: the role of the posterior parietal cortex

In this chapter, we consider the contribution of the posterior parietal cortex (PPC) to obstacle avoidance behavior and we define a model that identifies the major planning processes that are required for this task. A key aspect of this planning process is the need to integrate information concerning the obstacle, obtained from vision, together with an estimation of body and limb state. We suggest that the PPC makes a major contribution to this process during visually guided locomotion. We present evidence from lesion and single unit recording experiments in the cat that are compatible with this viewpoint.

Involvement of the corticospinal tract in the control of human gait

Given the inherent mechanical complexity of human bipedal locomotion, and that complete spinal cord lesions in human leads to paralysis with no recovery of gait, it is often suggested that the corticospinal tract (CST) has a more predominant role in the control of walking in humans than in other animals. However, what do we actually know about the contribution of the CST to the control of gait? This chapter will provide an overview of this topic based on the premise that a better understanding of the role of the CST in gait will be essential for the design of evidence-based approaches to rehabilitation therapy, which will enhance gait ability and recovery in patients with lesions to the central nervous system (CNS). We review evidence for the involvement of the primary motor cortex and the CST during normal and perturbed walking and during gait adaptation. We will also discuss knowledge on the CST that has been gained from studies involving CNS lesions, with a particular focus on recent data acquired in people with spinal cord injury.

Differences in movement mechanics, electromyographic, and motor cortex activity between accurate and nonaccurate stepping

What are the differences in mechanics, muscle, and motor cortex activity between accurate and nonaccurate movements? We addressed this question in relation to walking. We assessed full-body mechanics (229 variables), activity of 8 limb muscles, and activity of 63 neurons from the motor cortex forelimb representation during well-trained locomotion with different demands on the accuracy of paw placement in cats: during locomotion on a continuous surface and along horizontal ladders with crosspieces of different widths. We found that with increasing accuracy demands, cats assumed a more bent-forward posture (by lowering the center of mass, rotating the neck and head down, and by increasing flexion of the distal joints) and stepped on the support surface with less spatial variability. On the ladder, the wrist flexion moment was lower throughout stance, whereas ankle and knee extension moments were higher and hip moment was lower during early stance compared with unconstrained locomotion. The horizontal velocity time histories of paws were symmetric and smooth and did not differ among the tasks. Most of the other mechanical variables also did not depend on accuracy demands. Selected distal muscles slightly enhanced their activity with increasing accuracy demands. However, in a majority of motor cortex cells, discharge rate means, peaks, and depths of stride-related frequency modulation changed dramatically during accurate stepping as compared with simple walking. In addition, in 30% of neurons periods of stride-related elevation in firing became shorter and in 20-25% of neurons activity or depth of frequency modulation increased, albeit not linearly, with increasing accuracy demands. Considering the relatively small changes in locomotor mechanics and substantial changes in motor cortex activity with increasing accuracy demands, we conclude that during practiced accurate stepping the activity of motor cortex reflects other processes, likely those that involve integration of visual information with ongoing locomotion.

Muscle synergies during locomotion in the cat: a model for motor cortex control

It is well established that the motor cortex makes an important contribution to the control of visually guided gait modifications, such as those required to step over an obstacle. However, it is less clear how the descending cortical signal interacts with the interneuronal networks in the spinal cord to ensure that precise changes in limb trajectory are appropriately incorporated into the base locomotor rhythm. Here we suggest that subpopulations of motor cortical neurones, active sequentially during the step cycle, may regulate the activity of small groups of synergistic muscles, likewise active sequentially throughout the step cycle. These synergies, identified by a novel associative cluster analysis, are defined by periods of muscle activity that are coextensive with respect to the onset and offset of the EMG activity. Moreover, the synergies are sparse and are frequently composed of muscles acting around more than one joint. During gait modifications, we suggest that subpopulations of motor cortical neurones may modify the magnitude and phase of the EMG activity of all muscles contained within a given synergy. Different limb trajectories would be produced by differentially modifying the activity in each synergy thus providing a flexible substrate for the control of intralimb coordination during locomotion.

Modelling gait processes as a combination of sensory-motor and cognitive controls in an attempt to describe accidents on the level in occupational situations

In occupational situations, accidents referred to as accidents on the level (AoLs) occur most of the time when locomotion control fails. This control is determined by the interactions between the operator and the environment, the task and the used tools. Hence, AoLs prevention requires developing ways to optimise these interactions. More fundamentally, AoLs prevention requires understanding locomotion control in situations where this control is at sake, that is in situations involving one or more AoLs factors. The purpose of this article is to propose a comprehensive model for the control of locomotion in occupational environments. This model featuring the operator, the task and the working space should be an appropriate tool to understand AoLs in the scope of their prevention. Firstly, we describe what occupational AoLs are. In a second part, we present a review of the theoretical and experimental knowledge related to the locomotion system through the various means developed by the Central Nervous System to cope with perturbations of the environment and/or particular constraints from the task. Finally, we propose a simplified systemic model presenting the various levels of control (sensory-motor to cognitive levels) describing locomotion in occupational situations, and we suggest experiments likely to produce the appropriate data to construct the final comprehensive model.

Contribution of the motor cortex to the structure and the timing of hindlimb locomotion in the cat: a microstimulation study

We used microstimulation to examine the contribution of the motor cortex to the structure and timing of the hindlimb step cycle during locomotion in the intact cat. Stimulation was applied to the hindlimb representation of the motor cortex in 34 sites in three cats using either standard glass-insulated microelectrodes (16 sites in 1 cat) or chronically implanted microwire electrodes (18 sites in 2 cats). Stimulation at just suprathreshold intensities with the cat at rest produced multi-joint movements at a majority of sites (21/34, 62%) but evoked responses restricted to a single joint, normally the ankle, at the other 13/34 (38%) sites. Stimulation during locomotion generally evoked larger responses than the same stimulation at rest and frequently activated additional muscles. Stimulation at all 34 sites evoked phase-dependent responses in which stimulation in swing produced transient increases in activity in flexor muscles while stimulation during stance produced transient decreases in activity in extensors. Stimulation with long (200 ms) trains of stimuli in swing produced an increased level of activity and duration of flexor muscles without producing changes in cycle duration. In contrast, stimulation during stance decreased the duration of the extensor muscle activity and initiated a new and premature period of swing, resetting the step cycle. Stimulation of the pyramidal tract in two of these three cats as well as in two additional ones produced similar effects. The results show that the motor cortex is capable of influencing hindlimb activity during locomotion in a similar manner to that seen for the forelimb.

Cortical and brainstem control of locomotion

While a basic locomotor rhythm is centrally generated by spinal circuits, descending pathways are critical for ensuring appropriate anticipatory modifications of gait to accommodate uneven terrain. Neurons in the motor cortex command the changes in muscle activity required to modify limb trajectory when stepping over obstacles. Simultaneously, neurons in the brainstem reticular formation ensure that these modifications are superimposed on an appropriate base of postural support. Recent experiments suggest that the same neurons in the same structures also provide similar information during reaching movements. It is suggested that, during both locomotion and reaching movements, the final expression of descending signals is influenced by the state and excitability of the spinal circuits upon which they impinge.

Contributions of the motor cortex to the control of the hindlimbs during locomotion in the cat

Although the corticospinal tract is not essential for the production of the basic locomotor rhythm in cats, it does contribute to the regulation of locomotion, particularly in situations in which there is a requirement for precise control over paw placement or limb trajectory. Lesions of the dorsolateral funiculi at the low thoracic level (T(13)) that completely interrupted both the cortico- and rubrospinal pathways produced long-term deficits in locomotion on a level surface. These deficits included a paw-drag that was probably caused both by a loss of cortico- and rubrospinal input to motoneurones controlling distal muscles as well as by a change in the relative timing of muscles acting around the hip and knee. Smaller lesions produced similar deficits from which the cats recovered relatively quickly. Cats with the largest lesions of the dorsolateral funiculi were unable to modify their gait sufficiently to step over obstacles attached to the treadmill belt even 3-5 months postlesion. These results imply that the medial pathways, the reticulo- and vestibulospinal pathways, are unable to fully compensate for damage to the lateral pathways. Single unit recordings from identified pyramidal tract neurones (PTNs) within the hindlimb representation of the primary motor cortex (area 4) showed that a substantial proportion of neurones (67%) significantly increased their discharge frequency when the cats modified their gait to step over obstacles attached to the treadmill belt. Of those PTNs that showed increased activity during the swing phase, populations of neurones were activated at different times. A large proportion of PTNS discharged early in swing, in phase with knee flexors such as the semitendinosus. Others discharged slightly later, in phase with the activity of ankle flexors, such as tibialis anterior, while still others discharged at the end of swing, in phase with digit dorsiflexors, such as the extensor digitorum brevis. We suggest that different populations of cortical neurones may specifically modify the activity of selected groups of close synergistic muscles during different parts of the swing phase. We further suggest that these modifications are mediated, in part, by groups of interneurones that are involved in determining the base locomotor rhythm. This provides a means by which the changes specified by the descending signal from the motor cortex may be smoothly, and appropriately, incorporated into the locomotor cycle.

Discharge characteristics of neurons in the red nucleus during voluntary gait modifications: a comparison with the motor cortex

We have examined the contribution of the red nucleus to the control of locomotion in the cat. Neuronal activity was recorded from 157 rubral neurons, including identified rubrospinal neurons, in three cats trained to walk on a treadmill and to step over obstacles attached to the moving belt. Of 72 neurons with a receptive field confined to the contralateral forelimb, 66 were phasically active during unobstructed locomotion. The maximal activity of the majority of neurons (59/66) was centered around the swing phase of locomotion. Slightly more than half of the neurons (36/66) were phasically activity during both swing and stance. In addition, some rubral neurons (14/66) showed multiple periods of phasic activity within the swing phase of the locomotor cycle. Periods of phasic discharge temporally coincident with the swing phase of the ipsilateral limb were observed in 7/66 neurons. During voluntary gait modifications, most forelimb-related neurons (70/72) showed a significant increase in their discharge activity when the contralateral limb was the first to step over the obstacle (lead condition). Maximal activity in nearly all cells (63/70) was observed during the swing phase, and 23/63 rubral neurons exhibited multiple increases of activity during the modified swing phase. A number of cells (18/70) showed multiple periods of increased activity during swing and stance. Many of the neurons (35/63, 56%) showed an increase in activity at the end of the swing phase; this period of activity was temporally coincident with the period of activity in wrist dorsiflexors, such as the extensor digitorum communis. A smaller proportion of neurons with receptive fields restricted to the hindlimbs showed similar characteristics to those observed in the population of forelimb-related neurons. The overall characteristics of these rubral neurons are similar to those that we obtained previously from pyramidal tract neurons recorded from the motor cortex during an identical task. However, in contrast to the results obtained in the rubral neurons, most motor cortical neurons showed only one period of increased activity during the step cycle. We suggest that both structures contribute to the modifications of the pattern of EMG activity that are required to produce the change in limb trajectory needed to step over an obstacle. However, the results suggest an additional role for the red nucleus in regulating intra- and interlimb coordination.

Visual analysis and image motion in locomoting cats

During locomotion, observers see a characteristic pattern of motion referred to as an optic flow field. To investigate how they make use of this pattern, we have developed a paradigm for testing visual function during locomotion. Foot placement was recorded while cats walked down an alley cluttered with a high density of small objects; the task was to avoid stepping on any object. In the experiments reported here, motion cues were eliminated by the use of low-frequency strobe lighting. In bright continuous light cats performed with great accuracy, and likewise at scotopic light levels. However, in strobe lighting their error rates increased more than threefold. This deterioration could not be attributed to lower acuity, since the cats' performance remained excellent when the light level was reduced well below that afforded by the strobe light. When very dim continuous light was combined with low-frequency strobe lighting, performance was substantially better than under strobe light alone. We conclude that motion-sensitive neurons make a major contribution to visual guidance of foot placement during locomotion. When strobe lighting is combined with very dim continuous light, even the minimal motion information available in the intervals between bright strobe flashes improves performance significantly. Cats were also trained to discriminate between complex patterns, and this discrimination was not affected by strobe lighting, suggesting that motion-sensitive neurons are not critical for this analysis.

Contributions of the reticulospinal system to the postural adjustments occurring during voluntary gait modifications

To test the hypothesis that reticulospinal neurons (RSNs) are involved in the formation of the dynamic postural adjustments that accompany visually triggered, voluntary modifications of limb trajectory during locomotion, we recorded the activity of 400 cells (183 RSNs; 217 unidentified reticular cells) in the pontomedullary reticular formation (PMRF) during a locomotor task in which intact cats were required to step over an obstacle attached to a moving treadmill belt. Approximately one half of the RSNs (97/183, 53%) showed significant changes in cell activity as the cat stepped over the obstacle; most of these cells exhibited either single (26/97, 26.8%) or multiple (63/97, 65.0%) increases of activity. There was a range of discharge patterns that varied in the number, timing, and sequencing of the bursts of modified activity, although individual bursts in different cells tended to occur at similar phases of the gait cycle. Most modified cells, regardless of the number of bursts of increased discharge, or of the discharge activity of the cell during unobstructed, control, locomotion, discharged during the passage of the lead forelimb over the obstacle. Thus, 86.9% of the modified cells increased their discharge when the forelimb ipsilateral to the recording site was the first to pass over the obstacle, and 72.2% when the contralateral limb was the first. Approximately one quarter of the RSNs increased their discharge during the passage of each of the four limbs over the obstacle in both the lead (27.1%) and trail (27.9%) conditions. In general, in any one cell, the number and relative sequencing of the subsequent bursts (with respect to the lead forelimb) was maintained during both lead and trail conditions. Patterns of activity observed in unidentified cells were very similar to the RSN activity despite the diverse population of cells this unidentified group may represent. We suggest that the increased discharge that we observed in these reticular neurons reflects the integration of afferent activity from several sources, including the motor cortex, and that this increased discharge signals the timing and the relative magnitude of the postural patterns that accompany the voluntary gait modification. However, based on the characteristics of the patterns of neuronal activity in these cells, we further suggest that while individual RSNs probably contribute to the selection of different patterns of postural activity, the ultimate expression of the postural response may be determined by the excitability of the locomotor circuits within the spinal cord.

Central regulation of motor cortex neuronal responses to forelimb nerve inputs during precision walking in the cat

1. The responses of neurones in forelimb motor cortex to impulse volleys evoked by single pulse electrical stimulation (at 1.5 or 2 times the threshold for most excitable nerve fibres) of the superficial radial (SR) and ulnar (UL) nerves of the contralateral forelimb were studied in awake cats both resting quietly and walking on a horizontal ladder. Nerve volley amplitude was monitored by recording the compound action potential elicited by the stimulus. 2. In the resting animal 34/82 (41%) cells yielded statistically significant responses to SR stimulation, and 20/72 (28%) responded to UL stimulation. Some responses were confined to or began with an increase in firing probability ('excitatory' responses) and others with a decrease in firing ('inhibitory' responses), typically including a brief interruption of the spike train (zero rate). Cells responding to both nerves usually yielded responses similar in type. Most (78%) response onset latencies were less than 30 ms. Responses involved the addition or subtraction of from 3.4 to 0.1 impulses stimulus-1 (most <1 impulse stimulus-1). The distribution of response sizes was continuous down to the smallest values, i.e. there was no 'gap' which would represent a clear separation into 'responsive' and 'unresponsive' categories. Responses were commonest in the lateral part of the pericruciate cortex, and commoner among pyramidal tract neurones (PTNs) than non-PTNs. 3. During ladder walking most cells generated a rhythmic step-related discharge; in assessing the size of responses to nerve stimulation (20 studied, from 13 cells) this activity was first subtracted. Response onset latencies (90% <30 ms) and durations showed little or no change. Although most cells were overall more active than during rest both 'excitatory' and 'inhibitory' responses in both PTNs and non-PTNs were often markedly reduced in large parts of the step cycle; over some (usually brief) parts responses approached or exceeded their size during rest, i.e. response size was step phase dependent. Such variations occurred without parallel change in the nerve compound action potential, nor were they correlated with the level of background firing at the time that the response was evoked. When responses to both nerves were studied in the same neurone they differed in their patterns of phase dependence. 4. The findings are interpreted as evidence for central mechanisms that, during 'skilled', cortically controlled walking, powerfully regulate the excitability of the somatic afferent paths from forelimb mechanoreceptors (including low threshold cutaneous receptors) to motor cortex. Retention (or enhancement) of responsiveness often occurred (especially for ulnar nerve) around footfall, perhaps reflecting a behavioural requirement for sensory input signalling the quality of the contact established with the restricted surface available for support.

Organization of the projections from the pericruciate cortex to the pontomedullary reticular formation of the cat: a quantitative retrograde tracing study

Dextran-amines were used as retrograde tracers to investigate the organization of cortical projections to different cytoarchitectonic regions of the pontomedullary reticular formation of the cat. Injections into the nucleus reticularis pontis oralis resulted in labelling of neurones in the proreus cortex and area 6a beta of the premotor cortex, with little labelling in the motor cortex (area 4). This labelling was predominantly ipsilateral to the injection site. In contrast, injections into the nucleus reticularis pontis caudalis (NRPc), nucleus reticularis gigantocellularis (NRGc), and nucleus reticularis magnocellularis (NRMc) resulted in bilateral labelling--primarily in areas 6a beta, 6a gamma, and in the rostromedial region of area 4--with little labelling in the proreus cortex. In general, the cortical projections to the caudal NRGc and the NRMc were larger than those to the NRPc. More than 25% of the total projections to each of the latter three reticular regions arose from the medial part of area 4. Labelling in the hindlimb regions of area 4 was largest following the NRMc injections and smallest after injections in the NRPc. The projections to the NRPc originated from more medial parts of areas 4 and 6 than did the projections to the caudal region of the NRGc. These results suggest that areas 4 and 6 may be able to differentially activate different regions of the pontomedullary reticular formation depending on the movement that is made and perhaps also on the context of that movement.

Force and the motor cortex

The relation between the activity of cells in the motor cortex and static force has been studied extensively. Most studies have concentrated on the relation to the magnitude of force; this relation is more or less monotonic. The slope of the relation, however, shows considerable variation among different studies and seems to be inversely associated with the range of forces over which the cell activity has been studied. Cells in the motor cortex also show variation in activity with the direction of static force. When both the direction and the magnitude of static force are allowed to vary, a majority of cells show significant changes in activity with direction of force alone, an intermediate number relate to both direction and magnitude, while a small number relate purely to the magnitude. This suggests that the direction of static force can be controlled independently of its magnitude and that this directional signal is especially prominent in the motor cortex. In general, it has been more difficult to study the relations to dynamic force. There is a correlation between motor cortex cell activity and the rate of change of force. The direction of dynamic force is also an important determinant of cell activity. When both static and dynamic force output are required (for example, with arm movement in the presence of gravity) it is the dynamic signal that is most clearly reflected in motor cortex activity. The relations between motor cortex activity and static or dynamic force are not invariant, but may be modified by the behavioral context of the motor output.

Force and the motor cortex

The relation between the activity of cells in the motor cortex and static force has been studied extensively. Most studies have concentrated on the relation to the magnitude of force; this relation is more or less monotonic. The slope of the relation, however, shows considerable variation among different studies and seems to be inversely associated with the range of forces over which the cell activity has been studied. Cells in the motor cortex also show variation in activity with the direction of static force. When both the direction and the magnitude of static force are allowed to vary, a majority of cells show significant changes in activity with direction of force alone, an intermediate number relate to both direction and magnitude, while a small number relate purely to the magnitude. This suggests that the direction of static force can be controlled independently of its magnitude and that this directional signal is especially prominent in the motor cortex. In general, it has been more difficult to study the relations to dynamic force. There is a correlation between motor cortex cell activity and the rate of change of force. The direction of dynamic force is also an important determinant of cell activity. When both static and dynamic force output are required (for example, with arm movement in the presence of gravity) it is the dynamic signal that is most clearly reflected in motor cortex activity. The relations between motor cortex activity and static or dynamic force are not invariant, but may be modified by the behavioral context of the motor output.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Kinematic determinants of human locomotion

1. The aim of this study was to find kinematic patterns that are invariant across the normal range of locomotion speeds. Subjects walked at different, freely chosen speeds ranging from 0.9 to 2.1 m s-1, while motion and ground reaction forces on the right side of the body were recorded in three-dimensional space. 2. The time course of the anatomical angles of flexion-extension at the hip and ankle was variable not only across subjects, but even from trial to trial in the same subject. By contrast, the time course of the changes in the angles of elevation of each limb segment (pelvis, thigh, shank and foot) relative to the vertical was stereotyped across subjects. 3. To compare the waveforms across speeds, data were scaled in time relative to gait cycle duration. The pattern of ground reaction forces was highly speed dependent. Several distinct families of curves could be recognized in the flexion-extension angles at the hip and ankle. Instead, the waveforms of global length and elevation of the limb, elevation angles of all limb segments and flexion-extension at the knee were invariant with speed. 4. When gait trajectories at all speeds are plotted in the position space defined by the elevation angles of the limb segments, they describe regular loops on a plane. The statistical characteristics of these angular covariations were quantified by means of principal component analysis. The first two principal components accounted together for > 99% of the total experimental variance, and were quantitatively comparable in all subjects. 5. This constraint of planar covariation of the elevation angles is closely reminiscent of that previously described for the control of posture. The existence of laws of intersegmental co-ordination, common to the control of posture and locomotion, presumably assures the maintenance of dynamic equilibrium during forward progression, and the anticipatory adaptation to potentially destabilizing factors by means of co-ordinated kinematic synergies of the whole body.

Nicotinic cholinergic normalization of amphetamine-induced loss of auditory gating in freely moving rats

The impairment in normal sensory processing which is usually observed in schizophrenics has been demonstrated using a paired-stimulus paradigm. Normal individuals show a diminished midlatency evoked potential response to the second of a pair of clicks given at a 0.5-s interval. This phenomenon is termed auditory "gating". Schizophrenics routinely fail to suppress their response to the second click in this paradigm; thus, they do not gate. Heavy tobacco use is common among schizophrenics and it has recently been shown that nicotine causes a transient normalization of auditory gating in these individuals. Our laboratory has been utilizing animal models to investigate the sensory deficit observed in schizophrenia. In the present study, rats were administered amphetamine to produce a schizophrenia-like loss of auditory gating. They were then given nicotine, which resulted in a dose-dependent normalization of the amphetamine-induced loss of gating. This effect was blocked by concurrent central administration of d-tubocurarine. Neither nicotine nor d-tubocurarine had any effect on auditory gating when administered alone. These data are in agreement with the human studies showing normalization of auditory gating with nicotine administration and suggest a possible role for the nicotinic cholinergic receptor in the modulation of auditory gating in the rat model.

The role of the motor cortex in the control of accuracy of locomotor movements in the cat

1. The impulse activity of single neurones in the motor cortex (MC) was recorded extracellularly, using movable varnish-insulated tungsten microelectrodes, in six adult, freely moving cats. Neuronal activity was recorded while the cats walked on a flat floor, as they stepped over a series of barriers, and as they walked on the flat rungs of a horizontal ladder. The mean discharge rate (mR) and the depth of frequency modulation (dM) in each cell were estimated over 10-100 steps. 2. The activity of ninety-eight MC cells (Including thirteen pyramidal tract neurones (PTNs)) was recorded during stepping over barriers 25 cm apart. The mR in 66% and the dM in 61% of these cells changed by more than 20% during locomotion with barriers compared to locomotion on the flat (an increase was more often the case). 3. The activity of nine cells was recorded during stepping over barriers 12 cm apart, and the activity of twenty-seven cells (including five PTNs) during walking with barriers only 6 cm apart. The mR in 67% and in 59% of the cells, respectively, and the dM in 56% and in 67% of the cells, respectively, were greater in these locomotor tasks than during locomotion on the flat. 4. The activity of twenty cells was recorded during walking and compared in experiments with different distances between barriers. The mR in 50% and the dM in 75% of the neurones progressively increased when the distance between successive barriers was diminished. 5. The discharge rates of thirteen cells were compared in two different locomotor tasks: (i) when the cat stepped over barriers requiring hyperflexion of the limbs and (ii) when it walked on the flat with loads attached to the distal forelimbs causing a hyperactivity of flexor muscles. The activity of nine cells was different during stepping over the barriers compared to locomotion with loadings on the forelimbs. 6. The activity of 108 cells (twenty-four PTNs) was recorded during walking along a horizontal ladder with flat rungs. The mR of 61% and the dM of 72% of cells changed by more than 20% during locomotion on the ladder compared with that on the flat (most often they increased). 7. The position of the peak rate relative to the step cycle did not differ in the majority of cells (in 78-91% depending on the task) during locomotion on the flat, with the barriers or on the ladder.(ABSTRACT TRUNCATED AT 400 WORDS)