Automatic postural responses in the cat: Responses to headward and tailward translation

EMG responses, vertical and A-P shear forces and kinematics of "automatic postural responses" to unexpected translational perturbations in the headward and tailward directions were studied in cats. Muscles acting on the major joints of the forelimbs and hindlimbs were studied. Movement of the animals in response to perturbation were highly stereotyped and consisted of two phases: (1) motion of the feet during platform movement while the trunk remained relatively stationary followed by (2) active correction of posture by movement of the trunk in the direction of perturbation. Vertical force changes occurred after the perturbation was well underway (latency 65 ms) and were related to the displacement of the center of mass and active correction of trunk position. Shear forces showed both passive (inertial) and active components and suggested that the majority of the torque necessary for postural correction was generated by the hindlimb. EMG responses in forelimb and shoulder muscles were most correlated with increase in vertical force, showing a generalized co-contraction in tailward translation (when these limbs were loaded) and little activity when the forelimbs were unloaded. EMG responses in hindlimb showed reciprocal activation of agonists and antagonists during perturbation with strong synergies of thigh and foot flexors in tailward translation and thigh and foot extensors in headward translation. The forelimb EMG patterns were most consistent with the conclusion that the forelimb is used primarily for vertical support during perturbation. It was concluded that hindlimb EMG responses were appropriate for both vertical support and performance of the postural correction. The hindlimb muscle synergies observed during translation are the "mirror image" of those observed in humans by other workers.     

Effect of stance width on multidirectional postural responses

The effect of stance width on postural responses to 12 different directions of surface translations was examined. Postural responses were characterized by recording 11 lower limb and trunk muscles, body kinematics, and forces exerted under each foot of 7 healthy subjects while they were subjected to horizontal surface translations in 12 different, randomly presented directions. A quasi-static approach of force analysis was done, examining force integrals in three different epochs (background, passive, and active periods). The latency and amplitude of muscle responses were quantified for each direction, and muscle tuning curves were used to determine the spatial activation patterns for each muscle. The results demonstrate that the horizontal force constraint exerted at the ground was lessened in the wide, compared with narrow, stance for humans, a similar finding to that reported by Macpherson for cats. Despite more trunk displacement in narrow stance, there were no significant changes in body center of mass (CoM) displacement due to large changes in center of pressure (CoP), especially in response to lateral translations. Electromyographic (EMG) magnitude decreased for all directions in wide stance, particularly for the more proximal muscles, whereas latencies remained the same from narrow to wide stance. Equilibrium control in narrow stance was more of an active postural strategy that included regulating the loading/unloading of the limbs and the direction of horizontal force vectors. In wide stance, equilibrium control relied more on an increase in passive stiffness resulting from changes in limb geometry. The selective latency modulation of the proximal muscles with translation direction suggests that the trunk was being actively controlled in all directions. The similar EMG latencies for both narrow and wide stance, with modulation of only the muscle activation magnitude as stance width changed, suggest that the same postural synergy was only slightly modified for a change in stance width. Nevertheless, the magnitude of the trunk displacement, as well as of CoP displacement, was modified based on the degree of passive stiffness in the musculoskeletal system, which increased with stance width. The change from a more passive to an active horizontal force constraint, to larger EMG magnitudes especially in the trunk muscles and larger trunk and CoP excursions in narrow stance are consistent with a more effortful response for equilibrium control in narrow stance to perturbations in all directions.     

Activity of thoracic and lumbar epaxial extensors during postural responses in the cat

This study examined the role of trunk extensor muscles in the thoracic and lumbar regions during postural adjustments in the freely standing cat. The epaxial extensor muscles participate in the rapid postural responses evoked by horizontal translation of the support surface. The muscles segregate into two regional groups separated by a short transition zone, according to the spatial pattern of the electromyographic (EMG) responses. The upper thoracic muscles (T5-9) respond best to posteriorly directed translations, whereas the lumbar muscles (T13 to L7) respond best to anterior translations. The transition group muscles (T10-12) respond to almost all translations. Muscles group according to vertebral level rather than muscle species. The upper thoracic muscles change little in their response with changes in stance distance (fore-hindpaw separation) and may act to stabilize the intervertebral angles of the thoracic curvature. Activity in the lumbar muscles increases along with upward rotation of the pelvis (iliac crest) as stance distance decreases. Lumbar muscles appear to stabilize the pelvis with respect to the lumbar vertebrae (L7-sacral joint). The transition zone muscles display a change in spatial tuning with stance distance, responding to many directions of translation at short distances and focusing to respond best to contralateral translations at the long stance distance. Received: 2 January 1997 / Accepted: 23 September 1997     

Stance dependence of automatic postural adjustments in humans

Summary This study investigated the effect of initial stance configuration on automatic postural responses in humans. Subjects were tested in both bipedal and quadrupedal stance postures. The postural responses to horizontal translations of the supporting surface were measured in terms of the forces at the ground, movement of the body segments, and electromyographic (EMG) activity. Postural responses to the same perturbations changed with initial stance posture; these responses were biomechanically appropriate for restoring centre of mass. A change in stance configuration prior to platform movement led to a change in both the spatial and temporal organization of evoked muscle activation. Specifically, for the same direction of platform movement, during bipedal stance muscles on one side of the lower limb were activated in a distal to proximal sequence; during quadrupedal stance, muscles on the opposite side of the lower limb were activated and in a proximal to distal sequence. The most significant finding was an asymmetry in the use of the upper limbs and the lower limbs during postural corrections in quadrupedal stance. Whereas antagonists of the upper limb were either co-activated or co-inhibited, depending on the direction of translation, lower limb antagonists were reciprocally activated and inhibited. Human subjects in a quadrupedal stance posture used the lower limbs as levers, protracting or retracting the hips in order to propel the trunk back to its original position with respect to the hands and feet. Postural responses of the subjects during quadrupedal stance were remarkably similar to those of cats subjected to similar perturbations of the supporting surface. Furthermore, the same predominance of lower limb correction is characteristic of both species, suggesting that the standing cat is a good model for studying postural control in humans.     

Central programming of postural movements: adaptation to altered support-surface configurations

We studied the extent to which automatic postural actions in standing human subjects are organized by a limited repertoire of central motor programs. Subjects stood on support surfaces of various lengths, which forced them to adopt different postural movement strategies to compensate for the same external perturbations. We assessed whether a continuum or a limited set of muscle activation patterns was used to produce different movement patterns and the extent to which movement patterns were influenced by prior experience. Exposing subjects standing on a normal support surface to brief forward and backward horizontal surface perturbations elicited relatively stereotyped patterns of leg and trunk muscle activation with 73- to 110-ms latencies. Activity began in the ankle joint muscles and then radiated in sequence to thigh and then trunk muscles on the same dorsal or ventral aspect of the body. This activation pattern exerted compensatory torques about the ankle joints, which restored equilibrium by moving the body center of mass forward or backward. This pattern has been termed the ankle strategy because it restores equilibrium by moving the body primarily around the ankle joints. To successfully maintain balance while standing on a support surface short in relation to foot length, subjects activated leg and trunk muscles at similar latencies but organized the activity differently. The trunk and thigh muscles antagonistic to those used in the ankle strategy were activated in the opposite proximal-to-distal sequence, whereas the ankle muscles were generally unresponsive. This activation pattern produced a compensatory horizontal shear force against the support surface but little, if any, ankle torque. This pattern has been termed the hip strategy, because the resulting motion is focused primarily about the hip joints. Exposing subjects to horizontal surface perturbations while standing on support surfaces intermediate in length between the shortest and longest elicited more complex postural movements and associated muscle activation patterns that resembled ankle and hip strategies combined in different temporal relations. These complex postural movements were executed with combinations of torque and horizontal shear forces and motions of ankle and hip joints. During the first 5-20 practice trials immediately following changes from one support surface length to another, response latencies were unchanged. The activation patterns, however, were complex and resembled the patterns observed during well-practiced stance on surfaces of intermediate lengths.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vestibular and somatosensory contributions to responses to head and body displacements in stance

The relative contribution of vestibular and somatosensory information to triggering postural responses to external body displacements may depend on the task and on the availability of sensory information in each system. To separate the contribution of vestibular and neck mechanisms to the stabilization of upright stance from that of lower body somatosensory mechanisms, responses to displacements of the head alone were compared with responses to displacements of the head and body, in both healthy subjects and in patients with profound bilateral vestibular loss. Head displacements were induced by translating two 1-kg weights suspended on either side of the head at the level of the mastoid bone, and body displacements were induced translating the support surface. Head displacements resulted in maximum forward and backward head accelerations similar to those resulting from body displacements, but were not accompanied by significant center of body mass, ankle, knee, or hip motions. We tested the effect of disrupting somatosensory information from the legs on postural responses to head or body displacements by sway-referencing the support surface. The subjects' eyes were closed during all testing to eliminate the effects of vision. Results showed that head displacements alone can trigger medium latency (48–84 ms) responses in the same leg and trunk muscles as body displacements. Nevertheless, it is unlikely that vestibular signals alone normally trigger directionally specific postural responses to support surface translations in standing humans because: (1) initial head accelerations resulting from body and head displacements were in opposite directions, but were associated with activation of the same leg and trunk postural muscles; (2) muscle responses to displacements of the head alone were only one third of the amplitude of responses to body displacements with equivalent maximum head accelerations; and (3) patients with profound bilateral vestibular loss showed patterns and latencies of leg and trunk muscle responses to body displacements similar to those of healthy subjects. Altering somatosensory information, by sway-referencing the support surface, increased the amplitude of ankle muscle activation to head displacements and reduced the amplitude of ankle muscle activation to body displacements, suggesting context-specific reweighting of vestibular and somatosensory inputs for posture. In contrast to responses to body displacements, responses to direct head displacements appear to depend upon a vestibulospinal trigger, since trunk and leg muscle responses to head displacements were absent in patients who had lost vestibular function as adults. Patients who lost vestibular function as infants, however, had near normal trunk and leg response to head displacements, suggesting a substitution of upper trunk and neck somatosensory inputs for missing vestibular inputs during development.     

Individuals with non-specific low back pain in an active episode demonstrate temporally altered torque responses and direction-specific enhanced muscle activity following unexpected balance perturbations

Individuals with a history of non-specific low back pain (LBP) while in a quiescent pain period demonstrate altered automatic postural responses (APRs) characterized by reduced trunk torque contributions and increased co-activation of trunk musculature. However, it is unknown whether these changes preceded or resulted from pain. To further delineate the relationship between cyclic pain recurrence and APRs, we quantified postural responses following multi-directional support surface translations, in individuals with non-specific LBP, following an active pain episode. Sixteen subjects with and 16 without LBP stood on two force plates that were translated unexpectedly in 12 directions. Net joint torques of the ankles, knees (sagittal only), hips, and trunk, in the frontal and sagittal planes, were quantified and the activation of 12 muscles of the lower limb unilaterally and the dorsal and ventral trunk, bilaterally, were recorded using surface electromyography (EMG). Peaks and latencies to peak joint torques, rates of torque development (slopes), and integrated EMGs characterizing baseline and active muscle contributions were analyzed for group by perturbation direction (torques) and group by perturbation by epoch interaction (EMG) effects. In general, the LBP cohort demonstrated APRs that were of similar torque magnitude and rate but peaked earlier compared to individuals without LBP. Individuals with LBP also demonstrated increased muscle activity following perturbation directions in which the muscle was acting as a prime mover and reduced muscle activity in opposing directions, proximally and distally, with some proximal asymmetries. These altered postural responses may reflect increased muscle spindle sensitivity. Given that these motor alterations are demonstrated proximally and distally, they likely reflect the influence of central nervous system processing in this cohort.     

Three components of postural control associated with pushing in symmetrical and asymmetrical stance

A number of occupational and leisure activities that involve pushing are performed in symmetrical or asymmetrical stance. The goal of this study was to investigate early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs) during pushing performed while standing. Ten healthy volunteers stood in symmetrical stance (with feet parallel) or in asymmetrical stance (staggered stance with one foot forward) and were instructed to use both hands to push forward the handle of a pendulum attached to the ceiling. Bilateral EMG activity of the trunk and leg muscles and the center of pressure (COP) displacements in the anterior–posterior (AP) and medial–lateral (ML) directions were recorded and analyzed during the EPAs, APAs, and CPAs. The EMG activity and the COP displacement were different between the symmetrical and asymmetrical stance conditions. The COP displacements in the ML direction were significantly larger in staggered stance than in symmetrical stance. In staggered stance, the EPAs and APAs in the thigh muscles of the backward leg were significantly larger, and the CPAs were smaller than in the forward leg. There was no difference in the EMG activity of the trunk muscles between the stance conditions. The study outcome confirmed the existence of the three components of postural control (EPAs, APAs, and CPAs) in pushing. Moreover, standing asymmetrically was associated with asymmetrical patterns of EMG activity in the lower extremities reflecting the stance-related postural control during pushing. The study outcome provides a basis for studying postural control during other daily activities involving pushing.     

Head acceleration following linear translations in the freely-standing cat

Summary The aim of this study was to determine whether vestibular information related to head acceleration is available for triggering postural responses to perturbations of stance in the freely-standing cat. Linear accelerations of the head were recorded during postural responses evoked by linear translations of the support surface. A consistent initial peak of acceleration was observed at an average latency of 22 ms and magnitude of 0.03 g (g is acceleration due to gravity, 9.8 m/s/s). The acceleration peak preceded the first evoked EMG activity by an average of 24 ms. It was concluded that stimulation of the vestibular apparatus was both adequate and early enough for the vestibular system to have triggered the automatic postural response.     

The influence of natural body sway on neuromuscular responses to an unpredictable surface translation

Previous research has shown that the postural configuration adopted by a subject, such as active leaning, influences the postural response to an unpredictable support surface translation. While those studies have examined large differences in postural conditions, it is of additional interest to examine the effects of naturally occurring changes in standing posture. Thus, it was hypothesized that the normal postural sway observed during quiet standing would affect the responses to an unpredictable support surface translation. Seventeen young adults stood quietly on a moveable platform and were perturbed in either the forward or backward direction when the location of the center of pressure (COP) was either 1.5 standard deviations anterior or posterior to the mean baseline COP signal. Postural responses, in the form of electromyographic (EMG) latencies and amplitudes, were recorded from lower limb and trunk muscles. When the location of the COP at the time of the translation was in the opposite, as compared to the same, direction as the upcoming translation, there was a significantly earlier onset of the antagonists (10-23%, i.e. 15-45 ms) and a greater EMG amplitude (14-39%) in four of the six recorded muscles. Stepping responses were most frequently observed during trials where the position of the COP was opposite to the direction of the translation. The results support the hypothesis that postural responses to unpredictable support surface translations are influenced by the normal movements of postural sway. The results may help to explain the large variability of postural responses found between past studies.     

Attributes of quiet stance in the chronic spinal cat

Standing is a dynamic task that requires antigravity support of the body mass and active regulation of the position of the body center of mass. This study examined the extent to which the chronic spinal cat can maintain postural orientation during stance and adapt to changes in stance distance (fore-hindpaw separation). Intact cats adapt to changes in stance distance by maintaining a constant horizontal orientation of the trunk and changing orientation of the limbs, while keeping intralimb geometry constant and aligning the ground reaction forces closely with the limb axes. Postural adaptation was compared in four cats before and after spinalization at the T(6) level, in terms of the forces exerted by each paw against the support, body geometry (kinematics) and electromyographic (EMG) activity recorded from chronic, indwelling electrodes, as well as the computed net torques in the fore and hindlimbs. Five fore-hindpaw distances spanning the preferred distance were tested before spinalization, with a total range of 20 cm from the shortest to the longest stance. After spinalization, the cats were trained on a daily basis to stand on the force platform, and all four cats were able to support their full body weight. Three of the four cats could adapt to changes in stance distance, but the range was smaller and biased toward the shorter distances. The fourth cat could stand only at one stance distance, which was 8 cm shorter than the preferred distance before spinalization. All cats shifted their center of pressure closer to the forelimbs after spinalization, but the amount of shift could largely be accounted for by the weight loss in the hindquarters. The three cats that could adapt to changes in stance distance used a similar strategy as the intact cat by constraining the trunk and changing orientation of the limb axes in close relation with the forces exerted by each limb. However, different postures in the fore- and hindlimbs were adopted, particularly at the scapula (more extended) and pelvis (tipped more anteriorly). Other changes from control included a redistribution of net extensor torque across the joints of the forelimb and of the hindlimb. We concluded that the general form of body axis orientation is relatively conserved in the spinal cat, suggesting that the lumbosacral spinal circuitry includes rudimentary set points for hindlimb geometry. Both mechanical and neural elements can contribute toward maintaining body geometry through stiffness regulation and spinal reflexes.     

Epigenetic development of postural responses for sitting during infancy

This study examined whether postural responses emerge in children in a predetermined way before independent sitting is achieved, and in what respect postural responses in infants differ from those in adults. Children just able to sit independently and children not yet able to sit were exposed to surface perturbations (translation and rotation) while body movement and electromyographic (EMG) responses were recorded. Perturbations causing a backward sway of the body (i.e., forward translation and legs-up rotation), elicited consistent patterns of muscle activity in ventral hip, trunk, and neck muscles in the independently sitting children. A high tonic EMG background activity in trunk and neck extensor muscles was inhibited at the onset of the ventral muscle activity. Kinematic analysis revealed that backward rotation of the pelvis was the first detectable body movement, while head movements (linear and angular displacement) were irregular and occurred later than the pelvis movement. Perturbations in the opposite direction, causing a forward sway, evoked variable responses in dorsal trunk and neck muscles, suggesting that the excitability level for postural responses was set according to the stability limits of the body. Children not yet able to sit without support were tested when the support around the waist, given by the experimenter's hands, was released prior to the onset of the platform perturbation. Postural responses were elicited in ventral muscles following a backward sway in all children and in about 60% of all trials. Often, only some of the ventral muscles were activated. No distinct responses were evoked during perturbations imposing a forward sway. These results suggest that (1) backward rotation of the pelvis triggers the postural adjustments in the independently sitting children; (2) a basic form of the postural adjustment develops in a predetermined manner before children practice independent sitting; and (3) the basic structure of ventral muscle activation pattern resembles that of adults, while the activation of the dorsal muscles (inhibition) differs in several aspects. These findings are in agreement with a recent model of central pattern generators for postural responses consisting of two operative levels. At the first level, which is triggered by backward rotation of the pelvis, the basic activation pattern is generated. At the second level, the pattern is shaped and fine-tuned by multisensory interactions from all activated sensory systems. The basic pattern present in the youngest infants may be produced mainly by neural networks at the first level, while the shaping function develops during practice, the shaping function being subjected to a learning process in which appropriate responses are formed in conjunction with the establishment of an internal neural representation for sitting.     

Postural responses in the cat to unexpected rotations of the supporting surface: evidence for a centrally generated synergic organization

Summary Postural reactions to disruptions of stance are rapid and automatic in both quadrupeds and bipeds. Current evidence suggests that these postural responses are generated by the central nervous system as patterns involving muscle synergies. This study attempted to test this hypothesis of a centrally generated postural mechanism by determining whether the same postural response could be evoked in the freely-standing cat under two different biomechanical conditions. The present work is an extension of previous experiments in which the stance of cats was perturbed by a horizontal translation of the supporting surface in the anterior and posterior directions (Rushmer et al. 1983). We now tested whether simple rotation of the metacarpo- and metatarsophalangeal (M-P) joints that mimics the digit rotation occurring during platform translation, was sufficient to evoke the translation postural response. The rotational perturbations were biomechanically different from translations in that the rotation did not cause displacement of the centre of mass of the animal, nor did it result in any significant movement about any but the M-P joints. Even so, rotational perturbations did evoke the appropriate translational muscle synergies in all four animals. Both plantar flexion rotation and headward translation activated the posterior hindlimb synergy (which included gluteus medius, semitendinosus and lateral gastrocnemius). Similarly, dorsiflexion rotation and tailward translation both activated the same anterior hindlimb synergy (iliopsoas, vastus lateralis and tibialis anterior) together with the forelimb synergy. The postural responses elicited by rotational perturbations were biomechanically inappropriate, and caused the animal to displace its own centre of mass away from the stable, control position. The most striking finding was that the group of muscles in which the medium latency postural response was evoked was different than the group from which short latency reflex responses were elicited. These data support the hypothesis that postural reactions are not merely reflex responses to local sensory inputs associated with the perturbation but, instead, represent a centrally generated response, with the muscle synergy being the controlled unit.Supported by NIH grants NS19484 and RR05593 as well as Good Samaritan Hospital     

Identification of the plant for upright stance in humans: multiple movement patterns from a single neural strategy

We determined properties of the plant during human upright stance using a closed-loop system identification method originally applied to human postural control by another group. To identify the plant, which was operationally defined as the mapping from muscle activation (rectified EMG signals) to body segment angles, we rotated the visual scene about the axis through the subject's ankles using a sum-of-sines stimulus signal. Because EMG signals from ankle muscles and from hip and lower trunk muscles showed similar responses to the visual perturbation across frequency, we combined EMG signals from all recorded muscles into a single plant input. Body kinematics were described by the trunk and leg angles in the sagittal plane. The phase responses of both angles to visual scene angle were similar at low frequencies and approached a difference of approximately 150 degrees at higher frequencies. Therefore we considered leg and trunk angles as separate plant outputs. We modeled the plant with a two-joint (ankle and hip) model of the body, a second-order low-pass filter from EMG activity to active joint torques, and intrinsic stiffness and damping at both joints. The results indicated that the in-phase (ankle) pattern was neurally generated, whereas the out-of-phase pattern was caused by plant dynamics. Thus a single neural strategy leads to multiple kinematic patterns. Moreover, estimated intrinsic stiffness in the model was insufficient to stabilize the plant.     

Automatic postural responses in the cat: responses of hindlimb muscles to horizontal perturbations of stance in multiple directions

Summary The effect of the direction of unexpected horizontal perturbations of stance on the organization of automatic postural responses was studied in cats. We recorded EMG activity in eight proximal and distal muscles of the hindlimb along with vertical forces imposed by the limbs in awake behaving cats while they stood on an hydraulic platform. Postural responses to motion of the platform in 16 different horizontal directions were recorded. Vertical force changes were consistent with passive shifts of the center of mass and active correction of stance by the animals. When the perturbation was in the sagittal plane, vertical force changes began about 65 ms following initial platform movement. When the perturbation contained a component in the lateral direction, latency for vertical force changes was about 25 ms and an inflection in the vertical force trace was observed at 65 ms. No EMG responses were observed with latencies that were short enough to account for the early force component and it was concluded that this force change was due to passive shifts of the center of mass. The amplitude of the EMG responses of each muscle recorded varied systematically as perturbation direction changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscle's angular range of activation. No angular range of activation was oriented strictly in the A-P or lateral directions. Most muscles displayed angular ranges of activation that encompassed a range of less than 180°. Onset latencies of EMG responses also varied systematically with perturbation direction. The amplitude and latency relationships between muscles, which made up the organization of postural responses, also varied systematically as perturbation direction was changed. This result suggests that direction of perturbation determines organizational makeup of postural responses, and for displacements in the horizontal plane, is considered a continuous variable by the nervous system.     

Neck muscle responses to stimulation of monkey superior colliculus. II. Gaze shift initiation and volitional head movements

We report neck muscle activity and head movements evoked by electrical stimulation of the superior colliculus (SC) in head-unrestrained monkeys. Recording neck electromyography (EMG) circumvents complications arising from the head's inertia and the kinetics of muscle force generation and allows precise assessment of the neuromuscular drive to the head plant. This study served two main purposes. First, we sought to test the predictions made in the companion paper of a parallel drive from the SC onto neck muscles. Low-current, long-duration stimulation evoked both neck EMG responses and head movements either without or prior to gaze shifts, testifying to a SC drive to neck muscles that is independent of gaze-shift initiation. However, gaze-shift initiation was linked to a transient additional EMG response and head acceleration, confirming the presence of a SC drive to neck muscles that is dependent on gaze-shift initiation. We forward a conceptual neural architecture and suggest that this parallel drive provides the oculomotor system with the flexibility to orient the eyes and head independently or together, depending on the behavioral context. Second, we compared the EMG responses evoked by SC stimulation to those that accompanied volitional head movements. We found characteristic features in the underlying pattern of evoked neck EMG that were not observed during volitional head movements in spite of the seemingly natural kinematics of evoked head movements. These features included reciprocal patterning of EMG activity on the agonist and antagonist muscles during stimulation, a poststimulation increase in the activity of antagonist muscles, and synchronously evoked responses on agonist and antagonist muscles regardless of initial horizontal head position. These results demonstrate that the electrically evoked SC drive to the head cannot be considered as a neural replicate of the SC drive during volitional head movements and place important new constraints on the interpretation of electrically evoked head movements.     

Quantitative evidence for multiple widespread representations of individual muscles in the cat motor cortex

We sought to determine why a given muscle appears represented in widespread loci in the motor cortex (MCx). To this end, we microstimulated every 500 μm along medio-lateral rows and recorded the evoked electromyographic (EMG) responses of up to a dozen forelimb muscles of the cat. A consistent finding in all animals studied was that along a given row, distal muscle responses could be elicited from medially situated cortical loci and conversely, proximal muscles responses from laterally situated cortical loci. In many such cases, the evoked EMG responses were such that the largest responses from a distal muscle were obtained by stimulation at a medially situated point and those of a proximal muscle from a laterally situated point. A Spearman correlation analysis showed that there was no correlation between cortical position and where the peak response of a given muscle occurred. These quantitative results strongly support the view that in the forelimb area of the cat MCx there exists widespread ‘colonies’ of corticospinal neurons with common spinal cord targets.     

The importance of somatosensory information in triggering and scaling automatic postural responses in humans

To clarify the role of somatosensory information from the lower limbs of humans in triggering and scaling the magnitude of automatic postural responses, patients with diabetic peripheral neuropathy and agematched normal controls were exposed to posterior horizontal translations of their support surface. Translation velocity and amplitude were varied to test the patients' ability to scale their postural responses to the magnitude of the translation. Postural response timing was quantified by measuring the onset latencies of three shank, thigh, and trunk muscles and response magnitude was quantified by measuring torque at the support surface. Neuropathy patients showed the same distalto-proximal muscle activation pattern as normal subjects, but the electromyogram (EMG) onsets in patients were delayed by 20–30 ms at all segments, suggesting an important role for somatosensory information from the lower limb in triggering centrally organized postural synergies. Patients showed an impaired ability to scale torque magnitude to both the velocity and amplitude of surface translations, suggesting that somatosensory information from the legs may be utilized for both direct sensory feedback and use of prior experience in scaling the magnitude of automatic postural responses.     

Eye position modulates the electromyographic responses of neck muscles to electrical stimulation of the superior colliculus in the alert cat

Rapid gaze shifts are often accomplished with coordinated movements of the eyes and head, the relative amplitude of which depends on the starting position of the eyes. The size of gaze shifts is determined by the superior colliculus (SC) but additional processing in the lower brain stem is needed to determine the relative contributions of eye and head components. Models of eye–head coordination often assume that the strength of the command sent to the head controllers is modified by a signal indicative of the eye position. Evidence in favor of this hypothesis has been recently obtained in a study of phasic electromyographic (EMG) responses to stimulation of the SC in head-restrained monkeys (Corneil et al. in J Neurophysiol 88:2000–2018, 2002b). Bearing in mind that the patterns of eye–head coordination are not the same in all species and because the eye position sensitivity of phasic EMG responses has not been systematically investigated in cats, in the present study we used cats to address this issue. We stimulated electrically the intermediate and deep layers of the caudal SC in alert cats and recorded the EMG responses of neck muscles with horizontal and vertical pulling directions. Our data demonstrate that phasic, short latency EMG responses can be modulated by the eye position such that they increase as the eye occupies more and more eccentric positions in the pulling direction of the muscle tested. However, the influence of the eye position is rather modest, typically accounting for only 10–50% of the variance of EMG response amplitude. Responses evoked from several SC sites were not modulated by the eye position.     

Weight support and balance during perturbed stance in the chronic spinal cat

The intact cat maintains balance during unexpected disturbances of stance through automatic postural responses that are stereotyped and rapid. The extent to which the chronic spinal cat can maintain balance during stance is unclear, and there have been no quantitative studies that examined this question directly. This study examined whether the isolated lumbosacral cord of the chronic spinal cat can generate automatic postural responses in the hindlimbs during translation of the support surface. Responses to 16 directions of linear translation in the horizontal plane were quantified before and after spinalization at the T(6) level in terms of forces exerted by each paw against the support, motion of the body segments (kinematics), and electromyographic (EMG) activity. After spinalization, the cats were trained on a daily basis to stand on the force platform, and all four cats were able to support their full body weight. The cats usually required assistance for balance or stability in the horizontal plane, which was provided by an experimenter exerting gentle lateral force at the level of the hips. Three of the four animals could maintain independent stance for a brief period (10 s) after the experimenter stabilized them. The fourth cat maintained weight support but always required assistance with balance. Perturbations were delivered during the periods of independent stance in three cats and during assisted stance in the fourth. A response to translation in the spinal cats was observed only in those muscles that were tonically active to maintain stance and never in the flexors. Moreover, latencies were increased and amplitudes of activation were diminished compared with control. Nevertheless, flexors and extensors were recruited easily during behaviors such as paw shake and stepping. It is concluded that centers above the lumbosacral cord are required for the full elaboration of automatic postural responses. Although the spinal cat can achieve good weight support, it cannot maintain balance during stance except for brief periods and within narrow limits. This limited stability is probably achieved through spinal reflex mechanisms and the stiffness characteristics of the tonically active extensors.