Interlimb postural coordination in the standing cat

The dorsal-side-up body posture in standing quadrupeds is maintained by coordinated activity of four limbs. We studied this coordination in the cat standing on the platform periodically tilted in the frontal plane. By suspending different body parts, we unloaded one, two, or three limbs. The activity of selected extensor muscles and the contact forces under the limbs were recorded. With all four limbs on the platform, extensors of the fore- and hindlimbs increased their activity in parallel during ipsilateral downward tilt. With two forelimbs on the platform, this muscular pattern persisted in the forelimbs and in the suspended hindlimbs. With two hindlimbs on the platform, the muscular pattern persisted only in the hindlimbs, but not in the suspended forelimbs. These results suggest that coordination between the two girdles is based primarily on the influences of the forelimbs upon the hindlimbs. However, these influences do not necessarily determine the responses to tilt in the hindlimbs. This was demonstrated by antiphase tilting of the fore- and hindquarters. Under these conditions, the extensors of the fore- and hindlimbs appeared uncoupled and modulated in antiphase, suggesting an independent control of posture in the fore- and hindquarters. With only one limb supporting the shoulder or hip girdle, a muscular pattern with normal phasing was observed in both limbs of that girdle. This finding suggests that reflex mechanisms of an individual limb generate only a part of postural corrections; another part is produced on the basis of crossed influences.     

Visual cortical contribution to open-loop and feed-back control of convergence eye movements in the cat

Open-loop and closed loop controls in convergence eye movement have been reported by human psycho-physical studies. To investigate the visual cortical involvement in open-loop convergence eye movement, we trained a cat to elicit anticipatory convergence (convergence eye movement before the onset of target movement) by using an approaching visual target with a preceding alarm signal. After 1-2 weeks of training, anticipatory convergence was observed in more than half of the trials in seven cats. The frequency of occurrence of anticipatory convergence was significantly decreased after electrocoagulation in the convergence-related region of the lateral suprasylvian (LS) area, an extrastriate visual cortex of the cat. On the other hand, the localized injection of Muscimol, a GABA-A agonist, reduced visually evoked convergence, but caused no significant effects to anticipatory convergence. These differential results suggest that the LS plays a role in both open-loop and feed-back control of convergence eye movement, and a GABA-A sensitive subregion is involved in the feed-back control of convergence eye movement.     

Activity of pyramidal tract neurons in the cat during postural corrections

The dorsal side-up body orientation in quadrupeds is maintained by a postural control system. We investigated participation of the motor cortex in this system by recording activity of pyramidal tract neurons (PTNs) from limb representations of the motor cortex during postural corrections. The cat was standing on the platform periodically tilting in the frontal plane, and maintained equilibrium at different body configurations: with the head directed forward (symmetrically alternating loading of the left and right fore limbs), or with the head voluntary turned to the right or to the left (asymmetrical loading). We found that postural corrective responses to tilts included an increase of the contact forces and activity of limb extensors on the side moving down, and their decrease on the opposite side. The activity of PTNs was strongly modulated in relation to the tilt cycle. Phases of activity of individual PTNs were distributed over the cycle. Thus the cortical output mediated by PTNs appeared closely related to a highly automatic motor activity, the maintenance of the body posture. An asymmetrical loading of limbs, caused by head turns, resulted in the corresponding changes of motor responses to tilts. These voluntary postural modifications were also well reflected in the PTNs' activity. The activity of a part of PTNs correlated well with contact forces, in some others with the limb muscle activity; in still others no correlation with these variables was observed. This heterogeneity of the PTNs population suggests a different functional role of individual PTNs.     

The influence of postural threat on the cortical response to unpredictable and predictable postural perturbations

This study investigated the effects of postural threat on the cortical response associated with postural reactions to predictable and unpredictable perturbations to upright stance. Postural threat was manipulated by having individuals stand on an elevated surface to alter the context in which the postural task was performed. Ten healthy young adults experienced a series of predictable and unpredictable trunk perturbations when standing at ground level and at the edge of a platform located 3.2 m above the ground. Participants felt less confident, more fearful of falling, more anxious and less stable when standing at the high surface height. Unpredictable perturbations generated a large negative potential (N1) which was increased by 84% when standing at the high compared to low surface height. The magnitude of change in this potential was related to the magnitude of change in balance perceptions, such as confidence and fear. Predictable perturbations did not generate a N1 potential but instead produced an anticipation-related potential prior to the perturbation. This cortical activity observed in response to predictable perturbations was not influenced by postural threat. A large N1 potential was observed for a 'surprise' perturbation that followed a series of predictable perturbations. There was a trend for the amplitude of this potential to be increased when standing at the high compared to low surface height. The results of this study provide evidence for the modulating influence of psychological factors related to postural threat on the cortical activity associated with postural reactions to unpredictable perturbations.     

Stance and sensory feedback influence on postural dynamics

This study examined the effects of ice-induced plantar desensitization and the withdrawal of visual feedback on the magnitude and time-dependent structure of postural sway variability. The magnitude of variability was quantified as the area of an ellipse enclosing 95% of the center of pressure (COP) time-series during normal and tandem stances. The same time-series were also analyzed using Approximate Entropy (ApEn) and Cross-Approximate Entropy (CrossApEn) as indices of irregularity and asynchrony between the mediolateral and anteroposterior COP motions. Variability increased during tandem stance and this increase was compounded by both visual feedback withdrawal and cutaneous desensitization. Both ApEn (mediolateral and anteroposterior COP motion) and CrossApEn increased with the withdrawal of visual feedback during the tandem stance, but decreased significantly during normal stance. The results of the study demonstrate that plantar desensitization only affected the magnitude of sway variability but did not alter its time-dependent structure. Contrasting effects on the structure of postural sway variability with visual feedback withdrawal were observed during the different stances, highlighting the role of task demands in postural dynamics.     

Visual movement perception in the cat is directionally selective

Summary Behavioral experiments show that the visual system of cat contains mechanisms which are selective for direction of stimulus movement. The cat's contrast detection threshold for a drifting grating is unaffected by the addition of a grating moving in the opposite direction; this same pattern of results is found for human observers. The convergence of cat and human psychophysical data suggests that man's brain may hold direction-specific neurons, similar to those known to exist in the cat brain.This research was supported by grants from the National Institutes of Health and the National Science Foundation     

Effects of local fatigue of the lower limbs on postural control and postural stability in standing posture

Postural stability and postural control were studied before and after a fatigue protocol of soleus muscles. Postural stability was assessed by the centre of gravity motion, which was computed from the motion of the centre of pressure, evaluating the postural control. Ten healthy male subjects were asked to stand as still as possible with eyes open before and after the fatigue protocol, performed in a sitting position. Fatigue was assumed on the basis of a shortening of the exertion time of the soleus muscles at 60% of their maximal voluntary contraction. Results of the whole group showed that fatigue modified postural control but did not change postural stability. The same results were observed only for some subjects. However, these results indicate an increase of the neuromuscular activity in high frequencies.     

Sensorimotor integration in human postural control

It is generally accepted that human bipedal upright stance is achieved by feedback mechanisms that generate an appropriate corrective torque based on body-sway motion detected primarily by visual, vestibular, and proprioceptive sensory systems. Because orientation information from the various senses is not always available (eyes closed) or accurate (compliant support surface), the postural control system must somehow adjust to maintain stance in a wide variety of environmental conditions. This is the sensorimotor integration problem that we investigated by evoking anterior-posterior (AP) body sway using pseudorandom rotation of the visual surround and/or support surface (amplitudes 0.5-8 degrees ) in both normal subjects and subjects with severe bilateral vestibular loss (VL). AP rotation of body center-of-mass (COM) was measured in response to six conditions offering different combinations of available sensory information. Stimulus-response data were analyzed using spectral analysis to compute transfer functions and coherence functions over a frequency range from 0.017 to 2.23 Hz. Stimulus-response data were quite linear for any given condition and amplitude. However, overall behavior in normal subjects was nonlinear because gain decreased and phase functions sometimes changed with increasing stimulus amplitude. "Sensory channel reweighting" could account for this nonlinear behavior with subjects showing increasing reliance on vestibular cues as stimulus amplitudes increased. VL subjects could not perform this reweighting, and their stimulus-response behavior remained quite linear. Transfer function curve fits based on a simple feedback control model provided estimates of postural stiffness, damping, and feedback time delay. There were only small changes in these parameters with increasing visual stimulus amplitude. However, stiffness increased as much as 60% with increasing support surface amplitude. To maintain postural stability and avoid resonant behavior, an increase in stiffness should be accompanied by a corresponding increase in damping. Increased damping was achieved primarily by decreasing the apparent time delay of feedback control rather than by changing the damping coefficient (i.e., corrective torque related to body-sway velocity). In normal subjects, stiffness and damping were highly correlated with body mass and moment of inertia, with stiffness always about 1/3 larger than necessary to resist the destabilizing torque due to gravity. The stiffness parameter in some VL subjects was larger compared with normal subjects, suggesting that they may use increased stiffness to help compensate for their loss. Overall results show that the simple act of standing quietly depends on a remarkably complex sensorimotor control system.     

Visual cells in the pontine nuclei of the cat.

Two hundred and thirty-two visually activated neurones were recorded in a small area of the rostral pontine nuclei of cats. The location of visually activated neurones was coextensive with the input from visual areas of cat's cortex as determined by degeneration studies. 2. Pontine visual cells could only be driven by visual stimuli. Cells responsive to somatosensory or auditory stimuli were also found in different regions in rostral pontine nuclei. They too responded to only one modality. 3. 96% of the cells were directionally selective. 4. Pontine visual cells were responsive to a wide range of stimulus speeds. Some cells responded to targets moving as fast as 1000 degrees/sec without losing directional selectivity. No pontine visual cells gave a clearly sustained response to a stationary stimulus. 5. Exact stimulus configurations were not critical. Large fields containing many spots were the most effective stimuli for 50% of the cells. Inhibition of responses depending upon stimulus dimensions, direction of movement, or location in the visual field was found for many cells. 6. Receptive field dimensions were large, ranging in size from 3 degrees X 4 degrees to more than an entire hemifield. 7. 94% of the cells had receptive fields which were centred in the contralateral hemifield. 8. 98% of the cells could be driven from both eyes. 9. The properties of the pontine visual cells suggest a corticopontocerebellar pathway sensitive to a wide range of speeds and directions of movement, but not sensitive to precise form.     

Effects of immersion in virtual reality on postural control

In the present study, we examined the effects of the time lag between visual scene and the head movement in the virtual reality (VR) world on motion sickness and postural control in healthy volunteers. After immersion in VR with additional time lags (from 0 to 0.8 s) to the inherent delay (about 250 ms), the visual-vestibular conflict induced a slight motion sickness in experimental subjects, but no change was noticed in the body sway path with eyes open and closed. However, Romberg ratio of body sway path with eyes closed divided by that with eyes open after immersion in VR was significantly decreased in comparison with that before immersion in VR. Since Romberg ratio is an index of visual dependency on postural control, this finding indicates that the immersion in VR decreases the visual dependency on postural control. It is suggested that adaptation to visual-vestibular conflict in VR immersion increases the contribution of vestibular and somatosensory inputs to postural control by ignoring the conflicting delayed visual input in the VR world. VR may be a promising treatment for visual vertigo in vestibular patients with unsuccessful compensation by its ability to induce vestibular and somatosensory reweighing for postural control.     

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