Impact of orthognathic surgery on the body posture.

BACKGROUND: Postural control is classically described as being based on the visual, vestibular, and proprioceptive musculo-articular sensory systems. The influence of mandibular proprioception on postural stabilization remains controversial. Most previous studies analyzed how postural stability is influenced by partial changes in mandibular proprioception (dental occlusion and jaw position). RESEARCH QUESTION: In the present experiment, we asked whether drastic mandibular changes, resulting from orthognathic surgery (including dental, joint and muscular efferents), modify postural control. METHODS: The analyzes were performed in 22 patients tested before, and 2.5 months, after orthognathic surgery for treatment of dysmorphic jaws. Experiments were performed under 4 experimental conditions: 2 visual conditions: Eyes Open (EO) and Eyes Closed (EC), and 2 occlusal conditions: Occlusion (OC: mandible positioned by the contact of the teeth), and Rest Position (RP: mandible positioned by the muscles without tooth contact). The analyses focused on head orientation in the frontal plane and on postural stabilization in a static task, consisting of standing upright. RESULTS: The results show that, 2.5 months after orthognathic surgery, head orientation in the frontal plane was improved, since patient's external intercanthal lines became closer to the true horizontal line when they were tested EC and in OC condition. Postural responses, based on the wavelet transformation data, highlight an improvement in maintaining an upright stance for all the tested sensory conditions. However, such improvement was greater in the EC and RP conditions. SIGNIFICANCE: These results show, for the first time, that after drastic mandibular changes, the weight of proprioceptive cues linked to the mandibular system may be so enhanced that it may constitute a new reference frame to orient the head in space, in darkness, and improve static postural stabilization, even in the presence of visual cues.

Vestibular compensation following vestibular neurotomy.

OBJECTIVES: Four studies assessing vestibular compensation in Menière's disease patients undergoing unilateral vestibular neurotomy, using different analysis methods, are reviewed, with a focus on the different strategies used by patients according to their preoperative sensory preference. MATERIAL AND METHODS: Four prospective studies performed in a university tertiary referral center were reviewed, measuring the pattern of vestibular compensation in Menière's disease patients before and after unilateral vestibular neurotomy on various assessment protocols: postural syndrome assessed on static posturography and gait analysis; perceptual syndrome assessed on subjective visual vertical perception; and oculomotor syndrome assessed on ocular cyclotorsion. RESULTS: Vestibular compensation occurred at variable intervals depending on the parameter investigated. Open-eye postural control and gait/walking returned to normal one month after neurotomy. Fine balance analysis found that visual perception of the vertical and ocular cyclotorsion impairment persisted at long-term follow-up. Clinical postural disturbance persisted only when visual afferents were cut off (eyes closed). These impairments were the expression of a postoperative change in postural strategy related to the new use of visual and non-visual references. CONCLUSIONS: Understanding pre-operative interindividual variation in balance strategy is critical to screening for postural instability and tailoring vestibular rehabilitation.

Functional neuro-anatomy of egocentric versus allocentric space representation.

INTRODUCTION: The functional neuroanatomy of the egocentric and allocentric representations of space remains poorly studied with neuroimaging. Here we aim to determine brain structures subserving two different kinds of spatial representations centred on the main axis of either the body or the external scene. METHOD: Sixteen healthy participants evaluated the alignment of a bar relative to the middle of their body (Ego) or relative to another stimulus (Allo) during functional MRI. In a control task (Ctrl), they had to judge the colour of the bar. RESULTS: Correct response rates and response times were similar in the three tasks. fMRI data revealed a predominant role of the right hemisphere in the egocentric task (Ego vs. Allo): selective activity was found in the occipital, superior parietal, and inferior frontal cortices, as well as in the precuneus and supplementary motor area. On the left side, the insula, thalamus, and cerebellum were also activated. Conversely, the allocentric task (Allo vs. Ctrl) showed selective activity centred on the left temporal gyrus. DISCUSSION: This study demonstrates a right hemisphere dominance for representations centred on the longitudinal body axis, but more left-sided activity for scene/object-centred representations of space. These new data shed light on the unique role of several regions involved in spatial perception and help better understand spatial deficits in patients with right hemispheric lesions.

Posture and cognition in the elderly: interaction and contribution to the rehabilitation strategies.

In this paper we review the effects of aging on sensory systems and their impact on posture, balance and gait. We also address cognitive aging and attempt to specify which altered cognitive functions negatively impact balance and walking. The role of cognition in postural control is tested with dual-task experiments. This situation results in deleterious effects due to an attentional overload. Given the human cognitive system has limited capacities, we propose that simultaneously performing two tasks depends on the capacity of each individual to perform these tasks on a continuum between automatic execution to highly controlled performance. A level of maximum control exceeds the subject's attentional capacity, which makes it impossible to perform both tasks simultaneously. The subject therefore prioritizes one of the tasks. We use representative dual-task studies from the literature to illustrate the relationship between the different cognitive components and their impact on the control of posture and gait in elderly subjects with altered cognitive capacities and with elderly subjects who are fallers or who have altered sensory-motor capacities. Recently this postural-cognitive relationship was addressed with a new approach. We report how cognitive training can improve dual-task management and we attempt to define the cognitive mechanisms that may be responsible for better postural balance.

Electrical stimulation superimposed onto voluntary muscular contraction reduces deterioration of both postural control and quadriceps femoris muscle strength.

Fatiguing exercise of the quadriceps femoris muscle degrades postural control in human subjects. The aim of this work was to compare the effects of the fatigue of the quadriceps femoris induced by voluntary muscular contraction (VC), and by electrical stimulation (ES) superimposed onto voluntary muscular contraction (VC+ES), on postural control and muscle strength. Fourteen healthy young adults participated in the study. Postural control and muscle strength were evaluated using a stable force platform and an isokinetic dynamometer, respectively, before (PRE condition) and after the completion of each fatiguing exercise (immediately: POST condition; after a 5 min recovery time: POST 5 condition). In POST, both postural control and muscle strength were impaired by both fatiguing exercises. However, the impairment was higher for VC than for VC+ES. In POST 5, for both fatiguing exercises, postural control recovered its initial level while muscle strength did not. These results suggest that superimposing ES onto voluntary muscular contractions (VCs) impaired muscle strength and postural control less than did VCs alone. However the duration of recovery of these two neurophysiological functions did not differ for the two fatiguing exercises. For both exercises, postural control was restored faster than the ability to produce muscular strength.

Age-related changes in posture control are differentially affected by postural and cognitive task complexity.

The simple postural task of quiet standing, which requires minimal attentional resources, is generally paired with cognitive activity. Competition for attentional resources is a consequence of simultaneously performing balance tasks and cognitive tasks, and impairment of attentional resource allocation with aging leads to increased risks of fall. We investigated age-related changes in posture control during dual task performance, using a paradigm that crossed a static (quiet standing) and a dynamic (keeping balance on a translational force plate) postural task and cognitive tasks of low demand (mental arithmetic) and high demand (spatial memory). Postural performance was analyzed through center-of-pressure displacements using both statistical (body sway area/sway velocity) and nonlinear (wavelet transform) methods in three age groups (younger, middle-aged, and older healthy participants). Results showed that 1) the nonlinear analysis method was more sensitive than the traditional approach in distinguishing performance between age groups, a result that explains discrepancies in the dual-task literature; 2) dual-tasking costs were dependent on both postural task difficulty and cognitive task complexity, corroborating previous investigations; 3) younger adults improved their postural performance during dual-tasking, but older adults lowered their performance; 4) balance recovery strategies in the dynamic postural task appeared to differ in younger versus older adults. Together, our findings on dual-tasking can be interpreted within the conceptual frame of task prioritization: shifting attention away from postural task automates posture control in the younger adults, whereas prioritization of postural task and selection of compensatory strategy are the main characteristics of posture control in the older population.

Vestibular syndrome: a change in internal spatial representation.

The vestibular system contributes to a wide range of functions from reflexes to spatial representation. This paper reviews behavioral, perceptive, and cognitive data that highlight the role of changes in internal spatial representation on the vestibular syndrome. Firstly, we review how visual vertical perception and postural orientation depend on multiple reference frames and multisensory integration and how reference frames are selected according to the status of the peripheral vestibular system (i.e., unilateral or bilateral hyporeflexia), the environmental constraints (i.e., sensory cues), and the postural constraints (i.e., balance control). We show how changes in reference frames are able to modify vestibular lesion-induced postural and locomotor deficits and propose that fast changes in reference frame may be considered as fast-adaptive processes after vestibular loss. Secondly, we review data dealing with the influence of vestibular loss on higher levels of internal representation sustaining spatial orientation and navigation. Particular emphasis is placed on spatial performance according to task complexity (i.e., the required level of spatial knowledge) and to the sensory cues available to define the position and orientation within the environment (i.e., real navigation in darkness or visual virtual navigation without any actual self-motion). We suggest that vestibular signals are necessary for other sensory cues to be properly integrated and that vestibular cues are involved in extrapersonal space representation. In this respect, vestibular-induced changes would be based on a dynamic mental representation of space that is continuously updated and that supports fast-adaptive processes.

Kinematics of treadmill locomotion in rats conceived, born, and reared in a hypergravity field (2 g). Adaptation to 1 g.

The kinematics of treadmill locomotion in rats conceived, born, and raised in a hypergravity environment (HG: 2g) until the age of 3 months was investigated for 5 weeks after their exposition to earth's gravity. The locomotor performance of the HG rats (N=7) was compared to that of age-matched control rats (N=8) housed at 1g for the same period. Kinematic analysis of treadmill locomotion was performed up to 35 days of terrestrial life by an optoelectronic motion analyzer (ELITE system). Results showed that the HG rats exhibited a faster locomotor rhythm (increased number of steps/s), walked closer to the ground, and had a more dorsiflexed foot position. Also, HG rats had shorter steps. The data also highlight a fast adaptation to normal gravity since all the locomotor parameters returned to normal values within 3 weeks. The locomotor modifications may be seen as the persistence of a hypergravity-induced posturo-locomotor adaptation in the centrifuge and/or to more functional changes of sensorimotor systems. Because locomotor performance of HG rats is not severely affected, it is concluded that early development of locomotion processes is highly resistant to gravito-inertial changes.

Deficits and recovery of head and trunk orientation and stabilization after unilateral vestibular loss.

The aim of the study was to analyse changes in the orientation and stabilization of the head and trunk and their recovery after complete unilateral loss of vestibular information in humans. The ability of nine Ménière's patients to orient and stabilize their heads and trunks in space was investigated during a simple dynamic task of knee-bends and compared with the performance of 10 healthy subjects. Patients' performance was recorded before unilateral vestibular neurotomy (UVN) and during the time-course of recovery (1 week, 1 month, 3 months). Experiments were performed both in eyes open (EO) and eyes closed (EC) conditions to evaluate the role of visual cues in the recovery process. Head and trunk mean angular position (orientation) and mean maximal angular rotation (stabilization) in the roll plane and the yaw plane were recorded using a video motion analysis system. The results indicate that, in the acute stage after UVN (1 week), patients exhibit marked impairments in head and trunk orientation in both visual conditions. In the EC condition, head and trunk were deviated towards the operated side in the roll plane and the yaw plane. Head and trunk stabilization in space was impaired in the roll plane and associated with increased stabilization of the head on the shoulders. Interestingly, vision caused a complete inversion of the orientation pattern, with head and trunk rotations towards the intact side in the roll plane and the yaw plane. Relative to darkness, vision also reduced head and trunk oscillations. Recovery from abnormal head orientation in the light and impaired head stability in both visual conditions was achieved within 1 month and 3 months after UVN, respectively. However, head and trunk orientation in the dark and trunk stabilization in the roll plane remained uncompensated 3 months post-lesion. These results suggest that unilateral vestibular loss leads to a postural syndrome similar to that described previously for various animal species. They confirm the necessity of vestibular inputs for properly stabilizing head and trunk during self-generated displacements in healthy subjects. They also support the notion that vestibular compensation relies on visual cues whose substitution role gradually decreases after UVN.

How changes in vestibular and visual reference frames combine to modify body orientation in space.

The aim of this study was to analyse how changes in vestibular and visual reference frames combine to modify body orientation in space, and to determine the relationship between postural, oculomotor and perceptive parameters. Changes in vestibular and visual references were investigated by comparing controls and vestibular defective patients (Ménière's patients tested before and one week after unilateral vestibular nerve section) under three visual contexts (light with and without vertical and horizontal coordinates, darkness). Unilateral vestibular loss was responsible for postural and perceptive deviations whose direction depended on the presence of visual reference frame. We suggest these changes vary according to the spatial reference frame patients are based on. Postural changes were related to perceptive modifications but not to eye cyclotorsion.

Spatial performance of unilateral vestibular defective patients in nonvisual versus visual navigation.

The purpose of this study was to investigate the effects of unilateral vestibular neurotomy on humans ability to perform navigation tasks. These tasks provided self-motion feedback by way of either locomotor activity only (nonvisual navigation or "locomotor task") or visual motion cues only (visually simulated navigation or "visual task"). After exploration of an environment in which 4 locations were marked by different objects, subjects attempted to navigate to those locations either by reproducing the same paths as those followed during exploration, by reversing routes, or by making spatial inferences (shortcuts). Vestibular defective patients were tested one day before surgical treatment and during the recovery time course following unilateral vestibular nerve lesion (1 week, 1 month, and 3 month later). Their performance was assessed by measuring turn error and distance error in both navigation tasks and was compared to that of control subjects tested 4 times at similar time intervals. Turn error in the reproduction of previously explored routes in the locomotor task was lower in patients before surgery than in controls, suggesting the existence of compensatory processes. In the acute stage (1 week) after unilateral vestibular lesion, turn error was greater in patients than in controls for the highest level of mental representation (spatial inferences or reversing routes); impairment at making accurate rotations had disappeared by 1 month after vestibular lesion in both navigation tasks. These results point to the role of vestibular cues, in interaction with other sensory modalities, in the elaboration of an accurate internal representation of the environment. In addition, they suggest that unilateral suppression of vestibular information would induce transitory spatial memory disorganization at a high level of information processing.

Short-term changes in neck muscle and eye movement responses following unilateral vestibular neurectomy in the cat.

The purpose of this study was to investigate changes in neck muscle and eye movement responses during the early stages of vestibular compensation (first 3 weeks after unilateral vestibular neurectomy, UVN). Electromyographic (EMG) activity from antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles and eye movements were recorded during sinusoidal visual and/or otolith vertical linear stimulations in the 0.05-1 Hz frequency range (corresponding acceleration range 0.003-1.16 g) in the head-fixed alert cat. Preoperative EMG activity from the splenius and longus capitis muscles showed a pattern of alternate activation of the antagonist neck muscles in all the cats. After UVN, two motor strategies were observed. For three of the seven cats, the temporal activation of the individual neck muscles was the same as that recorded before UVN. For the other four cats, UVN resulted in a pattern of coactivation of the flexor and extensor neck muscles because of a phase change of the splenius capitis. In both subgroups, the response patterns of the antagonist neck muscles were consistent for each cat independently of the experimental conditions, throughout the 3 weeks of testing. Cats displaying alternate activation of antagonist neck muscles showed an enhanced gain of the visually induced neck responses, particularly in the high range of stimulus frequency, and a gain decrease in the otolith-induced neck responses at the lowest frequency (0.25 Hz) only. By contrast, for cats with neck muscle coactivation, the gain of the visually induced neck responses was basically unaffected relative to preoperative values, whereas otolith-induced neck responses were considerably decreased in the whole range of stimulation. As concerns oculomotor responses, results in the two subgroups of cats were similar. The optokinetic responses were not affected by the vestibular lesion. On the contrary, otolith-induced eye responses showed a gain reduction and a phase lead. Deficits and short-term changes after UVN of otolith- and semicircular canal-evoked collic and ocular responses are compared.

Sensory strategies in human postural control before and after unilateral vestibular neurotomy.

Vestibular inputs tonically activate the anti-gravitative leg muscles during normal standing in humans, and visual information and proprioceptive inputs from the legs are very sensitive sensory loops for body sway control. This study investigated the postural control in a homogeneous population of 50 unilateral vestibular-deficient patients (Ménière's disease patients). It analyzed the postural deficits of the patients before and after surgical treatment (unilateral vestibular neurotomy) of their diseases and it focused on the visual contribution to the fine regulation of body sway. Static posturographic recordings on a stable force-plate were done with patients with eyes open (EO) and eyes closed (EC). Body sway and visual stabilization of posture were evaluated by computing sway area with and without vision and by calculating the percentage difference of sway between EC and EO conditions. Ménière's patients were examined when asymptomatic, 1 day before unilateral vestibular neurotomy, and during the time-course of recovery (1 week, 2 weeks, 1 month, 3 months, and 1 year). Data from the patients were compared with those recorded in 26 healthy, age- and sex-matched participants. Patients before neurotomy exhibited significantly greater sway area than controls with both EO (+52%) and EC (+93%). Healthy participants and Ménière's patients, however, displayed two different behaviors with EC. In both populations, 54% of the subjects significantly increased their body sway upon eye closure, whereas 46% exhibited no change or significantly swayed less without vision. This was statistically confirmed by the cluster analysis, which clearly split the controls and the patients into two well-identified subgroups, relying heavily on vision (visual strategy, V) or not (non-visual strategy, NV). The percentage difference of sway averaged +36.7%+/-10.9% and -6.2%+/-16.5% for the V and NV controls, respectively; +45.9%+/-16.8% and -4.2%+/-14.9% for the V and NV patients, respectively. These two distinct V and NV strategies seemed consistent over time in individual subjects. Body sway area was strongly increased in all patients with EO early after neurotomy (1 and 2 weeks) and regained preoperative values later on. In contrast, sway area as well as the percentage difference of sway were differently modified in the two subgroups of patients with EC during the early stage of recovery. The NV patients swayed more, whereas the V patients swayed less without vision. This surprising finding, indicating that patients switched strategies with respect to their preoperative behavior, was consistently observed in 45 out of the 50 Ménière's patients during the whole postoperative period, up to 1 year. We concluded that there is a differential weighting of visual inputs for the fine regulation of posture in both healthy participants and Ménière's patients before surgical treatment. This differential weighting was correlated neither with age or sex factors, nor with the clinical variables at our disposal in the patients. It can be accounted for by a different selection of sensory orientation references depending on the personal experience of the subjects, leading to a more or less heavy dependence on vision. The change of sensory strategy in the patients who had undergone neurotomy might reflect a reweighting of the visual and somatosensory cues controlling balance. Switching strategy by means of a new sensory selection of orientation references may be a fast adaptive response to the lesion-induced postural instability.

Gaze strategies during linear motion in head-free humans.

1. Eye-head coordination strategies during horizontal displacements along the y (interaural) axis were investigated in human subjects seated on a sled (linear accelerator device) and tested in head-free conditions. They were instructed to stabilize their gaze, while in motion, on an earth-fixed memorized target and then, after cart immobilization, to look again at the real target. The last part of the test required a corrective saccade, which enabled us to evaluate the error of the subject's displacement estimation. Eye and head compensatory reflexes were tested within the 0.001-0.2 g acceleration range with a sinusoidal motion amplitude of 0.8 m peak to peak. 2. Fixation stabilization on a memorized target was achieved by different eye-head coordination strategies. According to the relative contribution of eye and head motion, a continuum among individual strategies was observed, covering a range of head contributions varying from 0 to almost 100%. All these strategies were well adapted because they contributed to the counteraction of the displacement and led to an optimal gaze accuracy. 3. The use of various gaze strategies during linear motion to achieve the same movement differed according to the subject, but also depended upon motion kinematics. As a rule, head contribution increased as the magnitude of linear acceleration was enhanced. 4. Different eye-head coordination strategies implicated either a linear vestibulo-ocular reflex (LVOR) or ocular responses composed of a combination of antagonistic angular and linear vestibulo-ocular reflexes (AVOR-LVOR). The slow phase direction of these two oculomotor responses for fixation stabilization on the target were compensatory and anticompensatory, respectively. 5. One of the major points of this study was the contribution of the saccadic system to gaze strategies, even in our experimental conditions where the head was free to move. We concluded that vestibular-saccadic cooperation appears to be a common feature in the elaboration of adequate fixation stabilization in daily life situations. 6. The functional coupling of these various subsystems involved in fixation stabilization depended on the range of motion: while the acceleration increased, the saccadic eye movements were replaced by vestibulo-ocular responses whose slow phase direction was opposite that of head motion and, therefore, directed away from the target. 7. Fast components of the nystagmic pattern of eye movements were able to improve gaze position accuracy, bringing the eyes toward the memorized target.(ABSTRACT TRUNCATED AT 400 WORDS)

Recovery of head postural control following unilateral vestibular neurectomy in the cat. Neck muscle activity and neuronal correlates in Deiters' nuclei.

Recovery of head postural control after unilateral vestibular neurectomy was investigated in the alert cat by chronically recording the spontaneous neck muscle EMG activity from splenius capitis on both sides and the vestibulocollic reflexes evoked during roll and pitch tilts. Neuronal correlates occurring within the lateral (Deiters) vestibular nuclei (LVN) were also recorded during the time-course of recovery. During the acute phase (1-2 weeks), the cats exhibited strong imbalance in spontaneous neck muscle activity, characterized by increased muscular tone in the ipsilateral splenius capitis muscle and hypoactivity in the contralateral one. At the same time, the mean resting activity of Deiters' neurons strongly decreased on the deafferented side, while a slight but significant decrease was observed on the intact side. Vestibulocollic reflexes were totally lacking during the acute phase, whatever the direction and the amplitude of tilt. Recovery developed in the following weeks, leading to complete rebalance of spontaneous EMG activity as well as near to normal static vestibulocollic reflexes 5 weeks after the lesion. However, compensation remained sub-normal during roll tilts while overcompensation was found during pitch tilts, suggesting that the intact labyrinth would play a leader role in the recovery process but that bilateral cooperation of the two labyrinths is required for proper head postural control. Five weeks are also needed for a partial rebalancing of resting activity between both LVN. These results indicate that changes in neck muscle activity observed in the acute cats and that recovery found in the compensated animals could result from modifications in neural networks controlling neck musculature, such as the LVN.

[Vestibular studies: current status, prospects]. / Explorations vestibulaires: actualités--perspectives.

Equilibrium is based on gaze stabilization. Clinical examinations currently test, individually the three components involved in equilibrium: vestibular, visual and proprioception. New investigations are being developed to test equilibrium dynamic conditions. After briefly reviewing vestibular physiology, under the authors discuss the various tests of vestibular function and equilibrium.

Functional coupling of the stabilizing eye and head reflexes during horizontal and vertical linear motion in the cat.

The otolith contribution and otolith-visual interaction in eye and head stabilization were investigated in alert cats submitted to sinusoidal linear accelerations in three defined directions of space: up-down (Z motion), left-right (Y motion), and forward-back (X motion). Otolith stimulation alone was performed in total darkness with stimulus frequency varying from 0.05 to 1.39 Hz at a constant half peak-to-peak amplitude of 0.145 m (corresponding acceleration range 0.0014-1.13 g). Optokinetic stimuli were provided by sinusoidally moving a pseudorandom visual pattern in the Z and Y directions, using a similar half peak-to-peak amplitude (0.145 m, i.e., 16.1 degrees) in the 0.025-1.39 Hz frequency domain (corresponding velocity range 2.5 degrees-141 degrees/s). Congruent otolith-visual interaction (costimulation, CS) was produced by moving the cat in front of the earth-stationary visual pattern, while conflicting interaction was obtained by suppressing all visual motion cues during linear motion (visual stabilization method, VS, with cat and visual pattern moving together, in phase). Electromyographic (EMG) activity of antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles as well as horizontal and vertical eye movements (electro-oculography, EOG) were recorded in these different experimental conditions. Results showed that otolith-neck (ONR) and otolith-ocular (OOR) responses were produced during pure otolith stimulation with relatively weak stimuli (0.036 g) in all directions tested. Both EMG and EOG response gain slightly increased, while response phase lead decreased (with respect to stimulus velocity) as stimulus frequency increased in the range 0.25-1.39 Hz. Otolith contribution to compensatory eye and neck responses increased with stimulus frequency, leading to EMG and EOG responses, which oppose the imposed displacement more and more. But the otolith system alone remained unable to produce perfect compensatory responses, even at the highest frequency tested. In contrast, optokinetic stimuli in the Z and Y directions evoked consistent and compensatory eye movement responses (OKR) in a lower frequency range (0.025-0.25 Hz). Increasing stimulus frequency induced strong gain reduction and phase lag. Oculo-neck coupling or eye-head synergy was found during optokinetic stimulation in the Z and Y directions. It was characterized by bilateral activation of neck extensors and flexors during upward and downward eye movements, respectively, and by ipsilateral activation of neck muscles during horizontal eye movements. These visually-induced neck responses seemed related to eye velocity signals. Dynamic properties of neck and eye responses were significantly improved when both inputs were combined (CS).(ABSTRACT TRUNCATED AT 400 WORDS)

Functional coupling of the stabilizing gaze reflexes during vertical linear motion in the alert cat.

Eye-head coordination is mainly achieved by means of stabilizing reflexes (VOR, VCR, OKR) and orienting movements (eye-neck surgery) underlying the close cooperation of the visual and vestibular systems in gaze stabilization. The functional coupling of these different sensorimotor subsystems has been principally analysed using rotatory stimulation of the whole body and/or of the visual surround. The aim of the present study was to investigate the dynamic properties of these stabilizing gaze reflexes and their coupling during linear motion in the vertical plane. These investigations were performed in the alert cat under open-loop conditions (head fixed). Otolith stimulation consisted of vertically translating the cat in total darkness using sinusoidal linear motion (0.025 Hz-1.39 Hz; 290 mm peak-to-peak amplitude). Optokinetic stimulation was provided by sinusoidally moving a pseudo-random visual pattern in front of the cat and in the vertical plane, with identical kinematic parameters. Normal visual-otolith interaction was performed by translating the cat in front of the stationary visual surround while conflicting interaction was provided by moving the animal and the visual pattern in phase and at the same velocity (visual stabilization). Results showed that the vertical otolith-neck reflex is very poorly developed or absent in the low frequency range of motion (0.025 Hz-0.25 Hz) while consistent EMG activity is found during pure optokinetic stimulation. EMG responses are in phase with the visual surround velocity in the upward direction and with the upward OKR velocity. A close correlation is observed between the EMG gain and the OKR gain, which both decrease in this low frequency range, indicating that gaze stabilization would be mainly ensured by the OKR and a functional oculo-collic coupling or eye-neck surgery in the vertical plane. On the contrary, gaze stabilization is principally achieved by way of the otolith-neck reflex in the higher frequency range of motion (above 0.25 Hz). EMG responses recorded during otolith stimulation exhibit a relatively constant gain and a phase lead with respect to motion velocity which progressively reduces as the stimulus frequency increases up to 1.39 Hz. When present, EMG responses evoked during the optokinetic stimulation show strong gain attenuation and phase lag. Normal visual-otolith interaction induces neck muscle activity which parallels the optokinetic and the otolith responses in the low and high frequency ranges, respectively. The motor responses are however improved in terms of gain and phase values in the whole frequency range when both sensory inputs are combined.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal coding of linear motion in the vestibular nuclei of the alert cat. II. Response characteristics to vertical optokinetic stimulation.

Extracellular activity from vestibular nuclei neurons and vertical eye movements were recorded in the alert cat during sinusoidal optokinetic stimulation in the vertical plane at frequencies varying from 0.0125 Hz to 0.75 Hz. Among a population of 96 vestibular units located in and around Deiters' nucleus, 73 neurons (76%) displayed a firing rate modulation which followed the input at the standard parameters of visual stimulation (0.05 Hz; 10.1 deg/s or 9.1 cm/s peak to peak velocity). Two different patterns of modulation were found. In 42 cells (57%) an increase in the firing rate was observed during motion of the visual scene in the downward direction, while 31 neurons (43%) showed the opposite behavior, with an enhanced firing rate during upward movement. The phase of the neuronal responses was close (+/- 45 degrees) to the velocity peaks (+90 degrees: downward and -90 degrees: upward) of visual scene motion for 65 among the 73 neurons. Mean values of phase was -6.1 +/- 19.5 degrees (SD) and -3.2 +/- 15.5 degrees (SD) with respect to the +90 degrees and -90 degrees velocity peaks, respectively. In the frequency range 0.0125-0.75 Hz, the phase of the neuronal responses remained almost stable, with only a slight lag which reaches -22 degrees at the 0.25 Hz visual stimulation. The firing rate modulation was found to be predominant at low frequencies (0.0125 Hz-0.25 Hz), with three distinct peaks of modulation occurring either at 0.025 Hz, 0.10 Hz or 0.25 Hz, depending on the recorded cells. Above 0.5 Hz, the cell modulation was very poorly developed or even absent. A gain attenuation was observed in all units, which was more important in cells showing a peak of modulation at 0.025 Hz as compared with the others (-20.7 dB vs -9.6 dB, respectively, in the 0.025 Hz-0.25 Hz decade). The gain of the optokinetic reflex (OKR) progressively decreased from mean values of 0.78 +/- 0.15 to 0.05 +/- 0.06 in the 0.025 Hz-0.5 Hz frequency range. A close correlation was observed between the OKR slow phase velocity and the modulation of the neuronal responses in the two cell populations with maximal modulations at 0.10 Hz or 0.25 Hz. No correlations were noticed in the third population characterized by a peak of modulation at 0.025 Hz. In all units, the phase of eye movement velocity and of neuronal responses were both related to the velocity of the visual surround motion. These correlations were also found when varying the amplitude of the visual stimulation at a fixed frequency.(ABSTRACT TRUNCATED AT 400 WORDS)