Biomedical subjects

A Berthoz

Publications and source records attributed to A Berthoz.

At least 127 records · Page 7Linked to original sources

PubMed1990

Head stabilization during various locomotor tasks in humans. I. Normal subjects.

Head kinematics were studied in ten normal subjects while they executed various locomotor tasks. The movement of the body was recorded with a video system which allowed a computer reconstruction of motion of joint articulations and other selected points on the body in three dimensions. Analyses focus on head translation along the vertical axis and rotation in the sagittal plane. This was done by recording the displacement of a line approximating the plane of horizontal semi-circular canals (the Frankfort plane: F-P). Four conditions were studied: free walking (W) walking in place (WIP) running in place (R) and hopping (H). In the 4 experimental conditions, amplitude and velocity of head translation along the vertical axis ranged from 1 cm to 25 cm and 0.15 m/s to 1.8 m/s. In spite of the disparities in the tasks regarding the magnitude of dynamic components, we found a significant stabilization of the F-P around the earth horizontal. Maximum amplitude of F-P rotation did not exceed 20 degrees in the 4 situations. Vertical angular velocities increased from locomotion tasks to the dynamic equilibrium task although the maximum values remained less than 140 degrees/s. Predominant frequencies of translations and rotations in all the tasks were within the range 0.4-3.5 Hz and harmonics were present up to 6-8 Hz. During walking in darkness, mean head position is tilted downward, with the F-P always below the earth horizontal. Darkness did not significantly influence the amplitude and velocity of head angular displacement during W, WIP and R, but during H the amplitude decreased by 37%. Residual head angular displacement is found to compensate for head translation during the 4 conditions. Our study emphasizes the importance of head stabilization as part of the postural control system and described as a basis for inertial guidance.

Adult↗

PubMed1989

A neurophysiological study of prepositus hypoglossi neurons projecting to oculomotor and preoculomotor nuclei in the alert cat.

The activity of 62 antidromically identified prepositus hypoglossi neurons was recorded in 10 alert cats during spontaneous, vestibular or visually induced eye movements. Neurons were antidromically activated from stimulating electrodes implanted in the ipsilateral medial longitudinal fasciculus (n = 24), the ipsilateral interstitial nucleus of Cajal (n = 6), the ipsilateral parabigeminal nucleus (n = 2), the contralateral superior colliculus (n = 6) and the contralateral cerebellar posterior peduncle (n = 24). Neurons were identified as eye-movement-related when their rate-position and/or rate-velocity plots showed correlation coefficients greater than or equal to 0.6. They were further classified as "position", "position-velocity" and "velocity-position" according to their relative eye position and velocity coefficients. However, they seemed to be distributed as a continuum in which a progressive decrease of eye velocity sensitivity was accompanied by a proportional increase in eye position sensitivity. "Position-velocity" neurons (n = 9) were mainly horizontal type II neurons projecting to the vicinity of the oculomotor complex; two of these neurons with vertical sensitivity were also activated from the interstitial nucleus of Cajal. Mean position and velocity sensitivity of these neurons were 5.2 spikes/s per degree and 0.62 spikes/s per degree per second, respectively. Pure "position" neurons (n = 7) also showed activation during ipsilateral eye fixations; their mean position gain was 7.3 spikes/s per degree and they projected to the ipsilateral oculomotor and Cajal nuclei, and to the contralateral superior colliculus. "Velocity-position" neurons (n = 18) were type I or II neurons with rather irregular tonic firing rates and a mean velocity gain of 0.75 spikes/s per degree per second. Type II "velocity-position" neurons projected mainly to the oculomotor area, while type I neurons projected preferentially to the cerebellum. A special type of "pause" neuron (n = 5), with very low firing rate and pausing mainly for contralateral saccades, was activated exclusively from the contralateral posterior peduncle. Many neurons with weak eye movement sensitivity (n = 22) were activated mainly (73%) from the cerebellum. It can be concluded that the prepositus hyperglossi nucleus distributes specific eye movement related signals to motor and premotor brainstem and cerebellar structures. The variability of interspike intervals of representative prepositus hypoglossi neurons of each class was compared to the discharge variability of identified abducens motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Abducens Nerve↗

PubMed1989

Head kinematic during various motor tasks in humans.

Head kinematic during various motor tasks was studied in ten subjects. The movement of the body was recorded with a video system (E.L.I.T.E.) which allows a computer reconstruction of three-dimensional motion of selected points on the body. Analysis is focused on head rotation in the horizontal and vertical planes. The results demonstrate that the amplitude and the maximum velocity do not exceed respectively 38 deg/s and 185 deg/s. However the head is intermittently stabilized and the angle of this stabilization is dependent upon the task and related to the direction of gaze. Darkness had no significant effect on head rotational velocity during walking but caused a decrease in velocity during running and hopping. The results suggest that head stabilization (1) is related to an ocular fixation point in the direction of gaze in space and (2) is probably regulated on the basis of a predictive mode of sensory motor control.

Adult↗

PubMed1989

Neural correlates of horizontal vestibulo-ocular reflex cancellation during rapid eye movements in the cat.

1. The aim of the present study is to describe the behaviour of identified second-order vestibular neurones in the alert cat during eye saccades. A selection of neurones which are involved in horizontal eye movements has been made. The activity has been compared with a selected sample of abducens motoneurones recorded in the same animals. 2. Alert head-fixed cats were used for this study. Eye movements were recorded by the scleral search coil technique. Abducens motoneurones were identified by antidromic stimulation from the VIth nerve with chronically implanted electrodes. They were recorded extracellularly. 3. Second-order vestibular neurones were identified by orthodromic stimulation from the vestibular organs. They were recorded intra-axonally and injected with horseradish peroxidase after recording of their physiological characteristics. Their morphology was reconstructed from frozen sections. 4. All the recorded vestibular neurones showed various amounts of eye position sensitivity. The firing rate (F) - horizontal eye position (H) characteristics are compared for abducens and vestibular neurones. The population average values are F = 33 + 4 H for motoneurones and F = 51 + 2.4 H for vestibular neurones. 5. All recorded vestibular neurones showed an increase of discharge rate during contralateral horizontal saccades and a strong decrease or pause during ipsilateral saccades. Firing rate - horizontal eye velocity sensitivity has been calculated. 6. Results suggest a strong inhibitory input on vestibular neurones from the saccadic generator. This mechanism underlies the suppression of the vestibulo-ocular reflex during saccades. Our results suggest that in the cat, for saccades of amplitude smaller than 20 deg, there is a variable degree of suppression which is provided by a projection of excitatory bursters (EBNs) on second-order vestibular neurones through inhibitory type II neurones. 7. We also conclude from this study that the eye position sensitivity of vestibular second-order neurones is in fact a motor signal indicating a motor error, i.e. the amount of head or eye movement which remains to be done in order to align gaze on target with the eyes centred in the orbit.

Abducens Nerve↗

PubMed1989

Contribution of the otoliths to the calculation of linear displacement.

1. The present work is a quantitative study of the eye movements induced by linear translation when the subject is instructed to stabilize his gaze on a memorized earth-fixed target. These experiments may allow a better understanding of the central processing of otolithic signals. 2. Human subjects were submitted to either sinusoidal or step-like horizontal linear displacements along the interaural (Y)-axis in darkness, seated in a cart moving along a linear track. Each subject's head was fixed by a helmet secured to the cart. They were asked to keep their eyes on an earth-fixed memorized target at 63 cm from them on the X-axis. 3. During sinusoidal motion, a combination of low smooth compensatory eye movements and of compensatory saccades allowed the subjects to track the memorized target. The linear model of the responses of five subjects (seven sessions) exhibited a near-ideal slope of 1.14 (range 0.84-1.58). Two subjects did not compensate properly for their displacement. The mean "vestibular-saccadic" (VS) gain (ratio of overall eye movement peak-peak amplitude versus head displacement amplitude) was 1.52 +/- 0.80 (SD), showing an overestimation of head displacement. 4. The otolith-ocular reflex (OOR) mean gain values (ratio of slow phase cumulated peak-peak amplitude versus head displacement amplitude) were about 0.13 degrees/cm. This value is 5 times higher than what has been reported in the literature, probably due to the fact that the target was at a short distance. 5. The number of saccades occurring during sinusoidal stimulations varied according to the different subjects. They were obviously compensatory saccades and not quick phases. They indicate that although the gain of the OOR was small, the brain has computed the adequate desired eye position. 6. During steplike head displacements in darkness, although the OOR gain was also small, seven of the eight subjects could stabilize their gaze with a mean VS gain of 1.01 +/- 0.70. The linear model for the pooled responses of these subjects exhibited a slope of 0.82. 7. When subjects were instructed not to move their eyes during the translation, three of the five examined could still correctly reproduce the head movement amplitude with saccades, even as late as 50 s after motion had stopped. This indicates that head displacement was stored with the adequate metrics and could be used to drive the saccadic system. 8. Bilabyrinthectomized patients could not perform any adequate gaze stabilization. This shows that the observed performance was of vestibular origin.

Adult↗

PubMed1989

[Cooperation and substitution of the saccadic system and the reflexes of vestibular origin: should the "reflex" concept be revised?].

Neuronal mechanisms of production of ocular saccadic movements are discussed, together with stabilizing movements of vestibular origin, and descriptions given of several neuronal elements constituting the tecto-reticulo-spinal system, the generator of horizontal saccadic movements in the brain stem, the vestibulo-ocular pathway, etc... Analysis of these pathways and of the signals travelling through them suggests that the overall apparatus is not constituted of isolated reflex modules but acts as a true functional unit for cooperation between orientation and stabilization mechanisms. In addition, coordination of synergies between eye and head movements appear to be based on the activity of clearly identifiable neurons. This neuronal implementation of the concept of synergy is completed further by a remarkable functional flexibility. The latter allows substitution by the brain, for example, fo saccades instead of vestibular reflex movements when the latter are deficient because of lesions or disturbances in the coherence of sensory information. The principal conclusion reached is that cognitive factors, which imply representations of body movements in space activated as a function of the subject's goals, must be considered when ocular movement, and gaze control is described.

Animals↗

PubMed1988

Changing patterns of eye-head coordination during 6 h of optically reversed vision.

1) This study investigates the early development of adaptive changes in oculomotor function associated with coordinated eye-head tracking of the optically reversed image of an earth-fixed target seen through horizontally reversing dove prism goggles attached to the skull. 2) Two tasks comprised a) fixation of a single target during head rotation which causes the seen target's image to move in the direction of head motion by an amount exactly equal to the head movement itself (the 1-Target task), and b) change of gaze onto a displaced target with head free to move (2-Target task). 3) The 1-Target task requires the eyes to move in a direction opposite to that of the normal vestibulo-ocular reflex (VOR). The 2-Target task is identical, except that reorientation onto the new target calls for an initial saccadic eye movement in a direction opposite to that of the ensuing head movement, which is contrary to the normal pattern of eye-head coordination during gaze shifts. 4) Eye (EOG) and head (potentiometer) movements were continuously recorded (0-250 Hz) in an apparatus which permitted sudden, unexpected, electromagnetic braking of the head movement, either just before or during the intended manoeuvre. 5) Early adaptive strategies employed reduction of VOR gain, rearrangement of timing, amplitude and shape of "catch-up" saccades and the introduction of centrally programmed eye movements uncovered by the braking manoeuvres. 6) All of these phenomena were detectable in an initial series of 60 trials, in which the total exposure to visual-vestibular conflict was less than 30 s. They became more systematized and more marked after 6 h of active reversed vision experience. 7) Specifically, mean VOR gain, measured within the first 80 ms of head movement (deemed free of visuomotor influence), became markedly attenuated (25% in the first test series; 66% after 6 h of active vision-reversed exercise). In addition (not included in the above percentages) there were numerous occasions of complete absence of measurable VOR during head rotation, in both the first and final test series. 8) In the 1-Target task, the latency of the first "catch-up" saccade (re onset of head movement) tended to offset residual VOR by becoming shortened to the point of synchrony with head movement onset. This saccade (not present in control tests) continued to occur on those occasions when the head was unpredictably prevented from moving, and when head movements were made in the dark.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗

PubMed1988

Eye-head coupling in humans. I. Simultaneous recording of isolated motor units in dorsal neck muscles and horizontal eye movements.

A tonic coupling between the horizontal component of eye position and dorsal neck muscle activity has been demonstrated in the cat and monkey. In order to demonstrate this synergy in humans and study its characteristics, we have measured the relation between the firing rate of individual motor unit of the splenius muscle and voluntary horizontal shifts of gaze using 5 degrees steps, in head-fixed subjects. Eye movement recording was achieved by conventional binocular electro-oculography and the activity of the right splenius muscle was recorded with Bronks coaxial bipolar electrodes inserted manually at the C4-C5 intervertebral level. The activity of 51 motor units in 10 subjects has been recorded. For all subjects, motor units firing rate increased when the gaze shifted to the ipsilateral side, and both increase in firing rate and recruitment were observed. These results demonstrate that the tonic eye head synergy is also present in man.

Action Potentials↗

PubMed1988

Vertical eye movements related signals in antidromically identified medullary reticular formation neurons in the alert cat.

Short and long lead burst neurons antidromically activated from the rostral mesencephalic reticular formation, and synaptically activated from the contralateral superior colliculus were recorded in the medullary reticular formation underlying the prepositus hypoglossi nucleus. These neurons were shown to be related to vertical eye movements, ranging from pure vertical to oblique planes. Vertical saccade coding was similar to that of horizontal short lead pontine cells. The presence of vertical short and long lead burst neurons in the medullary reticular formation raises new questions about the organization of the control of eye movements in the vertical plane.

Action Potentials↗

PubMed1988

Eye movements induced by off-vertical axis rotation (OVAR) at small angles of tilt.

Off-vertical rotation (OVAR) in darkness induced continuous horizontal nystagmus in humans at small tilts of the rotation axis (5 to 30 degrees). The horizontal slow eye velocity had two components: a mean velocity in the direction opposite to head rotation and a sinusoidal modulation around the mean. Mean velocity generally did not exceed 10 deg/s, and was less than or equal to the maximum velocity of optokinetic after-nystagmus (OKAN). Both the mean and modulation components of horizontal nystagmus increased with tilt angle and rotational velocity. Vertical slow eye velocity was also modulated sinusoidally, generally around zero. The amplitude of the vertical modulation increased with tilt angle, but not with rotational velocity. In addition to modulations in eye velocity, there were also modulations in horizontal and vertical eye positions. These would partially compensate for head position changes in the yaw and pitch planes during each cycle of OVAR. Modulations in vertical eye position were regular, increased with increases in tilt angle and were separated from eye velocity by 90 deg. These results are compatible with the interpretation that, during OVAR, mean slow velocity of horizontal nystagmus is produced by the velocity storage mechanism in the vestibular system. In addition, they indicate that the otolith organs induce compensatory eye position changes with regard to gravity for tilts in the pitch, yaw and probably also the roll planes. Such compensatory changes could be utilized to study the function of the otolith organs. A functional interpretation of these results is that nystagmus attempts to stabilize the image on the retina of one point of the surrounding world. Mean horizontal velocity would then be opposite to the estimate of head rotational velocity provided by the output of the velocity storage mechanism, as charged by an otolithic input during OVAR. In spite of the lack of actual translation, an estimate of head translational velocity could, in this condition, be constructed from the otolithic signal. The modulation in horizontal eye position would then be compensatory for the perceived head translation. Modulation of vertical eye velocity would compensate for actual changes in head orientation with respect to gravity.

Adult↗

PubMed1988

Motion perceptions induced by off-vertical axis rotation (OVAR) at small angles of tilt.

Off-vertical axis rotation in darkness induces a perception of body motion which lasts as long as rotation continues. Perceived body motion is the combination of two simultaneous displacements. The most easily perceived is a translation without rotation along a conical path, at the frequency of the actual rotation. Meanwhile, the subjects feel as if they were always facing towards the same direction. The summit of the cone is generally below the head, from the waist to below the feet, and subjects have a sense of progression in the direction opposite to actual spinning. Some subjects feel, on the contrary, the summit of the cone above their heads, and the progression in the direction of spinning. Subjects also perceived another body motion, although it was faint for some of them. It consists of a rotation at low velocity in the same direction as progression along the cone. The axis of the cone is perceived as slowly rotating along a larger cone. These motion perceptions increase with tilt angle and rotation velocity. They probably result from the analysis by the Central Nervous System of the acceleration acting on the otoliths. The perceived trajectory would be reconstructed from estimates of gravity, and kinematic variables such as head translational acceleration and velocity, and head rotational velocity. The same variables would account for OVAR-induced nystagmus. Motion sickness would result from the impossibility of reconstructing a consistent body movement from most sets of values of these variables.

Adult↗

PubMed1988

Contribution of eye positioning to control of the upside-down standing posture.

The present study attempts to clarify the relationship between eye and body positioning during an upside-down standing posture on the hands. In this posture the head was observed to be stable and the eyes were anchored to an earth-fixed target. We measured the variations of body sway when subjects displaced their gaze upward or downward from their anchoring position. They did this voluntarily under instruction, and involuntarily by means of base-down or base-up wedge prisms. Results show that the anchoring point chosen by the subjects corresponds to a perceived limit of their body sway. They suggest that vision is also used to convey the desired optimal zone for the center of gravity in cases where fine balance is required.

Eye Movements↗

PubMed1988