(73/868) Central positional nystagmus simulated by a mathematical ocular motor model of otolith-dependent modification of Listing's plane.
To find an explanation of the mechanisms of central positional nystagmus in neurological patients with posterior fossa lesions, we developed a three-dimensional (3-D) mathematical model to simulate head position-dependent changes in eye position control relative to gravity. This required a model implementation of saccadic burst generation, of the neural velocity to eye position integrator, which includes the experimentally demonstrated leakage in the torsional component, and of otolith-dependent neural control of Listing's plane. The validity of the model was first tested by simulating saccadic eye movements in different head positions. Then the model was used to simulate central positional nystagmus in off-vertical head positions. The model simulated lesions of assumed otolith inputs to the burst generator or the neural integrator, both of which resulted in different types of torsional-vertical nystagmus that only occurred during head tilt in roll plane. The model data qualitatively fit clinical observations of central positional nystagmus. Quantitative comparison with patient data were not possible, since no 3-D analyses of eye movements in various head positions have been reported in the literature on patients with positional nystagmus. The present model, prompted by an open clinical question, proposes a new hypothesis about the generation of pathological nystagmus and about neural control of Listing's plane. (+info)
(74/868) Neck muscles in the rhesus monkey. II. Electromyographic patterns of activation underlying postures and movements.
Electromyographic (EMG) activity was recorded in < or = 12 neck muscles in four alert monkeys whose heads were unrestrained to describe the spatial and temporal patterns of neck muscle activation accompanying a large range of head postures and movements. Some head postures and movements were elicited by training animals to generate gaze shifts to visual targets. Other spontaneous head movements were made during orienting, tracking, feeding, expressive, and head-shaking behaviors. These latter movements exhibited a wider range of kinematic patterns. Stable postures and small head movements of only a few degrees were associated with activation of a small number of muscles in a reproducible synergy. Additional muscles were recruited for more eccentric postures and larger movements. For head movements during trained gaze shifts, movement amplitude, velocity, and acceleration were correlated linearly and agonist muscles were recruited without antagonist muscles. Complex sequences of reciprocal bursts in agonist and antagonist muscles were observed during very brisk movements. Turning movements of similar amplitudes that began from different initial head positions were associated with systematic variations in the activities of different muscles and in the relative timings of these activities. Unique recruitment synergies were observed during feeding and head-shaking behaviors. Our results emphasize that the recruitment of a given muscle was generally ordered and consistent but that strategies for coordination among various neck muscles were often complex and appeared to depend on the specifics of musculoskeletal architecture, posture, and movement kinematics that differ substantially among species. (+info)
(75/868) Lateralizing value of early head turning and ictal dystonia in temporal lobe seizures: a video-EEG study.
To investigate early head turning, we retrospectively studied videotapes of 262 seizures from 82 patients who were seizure free after temporal lobectomy. Early head movements were arbitrarily classified into non-tonic turning, tonic turning, and absence of turning. Among the 222 seizures which showed early head turning, 168 (75.7%) had non-tonic turning and 54 (24.3%) had tonic turning. The direction of the first head turning was ipsilateral to the epileptogenic foci in 132 (78.6%) seizures with non-tonic turning and in 35 (64.8%) seizures showing tonic head turning. The proportion of seizures with turning towards the ipsilateral side in the presence of tonic and non-tonic head turning were significantly different (P= 0.04). Seventy-four seizures (28.2%) evolved to secondary generalization, more frequently found in seizures with early head turning (P= 0.0015) and especially those showing tonic turning (P< 0.0001). The direction of head turning immediately preceding secondary generalization was contralateral to the lesion side in 53 seizures (82.8%). Dystonic upper limb posturing occurred in 86 seizures (32.8%), exclusively contralateral to the seizure focus, whereas 65 (75.6%) were associated with initial head turning ipsilateral to the focus. In summary, temporal lobe seizures with tonic head turning tends to secondarily generalize and the direction of head turning before secondarily generalized was contralateral to the seizure foci. Earlier in the seizures the direction of non-tonic head turning tends to be towards the epileptogenic hemisphere. In addition, dystonic posturing of the extremities is a significant lateralizing sign to the contralateral hemisphere in temporal lobe seizures. (+info)
(76/868) Depth thresholds of motion parallax as a function of head movement velocity.
The lower parallactic depth threshold is determined by (a) the ratio of relative image velocity to head velocity when the head moves fast (>13 cm/s) and (b) the motion threshold when the head moves slow (<13 cm/s). These two results are explained by a single system that codes the ratio of relative image velocity to head velocity, using the same image velocity signal as that used for motion perception. In this explanation, ratios coded from low relative image velocities, which are slightly higher than the motion threshold, produce a perception of depth only when the head moves slowly. (+info)
(77/868) Experimental control of eye and head positions prior to head-unrestrained gaze shifts in monkey.
A coordinated movement of the eyes and head in the head-unrestrained condition is often used to change orientation between targets. Under natural conditions, these gaze shifts are typically generated with the eyes roughly centered in the orbits. To achieve experimental control of eye and head positions, a miniature laser was mounted on the head implants of monkeys that were trained to point the head to one target and direct gaze to another before generating a head-unrestrained gaze shift to a third target (dissociation paradigm). For comparison, monkeys were also required to make gaze shifts between stimuli, without any constraints on eye and head positions (standard paradigm). Analyses indicated that movement parameters, limited to horizontal gaze shifts, were similar for both behavioral conditions. Thus, the proposed technique and behavioral paradigm, when used in conjunction with electrophysiological and pharmacological experiments, may facilitate the study of neural control of gaze. (+info)
(78/868) Early head movements elicited by visual stimuli or collicular electrical stimulation in the cat.
During the course of previous recordings of visually-triggered gaze shifts in the head-unrestrained cat, we occasionally observed small head movements which preceded the initiation of the saccadic eye/head gaze shift toward a visual target. These early head movements (EHMs) were directed toward the target and occurred with a probability varying between animals from 0.4% to 16.4% (mean=5.2%, n=11 animals). The amplitude of EHM ranged from 0.4 degrees to 8.3 degrees (mean=1.9 degrees ), their latency from 66 to 270 ms (median=133 ms) and the delay from EHM onset to gaze shift onset averaged 183+/-108 ms (n=240). Their occurrence did not depend on visual target eccentricity in the studied range (7-35 degrees ), but influenced the metrics and dynamics of the ensuing gaze shifts (gain and velocity reduced). We also found in the two tested cats that low intensity microstimulation of the superior colliculus deeper layers elicited a head movement preceding the gaze shift. Altogether, these results suggest that the presentation of a visual target can elicit a head movement without triggering a saccadic eye/head gaze shift. The visuomotor pathways triggering these early head movements can involve the deep superior colliculus. (+info)
(79/868) Cortical and subcortical contributions to coordinated eye and head movements.
This paper summarizes recent experiments conducted by the authors - experiments that studied the behavioral characteristics of large gaze shifts and the neural bases of coordinated movements of the eyes and head. (+info)
(80/868) Temporal coordination of the human head and eye during a natural sequential tapping task.
The 'natural' temporal coordination of head and eye was examined as four subjects tapped a sequence of targets arranged in 3D on a worktable in front of them. The head started to move before the eye 48% of the time. Both the head and eye started to move 'simultaneously' (within 8 ms of each other) 37% of the time. The eye started to move before the eye only 15% of the time. Gaze-shifts required to perform the tapping task were relatively large, 68% of them were between 27 degrees and 57 degrees. Gaze-shifts were symmetrical. There were almost as many lefts as rights. Very little inter- or intra-subject variability was observed. These results were not expected on the basis of prior studies of head/eye coordination performed under less natural conditions. They also were not expected given the results of two rather similar, relatively natural, prior experiments. We conclude that more observations under natural conditions will have to be made before we understand why, when and how human beings coordinate head and eyes as they perform everyday tasks in the work-a-day world. (+info)