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T Vilis

Publications and source records attributed to T Vilis.

At least 55 records · Page 3Linked to original sources
PubMed1987

The pattern of changes produced in the saccadic system and vestibuloocular reflex by visually patching one eye.

The purpose of the present study was to determine whether, in the absence of visual input to one eye, saccades remained equal in the two eyes. The same question was addressed for the VOR gain of the two eyes. After 1 wk during which one eye was continuously patched, the saccadic properties of only the unseeing eye showed changes consisting of a change, usually a decrease, in saccadic step magnitude, postsaccadic drift with an exponentially decaying component in the temporal direction, and the appearance of a vertical component as well as vertical postsaccadic drift during horizontally directed saccades. Effects were also observed in the VOR consisting of a change in gain and a vertical component during horizontal head rotation. As with saccades, the vertical component in the patched eye was upward when the eye was deviated nasally. When the patch was removed, normal function was restored within 1 day to the previously patched eye without impairing the function of the unpatched eye. These results suggest that the conjugate nature of saccades and the VOR is in part the consequence of a selective, visually driven, calibration mechanism, which can alter commands to motoneurons of one muscle of a conjugate muscle pair without affecting commands to the other. The similarity of changes observed in the VOR and saccades after patching suggests that elements common to both are altered in the absence of vision.

Animals↗
PubMed1987

Implications of rotational kinematics for the oculomotor system in three dimensions.

1. This paper develops three-dimensional models for the vestibuloocular reflex (VOR) and the internal feedback loop of the saccadic system. The models differ qualitatively from previous, one-dimensional versions, because the commutative algebra used in previous models does not apply to the three-dimensional rotations of the eye. 2. The hypothesis that eye position signals are generated by an eye velocity integrator in the indirect path of the VOR must be rejected because in three dimensions the integral of angular velocity does not specify angular position. Computer simulations using eye velocity integrators show large, cumulative gaze errors and post-VOR drift. We describe a simple velocity to position transformation that works in three dimensions. 3. In the feedback control of saccades, eye position error is not the vector difference between actual and desired eye positions. Subtractive feedback models must continuously adjust the axis of rotation throughout a saccade, and they generate meandering, dysmetric gaze saccades. We describe a multiplicative feedback system that solves these problems and generates fixed-axis saccades that accord with Listing's law. 4. We show that Listing's law requires that most saccades have their axes out of Listing's plane. A corollary is that if three pools of short-lead burst neurons code the eye velocity command during saccades, the three pools are not yoked, but function independently during visually triggered saccades. 5. In our three-dimensional models, we represent eye position using four-component rotational operators called quaternions. This is not the only algebraic system for describing rotations, but it is the one that best fits the needs of the oculomotor system, and it yields much simpler models than do rotation matrix or other representations. 6. Quaternion models predict that eye position is represented on four channels in the oculomotor system: three for the vector components of eye position and one inversely related to gaze eccentricity and torsion. 7. Many testable predictions made by quaternion models also turn up in models based on other mathematics. These predictions are therefore more fundamental than the specific models that generate them. Among these predictions are 1) to compute eye position in the indirect path of the VOR, eye or head velocity signals are multiplied by eye position feedback and then integrated; consequently 2) eye position signals and eye or head velocity signals converge on vestibular neurons, and their interaction is multiplicative.(ABSTRACT TRUNCATED AT 400 WORDS)

Computer Simulation↗
PubMed1986

A reexamination of the gain of the vestibuloocular reflex.

The properties of the vestibuloocular reflex (VOR) when the axis of rotation is behind the eyes and fixation of a near target is required were studied in the monkey. The magnitude of VOR gain in each eye was found to be above 1.0 and near the ideal value for stabilizing a retinal image. Evidence that this large VOR gain was not visually mediated was provided by the observations that no reduction in gain and no phase lag were observed at high frequencies of head rotation (2 Hz), large gain was observed in the dark, and large gain was observed within 10-20 ms of the start of head rotation. The magnitude of VOR gain was found to increase with increasing radius of head rotation and also to increase with decreasing target distance. When the distances from the two eyes to the target were different the instantaneous velocities and VOR gains of the eyes were also different. The dependence on radius of rotation indicates that the VOR is mediated by a combination of otolith and canal inputs. A general model for otolith-canal interaction is proposed in which VOR gain is based on a computation of target location relative to the head. This model simplifies to the classical VOR reflex when a cyclopean eye is subjected only to angular displacement.

Adaptation, Ocular↗
PubMed1985

A two dimensional model for saccade generation.

A model for the generation of oblique saccades is constructed by extending and modifying the one dimensional local feedback model. It is proposed that the visual system stores target location in inertial coordinates, but that the feedback loop which guides saccades works in retinotopic coordinates. To achieve straight trajectories for centripetal and centrifugal saccades in all meridians, a comparator computes motor error as a vector and uses the vectorial error signal to drive two orthogonally-acting burst generators. The generation of straight saccade trajectories when the extraocular muscles are of unequal strengths requires the introduction of a burst-tonic cell input to motor neurons. The model accounts for the results of two-site stimulation of the superior colliculus and frontal eye fields by allowing simultaneous activation of more than one comparator. The postulated existence of multiple comparators suggests that motor error may be computed topographically.

Animals↗
PubMed1985

Adaptation of saccadic and vestibulo-ocular systems after extraocular muscle tenectomy.

Adaptation of saccadic magnitude and vestibulo-ocular reflex gain (VOR) was examined in six monkeys that had undergone a tenectomy of the medial and lateral recti of one eye. After the tenectomy, when the normal eye was patched, a conjugate increase in saccadic magnitude and VOR was observed in both eyes. Subsequent unpatching resulted in selective (nonconjugate) changes in the two eyes such that control values of saccadic magnitude and VOR gain were eventually reestablished in both eyes. Evidence that this return to normal was mediated by a selective adaptation of the neural innervation to one eye was provided by the following observations: (1) saccades in the normal eye were of a lower peak velocity and longer duration after normalization than prior to the tenectomy; and (2) repatching the tenectomized eye after normalization produced a selective return of deficits in that eye over a period of a few days. Thus, the saccadic and vestibulo-ocular systems can be adapted in both a conjugate and a selective or nonconjugate fashion.

Animals↗
PubMed1984

Dependence of cerebellar tremor on proprioceptive but not visual feedback.

We studied the influence of proprioceptive and visual feedback on cerebellar tremor which occurred after arm perturbations and after voluntary elbow flexions. Cerebellar tremor was produced in monkeys by reversibly cooling through two probes implanted lateral and medial to the dentate nucleus. Cerebellar tremor was synchronized in different trials to torque pulse onset and to the end, but not the start, of voluntary movements. Addition of loads to the handle held by the monkey (increases in spring stiffness, viscosity, constant torque, and inertial load) changed the amplitude and frequency of tremor that follows arm perturbations or voluntary movements in the same way. In both situations EMG activity in each cycle of tremor followed stretch of its own muscle and attained a peak near peak velocity irrespective of the mechanical load. Removal of visual feedback did not alter the characteristics of the tremor or the associated EMG activity. We concluded that cerebellar intention tremor, which occurs when attempting to hold the arm in an intended position, is driven by stretch-evoked peripheral feedback and not by voluntary corrections based on vision.

Animals↗
PubMed1984

Braking of fast and accurate elbow flexions in the monkey.

The processes responsible for braking fast and accurate elbow movements were studied in the monkey. The movements studied were made over different amplitudes and against different inertias . All were made to the same end position. Only fast movements that showed the typical biphasic or triphasic pattern of activity in agonists and antagonists were analysed in detail. For movements made over different amplitudes and at different velocities there was symmetry between the acceleration and deceleration phases of the movements. For movements of the same amplitude performed at different velocities there was a direct linear relation between peak velocity and both the peak acceleration (and integrated agonist burst) and peak deceleration (and integrated antagonist burst). The slopes of these relations and their intercept with the peak velocity axis were a function of movement amplitude. This was such that for large and small movements of the same peak velocity and the same end position (i) peak acceleration and phasic agonist activity were larger for the small movements and (ii) peak deceleration and phasic antagonist activity were larger for the small movements. The slope of these relations and the symmetry between acceleration and deceleration were not affected by the addition of an inertial load to the handle held by the monkey. The results indicate that fast and accurate elbow movements in the monkey are braked by antagonist activity that is centrally programmed. As all movements were made to the same end position, the larger antagonist burst in small movements, made at the same peak velocity as large movements, cannot be due to differences in the viscoelastic contribution to braking (cf. Marsden, Obeso & Rothwell , 1983).(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration↗
PubMed1984

Loss of set in muscle responses to limb perturbations during cerebellar dysfunction.

The properties of electromyograph (EMG) responses that enabled the arm to return accurately to target following limb perturbations were investigated in five Cebus monkeys. In particular, factors that affected the timing and magnitude of an early antagonist response that occurred prior to stretch of the antagonist muscle were examined. The early antagonist response was large and early (latency, 60 ms) when the perturbation was brief and a constant force assisted the return movement. In this situation, early contraction of the antagonist muscle was required to prevent the return movement from overshooting the target. To determine whether this early antagonist response was influenced by prior instruction (which in this case was the type of perturbation the monkey had previously received), two types of perturbations requiring different EMG responses were studied. When torque steps (duration, 2,000 ms) were expected and were applied, monkeys generated M1, M2, and M3 responses and later activity only in the agonist (the initially stretched) muscle. When torque pulses (duration, 40 ms) were expected and were applied, monkeys generated M1 and M2 responses in the agonist and an early antagonist response. EMG responses to torque pulses and steps were then compared when the type of perturbation was expected and when it was unexpected. These comparisons revealed that the early antagonist response only occurred when the monkey expected a torque pulse. Therefore, this response was dependent on set. Expectation of a torque step caused enhancement of the agonist M2 and M3 responses. These agonist and antagonist EMG responses that were dependent on set were also influenced by changes in afferent drive. Cerebellar nuclear cooling through probes implanted lateral and medial to the dentate abolished that component of EMG responses attributed to set. The residual EMG responses in agonists and antagonists appeared to be driven by stretch of their respective muscles. The results suggest that when the nature of an arm perturbation is correctly predicted, the cerebellum provides accuracy in repositioning the limb a) by adjusting the magnitude of the M2 agonist response and b) by enabling activity after a latency of 60 ms (e.g., the M3 and early antagonist response) to be switched to the agonist or antagonist as appropriate, irrespective of which muscle is being stretched. This latter mechanism provides the motor system with predictive ability.

Animals↗
PubMed1981

Characteristics of saccadic dysmetria in monkeys during reversible lesions of medial cerebellar nuclei.

1. The accuracy of saccadic eye movements made by trained Cebus monkeys was studied during reversible lesions produced by cooling through probes implanted between the interpositus and fastigial nuclei (medial probe) or lateral to the dentate nucleus (lateral probe). 2. Cooling through the lateral probe did not impair the accuracy of vertical or horizontal saccades, However, cooling through the medial probe produced a dysmetria whose magnitude was dependent on the position of the eye and on the direction of the saccade. 3. The amplitude/duration relation of dysmetria saccades was not significantly different from that of normal saccades. 4. The trajectories of the horizontal and vertical components of oblique saccades remained essentially straight during medial probe cooling in spite of unequal dysmetria in the two components. This suggests that the mechanism that produces a dysmetria in one component must interact with the gaze center that determines the duration of the other component. 5. Cerebellar nuclear cooling through either lateral of medial probes did not alter the saccadic reaction time to a randomly timed step change in target position. This result differs from that found for limb movements where cerebellar dysmetria was associated with increased reaction times. 6. These results provide evidence that the cerebellum through the medial nuclei normally plays a role in terminating, but not in initiating, saccades.

Animals↗
PubMed1980

Arm movement performance during reversible basal ganglia lesions in the monkey.

Arm motor performance of eight Cebus monkeys was examined during reversible cooling in the ventral lateral region of the putamen and globus pallidus (primarily the external segment), where neurons discharging during arm movements have been found (DeLong 1972). When attempting to hold a handle stationary during basal ganglia cooling, all monkeys developed flexion at the wrist and some developed a slow flexion drift of the arm at the elbow. The prominence of wrist flexion emphasizes that the basal ganglia may normally influence distal musculature. During basal ganglia cooling an increase in segmental stretch reflexes (15-30 ms) was sometimes observed following arm perturbations, but no consistent increase occurred in the later EMG responses (30-95 ms) in contrast to results obtained in Parkinsonian patients (Tatton and Lee 1975). No major changes were observed in the time of onset of the earliest EMG activity in the agonist muscle in a simple reaction time elbow movement task during basal ganglia cooling. Basal ganglia lesions produced major disorders in both flexion and extension movements including slowing of movements and rebound of the arm towards its initial position after onset of movement. These disorders were accompanied by an increase in tonic activity of both flexors and extensors while holding and by increased levels of cocontraction of agonists and antagonists during attempted movements. It is suggested that this basal ganglia disorder is due to a failure to achieve the correct balance of activity between agonists and antagonists that is appropriate for a particular motor act.

Animals↗
PubMed1978

Mechanical properties of succinylcholine activated muscle fibers in the inferior oblique muscle of the cat.

To determine the mechanical properties of the multi-innervated fibres in inferior oblique muscle of the cat, these fibers were selectively activated by means of an intravenous injection of succinylcholine chloride (Sch). For small sinusoidal length changes (+/- 1 mm) of low frequency, tension in the Sch activated muscle exhibited a component that was sensitive to the direction of stretch but was independent of the velocity. At higher frequencies of length change (16 Hz) a form of negative friction was observed. Negative friction had not been previously noted in mammalian muscles, although a similar friction was observed both in the muscles of insects and amphibians. These properties were expressed in the form of a simple mechanical model.

Animals↗
PubMed1976

Modulation of the functional stretch reflex by the segmental reflex pathway.

Electromyographic (EMG) reflex responses were examined in the biceps muscle of awake Cebus monkeys trained to resist perturbations of a handle with their forearm. In particular responses at latencies of 15-20 msec (M1) and 40-55 msec (M2), thought to correspond to segmental and suprasegmental reflex pathways respectively, were studied. The experiments demonstrated that the magnitude of the M1 response was large, as compared to M2, only when the muscle was tonically active and small perturbations were applied. For larger perturbations the magnitude of M1 saturated and the M2 response became functionally significant, its magnitude being directly related to the magnitude of the perturbation. By means of delayed reductions in torque, the magnitude of this M2 response was also shown to be very sensitive to changes in facilitatory drive provided by segmental pathways.

Afferent Pathways↗