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T Vilis

Publications and source records attributed to T Vilis.

At least 37 records · Page 2Linked to original sources
PubMed1994

Three-dimensional eye, head, and chest orientations after large gaze shifts and the underlying neural strategies.

1. The fixation orientations adopted by the eye, head, and chest were examined when all three were allowed to participate in gaze shifts to visual targets. The objective was to discover whether there are invariant, neurally determined laws governing these orientations that might provide clues to the processes of perception and motor control. This is an extension of the classical studies of eye-only saccades that determined that there is only one eye orientation for each gaze direction (Donders' law) and that the rotations necessary to take the eye from a reference orientation to all other orientations adopted are about axes that lie in a plane (Listing's law). 2. The three-dimensional orientations of the static eyes, head, and chest were measured after each gaze shift to a visual target, the targets having been fixed at positions ranging from 0 to 135 degrees to the left and right of center and 45 degrees up and down. These measurements were taken of seven human subjects by means of the search coil technique with coils attached to the sternum, head, and right eye. Orientations were plotted as quaternion vectors so that those orientations obeying Donders' law formed a surface and those obeying Listing's law formed a plane. 3. The orientations adopted by the eye, head, and chest were found to be a small subset of those possible under the biomechanical and task-imposed constraints. Thus there is a neurally implemented restriction, specifically of the rotation of the eye relative to space (i.e., the orientation variable es) and to the head (eh); also of the rotation of the head relative to space (hs) and to the chest (hc), and the rotation of the chest relative to space (cs). Plotted as quaternion vectors, the data for each orientation variable formed a characteristic surfacelike shape. In the case of es, hs, and hc these were twisted surfaces, whereas for eh the surface was planar and for cs it was nearly linear. Thus to a first approximation each of the orientation variables conformed to Donders' law. 4. The eye adopted a pointing (gaze) direction that has the ratio of vertical to horizontal components generally greater than one when fixating each of the corner targets. The chest, by contrast, moved almost entirely in the horizontal direction, whereas the head performed an intermediate role. 5. The es-, hs-, and hc-fitted surfaces and cs-fitted lines were titled remarkably little from the vertical axis (i.e., the gravity direction) despite larger tilts being possible.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1993

Modularity and parallel processing in the oculomotor integrator.

The neural signals that hold eye position originate in a brainstem structure called the neural integrator, so-called because it is thought to compute these position signals using a process equivalent to mathematical integration. Most previous experiments have assumed that the neural integrator reacts to damage like a single mathematical integrator: the eye is expected to drift towards a unique resting point at a simple exponential rate dependent on current eye position. Physiologically, this would require a neural network with uniformly distributed internal connections. However, Cannon et al. (1983) proposed a more robust modular internal configuration, with dense local connections and sparse remote connections, computationally equivalent to a parallel array of independent sub-integrators. Damage to some sub-integrators would not affect function in the others, so that part of the position signal would remain intact, and a more complex pattern of drift would result. We evaluated this parallel integrator hypothesis by recording three-dimensional eye positions in the light and dark from five alert monkeys with partial neural integrator failure. Our previous study showed that injection of the inhibitory gamma aminobutyric acid agonist muscimol into the mesencephalic interstitial nucleus of Cajal (INC) causes almost complete failure of the integrators for vertical and torsional eye position after approximately 30 min. This study examines the more modest initial effects. Several aspects of the initial vertical drift could not be accounted for by the single integrator scheme. First, the eye did not initially drift towards a single resting position; rapid but brief drift was observed towards multiple resting positions. With time after the muscimol injection, this range of stable eye positions progressively narrowed until it eventually approximated a single point. Second, the drift had multiple time constants. Third, multiple regression analysis revealed a significant correlation between drift rate and magnitude of the previous saccade, in addition to a correlation between drift rate and position. This saccade dependence enabled animals to stabilize gaze by making a series of saccades to the same target, each with less post-saccadic drift than its predecessor. These observations were predicted and explained by a model in which each of several parallel integrators generated a fraction of the eye-position command.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Rotation of Listing's plane during vergence.

When visually fixating targets on an isovergence surface, the position of each eye was constrained to a plane. Thus, Listing's law holds during vergence. The planes were, however, rotated temporally with respect to those when viewing distant targets. The effect of this rotation was to produce a torsion which depended on eye elevation; extorsion of the two eyes for downward gaze and intorsion for upward gaze. The saccadic velocity command was relatively unaffected during vergence. Computer simulations suggest that the saccadic tonic command and the vergence command interact multiplicatively in three dimensions.

Convergence, Ocular↗
PubMed1992

The conjugacy of human saccadic eye movements.

Binocular measurements of instantaneous velocity vectors in normal human subjects during saccades showed: (1) considerable trial to trial variation in peak velocity, saccade duration, and saccade curvature despite saccade accuracy; (2) variations in one eye were mirrored by similar variations in the other eye, with a high positive correlation. The high correlation between the peak velocities suggest that saccades in the two eyes are driven by a common saccade generator. Assuming that a local feedback loop guides saccades, the high correlation between saccade durations and between saccade curvatures suggests that both eyes are guided by common feedback. If so, monocular adaptation must occur downstream from the saccade generator.

Humans↗
PubMed1992

Violations of Listing's law after large eye and head gaze shifts.

1. Kinematic constraints were examined in static eye and head positions after large gaze shifts to visual targets. Three-dimensional eye and head rotations were measured in six adult human subjects by the use of the magnetic field search coil technique. 2. Eye positions in space were found to obey Donder's law; i.e., for any given gaze direction there was a unique three-dimensional orientation. In other words, angular eye positions in space (expressed as quaternions) were constrained to a two-dimensional surface. 3. When only the eye moved (head stationary), the shape of this surface resembled a plane and thus the eye position in space obeyed Listing's law. However, after gaze shifts involving both the eye and the head, the eye in space surface became twisted and thus nonplanar. This twist was similar to that achieved by a Fick gimbal model of rotations in which the horizontal axis is nested within a fixed vertical axis. During oblique gaze shifts, the head made predominantly horizontal movements whereas the eye made predominantly vertical movements. This, combined with the fact that the eye is mounted within the head, causes the eye in space surface to resemble that of a Fick gimbal. 4. The angular position of the head in space was also constrained to a two-dimensional surface. This surface was also not planar (Listinglike) and twisted in a manner similar to that of the eye in space. 5. Whereas the angular position of the eye in head was found to obey Listing's law after head-fixed gaze shifts, violations of Listing's law occurred after head-free gaze shifts. These violations showed significant intersubject variation in their magnitude and character. 6. Given that the eye in space violates Listing's law after head movements, the supposition that Listing's law serves the perceptual purpose of maintaining radial constancy is untenable. The Fick gimballike behavior of the head in space and eye in space may hold several advantages over a Listing's system. When the head in space behaves like a Fick gimbal, a horizontal line through the eyes remains parallel to the horizon. By having the eye in space behave like a Fick gimbal, the work done against gravity may be minimized by having the eye contribute more to vertical gaze shifts than does the head.

Adult↗
PubMed1992

Constraints on arm position when pointing in three dimensions: Donders' law and the Fick gimbal strategy.

1. While making saccades between targets with the head stationary, eye positions are constrained to two of the possible three degrees of freedom. Classically this constraint has been described by Donders' and Listing's laws. The objective was to determine whether these laws also apply for the straight arm when pointing between different targets. Thus we determined whether the arm adopts only one angular position for every pointing direction (Donders' law) and whether these positions can be described by rotations from a reference position about axes that lie in a plane (Listing's law). 2. The angular positions (orientations) of the arm in three-dimensional space were studied as subjects pointed with a straight arm at different targets. Arm position was measured with the search coil technique by means of coils attached to the back of the hand. Pointing was studied over a range of +/- 45 degrees in all directions from a central target located 45 degrees to the right of the straight-ahead position. 3. The positions of the arm in space were described by quaternion vectors, i.e., a particular position was described in terms of the axis and amplitude of a rotation from a reference position to that position. Using this description, it was found that the straight arm adopted a similar orientation (standard deviations ranged from 2.8 to 4.8 degrees) when pointing at a particular target irrespective of which target from which it had moved. 4. The angular position vectors for arm positions associated with relatively small movements (e.g., less than +/- 30 degrees) lay in a flat surface with minimal torsion. At first sight, this surface appeared to be similar to Listing's plane of the eye. However, for positions associated with larger movements (e.g., +/- 45 degrees) it became apparent that, unlike the eye, the surface deviated from one obeying Listing's law, i.e., it was twisted and showed torsion like that produced by rotations around the horizontal and vertical axes of a Fick gimbal. (The characteristic of a Fick gimbal is that the vertical axis is fixed, whereas the horizontal axis moves with the gimbal.) 5. Although there were differences between subjects, all showed a twisted position vector surface. The twist was always in the same direction, and it was always less than that of a Fick gimbal. 6. This position vector surface had a similar shape whether the arm was stationary or was moving between targets, whether subjects pointed with or without vision, and whether the pointing arm had moved between targets or from a bent-elbow position on the lap.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1992

Symmetry of oculomotor burst neuron coordinates about Listing's plane.

1. The purpose of this investigation was to determine the axes of eye rotation generated by oculomotor burst neuron populations and the coordinate system that they collectively define. In particular, we asked if such coordinates might be related to constraints in the emergent behavior, i.e., Listing's law for saccades. 2. The mesencephalic rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) was identified in four monkeys with the use of single-unit recording, and then explored with the use of electrical microstimulation and pharmacological inactivation with the inhibitory gamma-aminobutyric acid (GABA) agonist muscimol. Three-dimensional (3-D) eye positions and velocities were recorded in one or both eyes while alert animals made eye movements in response to visual stimuli and head rotation. 3. Unilateral stimulation of the riMLF (20 microA, 200 Hz, 300-600 ms) produced conjugate, constant velocity eye rotations, which then stopped abruptly and held their final positions. This is expected if the riMLF produces phasic signals upstream from the oculomotor integrator. 4. Units that burst before upward or downward saccades were recorded intermingled in each side of the riMLF. Unilateral stimulation of the same riMLF sites produced eye rotations about primarily torsional axes, clockwise (CW) during right riMLF stimulation and counterclockwise (CCW) during left stimulation. Only small and inconsistent vertical components were observed, supporting the view that the riMLF carries intermingled up and down signals. 5. The torsional axes of eye rotation produced by riMLF stimulation did not correlate to external anatomic landmarks. Instead, stimulation axes from both riMLF sides aligned with the primary gaze direction orthogonal to Listing's plane of eye positions recorded during saccades. 6. Injection of muscimol into one side of the riMLF produced a conjugate deficit in saccades and quick phases, including a 50% reduction in all vertical velocities and complete loss of one torsional direction. CW was lost after right riMLF inactivation, and CCW was lost after left inactivation. 7. The plane that separated the intact torsional axes from the missing axes correlated with the orientation of Listing's plane. Thus, during left or right riMLF inactivation, the vertical axes of intact horizontal saccades were abnormally aligned with Listing's plane. The orientation of these axes was not correlated with external anatomic landmarks. 8. As suggested by their alignment with Listing's plane, the intact vertical axes of horizontal saccades following riMLF inactivation were orthogonal to torsional riMLF stimulation axes.(ABSTRACT TRUNCATED AT 400 WORDS)

Algorithms↗
PubMed1991

Generation of torsional and vertical eye position signals by the interstitial nucleus of Cajal.

The neural integrator, which converts eye velocity signals into position signals, is central to oculomotor theory. Similar integrators are probably necessary in any neural system that changes and maintains muscular tension. The integrator for horizontal eye position is in the pons, but the locations of the vertical and torsional integrators have not been clearly defined. Recording three-dimensional eye movements in alert monkeys during microstimulation and pharmacological inactivation of midbrain sites showed that the interstitial nucleus of Cajal generates both the torsional and vertical eye position signals. Up and down signals are linked with clockwise signals in the right brain and counterclockwise signals in the left brain. This three-dimensional coordinate system achieves orthogonality and bilateral symmetry without redundancy and optimizes energy efficiency for horizontal visual scanning.

Animals↗
PubMed1991

Axes of eye rotation and Listing's law during rotations of the head.

1. The vestibuloocular reflex (VOR) was examined in four alert monkeys during rotations of the head about torsional, vertical, horizontal, and intermediate axes. Eye positions and axes were recorded in three dimensions (3-D). Visual targets were used to optimize gaze stabilization. 2. Axes of eye rotation during slow phases showed small but systematic deviations from collinearity with the axes of head rotation. These noncollinearities apparently resulted from vector summation of torsional, vertical, and horizontal VOR components with different gains. 3. VOR gain was lowest about a head-fixed torsional axis that was correlated with the primary gaze direction, as determined by Listing's law for saccades. As a result, rotation of the head about a partially torsional axis produced noncollinear slow phases, with axes that tilted toward Listing's plane. 4. During slow phases, eye position changed not only in the direction of rotation, but also systematically in other directions. Even axes of eye rotation within Listing's plane caused eye position to move out of the plane to a torsional position that was then held. Thus Listing's law for saccades cannot be a product of plant mechanics. 5. VOR slow phases were simulated with the use of a model that incorporated 3-D rotational kinematics into the indirect path and the oculomotor plant. This demonstrated that the observed pattern of position changes is the expected consequence of rotating the eye about a fixed axis and that to hold these positions the indirect path must employ a 3-D velocity-to-position transformation. 6. Quick phases not only corrected the violations of Listing's law produced by slow phases but anticipated them by directing the eye toward a plane rotated in the direction of head rotation. This was modeled by inputting the vestibular signal to a Listing's law operator that is shared by the quick phase and saccadic systems.

Animals↗
PubMed1991

Generation of vertical and torsional rapid eye movement in the rostral mesencephalon. Experimental data and clinical implications.

The riMLF is a nucleus in the rostral mesencephalon whose bilateral destruction leads to a palsy of vertical and torsional rapid eye movements. A unilateral lesion leads to a loss of torsional rapid eye movements in only one direction, but vertical rapid movements can still be generated with some reduction in their velocity. Single neuron studies in monkeys and anatomy support the concept that the riMLF together with the PPRF are the critical areas in the brainstem to generate rapid eye movements in 3 dimensions.

Animals↗
PubMed1990

Computing three-dimensional eye position quaternions and eye velocity from search coil signals.

The four-component rotational operators called quaternions, which represent eye rotations in terms of their axes and angles, have several advantages over other representations of eye position (such as Fick coordinates): they provide easy computations, symmetry, a simple form for Listing's law, and useful three-dimensional plots of eye movements. In this paper we present algorithms for computing eye position quaternions and eye angular velocity (not the derivative of position in three dimensions) from two search coils (not necessarily orthogonal) on one eye in two or three magnetic fields, and for locating primary position using quaternions. We show how differentiation of eye position signals yields poor estimates of all three components of eye velocity.

Adult↗
PubMed1990

Geometric relations of eye position and velocity vectors during saccades.

Measurements of angular position and velocity vectors of the eye in three human and three monkey subjects showed that: (1) position vectors lie roughly in a single plane, in accordance with Listing's law, between and during saccades; (2) primary position of the eye is often far from the centre of the oculomotor range. (3) saccades have nearly-fixed rotation axes, which tilt out of Listing's plane in a systematic way depending on current eye position. Findings 1 and 3 show that saccadic control signals accurately reflect the properties of three-dimensional rotations, as predicted by a new quaternion model of the saccadic system; models that approximate rotational kinematics using vectorial addition and integration do not predict these findings.

Adult↗
PubMed1989

Rapid eye movement generation in the primate. Physiology, pathophysiology, and clinical implications.

The trajectories of rapid eye movements are usually described in a Cartesian coordinate frame with a horizontal, vertical and torsional component. The sensory to motor coordinate transformations for horizontal components of rapid eye movements can be localized to neurons of the paramedian pontine reticular formation (PPRF), where long-lead and short-lead burst neurons are found. The equivalent area for recoding of vertical and torsional movement components is situated in the rostral interstitial nucleus of the MLF (rostral iMLF). Pause cells in caudal midline structures of the PPRF help to coordinate the various movement components. Experimental inactivation of these different neuron population lead to palsies of rapid eye movement generation. A unilateral PPRF lesion leads to a loss of all horizontal rapid eye movements towards the ipsilateral side. A bilateral PPRF lesion involving caudal midline structures leads to a bilateral horizontal gaze palsy in addition to a severe disruption of vertical and torsional eye movements. A bilateral rostral iMLF lesion leads to a loss of all rapid eye movements with a vertical or torsional movement component. A unilateral iMLF lesion leads to a loss of all rapid eye movements with an ipsilateral torsional component.

Animals↗
PubMed1988

A matrix analysis for a conjugate vestibulo-ocular reflex.

The technique of matrix analysis is used to compare the connectivity between vestibular neurons and oculomotor neurons of the two eyes that would generate a conjugate vestibulo-ocular reflex (VOR). The technique shows that the connectivity is normally anatomically symmetric. The technique is also used to determine the types and loci of adaptation within the VOR that will maintain conjugacy. Adaptation is divided into 1) that evoked by changes in visual feedback, which requires VOR or system-specific changes and 2) that produced by changes in the canals or muscles, which requires deficit-specific adaptation. In the former case, the adaptation could best be achieved by an additive alteration of the vestibular-motoneuron projections. In the latter case, the appropriate adaptations would be serial, multiplicative changes, applied at the level of the vestibular neurons when the canals are at fault or at the level of the motoneurons of the eye whose muscles are impaired. The analysis thus suggests multiple loci of plasticity within the VOR, specialized for adapting to different deficits.

Adaptation, Physiological↗
PubMed1988

Monocular adaptation of the saccadic system and vestibulo-ocular reflex.

This study asks whether or not adaptation of saccades and the VOR is constrained to be conjugate by Hering's Law. Changes in saccades and the VOR produced by surgically weakening one of the horizontal recti by recession or tenotomy were examined in monkeys with the affected eye patched. After the restoration of vision a rapid monocular recalibration of both saccades and the VOR was observed in preparations with a small or moderate muscle weakening. Preparations with a severe weakening exhibited little monocular adaptation but when the normal eye was patched exhibited a strong conjugate adaptation. Thus the results indicate that saccades and the VOR have the capacity for monocular recalibration but that this capacity is more limited than that for conjugate changes.

Adaptation, Physiological↗