Vallabh E. Das

Evaluation of a video tracking device for measurement of horizontal and vertical eye rotations during locomotion

We have evaluated a video-based method for measuring binocular horizontal and vertical eye movements of human subjects by comparing it with the magnetic search coil technique. This video tracking system (VTS) uses multiple infrared light sources and small video cameras to simultaneously measure the positions of reflected corneal images and the center of the pupil. The system has a linear range of approximately +/- 40 degrees horizontally and +/- 30 degrees vertically, a sampling rate of 120 Hz (180 Hz with the head fixed), and system noise with standard deviation of < 0.04 degree. The binocular eye-tracking system is light-weight (190 g), being mounted on goggles that, with the eyes in primary position, permit a field of view of 60 degrees horizontally and vertically. The VTS is insensitive to translations of the tracker relative to the eyes. By placing the video preprocessing unit on a cart, eye movements may be recorded while subjects walk through distances up to 100 feet. In comparison with the magnetic search coil technique, the VTS generally provides reliable measurements of horizontal and vertical eye position; eye velocity is noisier than corresponding coil signals, but superior to electro-oculography.

Comparison of horizontal, vertical and diagonal smooth pursuit eye movements in normal human subjects.

We compared horizontal and vertical smooth pursuit eye movements in five healthy human subjects. When maintenance of pursuit was tested using predictable waveforms (sinusoidal or triangular target motion), the gain of horizontal pursuit was greater, in all subjects, than that of vertical pursuit; this was also the case for the horizontal and vertical components of diagonal and circular tracking. When initiation of pursuit was tested, four subjects tended to show larger eye accelerations for vertical as opposed to horizontal pursuit; this trend became a consistent finding during diagonal tracking. These findings support the view that different mechanisms govern the onset of smooth pursuit, and its subsequent maintenance when the target moves in a predictable waveform. Since the properties of these two aspects of pursuit differ for horizontal and vertical movements, our findings also point to separate control of horizontal and vertical pursuit.

Animal Models for Visual Deprivation‐Induced Strabismus and Nystagmus

Abstract: The development of gaze‐stabilizing systems depends on normal vision during infancy. Monkeys reared with binocular lid suture (BLS) for the first 25‐40 days of life have strabismus, optokinetic nystagmus deficits, latent nystagmus, and decreased binocular cells in the visual cortex and nucleus of the optic tract. When BLS is extended to 55 days, pendular and congenital nystagmus also occurs. Eyelids in infant monkeys are hairless and thin, but BLS still degrades sensory fusion, motion, and form perception. To determine to what extent these visual properties are critical in the development of normal gaze stabilization, we examined infant monkeys reared with one opaque contact lens over one eye, alternated to the fellow eye every other day (AMO); and monkeys reared in a 3‐Hz strobe environment. Monkeys reared with AMO develop strabismus, but have normal optokinetic nystagmus and no spontaneous nystagmus. Area 17 is monocular, but the medial temporal area and the nucleus of the optic tract are binocular. Monkeys reared in strobe light develop pendular nystagmus but not strabismus. We were puzzled by the results of the AMO monkeys until we examined infant monkeys with BLS that were prevented from seeing form through the lids. This was done by leaving the tarsal plate intact behind the eyelid. They developed similar to the AMO monkeys. These results suggest that disruption of sensory fusion during infancy (BLS, AMO) causes strabismus. If strabismus occurs while the monkeys have some form vision from each eye (BLS without tarsal plate), then the nucleus of the optic tract becomes monocular, which causes optokinetic nystagmus deficits and latent nystagmus. Infant monkeys reared without visual motion develop pendular nystagmus.

Evidence for independent feedback control of horizontal and vertical saccades from Niemann-Pick type C disease

We measured the eye movements of three sisters with Niemann-Pick type C disease who had a selective defect of vertical saccades, which were slow and hypometric. Horizontal saccades, and horizontal and vertical pursuit and vestibular eye movements were similar to control subjects. The initial movement of oblique saccades was mainly horizontal and most of the vertical component occurred after the horizontal component ended; this resulted in strongly curved trajectories. After completion of the horizontal component of an oblique saccade, the eyes oscillated horizontally at 10-20 Hz until the vertical component ended. These findings are best explained by models that incorporate separate feedback loops for horizontal and vertical burst neurons, and in which the disease selectively affects vertical burst neurons.

A Neurobiological Approach to Acquired Nystagmus

Abstract: The development of animal and mathematical models for several forms of acquired nystagmus has led to more comprehensive knowledge of these disorders. In the best understood forms, such as periodic alternating nystagmus, our range of knowledge includes an animal model, the neurotransmitters involved, and effective treatment. For some other forms, such as downbeat nystagmus, we have an animal model, but reliable treatment is lacking. In other cases, exemplified by acquired pendular nystagmus, we have only a provisional hypothesis for pathogenesis to account for the oscillations, without an animal model, but effective treatment is possible in some patients. The present trend of studying all aspects of the neurobiology of nystagmus, from molecules to behavior, seems to be the best approach to extend our knowledge and to identify new treatments, but much remains to be done.

Cervico-ocular reflex in normal subjects and patients with unilateral vestibular hypofunction.

Objective To determine whether the cervico-ocular reflex contributes to gaze stability in patients with unilateral vestibular hypofunction. Study Design Prospective study. Setting Tertiary referral center. Patients Patients with unilateral vestibular hypofunction (n = 3) before and after vestibular rehabilitation and healthy subjects (n = 7). Interventions Vestibular rehabilitation. Main Outcome Measures We measured the cervico-ocular reflex in patients with unilateral vestibular hypofunction before and after vestibular rehabilitation and in healthy subjects. To measure the cervico-ocular reflex, we recorded eye movements with a scleral search coil while the trunk moved at 0.3, 1.0, and 1.5 Hz beneath a stabilized head. To determine whether the head was truly stabilized, we measured head movement using a search coil. Results We found no evidence of cervico-ocular reflex in any of the seven healthy subjects or in two of the patients with unilateral vestibular hypofunction. In one patient with chronic unilateral vestibular hypofunction, the cervico-ocular reflex was present before vestibular rehabilitation only for leftward trunk rotation (relative head rotation toward the intact side). After 5 weeks of placebo exercises, there was no change in the cervico-ocular reflex. After an additional 5 weeks that included vestibular exercises, cervico-ocular reflex gain for leftward trunk rotation had increased threefold. In addition, there was now evidence of a cervico-ocular reflex for rightward trunk rotation, potentially compensating for the vestibular deficit. Conclusion The cervico-ocular reflex appears to be a highly inconsistent mechanism. The change of the cervico-ocular reflex in one patient after vestibular exercises suggests that the cervico-ocular reflex may be adaptable in some patients.

Tracking of illusory target motion: differences between gaze and head responses.

We compared ocular and eye-head tracking responses to an illusion of diagonal motion produced when vertical movement of a small visual target was synchronized to horizontal movement of a background display. In response to sinusoidal movement, smooth ocular pursuit followed vertical target motion, with only a small horizontal component. In response to regular stepping movement, all anticipatory saccades were in the direction of the illusion; these erroneous oblique movements were followed by corrective horizontal saccades. When the head was free to move, it usually showed a diagonal trajectory that, for both sinusoidal and stepping target motion, was always in the direction of the illusion; no corrective movements were present. Thus, for our illusory stimuli, eye and head tracking showed qualitative differences that imply that ocular tracking was ultimately controlled by actual target motion but head tracking was controlled by illusory target motion.

Information processing by parafoveal cells in the primate nucleus of the optic tract

Abstract. We recorded from single units in the pretectal nucleus of the optic tract (NOT) of the nonhuman primate. Specifically, we examined units that are modulated during smooth tracking of a small laser spot against a dark background. We used a nonlinear optimization procedure to determine whether the unit responses of these parafoveal cells are better described by a model that incorporates retinal error motion parameters or by a model that incorporates eye motion parameters. Our main finding was that all the cells in our sample group were better fit with a three-component model that incorporated retinal error motion parameters of position, velocity and acceleration (average coefficient of determination = 0.84) than a model that used position, velocity and acceleration components of eye motion (average coefficient of determination = 0.68). Other analyses involved comparison of goodness of fit between the three-component retinal error model and two-component retinal error models that excluded position or acceleration related terms. We found that there was a statistically significant degradation in the fit when position and acceleration related terms were dropped from the retinal error based model (P<0.05). Unit data from experiments in which the laser spot was extinguished for a brief period of time during tracking showed that the unit response was decreased following the target blink. We conclude on the basis of this and previous experimental data and our dynamic modeling approach that the parafoveal cells in the NOT primarily encode retinal error motion. Further they encode position, velocity and acceleration components of retinal error that could be used by other downstream structures for synthesis of a smooth-pursuit eye movement.

Tests of two hypotheses to account for different-sized saccades during disjunctive gaze shifts

Abstract Rapid shifts of the point of visual fixation between objects that lie in different directions and at different depths require disjunctive eye movements. We tested whether the saccadic component of such movements is equal for both eyes (Hering’s law) or is unequal. We compared the saccadic pulses of abducting and adducting movements when horizontal gaze was shifted from a distant to a near target aligned on the visual axis of one eye (Müller paradigm) in ten normal subjects. We similarly compared horizontal saccades made between two distant targets lying in the same field of movement as during the Müller paradigm tests, and between targets lying symmetrically on either side of the midline, at near side of the midline, at near or far. We measured the ratio of the amplitude of the movements of each eye in corresponding directions due to the saccadic component, as well as corresponding ratios of peak velocity and peak acceleration. In response to a Müller test paradigm requiring about 17° of vergence, the change in position of the unaligned eye was typically twice the size of the corresponding movement of the aligned eye. The ratio of peak velocities for the unaligned/aligned eyes was about 1.5, which was greater than for saccades made between distant targets. The ratio of peak acceleration for unaligned/aligned eyes was about 1.0 during shifts from near to far and about 1.3 for shifts from far to near, these values being similar to corresponding ratios for saccades between distant targets. These measurements of peak acceleration indicate that the saccadic pulses sent to each eye during the Müller paradigm are more equal than would be deduced by comparing the changes in eye position. We retested five subjects to compare directly the peak acceleration of saccades made during the Müller paradigm with similar-sized ”conjugate” saccades made between targets at optical infinity. Saccades made during the Müller paradigm were significant slower (P<0.005) than similar-sized conjugate saccades; this indicated that the different-sized movements during Müller paradigm are not simply due differences in saccadic pulse size but are also influenced by the concurrent vergence movement. A model for saccade-vergence interactions, which incorporates equal saccadic pulses for each eye, and differing contributions from convergence and divergence, accounts for many of these findings.

Visual-vestibular interaction in progressive supranuclear palsy

We measured the stability of gaze during horizontal head rotations at 1-3 Hz in four patients with progressive supranuclear palsy (PSP), while they viewed a stationary target. Median gain of compensatory eye movements was 0.94, similar to control subjects. During rotation in darkness, median gain of vestibulo-ocular reflex (VOR) was 0.88, similar to controls. Conversely, the median gain of smooth-pursuit eye movements at 1.0 Hz was 0.23, lower than controls. A simple superposition model of smooth pursuit and the VOR could not account for the observed gaze stability during fixation. Our results are further evidence that a visually mediated mechanism, independent of smooth pursuit, optimizes eye movements to compensate for head rotations.