Ari Z. Zivotofsky

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.

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.

Torsional eye movements in patients with skew deviation and spasmodic torticollis Responses to static and dynamic head roll

Article abstract-We measured torsional eye movements induced by sinusoidal rotation or static tilt of the head in roll while viewing a far or near target in 4 patients with skew deviation due to brainstem lesions, 4 patients with spasmodic torticollis (ST), 2 patients with unilateral eighth nerve section (VIIIS), and 10 normal subjects. Torsional nystagmus was present in all 4 patients with skew deviation. In subjects and patients, responses to both sinusoidal and static roll were larger while viewing the far target, consistent with factors dictated by geometry. Response gains to sinusoidal roll were abnormal in 3 patients with skew (increased in one, decreased in two), abnormal in 3 with ST (increased in 1, decreased in 2), and in abnormal both VIIIS patients (decreased). Greater abnormalities were evident in 3 skew patients while rolling away from the side of their brainstem lesions and in both VIIIS patients while rolling toward their lesioned ears. There were similar but less pronounced changes during static head roll. We conclude that patients with skew, ST, and VIIIS may all have abnormal ocular counter-rolling that is more evident during dynamic testing while viewing a far target. Such abnormalities endure because of the limited influence exerted by vision on torsional eye movements. NEUROLOGY 1997;48: 506-514

Scaling of horizontal and vertical fixational eye movements.

Eye movements during fixation of a stationary target prevent the adaptation of the visual system to continuous illumination and inhibit fading of the image. These random, involuntary, small movements are restricted at long time scales so as to keep the target at the center of the field of view. Here we use detrended fluctuation analysis in order to study the properties of fixational eye movements at different time scales. Results show different scaling behavior between horizontal and vertical movements. When the small ballistic movements, i.e., microsaccades, are removed, the scaling exponents in both planes become similar. Our findings suggest that microsaccades enhance the persistence at short time scales mostly in the horizontal component and much less in the vertical component. This difference may be due to the need for continuously moving the eyes in the horizontal plane, in order to match the stereoscopic image for different viewing distances.

Comprehensive computerized assessment of cognitive sequelae of a complete 12-16 hour fast.

Food deprivation has been shown to deleteriously affect human cognition, but findings are equivocal, and few studies have examined several cognitive domains. In this study, the authors used computerized testing to describe the profile of shifts in cognition attributable to short-term religious fasting. Multiple cognitive domains were evaluated at midday and late afternoon following complete abstention from eating and drinking beginning at midnight. Cross-domain, fasting-related deficits were found for tasks requiring perception of spatial relations. Fasting-related information processing deficits were found for response time but not accuracy for test levels of intermediate difficulty. Time-of-day effects often reflected poorer afternoon performance. These findings provide a detailed profile of cognitive consequences of food deprivation, affected by time of day, task demands, and type of outcome.

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.

The Case of Samuel Golubchuk and the Right to Live

Samuel Golubchuk was unwittingly at the center of a medical controversy with important ethical ramifications. Mr. Golubchuk, an 84-year-old patient whose precise neurological level of function was open to debate, was being artificially ventilated and fed by a gastrostomy tube prior to his death. According to all reports he was neither brain dead nor in a vegetative state. The physicians directly responsible for his care had requested that they be allowed to remove the patient from life support against the wishes of the patients family. Concurrently the Manitoba College of Physicians and Surgeons released a statement which states that the final decision to withdraw life support lies with the physician. In our opinion the statement is ethically problematic for a number of reasons. 1. It is an affront to the guiding principles of Western medical ethics: patient autonomy and human freedom. 2. The position of Samuel Golubchuks physicians and the new statement lack cultural sensitivity towards other traditions. 3. In modern society there exists an erosion of a basic attitude towards the value of life. 4. The ability of physicians to predict life expectancy in terminally ill patients has been shown repeatedly to be quite limited.

Convergent-Divergent Pendular Nystagmus Possible Role of the Vergence System

Article abstract-We used the magnetic search coil technique to measure horizontal, vertical, and torsional components of convergent-divergent pendular nystagmus in three patients. All showed phase shifts of approximately 180 degrees between the two eyes in the horizontal and torsional planes, but the vertical components were conjugate. Viewing a near target increased the oscillations threefold in one patient and by 60% in a second patient. The waveform was sinusoidal in one patient, but in the other two it was complex, resembling either a sum of several sine waves or a cycloid. When the predominant frequency of the nystagmus was low (1.8 Hz), oscillation of visually mediated vergence might have been responsible; when the frequency was high (6 Hz), the nystagmus might have arisen from an internal instability in connections between nucleus reticularis tegmenti pontis and cerebellar nucleus interpositus, which are important for vergence control. NEUROLOGY 1995;45: 509-515

Modality-specific communication enabling gait synchronization during over-ground side-by-side walking

An attentive observer will notice that unintentional synchronization of gait between two walkers on the street seems to occur frequently. Nonetheless, the rate of occurrence and motor-sensory mechanisms underlying this phase-locking of gait have only recently begun to be investigated. Previous studies have either been qualitative or carried out under non-natural conditions, e.g., treadmill walking. The present study quantitatively examined the potential sensory mechanisms that contribute to the gait synchronization that occurs when two people walk side by side along a straight, over-ground, pathway. Fourteen pairs of subjects walked 70 m under five conditions that manipulated the available sensory feedback. The modalities studied were visual, auditory, and tactile. Movement was quantified using a trunk-mounted tri-axial accelerometer. A gait synchronization index (GSI) was calculated to quantify the phase synchronization of the gait rhythms. Overall, 36% of the walks exhibited synchrony. Tactile and auditory feedback showed the greatest ability to synchronize, while visual feedback was the least effective. The results show that gait synchronization during natural walking is common, quantifiable, and has modality-specific properties.