Simulation of pathological ocular counter-roll and skew-torsion by a 3-D mathematical model

Eye movements during path integration

Self-motion induces spontaneous eye movements which serve the purpose of stabilizing the visual image on the retina. Previous studies have mainly focused on their reflexive nature and how the perceptual system disentangles visual flow components caused by eye movements and self-motion. Here, we investigated the role of eye movements in distance reproduction (path integration). We used bimodal (visual-auditory)-simulated self-motion: visual optic flow was paired with an auditory stimulus whose frequency was scaled with simulated speed. The task of the subjects in each trial was, first, to observe the simulated self-motion over a certain distance (Encoding phase) and, second, to actively reproduce the observed distance using only visual, only auditory, or bimodal feedback (Reproduction phase). We found that eye positions and eye speeds were strongly correlated between the Encoding and the Reproduction phases. This was the case even when reproduction relied solely on auditory information and thus no visual stimulus was presented. We believe that these correlations are indicative of a contribution of eye movements to path integration.

Age differences in virtual environment and real world path integration

Accurate path integration (PI) requires the integration of visual, proprioceptive, and vestibular self-motion cues and age effects associated with alterations in processing information from these systems may contribute to declines in PI abilities. The present study investigated age-related differences in PI in conditions that varied as a function of available sources of sensory information. Twenty-two healthy, young (23.8 ± 3.0 years) and 16 older (70.1 ± 6.4 years) adults participated in distance reproduction and triangle completion tasks (TCTs) performed in a virtual environment (VE) and two “real world” conditions: guided walking and wheelchair propulsion. For walking and wheelchair propulsion conditions, participants wore a blindfold and wore noise-blocking headphones and were guided through the workspace by the experimenter. For the VE condition, participants viewed self-motion information on a computer monitor and used a joystick to navigate through the environment. For TCTs, older compared to younger individuals showed greater errors in rotation estimations performed in the wheelchair condition, and for rotation and distance estimations in the VE condition. Distance reproduction tasks (DRTs), in contrast, did not show any age effects. These findings demonstrate that age differences in PI vary as a function of the available sources of information and by the complexity of outbound pathway.

Latency and initiation of the human vestibuloocular reflex to pulsed galvanic stimulation

Cathodal galvanic currents activate primary vestibular afferents, whereas anodal currents inhibit them. Pulsed galvanic vestibular stimulation (GVS) was used to determine the latency and initiation of the human vestibuloocular reflex. Three-dimensional galvanic vestibuloocular reflex (g-VOR) was recorded with binocular dual-search coils in response to a bilateral bipolar 100-ms rectangular pulse of current at 0.9 (near-threshold), 2.5, 5.0, 7.5, and 10.0 mA in 11 normal subjects. The g-VOR consisted of three components: conjugate torsional eye rotation away from cathode toward anode; vertical divergence (skew deviation) with hypertropia of the eye on the cathodal and hypotropia of the eye on the anodal sides; and conjugate horizontal eye rotation away from cathode toward anode. The g-VOR was repeatable across all subjects, its magnitude a linear function of the current intensity, its latency about 9.0 ms with GVS of >or=2.5 mA, and was not suppressed by visual fixation. At 10-mA stimulation, the g-VOR [x, y, z] on the cathodal side was [0.77 +/- 0.10, -0.05 +/- 0.05, -0.18 +/- 0.06 degrees ] (mean +/- 95% confidence intervals) and on the anodal side was [0.79 +/- 0.10, 0.16 +/- 0.05, -0.19 +/- 0.06 degrees ], with a vertical divergence of 0.20 degrees . Although the horizontal g-VOR could have arisen from activation of the horizontal semicircular canal afferents, the vertical-torsional g-VOR resembled the vestibuloocular reflex in response to roll-plane head rotation about an Earth-horizontal axis and might be a result of both vertical semicircular canal and otolith afferent activations. Pulsed GVS is a promising technique to investigate latency and initiation of the human vestibuloocular reflex because it does not require a large mechanical apparatus nor does it pose problems of head inertia or slippage.

Skew Deviation Revisited

Skew deviation is a vertical misalignment of the eyes caused by damage to prenuclear vestibular input to ocular motor nuclei. The resultant vertical ocular deviation is relatively comitant in nature, and is usually seen in the context of brainstem or cerebellar injury from stroke, multiple sclerosis, or trauma. Skew deviation is usually accompanied by binocular torsion, torticollis, and a tilt in the subjective visual vertical. This constellation of findings has been termed the ocular tilt reaction. In the past two decades, a clinical localizing value for skew deviation has been assigned, and a cogent vestibular mechanism for comitant and incomitant variants of skew deviation has been proposed. Our understanding of skew deviation as a manifestation of central otolithic dysfunction in different planes of three-dimensional space is evolving. The similar spectrum of vertical ocular deviations arising in patients with congenital strabismus may further expand the nosology of skew deviation to include vergence abnormalities caused by the effects of early binocular visual imbalance on the developing visual system.

Mathematical model predicts clinical ocular motor syndromes

Clinical ocular motor syndromes were compared with ocular motor syndromes simulated by a mathematical model of the vestibuloocular reflex. The mathematical sensorimotor feedforward model of otolith control of three-dimensional binocular eye position is based on relevant anatomical connections of the vestibuloocular reflex from the utricles to extraocular eye muscles. This is the first attempt to simulate static ocular motor syndromes for unilateral utricular or vestibular nerve failure, lesions of the vestibular nucleus, and lesions of the ascending vestibuloocular reflex pathways. Comparison of the predicted syndromes with those found in patients with unilateral disorders of the vestibular nerve (herpes zoster neuritis), the vestibular nucleus (medullary infarction), and the medial longitudinal fasciculus (pontine infarction) showed good agreement as regards the direction of horizontal, vertical, and torsional eye deviations. The ability of the model to simulate complete or incomplete failures of single elements or entire pathways allows us to pose direct clinical questions about as yet unknown ocular motor syndromes or about the localization of the damage as well as the mechanism involved in syndromes already known.

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.

Ocular motor disorders

Our detailed understanding of the physiology and anatomy of the ocular motor system allows an accurate differential diagnosis of pathological eye movement patterns. This review covers important clinical studies and studies in basic research relevant for the neurologist published during the past year.