Modulation of human velocity storage sampled during intermittently-illuminated optokinetic stimulation

Study of the biomechanical mechanisms of benign paroxysmal positional vertigo

Abstract. From a biomechanical point of view, the process of Benign paroxysmal positional vertigo (BPPV) includes 2 fluid¯solid coupling effects: the interaction between particles and endolymph and the interaction between endolymph and cupula. The interaction between the canaliths and the wall would affect the coupling effects. This study aimed to investigate the entire process of cupula motion caused by canaliths motion and the influence of canalith particles composition. A biomechanical numerical model was established to simulate the canalith falling process and study the influence of canalith diameter, number, and initial falling position on cupula movement. Simultaneously, the relationship between cupula displacement and the nystagmus signal was analyzed in BPPV patients. The results revealed that the particle velocity was proportional to the particle diameter. The pressure difference between the two sides of the cupula was directly proportional to the canalith diameter and number. The degree of vertigo was positively related to the slow angular velocity of the nystagmus and, therefore, reflected canalith number and diameter. The BPPV latent period and vertigo duration were inversely related to particle diameter. Thus, the number of particles, particle radius, and initial falling position affected cupula movement, which was reflected in the nystagmus.

Effect of the Stimulus Duration on the Adaptation of the Optokinetic Afternystagmus

Observing a rotating visual pattern covering a large portion of the visual field induces optokinetic nystagmus (OKN). If the lights are suddenly switched off, optokinetic afternystagmus (OKAN) occurs. OKAN is hypothesized to originate in the velocity storage mechanism (VSM), a central processing network involved in multi-sensory integration. During a sustained visual rotation, the VSM builds up a velocity signal. After the lights are turned off, the VSM discharges slowly, with OKAN as the neurophysiological correlate. It has been reported that the initial afternystagmus in the direction of the preceding stimulus (OKAN-I) can be followed by a reversed one (OKAN-II), which increases with stimulus duration up to 15 min. In 11 healthy adults, we investigated OKAN following optokinetic stimulus lasting 30 s, 3-, 5-, and 10-min. Analysis of slow-phase cumulative eye position and velocity found OKAN-II in only 5/11 participants. Those participants presented it in over 70% of their trials with longer durations, but only in 10% of their 30 s trials. While this confirms that OKAN-II manifests predominantly after sustained stimuli, it suggests that its occurrence is subject-specific. We also did not observe further increases with stimulus duration. Conversely, OKAN-II onset occurred later as stimulus duration increased (p = 0.02), while OKAN-II occurrence and peak velocity did not differ between the three longest stimuli. Previous studies on OKAN-I, used negative saturation models to account for OKAN-II. As these approaches have no foundation in the OKAN-II literature, we evaluated if a simplified version of a rigorous model of OKAN adaptation could be used in humans. Slow-phase velocity following the trials with 3-, 5-, and 10-min stimuli was fitted with a sum of two decreasing exponential functions with opposite signs (one for OKAN-I and one for OKAN-II). The model assumes separate mechanisms for OKAN-I, representing VSM discharge, and OKAN-II, described as a slower adaptation phenomenon. Although the fit was qualitatively imperfect, this is not surprising given the limited reliability of OKAN in humans. The estimated adaptation time constant seems comparable to the one describing the reversal of the vestibulo-ocular reflex during sustained rotation, suggesting a possible shared adaptive mechanism.

Early behavior of optokinetic responses elicited by transparent motion stimuli during depth-based attention

When two visual patterns moving in different directions are superimposed on the same depth plane (transparent motion stimulus), observers perceive transparent surfaces sliding over each other on different depth planes. Simultaneously, an optokinetic response (OKR) occurs so that one of the visual patterns is stabilized on the retina. In this study, we investigated the early behavior of the OKR elicited by transparent motion stimuli while subjects focused their attention on either the near or far surface. Two random dot patterns were superimposed and moved in orthogonal or opposite directions. Subjects were instructed to report the motion direction of the surface on which their attention was focused. The mean latency of initiation of OKR in the case of motion in opposite directions (150 ms) was significantly longer than that in the case of motion in orthogonal directions (100 ms). In the case of motion in orthogonal directions, the distribution of directions of OKR during the initial period, from 100 to 150 ms, was biased toward the mean direction of the two stimulus motions. After 160 ms, the eyes started to pursue a particular motion pattern of which the direction agreed with the far-perceived motion regardless of depth-based attention. Depth-based attention changed the direction of eye movements after 200 ms and eventually made the eyes follow a pattern on which the attention was focused. These results suggest that pursuit eye movement immediately after 160 ms may determine perceptual depth order through change of retinal image motion, because the slow-moving retinal image may be perceived in the far depth plane. Following this process of determination of perceptual depth order, depth-based attention starts to affect OKR.

Localizing value of optokinetic afternystagmus

Previous reports have indicated that optokinetic afternystagmus (OKAN) becomes asymmetric after the occurrence of unilateral peripheral vestibular lesions, and suggested that OKAN may be used for localizing the side of the lesion. These studies did not take into account spontaneous nystagmus. We compared OKAN in 12 subjects with unilateral vestibular loss after resection of acoustic neuroma to OKAN in 30 normal subjects. After offsetting the data for spontaneous nystagmus, we calculated the initial amplitude, the time constant, and the slow-phase cumulative eye position (SCEP) parameters of OKAN. The directional asymmetry of parameters to rightward and leftward stimulation were also calculated. The mean SCEP, initial amplitude, and time constant parameters were reduced significantly in the patients, and each also showed a directional asymmetry, such that they were greater for stimulation toward the side of the lesion. The directional preponderance of the SCEP parameter had the highest sensitivity for the side of the lesion, being abnormally elevated in 58.3% of patients with unilateral loss. We conclude that OKAN might be useful in combination with other subtests of a battery, but that by itself OKAN is only moderately sensitive to unilateral peripheral vestibular loss.

Orientation of human optokinetic nystagmus to gravity: a model-based approach

Optokinetic nystagmus (OKN) was induced by having subjects watch a moving display in a binocular, head-fixed apparatus. The display was composed of 3.3 degrees stripes moving at 35 degrees/s for 45 s. It subtended 88 degrees horizontally by 72 degrees vertically of the central visual field and could be oriented to rotate about axes that were upright or tilted 45 degrees or 90 degrees. The head was held upright or was tilted 45 degrees left or right on the body during stimulation. Head-horizontal (yaw axis) and head-vertical (pitch axis) components of OKN were recorded with electro-oculography (EOG). Slow phase velocity vectors were determined and compared with the axis of stimulation and the spatial vertical (gravity axis). With the head upright, the axis of eye rotation during yaw axis OKN was coincident with the stimulus axis and the spatial vertical. With the head tilted, a significant vertical component of eye velocity appeared during yaw axis stimulation. As a result the axis of eye rotation shifted from the stimulus axis toward the spatial vertical. Vertical components developed within 1-2 s of stimulus onset and persisted until the end of stimulation. In the six subjects there was a mean shift of the axis of eye rotation during yaw axis stimulation of approximately 18 degrees with the head tilted 45 degrees on the body. Oblique optokinetic stimulation with the head upright was associated with a mean shift of the axis of eye rotation toward the spatial vertical of 9.2 degrees. When the head was tilted and the same oblique stimulation was given, the axis of eye rotation rotated to the other side of the spatial vertical by 5.4 degrees. This counterrotation of the axis of eye rotation is similar to the "Müller (E) effect," in which the perception of the upright is counterrotated to the opposite side of the spatial vertical when subjects are tilted in darkness. The data were simulated by a model of OKN with a "direct" and "indirect" pathway. It was assumed that the direct visual pathway is oriented in a body, not a spatial frame of reference. Despite the short optokinetic after-nystagmus time constants, strong horizontal to vertical cross-coupling could be produced if the horizontal and vertical time constants were in proper ratio and there were no suppression of nystagmus in directions orthogonal to the stimulus direction. The model demonstrates that the spatial orientation of OKN can be achieved by restructuring the system matrix of velocity storage. We conclude that an important function of velocity storage is to orient slow-phase velocity toward the spatial vertical during movement in a terrestrial environment.

Slow cumulative eye position to quantify optokinetic afternystagmus

In 30 normal subjects we computed the slow cumulative eye position (SCEP) of optokinetic afternystagmus (OKAN) that followed 60 seconds of full-field optokinetic stimulation at 60 degrees/s. The mean SCEP was 112.8 degrees +/- 65.0 degrees. The lower and upper fifth percentile limits for directional preponderance of the SCEP were -38.8% and 44.3%, respectively. The time constant, which we calculated by dividing the SCEP by the initial velocity, was 12.0 +/- 7.4 seconds. This value is nearly identical to the time constant obtained from semilogarithmic regression of the decay of OKAN slow-phase velocity versus time. We conclude that the SCEP is a good measure of OKAN and that it reflects the substantial amount of variability and directional asymmetry observed in the optokinetic responses of normal subjects.

Abolition of optokinetic afternystagmus by aminoglycoside ototoxicity

We studied optokinetic afternystagmus in eight subjects with loss of or impairment of vestibular function due to ototoxic antibiotics. We found that the initial amplitude, the time constant, and the slow-phase cumulative eye position of optokinetic afternystagmus were significantly reduced in the patients. Slow-phase cumulative eye position most reliably distinguished our patients' responses from those of a normal group.

Optokinetic afternystagmus in humans: normal values of amplitude, time constant, and asymmetry

It has been suggested that the appearance of directional asymmetry and/or a reduced time constant of optokinetic afternystagmus (OKAN) might be a clinical index of vestibular imbalance. However, we do not know the limits for OKAN parameters in normal humans. Accordingly, we studied OKAN in 30 normal subjects using a "sampling" method, in which a number of values of OKAN are obtained by turning out the lights periodically during optokinetic stimulation. We found that the initial velocity of OKAN has a large intrasubject variability. Accordingly, if precision is desired so as to obtain 95% confidence that the measured mean of the initial velocity of OKAN is within 25% of the true mean in an individual subject, at least eight measurements of the initial OKAN velocity must be taken. When 12 measurements are made, all subjects had a minimum value of 5 degrees/s initial OKAN, and there was little directional asymmetry (mean of -0.47 degree/s +/- 3.13 degrees/s). The intrasubject variability of the time constant of OKAN was similar to the variability of initial OKAN velocity. However, because it is not possible to obtain repeated measures of the time constant in a short period of time, the time constant of OKAN is less likely to be useful in clinical testing.