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Supplementation of the human vestibulo-ocular reflex by visual fixation and smooth pursuit.

In studies over the past 30 years, D. A. Robinson and colleagues established that the dynamic characteristics of smooth pursuit eye movements (SP) are different at the onset from those at the cessation of the response. They proposed that cessation of SP was due to a separate fixation system. During head movements, both fixation and SP may contribute to gaze stabilization. We investigated the relative contributions of fixation and SP to the "visually enhanced" vestibulo-ocular reflex (VVOR) using a paradigm requiring a transition from VVOR to combined eye-head tracking (CEHT). We found, in four normal subjects, that ringing typical of SP generally did not occur during VVOR, but that it often appeared after the transition to CEHT. The findings were different in two patients with absent peripheral vestibular function; ringing typical of SP occurred always during VVOR but disappeared during the onset of CEHT. These results can be explained by a model in which an internal representation of target velocity serves as input to parallel SP and fixation systems, and as the determinant of which of the two systems will provide the command signal. Interpretation of our data using this model indicates that either fixation or SP systems may "visually enhance" the VOR, depending on the magnitude of retinal error velocity that remains after vestibular eye movements have been generated.

Adult↗
PubMed2005

Maximum angle of ocular duction during visual fixation as a function of age.

PURPOSE: To measure and compare the maximum angle of ocular duction in healthy individuals as a function of age. METHODS: A calibrated arc perimeter was modified to display one of six randomly presented targets (high contrast Snellen equivalent letters), in both vertical (supra/infraduction) and horizontal (ab/adduction) gaze to the dominant eye of 204 healthy volunteers with best-corrected visual acuity. A bite-bar and headrest were employed to prevent head movement. Using a modified method of limits for discrimination threshold, a maximum mean angle of ocular duction was determined by stepping a target out in 5 degrees steps until an error was reported and thereafter bracketing around the limits of the target identification in 1 degrees steps. A mean threshold value was determined as the angle at which a subject obtained a correct response 75% of the time in two and as many as the trails in each of the four randomly presented directions of gaze (abduction, adduction, supraduction and infraduction). RESULTS: A decrease in mean maximum duction angle was found over all age groups in al four directions (p < 0.001), with a step decline beginning in the sixth decade and almost doubling in the oldest age group tested (80-95 years-olds). The percent change in mean maximum angle of duction due to age from the 14-19 to the 80-95 year-olds was: abduction 21%, adduction 24%, supraduction 35%, infraduction 26%. CONCLUSION: Baseline data are useful to differentiate normal changes occurring with age from early signs of disease. AdditIonally, disease progression and effects of treatment can be monitored.

Adolescent↗
PubMed1994

Extraocular muscle proprioception functions in the control of ocular alignment and eye movement conjugacy.

1. The function of extraocular muscle proprioception in the control of eye movements is uncertain. We tested the hypothesis that proprioception contributes to the long-term regulation of ocular alignment and eye movement conjugacy. 2. Eye movements were recorded in monkeys with unilateral extraocular muscle palsies, before and after proprioceptive deafferentation of the paretic eye. Following deafferentation, ocular alignment and saccade conjugacy gradually worsened over several weeks. In contrast, disconjugate adaptation induced by habitual binocular viewing with a prism (disparity-mediated adaptation) occurred normally after deafferentation. 3. These results provide the first evidence that proprioception functions in the control of eye movements in primates, and indicate that proprioception contributes to the long-term adaptive mechanisms that regulate ocular alignment during fixation and saccades. The error signal used in this process may be derived from a mismatch between the efference copy and proprioceptive afference.

Afferent Pathways↗
PubMed1991

Accommodation without pupillary constriction.

In this paper it will be shown that the association of pupillary contraction with even substantial accommodation breaks down under rigorous control of alignment. Conditions exclude other than "accommodative" (blur induced) pupillary responses by eliminating light reflexes due to temporal and spatial luminance differences, colour and polarization differences. The visual solid angles of the targets were also precisely equalized. A subtle procedure is described to obtain accurate alignment along a single line of sight of the stimulated eye. The pupillary system appears to be quite sensitive to alignment errors, but the "accommodative pupillary reflex" is totally absent under rigorous alignment. Pupillometry used an infrared television camera to monitor the non-stimulated eye in total darkness and silence. Occurrence of correct accommodation for every stimulus event was confirmed by observing accommodative vergence. Judging by the size of the accommodative vergence response and, occasionally, the size of Purkinje-Sanson images, the accommodative response was found to be essentially equal to the stimulation.

Accommodation, Ocular↗
PubMed1992

The remarkable saccades of asymmetrical vergence.

The saccades that usually arise near the onset of asymmetrical changes in vergence, when one eye is aligned with both targets, are remarkably different from ordinary saccades: (1) the excursions of the two eyes are typically very unequal, often differing by several fold from each other; (2) mean excursion (version) is extremely variable across replicate tests with identical targets; (3) at the end of the saccades, eye orientation is usually not even briefly stable: the aligned eye immediately reverses its movement, indicating that the pulse in muscular forces is apparently not followed by a corresponding step; and (4) a second saccade in the opposite direction can immediately follow the initial saccade of asymmetrical divergence, with no sign of refractoriness. These phenomena suggest that the pulse and step components of saccadic motoneuron activity may be generated by largely independent processes; that the step component for each eye depends only on that eye's visual input; and that the pulse components generated for each eye depend on weighted averaging of visual stimuli that impinge on both eyes. This interpretation is incompatible with most current models of saccade generation, but was anticipated in its essentials by Ditchburn [(1973) Eye movements and visual perception. Oxford: Clarendon Press]. A corollary of this hypothesis is that disparity-evoked vergence changes can be viewed as the general-case output from that system which produces fully conjugate saccades as a special case.

Adolescent↗
PubMed1995

Motion aftereffect after monocular adaptation to filled-in motion at the blind spot.

Although the blind spot encodes no visual information, one never perceives an odd blob or blank there, but sees a complete scene of the world even when viewing monocularly. This phenomenon called "filling-in" might be related to mechanisms essential to surface perception, but the neural representation has still been unclear. To determine at what stage the computation for filling-in is established in the visual system, whether prolonged observation of a filled-in motion including the blind spot of one eye could cause motion aftereffect at the corresponding visual field of the other eye was examined. The result was positive--interocular transfer of motion aftereffect was obtained at the tested eye. This finding suggests the possibility that real motion and filled-in motion share a common motion pathway in an early stage in the human visual system.

Adaptation, Ocular↗