Ocular oscillations on eccentric gaze.

Many normal individuals show ocular oscillations on eccentric gaze. This study was designed to investigate the effect of visual disengagement and visual feedback on the nature of these end point oscillations. Three test conditions were examined: target present, target absent and when the target position was determined by the subject's eye position via a variable feedback control system. Feedback gains (i.e. target velocity/eye velocity) ranged from 0, where the target position was decoupled from the subject's eye movements (i.e. the target is stationary on the screen), to +1.0 where the retinal image was stabilised (i.e. the target is driven by the subject's eye movements). Only subjects who exhibited sustained end-point oscillations with no latency were included in the study (n=6). Seven different oscillations including square-wave jerks were recorded in the abducting eye during eccentric gaze of a stationary target. The three most common oscillations were the jerk oscillations, with decelerating, linear or pendular slow phases. A number of additional previously unreported waveforms were also recorded. On removal of the target, the mean drift velocity of the slow phase was greatly reduced. The response to the introduction of a change in the visual feedback was specific to each subject, although in all cases, the end-point oscillations generally were of a lower velocity, and gaze was shifted by up to 8 deg in the direction of the slow phase within the first two seconds. The important role of slow eye movement control for maintaining gaze holding is discussed.

Waveform characteristics of manifest latent nystagmus.

PURPOSE: To examine the waveform characteristics of 37 subjects with manifest latent nystagmus (MLN) and determine the manner in which visual feedback influences the nature of the waveform. METHODS: Binocular recordings of the eye movements of all subjects were undertaken using an infrared tracking system. Subjects viewed the target binocularly and monocularly in primary gaze. The effect of visual feedback on the nature of the MLN waveform was examined by either removing the fixation target or by progressively stabilizing the target in relation to the retina. This progressive stabilization was achieved by feeding back the eye movement signal to move an otherwise stationary target. RESULTS: Four types of MLN were distinguished on the basis of the fixation characteristics seen during binocular and monocular viewing. First, under binocular viewing conditions, subjects could theoretically exhibit stable fixation (type 1 MLN). In addition, three other MLN types were recorded during binocular fixation: conjugate horizontal square-wave jerks (type 2 MLN), conjugate torsional nystagmus (type 3 MLN) and conjugate horizontal jerk MLN waveforms (type 4 MLN). Monocular viewing always gave rise to a conjugate horizontal jerk MLN waveform for each of the four types of MLN. More than 80% of the subjects exhibited either type 3 or type 4 MLN, both of which conform with previous classic descriptions of MLN. Much less common was type 2 MLN. Type 1 MLN (conventionally referred to as a latent nystagmus) appeared to be a rare occurrence. In addition to the two classic linear and decelerating MLN slow phases, four additional slow-phase shapes with either saccadic or pendular elements were recorded and described. Removing visual feedback generally reduced the mean slow-phase velocity and the number of fast phases. For each subject some variability of the slow-phase class was documented from session to session. CONCLUSIONS: Four types of MLN have been described. Their differences are based on their binocular oculomotor behavior, and it is proposed that type 1 MLN and type 4 MLN represent the absolute states and types 2 and 3 the intermediate levels of the MLN spectrum. All types of MLN appear to be strongly visually driven and are largely dependent on the attentional state of the subject and the target conditions. Six different classes of slow phase were found among the four MLN types. The introduction of visual feedback had an immediate effect on the subsequent slow phase or fast phase. It is likely that adaptation mechanisms are in play after a period of visual feedback.

The characteristics of dynamic overshoots in square-wave jerks, and in congenital and manifest latent nystagmus.

Dynamic overshoots are seen after voluntary re-fixation saccades. They are microsaccadic movements which follow primary saccades and have no delay. The purpose of this study was to examine the prevalence and metrics of the dynamic overshoots seen after involuntary saccades. Using infra-red oculography we demonstrate that dynamic overshoots are a common occurrence in physiological square-wave jerks, congenital nystagmus and manifest latent nystagmus and that these overshoots are saccadic in nature and have the same dynamic characteristics as those seen following voluntary saccades. It is therefore likely that they share common neural commands to those dynamic overshoots seen after a volitional saccade. All dynamic overshoots are postulated to be the unwanted consequence of making a saccade and are simulated in a model of fast oculomotor behaviour which is consistent with known experimental results.