Oscillopsia and tolerance to retinal image movement in congenital nystagmus.

PURPOSE: To determine the relationship between retinal image movement (RIM) and oscillopsia in subjects with idiopathic congenital nystagmus (CN). METHODS: Eye movements were recorded using an IRIS infrared system. The eye movement signal was fed back to move an otherwise stationary target on a screen and thereby modify the RIM experienced by each of the five CN subjects. The target was present with either no background (the absolute condition) or a textured background (the relative condition). Feedback gains were varied from -1.0 (i.e., 100% retinal image increase) to +1.0 (i.e., 100% retinal image decrease or complete stabilization), with 0 representing the zero feedback or stationary target condition. In the first experiment, RIM thresholds were determined for a range of feedback values. Using zero feedback, a second experiment measured the detection threshold for absolute and relative motions to a ramp-generated target movement for five CN and five control subjects. RESULTS: Under feedback control spatial constancy broke down for both increased and reduced RIM. The range of spatial constancy was greater for absolute (-0.56 to +0.44) compared with relative (-0.18 to +0.18) RIM. Motion detection thresholds for the CN group were 8 times less sensitive to the absolute and 17 times less sensitive to the relative motion of the target compared with the control group. CONCLUSIONS: These results suggest that in CN subjects perceptual stability is achieved primarily by extraretinal signals.

Retinal slip velocities in congenital nystagmus.

An analysis of waveform velocities was performed on 16 individuals with congenital nystagmus. The fast phases were shown to be saccadic in nature and their main sequence peak velocity was significantly slower than that of the normal control group (P less than 0.01). Peak slow phase velocities were found to reach values as high as 180 deg/sec; a relationship between the peak slow phase velocity and the amplitude of the slow phase was demonstrated. The range of the velocities that comprise the slow phase was generally in excess of 100 deg/sec. The correlation between the period of time spent at low retinal slip velocities (less than or equal to 10 deg/sec) and visual acuity was found to be significant (P less than 0.05). Implications of high slow phase velocities are discussed with reference to future experimental design.

Quick phase programming and saccadic re-orientation in congenital nystagmus.

Horizontal single-step stimuli were presented to 19 subjects with congenital nystagmus (CN). When the stimulus jump was in the same direction as the waveform beat, spatial and temporal measurements of the responses revealed that saccades occurring between 90 and 180 msec after the stimulus were directed to an average of the spatial locations of the initial and final target positions. Transition functions of response end position against delay resembled those collected from normal subjects responding to either double-step staircase or pulse-overshoot stimuli, according to the stimulus direction. An analogy is drawn between the response to a single-step stimulus in the presence of CN and responses made by normal subjects to double-step stimuli. These findings suggest that pathways involved in the computation of amplitude of visually guided saccades also participate in programming CN quick phases and that the time taken to programme CN quick phases is similar to that for voluntary saccades. Evidence of saccadic parallel processing was demonstrated in some, but not all, of the subjects. Responses to steps in the same direction as the nystagmus beat frequently overshot the target. The mean overshoot was proportional to the amplitude and intensity of the nystagmus. Responses to stimuli presented in a direction opposite to the nystagmus beat were as accurate as the responses of a normal control group.

Interaction of active and passive slow eye movement systems.

Independent target and background motions have been used to generate conflicting activity within the pursuit and optokinetic systems. Subjects were required to pursue a small target against a structured background which moved independently. Selective enhancement of the response to the target generated high-gain active pursuit which dominated the eye movements. Passive eye movements induced during relative target and background motion are not normally directly quantifiable due to their low gain. By reducing the gain of the active pursuit optokinetically induced eye movements were enhanced and quantified. Three techniques are described for degrading active pursuit: tachistoscopic, eccentric and pseudorandom methods of target presentation. Our results demonstrate the synchronous input of active and passive eye movement drives to the oculomotor system and illustrate their interaction.

Retinal fixation behavior in human albinos.

The retinal fixation behavior of 17 albinos was examined whilst they fixated a stationary target. An optical system was arranged so that the motion of the target on the retina, due to the nystagmus, could be video-recorded. Eye movements were monitored simultaneously. Eight of the 17 subjects were judged to use consistently a retinal location compatible with the anatomical position of the fovea. The implications of this for subjects with foveal hypoplasia are discussed.

Dynamical systems analysis: a new method of analysing congenital nystagmus waveforms.

Congenital nystagmus is an oculomotor disorder in which fixation is disrupted by rhythmical, bilateral involuntary oscillations. Clinically these eye movements have been described with some degree of success in terms of their peak-to-peak amplitude, frequency, mean velocity and waveform shape. However, it has not proved possible to diagnose any underlying pathology from the nystagmus characteristics. Here, we propose a new approach to understanding the nystagmus using dynamical systems theory. Our approach is based on the use of delay embedding techniques, which allow one to relate a time series of scalar observations to the state space dynamics of the underlying dynamical system. Using this approach we quantify the dynamics of the nystagmus in the region of foveation and present evidence to suggest that it is low-dimensional and deterministic. Our results put new constraints on acceptable models of nystagmus and suggest a way to make a closer link between data analysis and model development. This approach raises the hope that techniques originally developed to stabilise chaotic systems, by using small perturbations, may prove useful in the control of nystagmus.