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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.

We tested the effect of visual distractors presented monocularly and binocularly on saccade latency and accuracy to determine whether differences occur in saccadic planning with binocular or monocular visual input. For five participants with normal binocular single vision (BSV), saccade latency and accuracy were compared with distractors presented to the dominant eye, non-dominant eye or to both eyes. Eye movements of the dominant eye were recorded using a Skalar infra-red recorder. In the presence of normal BSV, the effect of distractors is significantly larger for saccade latency and accuracy with binocular distractor presentation than for monocular presentations, with no difference between distrators presented to the dominant or non-dominant eye. The implications of these results are discussed with regard to saccade programming.

Because the oscillatory eye movements of congenital nystagmus vary from cycle to cycle, there is no clear relationship between the waveform produced and the underlying abnormality of the ocular motor system. We consider the durations of successive cycles of nystagmus which could be (1) completely determined by the lengths of the previous cycles, (2) completely independent of the lengths of the previous cycles or (3) a mixture of the two. The behaviour of a deterministic system can be characterised in terms of a collection of (unstable) oscillations, referred to as periodic orbits, which make up the system. By using a recently developed technique for identifying periodic orbits in noisy data, we find evidence for periodic orbits in nystagmus waveforms, eliminating the possibility that each cycle is independent of the previous cycles. The technique also enables us to identify the waveforms which correspond to the deterministic behaviour of the ocular motor system. These waveforms pose a challenge to our understanding of the ocular motor system because none of the current extensions to models of the normal behaviour of the ocular motor system can explain the range of identified waveforms.

This paper reports two experiments to determine the contribution of the suppressing eye to the generation of saccadic eye movements in constant strabismus. Eye movements were recorded using a Skalar infra-red recorder. Experiment 1 tested six participants with constant strabismus, pathological suppression and no clinically demonstrable binocular single vision (BSV). We explored the effect of visual distractors presented monocularly (to either the fixing eye or the strabismic eye) and binocularly, on saccade latency and accuracy. Saccade latency significantly increased when distractors were presented to the strabismic eye compared to the no distractor condition. In all participants the effect on latency, with distractors presented to the strabismic eye, was maximum when distractors were presented towards the location of the anatomical fovea. Saccade accuracy was reduced with ipsilateral distractors to the target when presented binocularly or monocularly to the fixing eye but not affected by distractors presented to the strabismic eye. Experiment 2 investigated fast disconjugate saccade adaptations in six participants with constant strabismus, pathological suppression and no clinically demonstrable BSV and for comparison 8 with normal bifoveal BSV. Saccade disconjugacy was induced using an electronic feedback system in which the calibrated eye movement position signal could be scaled by a factor (the feedback gain) to move the target visible to one eye during binocular viewing. In all BSV participants and 3 of 6 participants with constant strabismus, saccadic adaptation occurred rapidly such that under conditions of visual feedback saccades became increasingly disconjugate. These disconjugacies persisted when normal viewing conditions were restored. The presence of an adaptive mechanism to adjust the binocular co-ordination of saccades in the presence of constant strabismus with suppression and no clinically demonstrable BSV has been demonstrated. Mechanisms that might explain such results are discussed.

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