Fixational and pursuit eye movements in infantile nystagmus: oculomotor control and perception

Infantile nystagmus (IN) is a pathological condition of continuous, horizontal eye oscillations. Despite ongoing eye movements, those with IN do not experience oscillopsia, i.e. the illusory perception that the environment is moving to-and fro. The correct use of neural compensation for their eye movements, just as in typical individuals, is hypothesised to account for this lack of oscillopsia. This neural mechanism requires that an estimate of eye velocity (‘extra-retinal signal’) be compared to an estimate of the motion of the retinal image (‘retinal signal’), and any difference due to object motion. Despite this hypothesis, there have been no previous investigations on how accurately those with IN can estimate object motion. \ud Even in typical adults, eye movement compensation is not perfect, which underlies a number of pursuit-based illusions. One such illusion, the Filehne illusion can be used to investigate the relative mismatch between the extra-retinal and retinal signals as individuals judge their perception of stationarity while attempting to follow a moving target. This illusion was used to investigate, through comparison with typical individuals, whether those with IN correctly recover object motion.\ud Experiments were conducted to characterise the ability of those with IN to fixate as well as follow targets, using a novel two-dimensional eye movement measure. Under repeated testing, the fixation gaze angle at which the IN oscillation is minimum (a null zone) was not consistent. Moreover, those with IN were unable to accurately or precisely follow targets using their slow phase. Results from a comparison of fixation and pursuit performance in those with IN suggested no difference in either the accuracy or precision. Importantly, the psychophysical judgements of those with IN pursuit eye movements are prone to similar compensation errors as typical controls. However, the mechanisms by which this is achieved will require further exploration.