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Matt J Dunn, Thomas H Margrain, Joy Margaret Woodhouse, Jonathan T Erichsen, Research Unit for Nystagmus; Visual processing in infantile nystagmus is not slow. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3997. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Therapeutic interventions aimed at slowing the incessant eye movements of infantile nystagmus (IN) have elicited subjective reports of improved visual function, even in cases where visual acuity was not significantly changed. The purpose of this study was to investigate the extent to which temporal aspects of vision affect the visual experience of IN; in particular whether the presumed ‘slow to see’ phenomenon is primarily due to delays in redirecting the gaze or an increase in visual processing time.
The time taken to fixate and respond to the orientation of peripherally presented Gabor patches was measured in 11 subjects with IN and 11 controls.<br /> <br /> Subjects viewed a fixation cross on a screen at a distance of 2m. After a random delay of 1-3s, a Gabor patch was presented 3° away from the primary position in one of four locations (up/down/left/right). Each Gabor patch was tilted to one of two orientations. As soon as subjects could locate and identify the targets, a response box was used to report the direction of tilt. This was repeated until at least 10 presentations in each direction had occurred. Spatial frequency of the Gabor patch was adjusted relative to each subject’s visual acuity in order to make the task equally difficult for all subjects. For targets presented either above or below the primary position, the time from presentation until the target acquiring saccade, and the time from the end of the saccade until the response was calculated. Due to the presence of horizontal nystagmus, it was not possible to detect the time to fixation for targets presented left or right of the primary position.
For vertically displaced targets, subjects with IN took significantly longer than controls to redirect their gaze towards the targets (IN: 0.25s [IQR 0.23 - 0.39s], controls: 0.19s [IQR 0.18 - 0.21s], P < 0.01). However, there was no significant difference between subject groups in the time between saccade termination and subject response (IN: 0.59 s [IQR 0.48 - 0.84s], controls: 0.63s [IQR 0.59 - 0.65s], P = 0.78).
The results of this study suggest that IN does not affect the time taken to process visual information; once a target has been foveated, it is processed at a ‘normal’ speed. This study furthers our understanding of the visual difficulties associated with the condition, showing that the ‘slow to see’ phenomenon arises as a result of delayed targeting saccades, rather than slowed visual processing.
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