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Vijay K Tailor, Maria Theodorou, Annegret H Dahlmann-Noor, John A Greenwood; The effect of eye movements on visual crowding in congenital nystagmus. Journal of Vision 2020;20(11):711. https://doi.org/10.1167/jov.20.11.711.
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Congenital nystagmus is a disorder characterised by involuntary eye movements (horizontal in direction), with associated deficits in visual acuity and elevations in crowding, a process whereby objects that are visible in isolation become indistinguishable in clutter. We sought to understand the origin of these foveal crowding effects. If nystagmic crowding derives from image motion, crowding should have a disproportionate effect with horizontally arranged flankers (due to horizontal eye motion) compared to vertically placed flankers. In contrast, if crowding was due to a sensory deficit derived from enlarged receptive fields, crowding effects in each dimension should be matched.
Observers judged the orientation of a target Landolt C presented foveally in 3 conditions: the target alone (to measure acuity), or crowded with either horizontal or vertical Landolt-C flankers. A QUEST procedure was used to vary gap size (with flankers and element spacing scaled accordingly). Thresholds were taken as the size required to reach 62.5% correct. Overall, we found elevations in both crowded conditions relative to acuity. These elevations were greater for nystagmats (n=8) than controls (n=10). Consistent with predictions based on image motion, crowding was significantly larger with horizontally- vs. vertically-positioned flankers for nystagmats, unlike controls where thresholds were both equally elevated relative to the acuity baseline.
We next simulated nystagmic crowding in controls (n=10) by moving stimuli in the above 3 conditions, according to eye-movement recordings of patients with nystagmus. Observers were required to either follow the stimulus or maintain fixation. Crowding thresholds were elevated, particularly in the fixation condition, with worse performance for horizontal vs. vertical flankers (as seen with nystagmus).
The presence of elevated horizontal crowding in nystagmats and controls (when stimuli moved with simulated nystagmus) suggests that the eye movement-induced smear of target and flanker elements is the cause of nystagmic crowding, rather than a long-term sensory deficit.
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