Tailor-Hamblin, Vijay K.; (2023) Visual crowding in Infantile Nystagmus Syndrome. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Infantile nystagmus syndrome (INS) is a disorder characterised by involuntary (predominantly horizontal) eye movements, leading to decreased acuity and visual function. A particularly disruptive symptom is visual crowding – a process whereby objects that are easily recognised in isolation become impaired by nearby flankers. Though crowding occurs in typical peripheral vision, it becomes elevated in the fovea with nystagmus. Similar elevations occur in amblyopia, linked to a sensory deficit produced from long-term neural changes. This thesis aims to determine whether crowding derives from a similar deficit, or image smear from eye movements, in two subgroups with nystagmus: idiopathic INS and albinism. Firstly, I measured the spatial dimensions of crowding in IINS. Consistent with image smear from eye movements, horizontally-placed flankers caused greater interference than vertically-placed flankers. Equivalent levels of crowding elevations with the same spatial dimensions were also found in typical observers with nystagmus simulated through image motion, consistent with image smear. Secondly, I measured the feature selectivity of crowding in IINS. Although contrast-polarity differences between target-flanker elements reduce crowding in typical peripheral vision and with simulated nystagmus, these differences had no effect for individuals with IINS. The difference between simulated nystagmus and IINS suggests that a sensory deficit may be present – image smear alone cannot explain this featural selectivity. Finally, the spatial dimensions and featural selectivity of crowding in albinism are shown to be like IINS. However, the magnitude of crowded elevations is larger and in excess of those in IINS and simulated nystagmus, providing clear evidence that crowding in albinism derives from a sensory deficit. Overall, I propose that nystagmic crowding originates from a sensory deficit that causes greater interference in the horizontal meridian, following the pattern of eye movements, with a featural selectivity and magnitude (in albinism) that is unlike that produced by image smear alone.

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