RESEARCH PROJECT

Towards a Distal Model of Congenital Nystagmus

Aims 

• To build a distal model of the development of Congenital Nystagmus in visually impaired infants.
• To predict waveforms and to predict any relationship between the nystagmus and the type/degree of visual impairment, and to provide insight into idiopathic congenital nystagmus.
• To test the model on real data (infants and adults).
• Our ultimate goal is to use the model to explore how CN development may be modified or prevented.

Background

Congenital Nystagmus (CN) (infantile nystagmus) is a spontaneous oscillation of the eyes with an onset in the first few months of life (early-onset nystagmus). The nystagmus is usually life-long and there is no cure. The cause of CN is unknown, but in about 90% of cases there is an underlying visual disorder (sensory defect nystagmus). In about 10% no underlying abnormality can be found, and the nystagmus is called idiopathic. 

In sensory defect CN, there is a very wide range of associated visual disorders, including albinism (ocular and oculocutaneous), aniridia, isolated foveal hypoplasia, cataracts, congenital stationary nightblindness, cone dysfunction, optic nerve hypoplasia, and many more... CN may also be associated with visual abnormalities acquired at birth (eg. corneal infections). Because of these wide associations it is difficult to argue that CN is caused by any single neural or visual abnormality, or any single genetic mutation. Instead it seems that any congenital sensory abnormality (whether inherited or not) that affects the central retina can lead to CN. Onset of a visual disorder after a few months of age (eg. cataracts) does not lead to CN. Thus, CN only emerges in early infancy, a time of peak visuomotor plasticity, with the development of the visual system (the formation of the fovea and visual cortex) and the eye movement systems (especially smooth pursuit). It seems plausible that CN arises because of some abnormality in this developmental plasticity. The puzzle is why does it occur, or equally puzzling, why does CN not occur in all infants?

There have been a number of attempts to model CN, or rather, to propose how a neural circuit might be arranged to yield some of the unique characteristics of CN (particularly increasing velocity waveforms). So far, these have been proximal models, and do not address the reason why the nystagmus develops, or equivalently, why the proposed neural circuit develops instead of the normal one. It is not clear how to test these models, or how they might help us understand CN development.

The purpose of this project is to build models that are developmental in nature, that is, models that are distal. Our assumption is that CN is a developmental behavioural strategy (albeit abnormal) that solves a distal goal, in the same way that normal eye movement development must solve some visual goal. More specifically, we propose that abnormal congenital vision drives early motor development towards oscillations rather than the steady eye movements. What is the cost function that is being minimised?

A good distal model will provide us with insight into why CN occurs and why it does not occur in normal development. It may explain idiopathic CN, and it might inform us on how to intervene to stop CN from developing.

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