Modelling of congenital nystagmus waveforms produced by saccadic system abnormalities

Broomhead, D. S.; Clement, Richard A.; Muldoon, Mark; Whittle, J. P.; Scallan, Columba; Abadi, Richard V.

doi:10.1007/s004220050593

Cited by 38 publications

(48 citation statements)

References 22 publications

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“…Previous models of CN have attempted to describe how the nervous system might generate such oscillations (actually simplified models of the oculomotor circuitry) [4][5][6][7][8][9]. Although ingenious, these are proximal models and cannot offer any explanation as to why CN might develop in the first place.…”

Section: Discussionmentioning

confidence: 99%

“…Although there have been numerous attempts to model how the oculomotor system might generate these unique waveforms (proximal models, see [4][5][6][7][8][9]), there has been no explanation for why CN might emerge in the first place (distal model). Taking our plasticity argument one stage further, how could these peculiar oscillations result from an adaptive process, or equivalently, what is the control objective of infant eye movement development?…”

Section: Introductionmentioning

confidence: 99%

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A Distal Model of Congenital Nystagmus as Nonlinear Adaptive Oscillations

Harris

Berry

2006

Nonlinear Dyn

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Abstract. Congenital nystagmus (CN) is an incurable pathological spontaneous oscillation of the eyes with an onset in the first few months of life. The pathophysiology of CN is mysterious. There is no consistent neurological abnormality, but the majority of patients have a wide range of unrelated congenital visual abnormalities affecting either the cornea, lens, retina or optic nerve. In this theoretical study, we show that these eye oscillations could develop as an adaptive response to maximize visual contrast with poor foveal function in the infant visuomotor system, at a time of peak neural plasticity. We argue that in a visual system with abnormally poor high spatial frequency sensitivity, image contrast is not only maintained by keeping the image on the fovea (or its remnant) but also by some degree of image motion. Using the calculus of variations, we show that the optimal trade-off between these conflicting goals is to generate oscillatory eye movements with increasing velocity waveforms, as seen in real CN. When we include a stochastic component to the start of each epoch (quick-phase inaccuracy) various observed waveforms (including pseudo-cycloid) emerge as optimal strategies. Using the delay embedding technique, we find a low fractional dimension as reported in real data. We further show that, if a velocity command-based pre-motor circuitry (neural integrator) is harnessed to generate these waveforms, the emergence of a null region is inevitable. We conclude that CN could emerge paradoxically as an 'optimal' adaptive response in the infant visual system during an early critical period. This can explain why CN does not emerge later in life and why CN is so refractory to treatment. It also implies that any therapeutic intervention would need to be very early in life.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

A Distal Model of Congenital Nystagmus as Nonlinear Adaptive Oscillations

Harris

Berry

2006

Nonlinear Dyn

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“…In terms of the present notation, the unstable NI model posits thatz has lost stability as a result of the time constant N T changing sign, giving a positive eigenvalue of D z F (z), rather than the eigenvalue −0.04 characteristic of normal subjects. The alternative -thatȳ undergoes a bifurcation in the OCSẏ = f (y) -was suggested in (Broomhead et al, 2000), where an instability of the saccadic system was considered. In both cases, it can be proved that the skew-product form ofż = F (z) implies that the corresponding bifurcation in the full system will be of the same type (for example, a Hopf bifurcation in the OCS will induce a Hopf bifurcation in the full system).…”

Section: Characterisation Of a Pathological Oculomotor Systemmentioning

confidence: 99%

“…As a consequence, the fast phases are forced to terminate close to the target position (Broomhead et al, 2000;Akman et al, 2005). Both types of models thus result in a fast phase which is a deterministic refixation of the target.…”

Section: Construction Of a Poincaré Map Incorporating A Fast Phase Momentioning

confidence: 99%

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Nonlinear time series analysis of jerk congenital nystagmus

Akman

Broomhead

Clement³

et al. 2006

J Comput Neurosci

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Nonlinear dynamics provides a complementary framework to control theory for the quantitative analysis of the oculomotor control system. This paper presents a number of findings relating to the aetiology and mechanics of the pathological ocular oscillation jerk congenital nystagmus (jerk CN). A range of time series analysis techniques were applied to both recorded jerk CN waveforms and simulated waveforms produced by an established model in which the oscillations are a consequence of an unstable neural integrator. The results of the time series analysis were then interpreted within the framework of a generalised model of the unforced oculomotor system. This work suggests that for jerk oscillations, the origin of the instability lies in one of the five oculomotor subsystems, rather than in the final common pathway (the neural integrator and muscle plant). Additionally, experimental estimates of the linearised foveation dynamics imply that a refixating fast phase induced by a near-homoclinic trajectory will result in periodic oscillations. Local dimension calculations show that the dimension of the experimental jerk CN data increases during the fast phase, indicating that the oscillations are not periodic, and hence that the refixation mechanism is of greater complexity than a homoclinic reinjection. The dimension increase is hypothesised to result either from a signal-dependent noise process in the saccadic system, or the activation of additional oculomotor components at the beginning of the fast phase. The modification of a recent saccadic system model to incorporate biologically realistic signal-dependent noise is suggested, in order to test the first of these hypotheses.

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Abadi¹,

Clement²,

Gowen³

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Modelling of congenital nystagmus waveforms produced by saccadic system abnormalities

Cited by 38 publications

References 22 publications

A Distal Model of Congenital Nystagmus as Nonlinear Adaptive Oscillations

A Distal Model of Congenital Nystagmus as Nonlinear Adaptive Oscillations

Nonlinear time series analysis of jerk congenital nystagmus

Levels of Fixation

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