Biologically relevant models of infantile nystagmus syndrome: the requirement for behavioral ocular motor system models.

Infantile nystagmus syndrome (INS) is a combination of several types of nystagmus, each representing dysfunction in one subsystem of the ocular motor system (OMS) and having characteristic waveforms. Eye-movement recordings are the only certain way to identify IN and differentiate it from other types. The waveform classification scheme in use for 30 years is both accurate, inclusive, and suggests the underlying subsystem instabilities. In different individuals, they may appear at birth (hard wired) or in early infancy (developmental). The primary subsystem instability in IN is hypothesized to lie in the normally underdamped smooth pursuit system; vestibular dysfunction (imbalance) may also be present. Less often, the nucleus of the optic tract may be involved. Ocular motility studies over the past 40 years have demonstrated that saccades and gaze holding are normal in the INS and saccades contained within IN waveforms are always corrective; i.e., they cannot be the initiating movement responsible for IN. Because there are an infinite number of solutions to simulating specific waveforms, models that merely generate waveforms that resemble IN in isolation are of little use, either clinically or to increase our understanding of the underlying mechanisms of IN. A biologically relevant model of the INS should be part of, and operate within, a complete OMS model, capable of reproducing the normal ocular motor behavior of these individuals while still oscillating; i.e., the model, like the patient, must not have oscillopsia and be able to respond correctly to various target inputs.