Effects of Target Motion Perturbation During Initiation and Steady-State Pursuit in Humans With Infantile Esotropia

Abstract

Purpose: : Patients with infantile esotropia exhibit a nasotemporal asymmetry of pursuit initiation favouring nasalward movements. The goal of this study is to investigate whether a nasotemporal asymmetry also exists during perturbation from steady-state pursuit.

Methods: : Seven patients with a history of infantile esotropia and 11 control subjects were tested. Eye movements were recorded using a video-based system while subjects tracked a red laser target during monocular viewing. For pursuit initiation, a "step-ramp" paradigm was used, with targets moving at 5, 10 or 15 deg/s nasally and temporally. During perturbation from steady-state pursuit, a "ramp-in-ramp" paradigm was used - the target moved at an initial velocity of 5 or 15 deg/s and then accelerated or decelerated to 10 deg/s after 500ms. Velocity gains were measured over the first 100ms after perturbation onset.

Results: : During pursuit initiation, large nasotemporal asymmetries were found in patients when compared to control subjects, with temporalward velocity gains 46% lower than nasalward velocity gains (p=0.0007). Patients had latencies of 6 ms longer than controls for nasalward initiation trials and 28 ms longer for temporalward initiation trials (p=0.0232). No directional asymmetries of velocity gains or latencies were found during perturbation from steady-state pursuit.

Conclusions: : This is the first study to demonstrate the absence of a nasotemporal asymmetry during steady-state pursuit in humans with infantile esotropia. During initiation, pursuit is driven primarily by retinal velocity error. Because patients with infantile esotropia exhibit a nasalward bias in estimating target velocity, this innate directional bias is manifest during pursuit initiation. During perturbation from steady-state pursuit, the normal retinal position error is also available to augment the retinal velocity error information. Together, these two error signals compensate for the innate nasotemporal pursuit bias, resulting in symmetric responses to perturbations during steady-state pursuit.

Keywords: eye movements: saccades and pursuits • esotropia and exotropia • strabismus