Measuring Eye Movements in Infant Monkeys

Abstract

Abstract: : Purpose: A primary impediment to examining oculomotor behavior in infant animals has been i) a lack of appropriate technology to measure calibrated eye movements and ii) a lack of cooperation from the infant animal in performing oculomotor tasks. The scleral search coil technique combined with small laser spot target stimuli is used commonly in oculomotor studies in adult monkeys but is not suitable for the infant. Here we describe the use of a non–invasive video–based eye tracking system in combination with preferential looking techniques to measure calibrated optokinetic and saccadic eye movements in infant monkeys. Methods: We measured monocular eye movements using a non–invasive video based eye tracker in one normal infant monkey and two infant monkeys reared under visual sensory deprivation conditions to induce strabismus. Calibration was performed offline using data when animals fixated novel stimuli presented at specific horizontal and vertical eccentricities. The stimulus image consisted of an animated video clip subtending 2 deg X 2deg. In optokinetic experiments, nasalward and temporalward optokinetic responses were measured for large–field random dot stimuli moving leftward or rightward at 20 or 40deg/sec. Results: The infant animals’ tended to direct their gaze at the novel stimulus when the stimulus suddenly appeared at fixed horizontal or vertical eccentric locations on an otherwise featureless background. These periods of post–saccadic fixation were easily identifiable in the offline records and were used to calibrate the eye movement data. In the optokinetic experiments, a naso–temporal asymmetry was observed both in the normal infant animal measured at 6 weeks of age and in the animals with strabismus measured at various time points upto 13 weeks of age. One of the animals with strabismus continued to show an asymmetry even at 30 weeks of age while the asymmetry was resolved in the other strabismic monkey. Analysis of spontaneous horizontal saccadic activity suggested that the main–sequence relationships were robust even in infant animals. Conclusions: In this study we have shown that we can capture calibrated eye movement data from infant animals. We are now in a position to attempt longitudinal studies of the development of the visual–oculomotor system in the infant animals and compare normal development with development in animals with strabismus.

Keywords: eye movements: recording techniques • infant vision • strabismus