Microsaccades in head-free high-acuity tasks

Abstract

Previous research has shown that during examination of fine spatial patterns, microsaccades relocate the center of gaze toward nearby objects of interest with remarkable precision. This behavior appears to compensate for inhomogeneous vision within the foveola and to take advantage of temporal and attentional enhancements. In these previous studies, to reliably measure the smallest eye movements, the head of the observer was strictly immobilized. Here we examine whether this microsaccade behavior persists during normal head-free viewing, when both head and eye movements contribute to retinal image motion. Building on seminal work (Steinman, 1995), we simultaneously measured head and eye movements using a custom apparatus that couples a motion capture system Optitrack with the oscillating field monitor (Eibenberger et al, 2016), a device that continuously tunes three orthogonal, oscillating magnetic fields, to maintain high uniformity in the central 1 m^3 region. This approach enables arcminute resolution localization of the line of sight when the head is free to move normally. Subjects (N=11) wore eye coils and a tightly fitting helmet with markers. They discriminated the orientation of tumbling E optotypes on the 20/25, 20/20, and 20/15 lines of a Snellen eye chart. In two separate conditions, their head was either free to move normally or restrained by a chin rest. We report that microsaccade behavior is preserved in head-free viewing. In both conditions, microsaccade amplitude distributions peaked at 10’, the average distance between adjacent optotypes. Distributions of microsaccade speeds, durations, and directions were also similar in the two conditions. As previously observed under head-fixed conditions, all microsaccades were binocular with greatly correlated amplitudes in the two eyes (R^2 = 0.94; p < 0.001). These results show that microsaccades play a critical role in the acquisition of fine spatial detail irrespective of head movements.