Abstract
During visual fixation, we constantly move our eyes. These microscopic eye movements are composed of tremor, drift, and microsaccades. Early studies concluded that microsaccades, like larger saccades, are binocular and conjugate, as expected from Hering's law of equal innervation. Here, we document the existence of monocular microsaccades during both fixation and a discrimination task, reporting the location of the gap in a foveal, low-contrast letter C. Monocular microsaccades differ in frequency, amplitude, and peak velocity from binocular microsaccades. Our analyses show that these differences are robust to different velocity and duration criteria that have been used previously to identify microsaccades. Also, the frequency of monocular microsaccades differs systematically according to the task: monocular microsaccades occur more frequently during fixation than discrimination, the opposite of their binocular equivalents. However, during discrimination, monocular microsaccades occur more often around the discrimination threshold, particularly for each subject's dominant eye and in case of successful discrimination. We suggest that monocular microsaccades play a functional role in the production of fine corrections of eye position and vergence during demanding visual tasks.
Highlights
The occurrence of microsaccades during fixation has been reported since 1907 (Dodge, 1907)
The main sequence diagram of identified microsaccades (Figure 1C) indicates that monocular and binocular microsaccades generally follow the same peak velocity versus amplitude relationship, in agreement with previous literature (Bahill et al, 1975; Zuber et al, 1965) and suggesting that both are driven by the same type of oculomotor command
The amplitude distributions of monocular and binocular microsaccades (Figure 1E, F; see Figure 2A) show a marked difference, with the size of monocular microsaccades exhibiting a peak at very small amplitudes and rarely exceeding 180
Summary
The occurrence of microsaccades during fixation has been reported since 1907 (Dodge, 1907). Zuber, Stark, and Cook (1965) demonstrated that saccades, microsaccades, and corrective saccades (see paragraph) lie on a single linear peak velocity versus amplitude relationship, the so-called main sequence, and deduced that these movements result from a common physiological mechanism. Boyce (1967) observed that the amplitude distribution of microsaccades during monocular fixation exhibits a cut-off around 120 with a few outliers around 200. A review of the fixation data from 14 experiments by Ditchburn and Foley-Fisher (1967) confirmed these results and indicated a median microsaccadic amplitude during fixation of 40–50. More recent studies reported a larger mean microsaccade size, between 13.70 and 38.40 (see the review by Martinez-Conde, Macknik, Troncoso, & Hubel, 2009).
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