Abstract
Human eyes move continuously, even during visual fixation. These “fixational eye movements” (FEMs) include microsaccades, intersaccadic drift and oculomotor tremor. Research in human FEMs has grown considerably in the last decade, facilitated by the manufacture of noninvasive, high-resolution/speed video-oculography eye trackers. Due to the small magnitude of FEMs, obtaining reliable data can be challenging, however, and depends critically on the sensitivity and precision of the eye tracking system. Yet, no study has conducted an in-depth comparison of human FEM recordings obtained with the search coil (considered the gold standard for measuring microsaccades and drift) and with contemporary, state-of-the art video trackers. Here we measured human microsaccades and drift simultaneously with the search coil and a popular state-of-the-art video tracker. We found that 95% of microsaccades detected with the search coil were also detected with the video tracker, and 95% of microsaccades detected with video tracking were also detected with the search coil, indicating substantial agreement between the two systems. Peak/mean velocities and main sequence slopes of microsaccades detected with video tracking were significantly higher than those of the same microsaccades detected with the search coil, however. Ocular drift was significantly correlated between the two systems, but drift speeds were higher with video tracking than with the search coil. Overall, our combined results suggest that contemporary video tracking now approaches the search coil for measuring FEMs.
Highlights
Gaze position can indicate the locus of attention and provide information about visual processing strategies [1,2,3,4,5]
The accurate measurement of eye movements is important to numerous research areas, including visual neuroscience, scene perception [1,10], machine vision [11], clinical research [6,12,13,14,15], and other applied fields [4,16,17,18,19], but it is perhaps most critical to studies on fixational eye movements (FEMs; the involuntary, smallmagnitude eye movements continuously produced during fixation: i.e. microsaccades, drift and tremor), due to the eye tracking sensitivity and precision required to obtain reliable FEM data
We found substantial agreement between the two recording systems, and some differences. 95% of microsaccades detected with the search coil were detected with the video tracker, and 95% of microsaccades detected with video tracking were detected with the search coil
Summary
Gaze position can indicate the locus of attention and provide information about visual processing strategies [1,2,3,4,5]. Some recent FEM studies have used dual Purkinje eye image (DPI) trackers, which track the fourth Purkinje image and the corneal reflection [35], but these typically require the use of a bite bar as well as an eye patch (since recordings are almost always monocular), in addition to a bulky and complicated setup [36]. Because of these difficulties, most contemporary researchers rely on video-oculography (i.e. video tracking) methods to record human eye movements—including FEMs—, as they are easiest to use, most flexible, and interfere with natural vision the least. The most common video tracking method tracks the pupil (with or without the corneal reflection)
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