Is apparent fixational drift in eye-tracking data due to filters or eyeball rotation?

Diederick C Niehorster,Kenneth Holmqvist,Raimondas Zemblys

doi:10.3758/s13428-020-01414-3

Abstract

Eye trackers are sometimes used to study the miniature eye movements such as drift that occur while observers fixate a static location on a screen. Specifically, analysis of such eye-tracking data can be performed by examining the temporal spectrum composition of the recorded gaze position signal, allowing to assess its color. However, not only rotations of the eyeball but also filters in the eye tracker may affect the signal’s spectral color. Here, we therefore ask whether colored, as opposed to white, signal dynamics in eye-tracking recordings reflect fixational eye movements, or whether they are instead largely due to filters. We recorded gaze position data with five eye trackers from four pairs of human eyes performing fixation sequences, and also from artificial eyes. We examined the spectral color of the gaze position signals produced by the eye trackers, both with their filters switched on, and for unfiltered data. We found that while filtered data recorded from both human and artificial eyes were colored for all eye trackers, for most eye trackers the signal was white when examining both unfiltered human and unfiltered artificial eye data. These results suggest that color in the eye-movement recordings was due to filters for all eye trackers except the most precise eye tracker where it may partly reflect fixational eye movements. As such, researchers studying fixational eye movements should be careful to examine the properties of the filters in their eye tracker to ensure they are studying eyeball rotation and not filter properties.

Highlights

When humans fixate a static object to stabilize its retinal image, their eyes still make smallDiederick C
We have reported analyses of the spectral color observed in gaze position signals recorded with video-based eye trackers during human fixation episodes and in data from artificial eyes
We examined whether color in the gaze position signal is likely due to fixational eye movements of the human participants, or is instead mostly caused by filters in the eye tracker

Summary

Introduction

When humans fixate a static object to stabilize its retinal image (see Hessels et al, 2018), their eyes still make small. As the SMI RED250’s noise magnitude is at least as large as that of the Tobii TX300, it should be wondered whether the smooth gaze position signals produced by this eye tracker reflect fixational eye movements As seen, these smooth signals exist in data recorded with static artificial eyes on both the low-noise EyeLink and the higher-noise SMI RED250. While measurement noise in an eye tracker is likely white if each output sample is processed independently, 1/f power law behavior in the signal can be introduced due to the dynamics of a rotating human eye and by applying temporal filters to the recorded gaze signals This was, for instance, shown by Coey et al (2012), who recorded an artificial eye ( exhibiting no fixational eye movements) with an eye tracker and examined the color of the resulting gaze position signal. Blignaut and Beelders (2012) did not perform frequency analyses, their findings suggest that eye trackers may produce gaze position signals that exhibit 1/f dynamics even in the absence of any physical eye movement, which is at odds with the findings of Wang et al (2016), but consistent with those of Coey et al (2012)

Aims of this paper

Participants and artificial eye

Results

Discussion