Simultaneous recordings of ocular microtremor and fixational microsaccades with a piezoelectric sensor and a commercial video tracking system

Abstract

Our eyes continually move even while we fix our gaze. Fixational eye movements include microsaccades, drifts and ocular microtremor (OMT), a high frequency tremor of the eye. Microsaccades and OMT are the fixational eye movements with respectively largest and smallest amplitudes (up to several dozen photoreceptors in the case of microsaccades, and approximately one photoreceptor in the case of OMT). We simultaneously recorded the eye movements of human subjects with a piezoelectric device (adapted from the piezoelectric transducer technique developed by Bengi and Thomas (1968)) and a commercial infrared video tracking system (EyeLink II). The piezoelectric system could detect very small changes in eye velocity, such as those due to OMT. Both recording systems moreover allowed microsaccade detection, thus allowing the direct comparison of microsaccade dynamics between systems. Here we present experimental results concerning the effect of the piezoelectric sensors on the amplitude and other parameters of microsaccades during fixation. The application of a piezoelectric sensor resulted in decreased microsaccade amplitudes (as measured with EyeLink II). Normal amplitudes were restored upon sensor removal. Application of the sensor in one eye only resulted in a simultaneous increase of microsaccade amplitudes in the other eye. We also conducted a perceptual experiment to test the potential role of tremor on target visibility. Whereas microsaccades are known to counteract visual fading, it is not known whether tremor also contributes to visibility during fixation. We found that microsaccades were effective in counteracting fading in both foveal and peripheral target locations, in agreement with previous studies (Martinez-Conde et al., 2006; Troncoso et al., 2008). There was no correlation between OMT and the reappearance of a faded stimulus.