Abstract
We introduce a novel, noninvasive retinal eye-tracking system capable of detecting eye displacements with an angular resolution of 0.039 arcmin and a maximum velocity of 300°/s across an 8° span. Our system is designed based on a confocal retinal imaging module similar to a scanning laser ophthalmoscope. It utilizes a 2D MEMS scanner ensuring high image frame acquisition frequencies up to 1.24 kHz. In contrast with leading eye-tracking technology, we measure the eye displacements via the collection of the observed spatial excursions for all the times corresponding a full acquisition cycle, thus obviating the need for both a baseline reference frame and absolute spatial calibration. Using this approach, we demonstrate the precise measurement of eye movements with magnitudes exceeding the spatial extent of a single frame, which is not possible using existing image-based retinal trackers. We describe our retinal tracker, tracking algorithms and assess the performance of our system by using programmed artificial eye movements. We also demonstrate the clinical capabilities of our system with in vivo subjects by detecting microsaccades with angular extents as small as 0.028°. The rich kinematic ocular data provided by our system with its exquisite degree of accuracy and extended dynamic range opens new and exciting avenues in retinal imaging and clinical neuroscience. Several subtle features of ocular motion such as saccadic dysfunction, fixation instability and abnormal smooth pursuit can be readily extracted and inferred from the measured retinal trajectories thus offering a promising tool for identifying biomarkers of neurodegenerative diseases associated with these ocular symptoms.
Highlights
The human eye is an optical instrument in constant motion
Errors introduced during stable fixation and fixation periods in between saccades, when eye displacements and velocities are relatively small, and errors introduced during the larger excursions of saccades
We have demonstrated a novel, noninvasive eye tracking system capable of detecting retinal displacements as small as 0.028° with an angular resolution of 0.039 arcmin and a maximum velocity of 300°/s across an angular span as wide as 8°
Summary
The human eye is an optical instrument in constant motion. Even during stable fixation, eye movements exhibit a broad range of magnitudes and frequencies [1,2]. Research on eye movement was pioneered by Hering [3] and Lamare [4] at the end of the 19th century Emerging techniques such as mechanical recording and photography were subsequently replaced by suction caps [5], scleral search coils [6], and the dual Purkinje image eye tracker [7]. The scleral search coil approach has excellent signal-to-noise ratio (SNR) and a typical resolution of 0.25 arcmin [2,6] It is a highly invasive method involving the use of topical anesthesia and specialized contact lenses. Owing to their non-invasive nature and its easiness of use, the most popular eye trackers are video-based devices that utilize anterior eye features. Their typical tracking accuracy ranges within 18–30 arcmin [8,9]
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