A self-calibrating, camera-based eye tracker for the recording of rodent eye movements (Zoccolan et al, 2010)

Came across an interesting methods article in "Frontiers in Neuroscience" published in late November last  year which involves the development of a fully automated eye tracking system which is calibrated without requiring co-operation from the subject. This is done by fixing the location of the eye and moving the camera to establish a geometric model (also see Stahl et al, 2000, 2004). Apparently they attempted to use a commercial EyeLink II device first but found it not suitable for rodent eye tracking due to thresholding implementation, illumination conditions and failing corneal reflection tracking when the rodent was chewing. So the authors built their own solution using a Prosilica camera and a set of algorithms (depicted below). Read the paper for implementation details. I find it to be a wonderful piece of work, different  from human eye tracking for sure but still relevant and fascinating.

Schematic diagram of the eye-tracking system

 Illustration of the algorithm to track the eye’s pupil and corneal

reflection spot.

Eye coordinate system and measurements

Horizontal and vertical alignment of the eye with the center of

the camera’s sensor.

Abstract:

"Much of neurophysiology and vision science relies on careful measurement of a human or animal subject’s gaze direction. Video-based eye trackers have emerged as an especially popular option for gaze tracking, because they are easy to use and are completely non-invasive. However, video eye trackers typically require a calibration procedure in which the subject must look at a series of points at known gaze angles. While it is possible to rely on innate orienting behaviors for calibration in some non-human species, other species, such as rodents, do not reliably saccade to visual targets, making this form of calibration impossible. To overcome this problem, we developed a fully automated infrared video eye-tracking system that is able to quickly and accurately calibrate itself without requiring co-operation from the subject. This technique relies on the optical geometry of the cornea and uses computer-controlled motorized stages to rapidly estimate the geometry of the eye relative to the camera. The accuracy and precision of our system was carefully measured using an artificial eye, and its capability to monitor the gaze of rodents was verified by tracking spontaneous saccades and evoked oculomotor reflexes in head-fixed rats (in both cases, we obtained measurements that are consistent with those found in the literature). Overall, given its fully automated nature and its intrinsic robustness against operator errors, we believe that our eye-tracking system enhances the utility of existing approaches to gaze-tracking in rodents and represents a valid tool for rodent vision studies."

• Zoccolan DF, Graham BJ, Cox DD (2010) A self-calibrating, camera-based eye tracker for the recording of rodent eye movements. Frontiers in Neuroscience Methods. doi:10.3389/fnins.2010.00193 [link]