From human perception to primate neurophysiology, monitoring eye position is critical to the study of vision, attention, oculomotor control, and behavior. Two principal techniques for the precise measurement of eye position—the long-standing sclera-embedded search coil and more recent optical tracking techniques—are in use in various laboratories, but no published study compares the performance of the two methods simultaneously in the same primates. Here we compare two popular systems—a sclera-embedded search coil from C-N-C Engineering and the EyeLink 1000 optical system from SR Research—by recording simultaneously from the same eye in the macaque monkey while the animal performed a simple oculomotor task. We found broad agreement between the two systems, particularly in positional accuracy during fixation, measurement of saccade amplitude, detection of fixational saccades, and sensitivity to subtle changes in eye position from trial to trial. Nonetheless, certain discrepancies persist, particularly elevated saccade peak velocities, post-saccadic ringing, influence of luminance change on reported position, and greater sample-to-sample variation in the optical system. Our study shows that optical performance now rivals that of the search coil, rendering optical systems appropriate for many if not most applications. This finding is consequential, especially for animal subjects, because the optical systems do not require invasive surgery for implantation and repair of search coils around the eye. Our data also allow laboratories using the optical system in human subjects to assess the strengths and limitations of the technique for their own applications.
Keywords: eye position, eye tracking, eyelink, fixation, infrared optical eye tracker, microsaccade, saccade, scleral search coil
Citation: Kimmel DL, Mammo D and Newsome WT (2012) Tracking the eye non-invasively: simultaneous comparison of the scleral search coil and optical tracking techniques in the macaque monkey. Front. Behav. Neurosci. 6:49. doi: 10.3389/fnbeh.2012.00049
Received: 07 May 2012; Paper pending published: 28 May 2012;
Accepted: 16 July 2012; Published online: 14 August 2012.
Edited by:Carmen Sandi, École Polytechnique Fédérale de Lausanne, Switzerland
Reviewed by:Nan Sui, Chinese Academy of Sciences, China
Copyright © 2012 Kimmel, Mammo and Newsome. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.
*Correspondence: Daniel L. Kimmel and Dagem Mammo, Department of Neurobiology, Stanford University, 299 W. Campus Drive, Rm. D200, Stanford, CA 94305, USA. e-mail: email@example.com
†These authors contributed equally to this work.