Development of a minimally-invasive magnetic system for high-quality wireless eye movement tracking in non-human primates

• Applicant: Professor Ziad Hafed, Ph.D.
• Subject Area: Cognitive, Systems and Behavioural Neurobiology
• Term: from 2020 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 446887860

Precise tracking of eye position is critical in a great variety of sensory, cognitive, and motor control investigations of brain function. In human and non-human primates, the majority of experiments nowadays rely on non-invasive video-based eye trackers, which have experienced significant improvements in performance during the last few decades. However, for tiny fixational eye movements, which are emerging to play important roles in cognition and in reformatting of the spatio-temporal statistics of images entering into the visual system, video-based eye trackers are still lacking. For example, for drift eye movements that continuously occur in between saccades and microsaccades, the position variation of the eye is on the order of approximately 1 minute of arc. Factors as simple as pupil diameter variations (due to varying light intensities or changes in internal state) make such small scales of eye movements simply unmeasurable with trackers that rely on pupil images. This has rendered human investigations of ocular drifts rare. In non-human primates, which constitute our focus here, the gold standard for eye tracking remains the scleral search coil technique, first described in the middle of the previous century. While this technique is suitable for studying even the tiniest of fixational eye movements, it is a highly invasive technique that requires surgical implantation of a coil of wire around the eye orbit, threading of such wire under the skin for several centimeters to a connector on the skull, and a chronic “open wound” in the skin of the head at the site of the connector. Besides the difficulty of the surgical procedure itself, potentials for wire breakage (due to hundreds of thousands of ballistic eye movements every day) and infections (due to a chronic open wound at the connector) result in needs for additional re-implantation surgeries. Our goal here is to develop a minimally-invasive eye tracking system for non-human primates. The technique uses wireless sensing of magnetic fields that rotate with the eye, and it has been demonstrated previously for small animals (e.g. rodents). It is minimally-invasive because it requires only a single implantation surgery, to implant tiny magnets around the eye orbit for creating the magnetic field. The 1-time surgery is much simpler than the implant surgery needed for scleral search coils. Using extensive theoretical and simulation preliminary experiments, we show that approaching the quality of scleral search coils is possible with this proposed technique. Our purpose in the proposed research is to demonstrate successful in vivo performance, and to exhaustively benchmark it against the two ends of the spectra of eye tracking: video-based and invasive search coils. We will make our method, which is much cheaper than both video-based and search coil techniques, openly available to the wider non-human primate community as soon as possible.

• DFG Programme: Research Grants