The double magnetic induction (DMI) method has successfully been used to record head-unrestrained gaze shifts in human subjects (Bremen et al., J Neurosci Methods 160:75–84, 2007a, J Neurophysiol, 98:3759–3769, 2007b). This method employs a small golden ring placed on the eye that, when positioned within oscillating magnetic fields, induces orientation-dependent voltages in a pickup coil in front of the eye. Here we develop and test a streamlined calibration routine for use with experimental animals, in particular, with monkeys. The calibration routine requires the animal solely to accurately follow visual targets presented at random locations in the visual field. Animals can readily learn this task. In addition, we use the fact that the pickup coil can be fixed rigidly and reproducibly on implants on the animal’s skull. Therefore, accumulation of calibration data leads to increasing accuracy. As a first step, we simulated gaze shifts and the resulting DMI signals. Our simulations showed that the complex DMI signals can be effectively calibrated with the use of random target sequences, which elicit substantial decoupling of eye- and head orientations in a natural way. Subsequently, we tested our paradigm on three macaque monkeys. Our results show that the data for a successful calibration can be collected in a single recording session, in which the monkey makes about 1,500–2,000 goal-directed saccades. We obtained a resolution of 30 arc minutes (measurement range [−60,+60]°). This resolution compares to the fixation resolution of the monkey’s oculomotor system, and to the standard scleral search-coil method.
We are grateful to Hans Kleijnen, Dick Heeren, Günter Windau, and Stijn Martens for their valuable technical assistance, and to Alex Hanssen and the central animal facility for excellent monkey care. This research was supported by a Marie Curie Early Stage Training Fellowship of the European Community’s Sixth Framework Program (MEST-CT-2004-007825 PB), a VICI grant within the Earth and Life Sciences of NWO (ALW 865.05.003; AJVO, RFVDW, MMVW, TJVG), The Radboud University Medical Centre (DFS), and the Radboud University Nijmegen (AJVO).
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