Low-Noise Magnetic Coil System for Recording 3-D Eye Movements

Vestibular and oculomotor research often requires measurement of 3-D eye orientation and movement with high spatial and temporal precision and accuracy. We describe the design, implementation, validation, and use of a new magnetic coil system optimized for recording 3-D eye movements using small scleral coils in animals. Like older systems, the system design uses off-the-shelf components to drive three mutually orthogonal alternating magnetic fields at different frequencies. The scleral coil voltage induced by those fields is decomposed into three signals, each related to the coil's orientation relative to the axis of one field component. Unlike older systems based on analog demodulation and filtering, this system uses a field-programmable gate array (FPGA) to oversample each induced scleral coil voltage (at 25 Msamples/s), demodulate in the digital domain, and average over 25 ksamples per data point to generate 1-ksamples/s output in real time. Noise floor is <0.036° peak-to-peak and linearity error is <0.1° during 345° rotations in all three dimensions. This FPGA-based design, which is both reprogrammable and freely available upon request, delivers sufficient performance to record eye movements at high spatial and temporal precision and accuracy using coils small enough for use with small animals.