A magnetic field system using implanted sensors to track limb movements in the monkey

This paper describes the design and construction of a magnetic field system with implanted sensors, capable of tracking limb orientation during unconstrained movements of awake, behaving monkeys. Tiny sensors (approximately 5 mm diameter) made from two orthogonal coils of wire are implanted in the monkey's upper arm, forearm, and hand. Three mutually orthogonal magnetic fields are detected by each sensor, providing sufficient information to determine the orientation of each limb segment. The use of implanted sensors and magnetic fields overcomes the two main difficulties associated with the use of existing tracking systems with monkeys. (1) It is impossible for the monkey to interfere with the sensors. (2) The sensors are never obscured because the body is permeable to magnetic fields. The tracking system provides highly accurate yaw, pitch, and roll orientation information for each limb segment within a cubical workspace approximately 50 cm on a side. Noise is less than 0.3 degrees, and long-term drift is on the order of 0.2 degrees/h. The system is insensitive even to large pieces of steel within the field, provided the sensors remain at least 10–15 cm from the metal.