Magnetic Tracking of Eye Motion in Small, Fast-Moving Animals

Here, we present a new approach for the magnetic tracking of eye motion in small, fast-moving animals. We employ a thin, flat magnetic tracking transmitter instead of the conventional bulky, cubic-shape transmitting frame. The new transmitter enables convenient access to the tracked animal, causes no visual distractions, and occupies much less space. We also employ a tiny solenoidal search coil instead of the conventional scleral search coils. Such a small solenoidal search coil attached laterally to the eye does not limit the peripheral field of view and allows the animal to perform its standard behavioral tasks. The flat transmitter comprises eight transmitting coils that allow us to monitor not only the orientation of a search coil but also its location. To test the efficiency of the new approach, we have measured the location and orientation of the solenoidal search coils attached to the eyes and head of an archer fish during swimming, targeting, and shooting. The size of the coils attached to the fish eyes was 2 mm in diameter and 2 mm in length, and the size of the coil attached to the fish head was 4 mm in diameter and 4 mm in length. The transmitter size was 60 cm times 60 cm times 2 cm. At a 25 cm from the transmitter, we have obtained the tracking resolution of 3 millidegree and 8.3 mu m rms for a 200-Hz bandwidth. Such a performance is good enough to precisely monitor the fastest component in the fish eyes movements. The fish with the search coils on the eyes and head correctly hits the target up to 20 times during an experimental session, which is similar to the shooting rate of the fish without the search coils. This implies that our new design does not introduce much discomfort for the fish.