Fixation was monitored using an infrared eye tracking system (ISCAN, Inc., Burlington, Massachusetts). Infrared illumination of the eye was supplied by an array of LEDs mounted on the head coil and arranged so as to minimise the external magnetic field due to current flow. The eye image was recorded by an infrared camera mounted at the rear of the magnet bore, and transmitted to a computer outside the scanner room for analysis. The eye tracker was calibrated at the beginning of each scanning session, and recalibrated as necessary between trials. Excursions of 1.25° (25% of the distance from fixation to the target location) or more were designated as breaks in fixation. (Noise in the tracking system prevented reliable detection of saccades less than 1.25° in amplitude.)
In order to avoid magnetic transients associated with electrical switching, an optical signalling device was constructed to transduce behavioural data. A high-output red LED (Radio Shack #276-086A, 5cd at 660nm peak) was mounted at the top of a wooden enclosure, powered from an external supply via a twisted-pair cable so as to minimise external magnetic field. Two strands of DB-1000 1mm plastic optical fibre were mounted in holes drilled at the bottom of this enclosure, so as to be illuminated or shaded depending on whether the subject's finger was positioned at the enclosure's left or right side. Outside the scanner room, a fast-acting Type 7H cadmium selenide photocell (Mouser Electronics #621-CL707H) in parallel with a 100M
resistor transduced the resulting optical signal into a standard PC game port, which was sampled at 18s-1.