Stimuli were displayed against a white background on a 19-inch computer monitor, controlled by a Dell PC, which also recorded the experimental data. Participants were seated at a distance of 57 cm from the screen, with their chins placed on a specially adapted, adjustable chin rest to reduce head movements. Testing was conducted in a quiet, darkened room, to which participants were acclimatized before testing commenced. Eye movements were recorded using an EyeLink II high-speed camera eye tracking system (500 Hz, < 0.5° accuracy) running on a Dell PC. A hand-held computer controller was also connected to the host computer. This permitted participants to control the pace of the presentation of the stimuli for each trial. Participants were required to wear a non-invasive headband supporting three small cameras throughout the experiment. Two cameras recorded eye movements using infra-red technology whereby differential absorbencies of the infra-red energy by the pupil and the sclera permitted the tracking of micro-movements of the pupil. The third tracking camera maintained optimal alignment by monitoring the positioning of four infra-red markers on the computer screen.