Overview of the experimental set-up and ray tracing simulation scheme.

<p>Upper panel: An eye model reflects light rays from a semicircular arrangement of point sources (simulating the array of illuminating LEDs). The reflected images from the point sources are captured through a teleobjective (working distance is 11 cm). The procedure to simulate the image formation of the corneal and posterior lens specular reflection images (PI and PIV images) consists of two sequential steps where the corresponding surfaces of the model eye are converted to mirrors. Lower panel, left: Picture of the actual set up at the lab with its main components: Camera, teleobjective, illumination and fixation LEDs. White LEDs (emitting IR light) in the image form the illumination semicircular source. Red (visible) fixation LEDs are used to generate the saccade movement of the subject by flickering from the center to a peripheral position alternatively (flickering frequency in this experiment was 1 Hz). Lower panels, right: Comparing the outcome of the computer model (left) to a real frame picture. This particular position corresponds to a 9 degrees upwards stationary position. Notice that PIV is optically inverted with respect to PI (negative magnification) and it is generally smaller than PI. The use of a semicircular source of light permits a very simple identification of each reflection.