Today, several techniques for measuring eye movement behaviour are available (see Carpenter, 1988, and more recently Collewijn, 1999, for reviews of oculomotor recording techniques). In our lab we make use of the “double-Purkinje-image (DPI) eye tracking system” designed by Crane and Steele (1985). This technique is based on capturing reflected infrared light that is projected on the eye.

For an extensive description of the Leuven dual-PC eye-tracking system, see van Diepen (1998), and Van Rensbergen & De Troy (1993).

van Diepen, P. M. J. (1998). New data-acquisition software for the Leuven dual-PC controlled Purkinje eye-tracking system (Psych. Rep. No. 246). Leuven, Belgium: Laboratory of Experimental Psychology, University of Leuven.

Van Rensbergen, J., & De Troy, A. (1993). A reference guide for the Leuven dual-PC controlled Purkinje eyetracking system (Psych. Rep. No. 145). Leuven, Belgium: Laboratory of Experimental Psychology, University of Leuven.

The DPI eye tracker illuminates the eye with an infrared beam and employs a complex combination of lenses and servo-controlled mirrors to continuously locate the positions of thefirst and fourth Purkinje images. These Purkinje images are formed by light reflected from surfaces in the eye.

As shown above, the first reflection (P1) takes place at the anterior surface of the cornea, while the fourth (P4) occurs at the posterior surface of the lens (see below for a frontal view of P1 and P4).

These two images move similarly under translation of the eye, that is, they move through the same distance and in the same direction as the eye. During rotation of the eye, the separation between the two images changes proportionally with the sine of the angle of rotation. Thus angular eye position can be obtained from the relative positions of the two images (disregarding their absolute position), free of error induced by translation. The positions of the Purkinje images are continuously tracked by servo-controlled mirrors that keep images of P1 and P4 stabilized on quadrant photo-detectors. The horizontal and vertical output signals, representing eye position are derived from the angles of these mirrors. The major strength of this system is that it allows measurement of eye position with both high spatial and temporal resolution.

The Generation 5.5 DPI eye trackers used in our lab have an accuracy of 1 min of arc and a 1000-Hz sampling rate. In addition, the DPI system, unlike other accurate techniques such as the search-coil technique, is a no-contact measurement technique, which makes it relatively non-invasive. The downside of this instrument is that it requires rigorous head stabilization, therefore the use of a bite-bar and headrest are essential. Another restriction is the limited range in which eye position can be measured. Eye deviations of more than 10 to 15 degrees of visual angle from the centre, cause occlusion of P4 by the iris.

The figure below shows the horizontal output of the DPI system during a typical saccade of about 8 deg. As can be seen, saccades measured with the DPI eye tracker show a minor ‘backshoot’ in the beginning and a considerable ‘overshoot’ at the end. These back- and overshoots are caused by the fact that the fourth Purkinje image stems from the back of the lens. The lens will lag behind somewhat in the beginning of the saccade and will not stop directly at the end of the rotational movement, because of inertia of the lens and the elasticity of the zonula. Although the actual saccade ends somewhat before a stationary output signal is received (as compared to search coil data), the overshoot corresponds to real instability of the retinal image following the actual saccade (Deubel & Bridgeman, 1995a, 1995b, Van Rensbergen, De Troy, Cavegn, De Graef, van Diepen, & Fias, 1993).

• Carpenter, R. H. S. (1988). Movements of the eyes. London: Pion.

• Collewijn, H. (1999). Eye movement recording. In R. H. S. Carpenter & J. G. Robson (Eds.), Vision research: A practical guide to laboratory methods (pp. 245-285). Oxford: Oxford University Press.

• Crane, H. D., & Steele, C. M. (1985). Generation-V dual-Purkinje-image eyetracker. Applied Optics, 24, 527-537.

• Deubel, H., & Bridgeman, B. (1995a). Fourth Purkinje image signals reveal eye-lens deviations and retinal image distortions during saccades. Vision Research, 35, 529-538.

• Deubel, H., & Bridgeman, B. (1995b). Perceptual consequences of ocular lens overshoot during saccadic eye movements. Vision Research, 35, 2897-2902.

• Van Rensbergen, J., De Troy, A., Cavegn, D., De Graef, P., van Diepen, P. M. J., & Fias, W. (1993). The consequences of eye-lens movement during saccades for a stable retinal image. Poster presented at the Seventh European Conference on Eye Movements. Durham, UK.

• Peter De Graef
• Karl Verfaillie
• Filip Germeys
• Jan Vanrie