Objective Calibration of Eye Movement Measurement Using the Brückner Effect

Strabismus ('squinting') affects about 5% of the western population. It can be treated through surgery. The most important determinant of the outcome of strabismus surgery is the measurement of the angles of strabismus. Currently this measurement is performed manually using prisms. Inaccuracies in the measurement of the angles of strabismus account for about 20% of the total variance of the angles of strabismus after surgery. To improve the outcome of surgery, the Delft Assessment Instrument for Strabismus in Young children (DAISY) is under development. DAISY measures the angles of strabismus using two optical methods that determine the eye- and head position. In eye movement measurement, conventional calibration methods require the subjects to fixate at known points. The problem is that it will remain uncertain if the young subjects fixated at these points. In this study an objective calibration method for eye movement measurements was presented. The proposed method used the Bruckner effect: When an eye is illuminated by a light coaxial with the observing eye or camera, the light is reflected by the back of the eye and a red reflex will be observed through the pupil. The intensity of this red reflex varies with the angle of eccentric gaze. The red reflex darkens as the eye fixates at the light source. To establish the Bruckner effect as a valid tool for objective calibration of the projection of the fovea in optical eye movement measurement, a number of preconditions were drawn up. These resulted in the main research questions: Is the red fundus reflex intensity at central foveal fixation significantly lower than the red fundus reflex intensity at eccentric fixation? Is the method repeatable on a inter-individual level and on a intra-individual level? Remaining interesting topics resulted in secondary research questions: What characterises the relation between eye rotation and red fundus reflex intensity? Is the method repeatable on a inter-ocular level? What is the estimated accuracy of the calibration method? The intensity of the red fundus reflex and the eye movements were measured continuously while the subject followed a fixation object that moved back and forth horizontally in front of a camera. The eyes were illuminated coaxially using an infrared light source and a beamsplitter. The result was a 'reflex profile': the tendency of the relation between horizontal eye rotation and red fundus reflex intensity. By varying the eye rotation in vertical direction by changing the height of the fixation object, also two dimensional profiles of the red fundus reflection were made ('scans'). Although the scan images were coarse, it is the first known method to attain images of the retina from a distance. The red fundus reflex at central foveal fixation was significantly darker than the red fundus reflex at eccentric fixation. The right eye of 5 out of 6 subjects showed a clearly distinguishable minimum red fundus reflex with foveal fixation on or very near the center of the observing lens and coaxial infrared light. The remaining subject had difficulties with fixating at the moving fixation object. Optimisation of the speed of the fixation object will likely prevent similar future difficulties. The characteristics of the relation between horizontal eye rotation and red fundus reflex intensity showed interesting unexpected reproducible local effects left and right of the fovea for all eyes of all subjects. These effects probably have an anatomical nature. To determine the intra-individual repeatability, repeated measurements were performed on 3 subjects. The repeated reflex profiles and local anatomical effects were very similar. The small differences in eye rotation at which the local extrema were found are probably caused by inaccuracies in the eye movement measurement. To determine the inter-ocular repeatability, both the left and right eyes of 3 subjects were measured. Although similar mirrored reflex profiles were expected for the two eyes of one subject, the local extrema did not correspond and the course of both reflex profiles was different. It seems plausible that the two eyes of one subject differ anatomically. The calibration accuracy was determined to be 0.343°±0.338° (worst case scenario) in 6 subjects, a third of the overall accuracy DAISY aims for. As all stated preconditions were achievable, the Bruckner effect can be used as objective calibration method for eye movement measurement. This is the first known objective calibration method. The findings of this study exclude two of five possible causes of the Bruckner effect and support Bruckners original reasoning behind the darkening of the red fundus reflex with foveal fixation: the macular pigmentation being densest at the fovea.