The Mechanism Underlying the Brückner Effect Studied with an Automated, High-Resolution, Continuously Scanning...

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. Brückner described this principle in 1962. Although this effect has been used as a screening test for strabismus ever since, the mechanism that causes this effect is only partly known. None of the current explanations are accepted as conclusively proven. We developed an automated, high-resolution, continuously scanning Brückner device to study the mechanisms underlying the Brückner effect.

The right eyes of 6 subjects, students aged 22-31 years, were measured. A subject sat in front of a camera at a distance of about 0.75 m. The luminance of the red fundus reflex and the eye movements were measured continuously while the subject followed a fixation target that moved in two dimensions. Eye movement was also recorded objectively and in the analysis this was used in addition to the position of the fixation target. The eyes were illuminated coaxially using an infrared light source and a beam splitter.

By using two methods to record eye rotation and relating this to the continuously measured pupillary luminous intensity and pupillary area, we were able to construct 2-dimensional maps of pupillary luminance. All 6 subjects showed the expected general darkening of the pupil as fixation changed from eccentric fixation to foveal fixation at the center of the camera. However, there were many local fluctuations. The global minimum in pupillary luminance, the "dark spot," was often roughly circle-shaped with a radius of about 1°. The pupillary luminance kept increasing beyond 2° eccentricity.

We measured the Brückner effect continuously in two dimensions. Brückner's original explanation--absorption by the macular pigment--cannot explain our findings as the macular pigment hardly absorbs light beyond 600 nm wavelength. If one assumes that reflection by the inner limiting membrane or diffuse backscatter in the thick retinal nerve fiber layer increase the luminance of the pupil, one would expect the pupil to be brightest at 2° eccentricity, but the pupillary luminance continued to increase beyond that. We used a beam splitter and still found a dark pupil on coaxial fixation, implying that blocking the image that is projected back from the eye by the funduscope only plays a contributory role.