Pupil brightness variation as a function of gaze direction

Pupil detection represents one of the most critical aspects for eye tracking systems based on video oculography. A robust segmentation of the aforementioned feature determines to a large extent the degree of performance of the system. However, a question remains unsolved... why does the pupil gray level change in the image? Apart from the possible room lighting variation, can the eyeball physiology influence its final level in the image by itself? The answer is yes. In this paper a further step in the work by Nguyen et al. [Nguyen et al. 2002] is proposed in which this eyeball characteristic was noticed but not explained. This paper gives some enlightenment to this effect finding a physiological reason for it. From the results it is clear that the pupil brightness can be a valid image feature and can contribute together with alternative ones to improve the tracking [Hammoud 2005]. A deep knowledge about its behavior is undoubtedly highly interesting. The matter should be to study how the retina reacts to the light in order to know how it can influence its final level in the image. The retina is not a uniform surface; in the fovea there is a higher density of cones and the ganglion cells are highly packed. When the eye is entered with a beam of light, the light can be reflected and absorbed at the various layers of the retina. Normally near infrared lighting is used because it is not visible for humans. Actually in this range of wavelength the reflected light is dominated by the light scattered back from the choroid: the last layer before the sclera that supports the retina, and it is precisely for this wavelength for which the retina presents the highest reflectance. A bright pupil tracking is conducted following the same method as in the works by Nguyen et al. [Nguyen et al. 2002] and Miller [Miller et al. 1995] but more exhaustive experiments are conducted. A ray of light directed to the fovea needs to cross a thicker layer in order to reach the choroid which produces a decrease of effective light intensity, a stronger reflection and consequently a brighter pupil can be expected if the most eccentric part of the retina is reached. Vertical rotations of the eyeball about its center are sketched in figure 1. From the figure it is clear that the pupil will appear brighter if the subject is looking at the upper part of the screen than for cases in which points in the lower part are fixated. Regarding to left and right eye rotations the fovea is horizontally and temporally displaced from the eyeball back pole. That means that the visual axis and the fovea present an angular offset with respect to the symmetry axis of the eye but with opposite sign depending on the eye. Following the same reasoning as the one used for vertical rotations it is clear that a brighter pupil could be expected for points on the right part of the screen for the left eye. A symmetrical behavior appears for the right eye being the points on the left part of the screen the ones with higher pupil levels.