Measuring visual axis as an independent variable from a fixation distance

Our pilot study suggested that while fixating on a stationary object binocularly, each line of sight of the two eyes does not always go directly toward the object. Therefore, we experimentally investigated the amount of the displacement of the pupil center while the fixation point was changed transversally. The subjects were instructed to fixate on three targets arranged horizontally with intervals of nine degrees. The distance from the subject to the targets was any of 0.5 m, 1.0 m and 1.5 m. A significant difference of the amount of the displacement of the pupil center was observed relative to the fixation distance. The positions of the node of the line of sight, which provide no relationship between the fixation distance and the pupil center displacement, were calculated. The node of the line of sight of the non-dominant eye was nearer to the fixation panels than that of the dominant eye. Considering this, such a node would be effective for the precise measurement of gazing point.