Horopter
The theoretical point horopter (from the Greek hóros , “border”, and optēr , “scout”) is the set of points that are mapped to corresponding points on the retina with a fixed eye position in both eyes .
Points or objects that lie on the surface of the horopter or at a short distance from it, in the so-called panum area, are simply perceived (fused), those in front of it (= crossed transverse disparity ) or behind (= uncrossed transverse disparity) are seen twice (physiological diplopia ), but their specific perception is usually suppressed.
If corresponding retinal points are defined by identical angles to the visual axis of the eye, the horopter consists of a segment of a circle, the Vieth-Müller segment, which runs through the nodes of the two eyes and the fixation point and ends at the optical nodes. The nodes are the points of intersection of the straight connection between the object and its image with the optical axis. Outside of the area defined by the nodal points and the fixation point, this horopter only exists along an approximately vertical line that runs in the plane that bisects the interocular axis and is perpendicular to it (mid-sagittal plane). This line is often called the vertical horopter. Outside these two lines there are no points in space that stimulate corresponding (i.e. equiangular) retinal locations.
If the roll angle of the eyes deviates from one another around the viewing axis (torsion angle) of the two eyes, the horopter consists of two continuous helical lines that approach the visual plane from above or below from the direction of the vertical horopter, and then towards the optical plane Bring the nodes of the eyes closer to the Vieth-Müller segment.
Empirically, corresponding points deviate from the definition of equality of angles given above, which leads to deformations of the so-called empirical horopter compared to the theoretical one. Within the visual level, which is defined by the two nodes and the fixation target, the curve radius of the horopter changes depending on the distance to the fixation point. There is a fixation distance, the abathic distance, for which the horopter is approximately flat, i.e. a straight line within the visual plane. For fixations beyond this abathic distance, the horopter is hyperbolically deformed and bends away from the viewer. For fixation closer than the abathic distance, the horopter is flatter than the Vieth-Müller segment. This deviation from the theoretical horopter is called the Hering-Hillebrandt deviation after its two discoverers, Ewald Hering and Franz Hillebrandt .
Outside the visual plane, the empirical vertical horopter leans away from the observer. This finding can be explained by a shear of the corresponding retinal points against the equality of angles and is called Helmholtz shear after its discoverer Hermann von Helmholtz .
The term horopter itself goes back to the Belgian Jesuit monk Franciscus Aguilonius , who introduced it in the second book of his six books on optics, published in 1613 , as the surface in which objects seen monocularly are located.
See also
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- Hermann von Helmholtz , Manual of Physiological Optics, Voss, Hamburg, 1867
- Wilhelm Ludwig , FP Fischer and R. Wartmann: The optimal horopter. With a consequence of the subjective curvature of the sky . In: Pflugers Archiv-European Journal of Physiology. Vol. 254, No. 5. 1952. pp. 377-392.
- Christopher Tyler: The Horopter and Binocular Fusion . In: D. Regan (Ed.): Binocular Vision. CRC Press, Boston, 1991.
- Brian Rogers and Ian Howard: Seeing in Depth . I. Porteus, Thornhill, 2002.
