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Published in Philip Winn, Dictionary of Biological Psychology, 2003
The horopter is the locus of points that stimulate CORRESPONDING RETINAL POINTS in the two eyes. If one fixates and converges on a point in space, that point will be foveated (see FOVEA) in the two eyes and will, by definition, have zero RETINAL DISPARITY . There will also be points in space that are not at the fixation point that appear to lie in the same visual direction in the two eyes, and appear to be at the same visual distance as the fixation point. These therefore stimulate corresponding retinal points. The surface that could be plotted through them is known as the horopter. The importance of this for vision is that anything that does not lie on the horopter will not stimulate corresponding retinal points and therefore the two retinal images will be disparate. Retinal disparity is the basis of stereoscopic vision. Over a limited range of values (which define PANUM'S FUSIONAL AREA) disparities are interpreted by the brain as depth (see DEPTH PERCEPTION). The geometry of the normal viewing situation suggests that the horizontal horopter should be a circle (the VEITH-MULLER CIRCLE). In reality the shape of the horopter depends on fixation distance.
Vision with two eyes, the doctrine of the identical retina points
Published in Strabismus, 2020
The closer the points are to each other in the image plane, the closer the stereoscopic image is to the eye and vice versa. From this rule it follows also, and further observations will confirm this, that a pair of points, with a distance between them in one single image that is equal to that in the other single image, is seen in the stereoscopic image at the same distance from the eye, in a plane that is parallel to the surface of the face. Incidentally, I notice that this expression is not a very strict and correct one; this point will be explained later, but permit that I anticipate here. I call the same distances at which congruent images are seen the horopter. The main distance, which corresponds to the main lateral distance of the images, I call the main horopter. The importance of what I call horopters for the time being is that on their basis the distance, the dimension of the depth, is determined. The main horopter denotes that distance which is determined by the position of the eyes and to which we refer primarily in stereoscopic phenomena when we speak of in front of and behind.
About the stereoscopic phenomena and Wheatstone’s attack on the theory of the identical points of the retinas: Part 2
Published in Strabismus, 2022
Applying now the above theory to this phenomenon, it is evident that when the variations in visual distance shown to be present are as large as the distance from the horopter for the most distant of the visible points of the object drawn, up to the nearest point of the object, all corresponding points of the images are projected once on identical points of the retinas, and we thereby see single for this reason as if the object were actually standing in front of us. This necessary change in visual distance is on average about 2 inches for the objects visualized by Wheatstone, the visual distance of an ordinary stereoscope is 8 inches. We will therefore calculate by how much each eye should rotate so that the visual distance increases from 7 to 9 inches.
The stereoscopic phenomena in relation to the doctrine of identical retinal points
Published in Strabismus, 2020
It should be noted that Brücke also devised control experiments to test his proposition, the success of which he found important. He says: as soon as one gets control over the fluctuations of the range of vision and fixates a certain point of the collective stereoscopic image, one sees all points of the image that are remote from the horopter double. Brücke assures that, in the sample images given by Wheatstone, e.g. in the circles which fuse into a truncated cone, he sees only the fixated circle singly, and the non-fixated circle double, and asserts that if Wheatstone saw otherwise, the reason for this should probably only be sought in inexperience of the English physicist in fixating.