Horopter

The horopter is a theoretical construct in binocular vision that represents the locus of all points in space that are imaged on corresponding points on the two retinas when the eyes are fixated on a particular point. In simpler terms, it's the set of locations in the visual field that appear to be a single point when viewed with both eyes, meaning there is zero binocular disparity.

Several types of horopters are defined based on how corresponding points are determined. The theoretical horopter is geometrically defined based on the positions of the eyes and the fixated point, typically assuming perfect symmetry and ideal retinal correspondence. This rarely matches the empirical horopter, which is determined experimentally by asking subjects to judge the relative depth of points in the visual field. The empirical horopter often deviates from the theoretical horopter due to factors such as anatomical asymmetry, inaccuracies in eye movements, and neural processing.

Different types of empirical horopters include the Vieth-Müller circle (a specific theoretical horopter under ideal conditions), the longitudinal horopter (describing points along the horizontal plane), and the vertical horopter (describing points along the vertical plane).

The shape of the horopter provides insights into the mechanisms of stereopsis (depth perception based on binocular vision) and the neural processing of binocular disparity. Deviations of the empirical horopter from the theoretical horopter can indicate perceptual biases or abnormalities in binocular vision. The concept is critical in understanding how the brain integrates the slightly different images from each eye to create a unified and three-dimensional perception of the world. Points lying outside the horopter fall on non-corresponding retinal points and thus create binocular disparity, which is the basis for stereoscopic depth perception. The amount of disparity determines the perceived depth relative to the fixation point.