The Role of Convergence in Stereoscopic Vision

Abstract

An apparatus for testing stereoscopic acuity has been constructed in which the distance of a reference aperture P is compared with that of a test aperture Q selected from one of a tapering series of aperture plates. The apertures are seen as illuminated discs in a black field and all monocular aids to depth perception are eliminated. The observer has to state whether Q is in front or behind P, and the acuity is derived by plotting the percentage estimates against the distance of Q. Observations have been made for condition A, in which the observer is allowed to look first at P and then at Q, and for condition B, in which the observer fixates on P and views Q extra-foveally. From a comparison of the acuity for the two conditions for various angular separations θ between P and Q, deductions can be made about the relative contributions to stereoscopic vision of convergence and retinal disparity The results suggest that convergence makes a significant contribution and has become the predominant factor at θ = 20° For θ = 14°, the acuity for condition A is due entirely to convergence, since the blind spot interferes with the retinal disparity. Tests with flash exposures suggest that small eye movements do not play any important part in depth perception The presence of depth perception under conditions of gross diplopia was confirmed, indicating that fusion is not an essential item for three-dimensional vision.