Accommodative tracking: A trial-and-error function

Abstract

The directional tracking of the accommodative system of three subjects has been analysed by following associated accommodative-convergence movements. An attempt was made to eliminate extraneous clues such as apparent vertical and horizontal target movement, size change, blur asymmetry and illumination differences, and also the possibility of learning patterns of stimulus presentations. Spherical and chromatic aberrations of the eye were left intact and astigmatism was uncorrected. For one subject the initial direction of change in accommodation was in error about fifty per cent of the time. The gross clue of the blurred image on the retina was always sufficient stimulus to allow the system eventually to reach the correct final level of accommodation. Statistical verification of the randomness of these data led to the conclusion that it is possible for the accommodative system to operate on an even-error signal with spherical and chromatic aberration intact, with uncorrected astigmatism, and in the presence of normal noise oscillations of the lens. Two other subjects made one hundred per cent correct responses even under conditions where chromatic aberration was removed as a possible clue. It is felt that major clues as to the direction of target movement can result from less than absolutely perfect alignment of the target on the visual axis and from blur asymmetry.