Aniso-accommodation as a possible factor in myopia development.

It is shown that the asymmetric convergence caused by the changes in fixation required to read a line of text results in unequal accommodation demands to the two eyes. Since experimental evidence suggests that the required aniso-accommodation response cannot be achieved, and that accommodation in both eyes tends to match the response to the lower of the two demands, in general the retinal image in at least one eye must be out-of-focus. The resultant blur increases as the eccentricity of gaze is increased and the reading distance is reduced, and also depends on the posture and position of the head, together with the balance between eye and head movements. By analogy with the results of animal experiments, it is argued that the defocused retinal imagery involved in reading and similar tasks could be a factor in myopia development, particularly in children who adopt unfavourable reading postures.