Effect of a mental arithmetic task on dark focus of accommodation

Three subjective techniques for measuring accommodation (polarized vernier optometer, laser optometer, and a hand optometer based on the vernier principle) were used to measure subjects' dark focus while they performed a short-term mental arithmetic task. Compared with the initial values measured without the arithmetic task, significant changes in dark focus occurred when the task was performed. The effects depended on the initial dark focus of the subjects: in far-dark-focus subjects, inward shifts were generally observed; in near-dark-focus subjects, the dark focus shifted in different directions when different measuring techniques were used. The individual changes in dark focus due to the mental task were reproducible when the mean dark focus of the group shifted inward and when the same technique for measuring accommodation was used. The results have implications for the interpretation of dark focus shifts that are found following prolonged visual work at close distances, e.g. at visual display terminals.