The effect of mental effort on open- and closed-loop accommodation.

The accommodative response to stimuli in normal visual environments is determined by a complex and subtle integration of optical and non-optical factors. Mental effort associated with the visual task can modify significantly the steady-state accommodative level, but, owing to the diversity of experimental designs, there is no clear consensus on the mechanisms involved. Changes in the accommodation response of ten emmetropic subjects (mean (+/- SD) age = 20.4 +/- 4.5 years) under open- and closed-loop conditions were investigated for three levels of mental activity. (1) A passive task whereby subjects simply read letters to themselves. (2) A stimulus-dependent task (SDT) whereby subjects are instructed to respond only when the letter 'e' appears in one of a series of presentations. (3) A stimulus-independent task (SIT) whereby subjects count backwards in sevens to themselves while viewing the target. An objective infra-red (IR) optometer was used in its static mode of operation to make monocular measurements of accommodation under monocular viewing conditions. Open-loop conditions were achieved by placing a pinhole (0.5 mm diameter), drilled into an IR filter, 12 mm in front of the eye. Under closed-loop conditions the mean accommodation response for passive viewing of the near target was +3.08 D. A significant (F = 5.45 d.f. 9,18 P less than 0.005) accommodative shift induced by mental effort in the mean response of +0.17 D occurred for the SDT. The SIT induced a mean shift of -0.05 D which was not significantly different to the passive viewing response.(ABSTRACT TRUNCATED AT 250 WORDS)