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 (0.5mm diameter), drilled into an IR filter. 12mm in front of the eye. Under closed-loop conditions the mean accommodation response for passive viewing of the near target was +3.08D. A significant (F= 5.45 d.f. 9,18 P < 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. A significant negative correlation between the magnitude of the passive open-loop response and the size of the shift induced by mental effort was found for both tasks (SIT: y = 1.314 − 0.815x R = 0.86. SDT: y = 2.169 − 0.821xR = 0.70). With respect to zero shift, the relationship was pivotal about 1.61 for SIT and 2.64 D for SDT. It is proposed that when the effect of optical feedback is minimized a significant interaction occurs between mental effort and perceived proximity.