Does over-accommodation occur when using aircraft head-up displays?

Background: Whether over-accommodation is caused by the use of head-up disp lays is still under debate. Most prior experimentation has involved cogniti vely demanding tasks, which are known to affect the accommodation response. The simulations have often been unrealistic and involved short working dis tances. Hypothesis: Over-accommodation is caused not by the presence of a h ead-up display per se, but rather by the cognitive demand of the task. Meth ods: The effect of increasing the task cognitive load and the use of forwar d looking infra-red imagery (FLIR) on the ocular accommodative response and task performance was assessed with a realistic head-up display assisted fl ying task. FLIR increases cognitive load due to its poor resolution and the need for interpretation of the images. Results: Over-accommodation was fou nd to be small in magnitude (0.17 +/- 0.03 D; range -0.02-0.45 D) occurring only with cognitively demanding tasks and with forward looking infrared im agery. Response times to detect tanks in the outside world scene were slowe r with increased cognitive load and forward looking infra-red imagery, alon g with a reduced detection rate, decreased accuracy of tracking tanks in th e outside world and poorer control of the head-up display pitch ladder. Whe n discrimination was added to detection in an outside world task, decisions were delayed until they could be accurately made, rather than performance degraded. Conclusion: The use of a virtual head-up display in a simulated a ircraft environment did not adversely affect ocular accommodation. However, increased cognitive demand or FLIR imagery caused significant inward shift s of accommodation.