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

Abstract

BACKGROUND: Whether over-accommodation is caused by the use of head-up displays is still under debate. Most prior experimentation has involved cognitively demanding tasks, which are known to affect the accommodation response. The simulations have often been unrealistic and involved short working distances. HYPOTHESIS: Over-accommodation is caused not by the presence of a head-up display per se, but rather by the cognitive demand of the task. METHODS: The effect of increasing the task cognitive load and the use of forward looking infra-red imagery (FLIR) on the ocular accommodative response and task performance was assessed with a realistic head-up display assisted flying task. FLIR increases cognitive load due to its poor resolution and the need for interpretation of the images. RESULTS: Over-accommodation was found to be small in magnitude (0.17+/-0.03D; range -0.02-0.45D) occurring only with cognitively demanding tasks and with forward looking infrared imagery. Response times to detect tanks in the outside world scene were slower with increased cognitive load and forward looking infra-red imagery, along with a reduced detection rate, decreased accuracy of tracking tanks in the outside world and poorer control of the head-up display pitch ladder. When 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 aircraft environment did not adversely affect ocular accommodation. However, increased cognitive demand or FLIR imagery caused significant inward shifts of accommodation.