Dual‐mode behaviour in the human accommodation system

The human accommodation system was investigated using a dynamic infra‐red optometer to determine whether its feedback control process exhibited dual‐mode behaviour. Ramp changes of target accommodative demand presented monocularly at various velocities (0.5‐0.6 D s‐1) with a fixed 2 D amplitude elicited two modes of behaviour. Smooth ramp movements were observed for low‐velocity stimuli, whereas step responses predominated for high‐velocity stimuli. At intermediate velocities, combined step‐ramp responses were more prevalent. The characteristic of these step‐ramp responses showed two important properties that suggest a pre‐programming mechanism. The amplitude of the step portion of a step‐ramp response approximated the value of the ramp stimulus at the time corresponding to the end of the step movement. Also, the initial step amplitude increased with increasing target ramp velocity. It appears that the amplitude of the preprogrammed accommodative movement was based on an estimate of the expected ramp target position at the end of the response. These findings are consistent with a recent unifying theory of oculomotor control in which the various oculomotor subsystems (versional and vergence eye movements and accommodation) each has fast and slow components. The fast component exhibits preprogramming, while the slow component shows continuous feedback control.