Accommodation responses and ageing

PURPOSE. To study the impact of age on accommodation dynamics. METHODS. Monocular accommodation responses were measured continuously using a modified Canon Auto Ref R1 infrared optometer. The stimulus was a single letter oscillating sinusoidally between 2.38 and 1.33 D providing a stimul us amplitude of 0.52 D, about a mean level of 1.86 D, Response characterist ics were used to quantify gain and phase. Step responses were also recorded between these stimulus vergence levels for calibration purposes and to mea sure reaction and response times. Nineteen visually normal subjects 18 to 4 9 years of age participated, and 11 frequencies were used in the range 0.05 to 1.0 Hz. A key feature of the experimental design was to use a stimulus vergence range that la) within the amplitude of accommodation of all the ob servers. RESULTS. Accommodation gain reduced and phase lag increased with age, parti cularly at the higher frequencies used. No strongly significant change with age was found for reaction and response times or accommodation velocity, a nd results were similar for both far-to-near and near-to-far responses. Res ponse amplitude for the step change in target vergence declined with age, a nd substantial differences were found between the measured and predicted (f rom reaction time) phase lags at 1.0 I-It as a function of age. Young obser vers showed a phase lag th:lt was shorter than predicted, whereas older obs ervers' measured phase lags were considerably larger than predicted, CONCLUSIONS. Results show that for a target oscillating sinusoidally in a p redictable manner at :I modest amplitude, the main ageing effects occur in phase lag, which is appreciably longer than predicted from reaction times i n the older observers. The effects of ageing on gain were not :Is marked. A lthough responses to sm:lll step changes do reduce with age, there is no ev idence of increased response times with ageing. In general, accommodation f unction in the middle-aged eye is quite robust despite a dwindling amplitud e of accommodation. These results provide evidence of accommodative rigor i n youth and a slowing of accommodation with ageing.