The objective of this proposal is to investigate changes in the interaction of accommodation and vergence during the development of presbyopia. Previous studies have indicated that the accommodative amplitude reduces steadily after 15 years of age, with a minimum level being attained around 50 years of age. However, the vergence amplitude, as assessed by the near-point of convergence, remains relatively constant with age. These findings suggest that changes in the accommodation-vergence synkinesis take place during presbyopic development. Furthermore, the low frequency of asthenopic symptoms during the development of presbyopia would imply that adaptive processes exist which enable the oculomotor system to adjust to these ongoing, age-related changes in accommodation-vergence interaction. Six experiments have therefore been designed to examine these age- related changes in the near-vision system. In experiments 1-4, subjects will be divided into groups or "bins" on the basis of their age at the time of examination. Subjects between 20 and 70 years of age will be examined. (1) A series of open-loop parameters, namely response AC/A and CA/C ratios, as well as tonic accommodation and tonic vergence, will be measured to determine the effect of age on these components. (2) Accommodative and vergence adaptation will be examined in these subjects by determining the post-task shift in the respective tonic component immediately following a 5 min near-vision task. (3) The effect of cross- link activity on oculomotor adaptation will be measured by examining the effect of disparity-induced accommodation and blur-driven vergence, under closed loop conditions, on accommodative and vergence adaptation respectively. (4) Age-related changes in proximally-induced accommodation and vergence will be determined by assessing these parameters in the population previously described. (5) The initial effects of either the first presbyopic correction, or an increase in the refractive power of the presbyopic correction, on accommodative-vergence synkinesis will be assessed in 32 subjects. The oculomotor synkinesis will be investigated before and immediately after the dispensing of the new prescription. (6). It is anticipated that data collected from these experiments will allow the construction of a comprehensive, quantitative control model which will include these age-related changes in accommodation-vergence interaction. This would represent the first model to include optical and non-optical stimuli, adaptive components and the oculomotor changes associated with the development of presbyopia.
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