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Abstract
Previous studies have used Badal laser optometry to demonstrate significant variance in the distribution of tonic (or "dark-focus") resting positions of accommodation (TA) for groups of observers. This study investigates whether individual differences in TA are due to individual differences in autonomic tone of the ciliary muscle by comparing separately the effects of a muscarinic receptor antagonist (Tropicamide 0.5%) and a beta receptor agonist (Isoprenaline 3%) on the distribution of TA. Ten subjects were used for each study with mean ages of 23.2 and 23.0 yr, respectively. The distribution of TA within each subject group was equivalent to the large sample distributions published in previous literature. Darkroom measurements of TA and pupil diameter were determined with a Badal laser optometer and infrared photography. Distance correction and amplitude of accommodation were determined by standard optometric techniques. Isoprenaline produced a significant hyperopic shift in TA of 0.47 D over 22 min but this was not accompanied by a significant change in standard deviation of the TA distribution. This finding was in accord with the authors' previously reported investigation using the beta receptor antagonist Timolol Maleate. Tropicamide also produced a significant hyperopic shift in TA of 1.24 D over 24 min, and this was accompanied by a significant change in standard deviation of the distribution from 0.87 to 0.17. The results indicate that the parasympathetic system plays a significant role in determining the TA position and that the variations in TA between individuals is a consequence of parasympathetic rather than sympathetic ciliary muscle tone.