Abstract
Purpose::
Prior studies of voluntary accommodation in human subjects have shown that accommodative and disaccommodative dynamics depend on response amplitude and starting point. The aim of this study was to determine how amplitude and starting point influence open-loop EW stimulated accommodative dynamics in anesthetized rhesus monkeys.
Methods::
One eye each of four iridectomized rhesus monkeys, (aged 6.8, 8.9, 15.0 and 16.3 years) were studied. The experiment was repeated in one eye. The monkeys had previously had stimulating electrodes implanted into the EW nucleus. Stimulus amplitudes were chosen to produce desired accommodative response amplitudes. Changes in lens thickness were recorded by dynamic ultrasound biometry at 100 Hz under three conditions: 1. Fixed Far: Accommodation starting from baseline with amplitudes increasing in 1 D steps up to maximum. 2. Fixed Near: Accommodation to maximum with starting points increasing in 1 D steps from baseline. 3. Fixed Amplitude: Accommodation with an amplitude of 1 D and starting points increasing in 1 D steps from baseline. Dynamic A-scan lens thickness measurements were converted into accommodation and the velocities of the responses were determined using a two-point difference algorithm.
Results::
Maximum accommodative amplitudes in the five experiments ranged from 4.68 D to 11.59 D. For all three conditions, peak velocity of accommodation and disaccommodation increased linearly with response amplitude. Cumulative regression slopes for accommodation were 2.36s-1 for condition 1, 2.70s-1 for condition 2 and 2.22s-1 for condition 3. For disaccommodation, the slopes were 6.58s-1 for condition 1, 6.05s-1 for condition 2 and 5.22s-1 for condition 3 (p < 0.0001 for all linear regressions). For condition 2, peak velocity of accommodation decreased linearly with response starting point in each experiment. The regression slopes varied from -2.93s-1 to -1.30s-1 dependent on the maximum accommodative amplitude. Peak velocity of disaccommodation showed no dependence on disaccommodative response starting point for any of the three conditions.
Conclusions::
Peak velocity of EW stimulated accommodation and disaccommodation in anesthetized rhesus monkeys is mainly dependent on response amplitude. Unlike shown previously in conscious human subjects, starting point appears to have little influence on EW stimulated accommodative dynamics in anesthetized monkeys. This suggests that in anesthetized monkeys, biomechanics related to starting configuration of the accommodative plant may have little influence on accommodative dynamics.
Keywords: presbyopia • refraction