Abstract
Purpose: :
Edinger–Westphal (EW) stimulated accommodation in anesthetized rhesus monkeys produces peak velocities of accommodation (far–to–near) and disaccommodation (near–to–far) that are linearly related to response amplitude. These response dynamics may be dictated by the neural signal and/or by the biomechanics of the accommodative plant. A constant stimulus pulse train frequency of 72 Hz has been used in prior studies. This study was undertaken to determine the effects of stimulus pulse train frequency on the dynamics of accommodation and disaccommodation.
Methods: :
The eyes of 3 adolescent iridectomized rhesus monkeys (5 years, 8 years and 8 years old) were studied with experiments repeated on one eye. Accommodation was stimulated with 4 second long stimulus trains of increasing amplitude with a pulse train frequency of 72 Hz. Five half–max. and full–max. amplitude stimuli were then delivered with pulse train frequencies ranging from 60 to 100 Hz in 5Hz increments. Stimulus pulse width remained constant at 600µs. Accommodative responses were recorded dynamically at 30 Hz with infrared photorefraction. Exponential functions were fit to the dynamic accommodative and disaccommodative responses and peak velocities were calculated from the derivatives of the fitted functions.
Results: :
72 Hz stimuli of increasing amplitude produced linear increases in peak velocity as a function of response amplitude for accommodation (slope: 1.359, intercept: 0.906, r2: 0.901) and disaccommodation (slope: 7.780, intercept: 1.508, r2=0.895). For the half max. stimulus amplitudes, increasing stimulus pulse train frequency caused a small, systematic, linear increase in accommodative response amplitude of 1.05%/Hz. For the full–max. stimulus amplitudes, no systematic change in response amplitude was seen. Peak velocity of accommodation increased with increasing stimulus frequency for both full–max, and half–max. stimulus amplitudes. Amplitude of disaccommodation increased with increasing stimulus frequency at the half–max. stimulus amplitudes, but with no corresponding change in peak velocity.
Conclusions: :
While increasing the stimulus frequency caused slight increases in accommodative response amplitudes and slight increases in peak velocities, the dynamic characteristics such as peak velocity as a function of response amplitude and peak velocity as a function of stimulus frequency were on the whole similar to the response dynamics recorded with a 72 Hz stimulus frequency. Accommodative dynamics in anesthetized rhesus monkeys are not appreciably or systematically affected by stimulus pulse frequencies between 60 and 100 Hz.
Keywords: refraction • ocular motor control • aging: visual performance