Abstract
Purpose: :
The main sequence (MS) of accommodation (ACC) and disaccommodation (DAC) when relating peak velocity and amplitude have been shown to be dependent on starting position1,2. If multiple amplitudes of ACC and DAC are generated by changing focus from a fixed far target to differing near targets then their respective MS will be asymmetric. Specifically, the MS of ACC has been found to be linear for small amplitudes2 and exponential for larger amplitudes up to 6D1. However, the MS of DAC remained linear across all amplitudes. It is not clear for a given individual whether these asymmetric functions represent completely separate velocity–amplitude relationships or share similar velocities at small amplitudes which in the case of ACC asymptote at larger amplitudes. We investigated this question using high speed photorefraction.
Methods: :
Three subjects were studied (Mean age = 26.6±2.3). Monocular accommodative responses were recorded by a digital high speed photorefractor at 75Hz while subjects changed focus from a high contrast letter (far) target fixed at 4m to various near positions (1 to 5D amplitude). The accommodative responses were analyzed offline using image analysis software (Image Pro Plus, Media Cybernetics). The functions best describing the MS for ACC and DAC were defined by curve fitting (Prism, GraphPad Inc.). The response amplitudes were matched in 1D bins and the differences in the peak velocity of ACC and DAC for a given amplitude were compared using a two–way ANOVA and Bonferroni post–tests.
Results: :
Main sequence of ACC was best described by an exponential (y = 9.96 * (1–e (0.546x)), R2=0.41, p<0.001). Peak velocity of DAC changed linearly with response amplitude (y = 3.398x–0.046, R2=0.68, p<0.001). When response amplitudes were matched, the differences in the peak velocity of the ACC and DAC were significant beyond 4D (ANOVA, p<0.001).
Conclusions: :
When amplitudes of accommodation are varied by holding the far target and varying the near, the MS of ACC asymptotes and is better described by an exponential that shows saturation effects at intermediate stimulus amplitudes (2.5–3D). The peak velocity at lower amplitudes was similar between ACC and DAC until this roll off such that, DAC was significantly faster than ACC at higher amplitudes. Since natural viewing mostly comprises variable endpoints, the MS of ACC would show an asymptotic form more commonly with a typical saturation velocity of 10D/s. (1) Kasthurirangan & Glasser. IOVS 2005.46(9):3463–72.
(2) Bharadwaj & Schor. VisRes. 2005 In Press
Keywords: refraction • ocular motor control • presbyopia