Abstract
Purpose: :
To quantify the size change threshold required for proximally induced accommodation in the absence of retinal image blur.
Methods: :
Four healthy emmetropic subjects (mean age: 24.7±4.7) with normal vision and accommodation viewed a range of targets, monocularly, through a 0.5 mm pinhole while their accommodation response was recorded continuously with a modified open field optometer (Shin Nippon SRW 5000). The stimulus was one of three targets: Maltese cross (T1), playing card of known size (T2), cartoon figure of unknown size (T3). These were presented in random order and viewed in free space, against a plain background. The targets oscillated sinusoidally at a frequency of 0.6 Hz. The amplitude of the axial oscillations was varied randomly so that changes in apparent target size ranging from -125% to +125% were induced in steps of 25%. All sinusoidal stimuli were presented randomly around two mean target vergence levels: 2D and 3D. For each trial, 10 continuous measures of accommodation of 16s duration were obtained and a mean power spectrum calculated. A response to the stimulus was defined as a significant increase in power at the driving frequency compared to the power obtained from the stationary target.
Results: :
The mean size change threshold for all conditions was found to be 68.6±26.8%. The size change threshold was found to be highly subject dependent. For 3D mean target vergence level, the mean (±SEM) size change threshold for targets T1, T2 and T3 was 66.0±36.5%, 93.3±30.1%, and 26.0±3.4% respectively. At the 2D mean target vergence level, the size change threshold was: T1=93.3±30.1%, T2=83.3±28.8% and T3= 50.0±0%. The size change threshold was significantly larger for the known target (T2), than for the unknown size target (T3, p<0.05). The size change threshold was greater at the mean target vergence level of 2D for both the unknown target (T3) and the Maltese cross target.
Conclusions: :
The size change threshold for proximally induced accommodation depends upon prior knowledge of the target dimensions.In conditions where a recognisable target is presented the size change threshold varies little with initial target position presumably due to some internal calibration of object proximity.In contrast where no prior knowledge of the target exists the size change threshold is found to be dependent upon the initial target position, presumably because any internal proximal calibration cannot be made for such targets.
Keywords: accommodation • ocular motor control • perception