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Integration of a sensory component into the accommodation model reveals differences between emmetropia and late-onset
1College of Optomtry, University of Houston, Texas 77204-6052, USA.
Purpose:
To evaluate the differences in accommodative function between subjects with emmetropia and those with late-onset myopia (LOM).
Methods:
This study suggests a modified model of static accommodation, in which an accommodative sensory gain as a linear operator is added to simulate the sensory part of the system. Results derived from the model show that the sensory part not only affects the slope of the accommodative response function but also increases the system's effective threshold (ET) to the blur signal. This method expands the utility of using the control model to evaluate accommodation behavior. Thirteen emmetropic and 10 LOM subjects participated in this study. The subject's accommodative responses to one-, two-, three-, and four-diopter stimuli were measured by the Canon R-1 optometer, and the differences in dark focus, the slope of the accommodative response function, and the ET were compared between the emmetropic and the LOM subjects.
Results:
The results show that although the dark-focus values and the slopes of the accommodative response function are not significantly different in emmetropia and LOM, the ETs are significantly different.
Conclusions:
The higher ET found among subjects with LOM suggests that either the blur (or the error) signal is degraded significantly in the sensory part of the system, the dead space as an internal threshold of the system is high, or both factors are important. On the basis of further analysis of the data, we speculate that the sensory system in LOM subjects was less sensitive to blur than that of the emmetropic subjects.