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Influence of Amplitude and Starting Point on Accommodative Dynamics in Humans

From the College of Optometry, University of Houston, Houston, Texas.

PURPOSE. The effects of amplitude and the starting point of an accommodative response on the dynamics of far-to-near (accommodation) and near-to-far (disaccommodation) focus were studied.

METHODS. Step responses were recorded with a dynamic optometer in nine 22- to 30-year-old subjects, under three conditions: (1) Fixed far: accommodative demands from 1 to 6 D were created by placing the far target at 6 m and the near target at various proximal distances. (2) Fixed near: accommodative demands from 1 to 5 D were created by placing the near target at 16.7 cm and the far target at various distal positions. (3) Fixed amplitude: far and near target positions were changed to create an accommodative demand of 1.5 D from starting positions of 1 to 4.5 D in 0.5-D steps. Each recorded response was fitted with an exponential function to calculate response amplitude, peak velocity, time constant, and starting point.

RESULTS. The relationship between starting point and amplitude of accommodation and disaccommodation was effectively manipulated in the three conditions. For accommodation and disaccommodation, peak velocity increased linearly with response starting point, whereas the peak velocity versus response amplitude relationship differed according to the condition. Similar amplitude responses were associated with a higher peak velocity and a smaller time constant at proximal starting points than at distal starting points. Low-amplitude responses were influenced by both starting point and amplitude.

CONCLUSIONS. The dynamics of accommodation and disaccommodation are strongly influenced by the starting point, but less so by amplitude.

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