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Y.S. Kim, S.R. Bharadwaj, C.M. Schor; Effect of Altering Dark Focus of Accommodation on the Dynamics of Disaccommodation . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5841.
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Peak velocity & peak acceleration of disaccommodation step responses are determined by the dioptric separation between response starting position & initial destination of disaccommodation. If the dark focus of accommodation corresponds to the initial destination, then a hyperopic shift in dark focus should increase the dioptric separation between the response starting position & initial destination. This should systematically increase the peak velocity of disaccommodation. Here, we tested this prediction by hyperopically shifting the dark focus using ‘accommodative fatigue’ & concurrently measuring its influence on the dynamics of disaccommodation.
Fatigue was induced in six young adults by repeated optical stimulation of accommodation & disaccommodation presented in two different pulse configurations (configuration I: pulse magnitude: 4 D, pulse duration: 6 sec, duty cycle: 0.8; configuration II: pulse magnitude: 2 D, pulse duration: 4 sec; duty cycle: 0.8). The two configurations were tested on separate days. The resultant step responses were differentiated to compute velocity & acceleration profiles. The peak velocity & peak acceleration from each set of 20 pulse stimuli were grouped & compared for statistical significance using a single–factor ANOVA test. Dark focus was measured for a period of 1 min, once before the start of fatigue session & once after every set of 20 pulse stimuli. Dark focus was defined by the steady–state dioptric state of the eye following the initial decay phase. All response characteristics were measured using the SRI dynamic optometer.
Reliable dark focus measures were obtained from five of six subjects. For both configurations, significant hyperopic shift in the dark focus occurred in four of five subjects as the experiment progressed. The hyperopic shift was larger & more robust in configuration I. For either configurations, none of the subjects showed a significant change in peak velocity or peak acceleration of disaccommodation as the experiment progressed (ANOVA: P–value: >0.5).
These results contradict the aforesaid predictions & suggest that the initial destination of disaccommodation does not correspond to the dark focus of accommodation.
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