Peak Velocity of Accommodation Depends on the Duration of Acceleration

Abstract

Abstract: : Purpose: Response amplitude-Peak velocity relationship (main sequence) has traditionally been used to characterize the dynamics of accommodation. Peak velocity is known to increase with response amplitude. But the question of how the accommodative system attains its peak velocity for different response amplitudes has not been addressed. There are two possibilities: acceleration amplitude could increase in proportion to the peak velocity (acceleration amplitude criterion) or acceleration duration could be increased to attain the peak velocity (acceleration duration criterion). We investigated these two possibilities. Methods: Accommodative responses were measured in the Far-Near and Near-Far directions using a dynamic infrared optometer (SRI Dual Purkinje image Eye tracker and Dynamic Infrared Optometer). Three subjects (19-23yrs; mean age: 21yrs) took part in the experiment. Subjects accommodated to a black & white Maltese cross stepped from 2D for Far-Near accommodation and from 4D for Near-Far accommodation. Accommodative step stimuli ranging from 0.5D – 4D were presented monocularly (left eye) in 0.5D steps each for a period of 4secs. Post latency velocity profiles were plotted as a function of time to assess the duration of acceleration and as a function of response accommodation (Phase-Plane plots) to assess the response magnitude at the peak velocity. Response acceleration was derived from the slopes of the velocity profiles. Results: Amplitude of acceleration was independent of the step response magnitude while the duration of acceleration increased as a function of step response magnitude. Responses with higher peak velocities required longer acceleration duration (Figure 1) and larger response magnitudes (Figure 2) to reach the peak velocity. The acceleration amplitudes (slope of the velocity profiles) were similar irrespective of the peak velocity (Far-Near std dev: <16diopters/sec²; Near-Far std dev: <17diopters/sec²). Similar trends were seen for both Far-Near and Near-Far directions. Conclusions: These results suggest that the accommodation system uses an acceleration duration criterion to reach the peak velocity for different responses.  

Keywords: accommodation • ocular motor control • refraction