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S.R. Bharadwaj, Y. Su, C.M. Schor; Short–Term Adaptive Modification of Dynamic Ocular Accommodation . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5842.
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© ARVO (1962-2015); The Authors (2016-present)
Accommodation step responses are controlled by an initial open–loop phase that determines the response dynamics followed by a feedback–modulated closed–loop phase that determines the response magnitude. Adaptive modification of the open–loop phase could compensate for age–related increase in visco–elastic properties of the lens to maintain youthful speed of accommodation & also restore ‘normal’ response dynamics following implantation of a prosthetic accommodating intraocular lens. Here we investigated the modification of dynamic accommodation following short–term adaptation to conditions that mimic increased or decreased lens elasticity.
The experiment, conducted on 5 young–adult subjects (ages 16–28 yrs), was divided into four sequential stages: pre–adapt, adapt–sustain & post–adapt. Pre– & post–adapt stages each contained 50 single–step accommodation stimuli with magnitude ranging from 1.5 – 2.0 D across subjects. Adaptation was induced with repeated double–step dioptric stimulation of accommodation which produced either hypermetric or hypometric response aftereffects. In the gain increasing paradigm, the first–step was incremented by 100% of its magnitude & in the gain–decreasing paradigm, the first–step was decremented by 50% of its magnitude. In both paradigms, first–step magnitude equaled that of single–step stimuli & inter–step–interval equaled the average latency of each subject’s single–step responses. The adapt–sustain stage contained 300 trials, composed of double–step stimuli interleaved with single step stimuli in a ratio of 5:1 respectively. The magnitude of adaptation was defined as the largest change in peak velocity (PV) & peak acceleration (PA) of single–step responses with respect to the averaged pre–adapt responses. Accommodation responses were stimulated & recorded monocularly using an SRI Optometer.
For the same response magnitude, 4 subjects showed concurrent changes in PV & PA following adaptation. Two subjects showed both increments (PV: 47 & 42%; PA: 38 & 34%) & decrements (PV: 51 & 57%; PA: 49 & 52%) in dynamics for gain increasing & gain decreasing paradigms respectively. The other 2 subjects showed only increment (PV: 81 & 25%; PA: 55 & 40%) but no decrement in dynamics for gain increasing & gain decreasing paradigms respectively. Post–adapted single–step responses with elevated PV & PA showed overshoots before attaining a steady–state.
These results reflect the capacity for accommodation to adapt the open–loop phase of the step responses to compensate for biomechanical changes in the lens in order to maintain optimal dynamic response characteristics.
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