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Vivek Labhishetty, William R Bobier, Vasudevan Lakshminarayanan; Defining the Nyquist Sampling limit for Dynamic Accommodation. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3955. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Main sequence characteristics of velocity and acceleration (as a function of response amplitude) often serve to define the dynamic aspects of oculomotor systems like accommodation. Accommodation is often measured using autorefractors designed to measure static refractive error. As a result it is important to determine if the resolution of an autorefractor is sufficient enough to meet the Nyquist sampling frequency required to measure accommodation accurately. The present study provides both theoretical and empirical evidence for the ideal sampling frequency required to measure the first and second order dynamics of accommodation and disaccommodation.
Varying accommodative (ACC) and disaccommodative (DA) step stimuli ranging from 1D to 3D (1D step) were presented using a Badal optical system on 12 children (8-13 years) and 6 adults (20-35 years). Accommodation was recorded using a custom built photorefractor (70Hz). Velocity and acceleration traces were obtained by differentiating the position traces. Fast Fourier transformation (FFT) was used to identify the Nyquist frequency exhibited by the position, velocity and acceleration traces. Velocity and acceleration main sequence characteristics were compared between the 70Hz photorefractor and the commercially available Power refractor (Multichannel systems, Germany, 25Hz) on 2 adult subjects.
FFT analysis showed that the ideal frequency required for measuring the accommodative (disaccommodative) position, velocity and acceleration were 0.1 ± 0.26 Hz (0.16 ± 0.28), 25.8 ± 26.6 Hz (27.54 ± 27.80) and 52.84 ± 14.12 Hz (53.24 ± 13.62) respectively. Empirically, slopes of accommodative and disaccommodative main sequence were similar between the two photorefractors for both velocity (ACC p=0.97; DA p=0.66) and acceleration traces (ACC p=0.66; DA p=0.12). However, the intercepts of velocity (ACC p=0.01; DA p<0.0001) and acceleration (ACC p=0.04; DA p<0.0001) were significantly lower with a 25Hz Power refractor.
In agreement with the previous studies, our data shows that 25Hz is adequate for measuring accommodative and disaccommodative position. However, peak velocity and acceleration were significantly under-estimated for a specific stimulus amplitude in both accommodation and disaccommodation with lower sampling rate. Interestingly, the main sequence slopes of velocity and acceleration were invariant with sampling frequency.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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