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L. Diaz-Santana, V. Guériaux, G. Arden, S. Gruppetta; Methodology to Study Dynamics of Eye Wavefront Aberrations During Small Amplitude Changes of Accommodation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2772.
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© ARVO (1962-2015); The Authors (2016-present)
The dynamics of eye aberrations during accommodation has been the subject of a few studies. A number of authors have proposed that one of the cues for accommodation may be based on changes of higher order aberrations (HOA) when the focus changes from in front of, to on, or behind, the retina. However, the HOA are small signals and the signal to noise ratio decreases with increasing (Zernike) order, limiting the information obtainable from wavefront data. We propose a methodology based on the idea of "running averages" routinely used in electrophysiology, to study accommodation and the HOA dynamics.
A Shack-Hartmann wavefront sensor was built which permits binocular viewing of a stimulus through a window, while measuring aberrations at 24Hz in one eye. The stimulus was an alternating chromatic chequerboard (red/blue), flickering at 0.25 Hz so accommodation stabilized between reversals. Chromatic aberration of the eye for these two colours is ~1D, producing an expected response <1D and even smaller HOA changes. 4 normal adults (27-34) participated in the study. Data was collected for 5 minutes. Wavefronts were reconstructed using a Zernike polynomial expansion.
Amplitudes of accommodation of 0.26D+/- 0.1D were recorded. The standard deviation of the wavefront RMS over time for HOA was typically 0.04um. Running averages allowed detecting changes in HOA of 0.006um to 0.02um correlated with accommodation. The Figure shows the 4s time evolution of Zernike coefficient c5-1. By averaging 10 epochs a signal starts appearing (a). Averaging 30 epochs or more shows cleaner signals (b). This technique permits to better quantify correlations between the Zernike terms and the magnitude and timing of accommodation. For instance for c5,-1, the correlation with accommodation increased from 0.25 for one epoch to 0.70 for 30 epochs.
Signal improvement was found in all Zernike terms. We were able to extract valid signals below the noise level for most HOA terms. This was of particular importance for terms with low and moderate correlation values, as these appear as negligible if no averaging is done.
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