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Carlos Dorronsoro, Aiswaryah Radhakrishnan, Lucie Sawides, Susana Marcos; Optical quality and subjective judgments of blur under pure simultaneous vision. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4068.
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© ARVO (1962-2015); The Authors (2016-present)
To compare retinal image quality and subjective image sharpness under pure bifocal simultaneous vision (PSV) across a wide range of addition values.
Four subjects performed a weighted rank psychophysical experiment to grade (from totally blurred to totally sharp, 0 to 5) the perceptual quality of PSV images. As a control condition, the perceptual quality of purely defocused (PD) images was also assessed. Computer generated images of a face were observed, for 1.5s in random order, through adaptive optics to correct the aberrations of the eye. The PSV images (50/50 energy content) were obtained from the combination of a focused image superimposed to a defocused version of the same image. At least 18 defocus conditions (0-3D) were subjectively ranked (10 repetitions). In addition, optical quality was evaluated as a function of defocus, from the PSV and PD Point Spread Functions (PSF), using the Modulation Transfer Function (MTF) at different frequencies as a metric.
As expected, images without defocus were perceived as sharp (score s=5) in all subjects and conditions. The perceptual weighted rank systematically dropped for PD at a rate of 9.0±4.0 s/D, with the images perceived as completely blurred (s=0) from 0.6 D onwards. PSV images provided a similar decrease as their defocus component increased, in the 0-0.3D range. However, in 3 subjects the score reached a minimum at 0.5D (s=2.1±0.4) and then it was partially recovered back towards sharpness (s=3.5 at 1.2D), remaining stable afterwards. The 4th subject showed a similar trend judging PSV images but without a minimum. The objective simulations predicted the perceptual judgment trends, as well as the differences between judgments of PD and PSV images. MTF@20 c/deg decreased sharply with defocus in PD, but reached a minimum (0.42 at 0.5 D) and recovered a stable level (0.50 from 1.5D) in PSV. Averaged MTFs (15-25 c/deg) provided the most accurate predictions.
As opposed to the common idea that simultaneous vision retinal images (as those found in multifocal contact or intraocular lenses) are severely optically degraded, but later restored in neural processes, we have found a good correspondence between pure subjective perception experiments and pure optical simulations. Although our findings do not preclude for possible effects of neural adaptation, neural effects seem to be secondary in the perceptual judgment of sharpness in PSV.
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