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Siegfried Wahl, David Kern, Alexander Leube, Arne Ohlendorf; Neuronal influences on the sensitivity to defocus for contrast vision. Invest. Ophthalmol. Vis. Sci. 2018;59(9):4754. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The level of defocus on the retinal plane continuously changes due to different viewing distances and the resulting level of accommodation. It is known that myopes have a reduced accommodative response, possibly caused by their reduced sensitivity to induced negative spherical defocus in terms of visual acuity. Since it was additionally observed that the sensitivity to contrast is depending on the sign of defocus, the study investigated if this behavior is caused by optical and/or neural influences.
Contrast sensitivity function (CSF) was tested at spatial frequencies (SF) of 0.5 cpd and 1 – 12 cpd in 1cpd step size, using the Tuebingen Contrast sensitivity test (TUECST) that incorporates Gabor Patches as stimuli and an adaptive staircase (PSI) to assess contrast thresholds. 10 subjects participated in the course of the study (mean spherical equivalent refractive error: -2.85 D ±2.19D) and CSF was measured, while spherical defocus of ± 3D in 1D step size was induced. Individual neuronal contrast sensitivity (NCSF) was calculated as a function of defocus, using the ratio of the CSF and the modulation transfer function (MTF) and is defined as a black box model of the non-optical influenced contrast sensitivity. Accommodation was paralyzed using three drops of 1% cyclopentolat and a 4mm artificial pupil was used.
The reduction of sensitivity to contrast was significantly higher with positive defocus when compared to the negative defocus conditions (p < 0.001, ANOVA) for SF from 1 to 12 cpd, whereas the MTF was not (p > 0.05, ANOVA). Regarding the NCSF, a sign dependency for the same magnitude of defocus was found (p < 0.05, ANOVA). Additionally, a significant increase of the NCSF was found for increasing power of negative induced defocus (p < 0.05, ANOVA).
The neuronal sensitivity to contrast show a significant sign-dependency in myopes and the observed sign dependency can’t be explained by optical factors. This finding suggest that the visual system is able to detect the sign of defocus without any optical cue and adjust neural tuning dependent on the sign of spherical defocus.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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