Symmetric visual response to positive and negative induced spherical defocus under monochromatic light conditions

The purpose of the study was to investigate the sign-dependent response to real and simulated spherical defocus on the visual acuity under monochromatic light conditions. The investigation included 15 myopic participants with a mean spherical equivalent error of -2.98 ± 2.17 D. Visual acuity (VA) was tested with and without spherical defocus using the source method (simulated defocus) and the observer method (lens-induced defocus) in a range of ±3.0 D in 1.0 D steps. VA was assessed using Landolt C's, while the threshold was determined with an adaptive staircase procedure. Monochromatic light conditions were achieved using band pass filters with a wavelength of 450 ± 2 nm, 530 ± 2 nm and 630 ± 2 nm. Results showed that the reduction of VA was significantly different under blue lighting conditions, when compared to the green and red light conditions. No significant difference in the reduction of the VA was found between the positive and the negative sign of defocus for all lighting conditions. The agreement for the VA between the source and observer method was significantly dependent on the wavelength as well as on the level of defocus. To conclude, under monochromatic light conditions, myopes show a symmetric sign-dependency regarding the influence of spherical defocus on visual acuity. The observed results indicate that the human visual system is capable of integrating the chromatic differences in refraction to distinguish between the signs of defocus.