Control of accommodation by longitudinal chromatic aberration and blue cones

To better understand the striking overaccommodation that is triggered at wavelengths below 430 nm (below referred to as OAB).

Fourteen students served as subjects, 6 emmetropic and 8 mildly myopic. They fixated a reading target or a Landolt C at 33-cm distance while the wavelength of light illuminating the target was varied. Their accommodation was continuously monitored with the PowerRefractor (Multichannel Systems, Reutlingen, Germany, 1995). Luminances were matched using a candela meter (Minolta LS100) and neutral density filters. The following experiments were done: (1) confirmation of the effect at 10 cd/m(2), (2) comparing 10 and 1 cd/m(2), (3) foveal stimulation, (4) parafoveal stimulation, (5) testing independent combinations of the wavelength in the center and periphery, (6) testing accommodation tonus without fixation target while the wavelength is varied.

(1) OAB was nicely confirmed as initially described by F. J. Rucker and P. B. Kruger (2004a, 2004b) and A. Seidemann and F. Schaeffel (2002). (2) OAB remained stable at target luminances between 10 and 1 cd/m(2). (3, 4) OAB was found to be more pronounced when the parafoveal region was stimulated than when mainly the fovea was stimulated. (5) When fovea and parafovea were illuminated by light of different wavelengths, the wavelength in the foveal region had greater impact on OAB. (6) OAB was not elicited in the absence of an accommodation target.

OAB appears to be controlled more by the parafovea than by the fovea. The findings are in line with the assumption that OAB is mediated by the short-wavelength-sensitive cones that are absent from the central fovea in most subjects.