The interaction between accommodation and monochromatic aberrations of the human eye

The quality of the retinal image is influenced by the monochromatic aberrations of the eye and these aberrations are known to change with accommodation level. Since the\ud monochromatic aberrations of the eye alter the focus of light at the retina, they might also be expected to influence the characteristics of the steady-state accommodation response. The work reported in this thesis has demonstrated a variety of complex interactions between steady-state accommodation and monochromatic aberrations.\ud The effect of accommodation on the nature and magnitude of monochromatic aberrations was studied using the aberroscope technique. The aberrations were measured in single dioptre steps of accommodation up to the subject's maximum\ud amplitude of accommodation and were recorded in both eyes of eleven young subjects. The longitudinal spherical aberration of the subjects became progressively\ud more negative in power with increasing accommodation (p=0.0001). The aberration data showed a trend towards a higher mean-square-error of the wavefront (p=0.06)\ud with increasing accommodation levels. The third-order components of the aberrations were found to be of greater magnitude than fourth-order components (about 4: 1 for a\ud 5 mm pupil). There was substantial correlation in the magnitude and sign of the spherical aberration present in the fellow eyes of individuals, however other\ud components of the wavefront showed little correlation between fellow eyes. To eliminate the possibility that changes in monochromatic aberrations associated with\ud accommodation could be at least partly caused by changes in anterior corneal topography, a study was undertaken with ten subjects using a videokeratoscope\ud capable of measuring corneal topography over a wide corneal region (the PAR Corneal Topography System). An accommodation stimulus (0, 2, 4 and 6 D) was presented to the left eye while corneal topography measurements were taken from the right eye. The results suggested that no major changes occurred in the topography of\ud the anterior cornea during accommodation up to 6 D.\ud The Canon Autoref R-1 was used to investigate various aspects of the interactions between monochromatic aberrations and the accommodation response. It was\ud therefore important to establish the region of the pupil used by the Autoref R-1 in deriving a reading of the refractive power of the eye. This region was estimated to be\ud centred around a 3.5 mm diameter, which meant that the Autoref R-1 readings at different accommodation levels included the effect of the monochromatic aberrations\ud of the eye (from this region). Stimulus-response relationships were measured for five subjects and then analysed to investigate if the subjects' existing aberrations may have influenced the magnitude of\ud the accommodation response. There was no evidence of an association between accommodation error and the root-mean-square deviation of the wavefront for any of\ud the subjects tested. The results for spherical aberration and accommodation error were also unconvincing, with possible associations for some subjects.\ud Known levels of spherical aberration were introduced to the tested eyes of three young subjects through the use of aspheric rigid contact lenses (in air). There was a clear\ud effect of spherical aberration on accommodation response, in the ratio of 1 D accommodation response to 4 - 5 D of spherical aberration (edge of a 5 mm pupil). Spherical aberration was systematically varied (induced spherical aberration of+ 1.1 D and +2.2 D at the edge of a 5 mm pupil) to investigate its influence on the accommodation microfluctuations of two young subjects. At the same time the subjects' pulse rates were measured and the subjects breathed at a fixed respiration rate to accurately identify their contributions to the accommodation rnicrofluctuations. The results showed the expected correlation between the high and low frequency components of the microfluctuations and the subjects' pulse and respiration rates. However there was no significant change in the microfluctuation power spectra associated with changes in spherical aberration