Cone directionality from laser ray tracing in normal and LASIK patients

Laser ray tracing, a technique originally developed to measure ocular aberrations from the deviations of the local ray aberrations as a function of entry pupil, was used to assess cone directionality in 29 normal eyes (seven of which underwent LASIK surgery) and seven eyes after LASIK corneal refractive surgery for myopia. The total intensity of the retinal aerial images was computed as a function of the entry location of the illuminated beam. The measured intensity distribution was fit to a two-dimensional Gaussian function plus a constant background. The maximum of the distribution represents the pupillary location toward which the cone photoreceptors are oriented (peak of the optical Stiles-Crawford, SCE, function). We found the average SCE peak location was located 0.43 ± 0.96mm nasally and 0.60 ± 0.87mm inferiorly to the center of the pupil. In general, there was not a relation between the pupillary area of best quality and SCE peak location, either pre-operatively or post-operatively. The cone directionality shape factor was also unchanged by surgery. However, in two eyes, pre- and post-operative SCE peak location changed significantly. LASIK refractive surgery decreased the MTF in all eyes, even when the actual SCE directionality of the subject is considered. In the two eyes that showed significantly different SCE peak location, the apodized post-operative MTF with the post-operative SCE peak exceeded the simulated post-operative MTF assuming no shift in the SCE peak. However, the statistical power of these two cases is low, and the general findings are consistent with the hypothesis that differences in optical quality are not a major driving mechanism for cone orientation.