Abstract
We investigated the influence of the Stiles-Crawford (SC) peak location on visual acuity and contrast sensitivity with 6mm diameter pupils in two subjects (SM and DAA). We also measured phase transfer in one subject. Peaks were moved with apodizing filters: from (0, −0.9) to (0, +0.9) mm for SM, and from (+0.3, −0.6) to (−2.0, −0.5) and (+2.9, −0.6) mm for DAA. Visual performance was measured with the apodizing filters and normal SC (and matching ND filters). The peak position had definite influence on performance, especially prominent when subjects were defocused. For example SM's visual acuity was reduced by 0.13 log units under her peak-shifted condition at −2D (hypermetropic) defocus and DAA's visual acuity was reduced by 0.14 log units when his peak was shifted (+)2.6mm nasally at +2D (myopic) defocus. Agreement between changes in contrast sensitivity and visual acuity with respect to SC peak location was poor in a number of situations, but here were no situations where decentering the peak from its normal position resulted in worsening of one visual performance measure and an improvement in the other. Notches in the defocused contrast sensitivity function corresponded well to rapid changes in the phase transfer function.
While this study does not address mechanisms for cone orientation, it explores how visual function may change if cones are oriented toward a different pupil location than the natural location. Visual acuity, contrast sensitivity and phase transfer change when the Stiles-Crawford peak location is altered. This indicates that not only the presence of the Stiles-Crawford effect, but also the actual location of the peak influences visual performance. Although with some variations with defocus state, our subjects' performances tended to be better with the natural Stiles-Crawford effect than when its peak was shifted to a considerably different location.