Rapid changes in the Stiles Crawford function in response to a decentred aperture

Purpose: Light guided photoreceptor realignment (phototropism) has been previously reported in the human eye over a period of days or weeks. In this experiment, we examined the capacity for rapid changes of directionality of photoreceptors, by locally measuring the Stiles Crawford function (SCF) during 10 minutes of viewing through a decentred aperture. Methods: Seven young adult subjects (27 ± 7 yrs) participated in the experiment. Testing was conducted monocularly, for two pupil conditions: (1) natural pupil (control condition), and (2) a 2 mm limiting aperture, offset by 2 mm in the temporal direction, and conjugate with the natural pupil (Figure 1). As a “washout”, the participants viewed a movie on a microdisplay for 10 mins through the natural pupil, with the refractive error optimally corrected. Then one of the two pupil conditions was introduced and the participant continued to watch the movie for a further 10 mins. SCF measurements of the eye were undertaken at two pupil locations (pupil center and 1.5 mm temporal offset), at 0, 5 and 10 mins with a 0.6 degree spot stimulus. A pupil viewing camera ensured steady fixation. Then the natural pupil was reintroduced for a further 10 mins (“recovery”). Results: No significant changes of relative brightness were found (1.5 mm periphery vs center) when the subject viewed through the natural pupil (all p>0.5). For the decentered aperture condition, there was an increased relative brightness of the test stimulus at the location near the induced aperture (by 9% and 10% at 5 mins and 10 mins, respectively) compared to the baseline. Repeated measures analysis of variance showed a significant interaction between time and the relative brightness of the test stimulus (periphery vs centre), after viewing through the decentred aperture for 5 and 10 mins (p=0.02 and p=0.04, respectively), which returned to near baseline levels after 10 mins “recovery” (p=1.00) (Figure 1). Conclusions: After viewing through a decentred aperture for only 5 mins, an increased relative brightness (SCF) was found for a test stimulus at a similar decentred location. This change could be generated from a rapid phototropic mechanism that alters the alignment of photoreceptors.