Densitometry of Cone Photoreceptors in Human Eyes by Adaptive Optics Scanning Laser Ophthalmoscopy

Abstract

Purpose: To measure the changes in the reflectance of human cone photoreceptors in situ with an adaptive optics scanning laser ophthalmoscope (AO-SLO) before and after bleaching.

Methods: Eight eyes of 8 healthy young adult subjects (age: 21-33 years) were studied. The changes in the cone reflectance were measured with a custom-built AO-SLO (developed by Osaka University and Topcon) with a sampling rate of 5-Hz. The AO-SLO had a resolution of 512 × 512 pixels and had a 12-bit gray scale resolution. The field of view was set at 7×7 degrees with the center at 1 or 3 degrees temporal to the fovea. We calculated the cone density in this area. For bleaching, we used 630-nm red light emitting diodes (LEDs, brightness of 3600 cd/m²). Twenty-one red LEDs were set in a circular pattern and placed 7 cm in front of the cornea of the right eye. After mydriasis and 10 min of dark-adaptation, images of the cone mosaics were recorded continuously for 300 seconds without light stimulation, then after bleaching by the LEDs for 300 seconds, and then for 300 seconds after the cessation of light stimulation. The measurements were made in a dark room of 0.01 lx. The average cone reflectance was computed by the ImageJ software (Rasband W, NIH, USA), and the average cone reflectance before light stimulation was set as the baseline. The reflectance at the baseline was compared to that during and after light stimulation at 1 and 3 degrees.

Results: The average cone density was significantly higher at 51283.87±2736.97 cells/mm² at 1 degree than the 30388.38±2176.05 cells/mm² at 3 degrees (P<0.001 Wilcoxon). After light-adaptation, the reflectance increased relative to that at the baseline by 134.3±13.8% at 1 degree and by 126.0±10.2% at 3 degrees (P<0.001, Wilcoxon). The increase in the cone reflectance after bleaching at 1 degree was significantly greater than that at 3 degrees (P<0.05, Wilcoxon).

Conclusions: The significantly greater reflectance change where the cone density was higher (1 degree) indicates that the amount of reflectance change is related to the cone density.