Psychophysical and Image-Based Characterization of Macular Pigment Using Structured Light

Macular pigment is thought to underlie entoptic percepts that enable human discrimination of polarized light. Therefore, polarization sensitivity may be a useful probe of macular pigment density and the risk of future macular disease. Structured light beams formed through the spin coupled orbital angular momentum states exhibit spatially dependent polarization and are therefore ideally suited to quantify human polarization sensitivity. When fixating at the center of these beams, a polarization-defined entoptic percept resembling radial spokes is observed. Using such structured light, we investigated whether we could characterize the portion of retina sensitive to polarization in healthy observers and observers with subclinical macular degeneration. In Experiment 1, the beam was presented to 23 healthy participants for 500ms per trial, rotating clockwise or counterclockwise. Participants indicated the direction of rotation. A circular mask with a varying radius was placed at fixation, thus the task was performed at varying eccentricities. The radius of the mask was controlled by a 2-up, 1-down thresholding staircase to estimate the size of the mask eliciting 71% accuracy. In addition, a fundus image using structured light was taken alongside a standard fundus image to register the psychophysical threshold to retinal landmarks, allowing the threshold to be expressed in visual angle units. In Experiment 2, normal participants and participants with subclinical macular degeneration performed a similar task using multiple mask shapes to characterize polarization sensitivity more fully. In healthy eyes, the mask size threshold ranged between 1° and 9° (mean = 4.6° ± 0.6°). In eyes exhibiting subclinical macular degeneration, a full-field mask with a ring of visible structured light elicited the best and most reliable performance, consistent with a selective pigment deficit in the central macula. Overall, our results indicate that structured light may be a useful tool for probing human macular pigment via polarization sensitivity.