For more than a century, entoptic phenomena have offered intriguing glimpses into the eye's internal optics. Haidinger's brushes and Boehm's brushes are among the few ways humans can directly perceive polarized light, revealing subtle interactions between light and retinal tissue. Now, researchers have shown that structured light can dramatically reshape one of these classic percepts, creating complex, multilobed patterns that could eventually provide a novel, non-invasive probe of retinal health.
In a study published in PNAS, investigators from the University of Waterloo and collaborators report that the familiar two-lobed appearance of Boehm's brushes can be transformed into intricate multi-lobed structures by illuminating the retina with spin-orbit-coupled light. Unlike conventional polarized light, these beams combine circular polarization with orbital angular momentum, producing spatially varying polarization patterns whose topology determines the perceived retinal image.
The work builds on the distinction between two well-known polarization phenomena. Haidinger's brushes arise in the macula through dichroic absorption by macular pigment, whereas Boehm's brushes are thought to originate from polarization-sensitive scattering in the inner retina, particularly within the ganglion cell and inner plexiform layers. By exploiting this scattering mechanism, the researchers demonstrate that structured light can generate entirely new entoptic percepts beyond the classical bowtie pattern.
Using an optical system incorporating a q-plate, annular apertures and narrowband 530 nm illumination, the team projected structured polarization fields directly onto the retina. Remarkably, the number of perceived lobes depended predictably on the topological properties of the incident light, with the lobe count following a simple relationship based on the orbital angular momentum difference between polarization components. Depending on the beam's topology, the bright lobes could even appear either inside or outside the illuminated annulus – a previously unreported behaviour.
To investigate retinal sensitivity, the researchers tested 11 healthy participants using a psychophysical contrast-threshold paradigm across retinal eccentricities ranging from 0.5° to 4°. Detection thresholds fell exponentially with increasing eccentricity, indicating that the structured-light percept becomes progressively easier to detect in the peripheral retina. On average, the multilobed pattern became robustly visible at approximately 1° retinal eccentricity, consistent with the known distribution of polarization-sensitive retinal scatterers outside the fovea.
While the study is fundamentally an exploration of human polarization vision, its translational potential may be of particular interest to ophthalmologists. Because the phenomenon depends on scattering rather than macular pigment absorption, it could provide complementary functional information about retinal microstructure. The study authors suggest that future studies could correlate these psychophysical measurements with OCT-derived retinal thickness and investigate whether diseases such as glaucoma, age-related macular degeneration or diabetic retinopathy alter these structured-light percepts in clinically meaningful ways.
Although considerable validation remains before any clinical application, the study demonstrates how advances in optical physics continue to uncover new ways of interrogating retinal function. By linking the topology of light with human visual perception, structured-light entoptic testing may ultimately provide a new functional biomarker for assessing retinal integrity – using nothing more invasive than what patients themselves can see.
