arXiv2025
We report a novel entoptic phenomenon in which the classical two-lobed Boehm's brushes are transformed into a multi-lobed structure by projecting spin-orbit coupled light onto the human retina. These structured beams, composed of non-separable superpositions of circular polarization and orbital angular momentum (OAM), produce azimuthally modulated entoptic patterns through polarization-dependent scattering in the retina. Unlike Haidinger's brushes, which arise from dichroic absorption in the macula, the observed effect is driven by angular variations in scattering strength relative to the local polarization direction. In regions where scattering centers exhibit polarization orientations that converge toward a common point, their combined contributions reinforce one another, producing brighter and more sharply defined entoptic lobes whose number and orientation vary systematically with the topology of the spin-orbit stimulus. Psychophysical measurements across retinal eccentricities from 0.5$^\circ$ to 4$^\circ$ in eleven participants revealed that contrast detection thresholds decreased exponentially with eccentricity, consistent with polarization-sensitive scattering by isotropic structures in the non-foveal retinal regions. From the psychophysical fits, the mean eccentricity at which the entoptic pattern reached a 50 % threshold was $r_{50} = 1.03^\circ$ with a 95 % confidence interval of [0.72, 1.34]$^\circ$, indicating that the spin-orbit-induced entoptic structure becomes perceptually robust at approximately 1$^\circ$ retinal eccentricity. Together, these findings demonstrate that spin-orbit light modulates scattering-based visual phenomena in previously unrecognized ways, enabling new approaches for probing retinal structure and visual processing using topological features of light.