Direct discrimination of structured light by humans

We predict and experimentally verify an entoptic phenomenon through which humans are able to perceive and discriminate optical spin–orbit states. Direct perception and discrimination of these particular states of light with polarization-coupled spatial modes is possible through the observation of distinct profiles induced by the interaction between polarization topologies and the radially symmetric dichroic elements that are centered on the foveola in the macula of the human eye. A psychophysical study was conducted where optical states with a superposition of right and left circular polarization coupled to two different orbital angular momentum (OAM) values (`<sub>1</sub> and `₂) were directed onto the retina of participants. The number of azimuthal fringes that a human sees when viewing the spin–orbit states is shown to be equal to the number (N) of radial lines in the corresponding polarization profile of the beam, where N = |(`<sub>1</sub> − `₂) − 2|. The participants were able to correctly discriminate between two states carrying OAM =7 and differentiated by N = 5 and N = 9, with an average success probability of 77.6% (average sensitivity d⁰ = 1.7, t(9) = 5.9, p = 2 × 10−⁴). These results enable methods of robustly characterizing the structure of the macula, probing retina signaling pathways, and conducting experiments with human detectors and optical states with nonseparable modes.