  We predict and experimentally verify an entoptic phenomenon through which
humans are able to perceive and discriminate structured light with
space-varying polarization. Direct perception and discrimination is possible
through the observation of distinct profiles induced by the interaction between
the polarization gradients in a uniform-intensity beam 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
coupled polarization and orbital angular momentum (OAM) were directed onto the
retina of participants. The participants were able to correctly discriminate
between two states, differentiated by OAM =\pm7, with an average success
probability of 77.6 % (average sensitivity d^\prime=1.7, t(9) = 5.9, p =
2\times 10^{-4}). These results enable new methods of robustly characterizing
the structure of the macula, probing retina signalling pathways, and conducting
experiments with non-separable optical states and human detectors.
