Human Polarization Sensitivity: An Update

The human eye can detect the polarization of light by at least two separate and distinct mechanisms: differential absorption by a radial diattenuator and polarization by scatter. Both mechanisms transform incident polarized light into luminance signals that can be detected by polarization-insensitive photoreceptors. These pre-receptoral mechanisms are fundamentally different from those of polarization sensitivity in organisms with intrinsically polarization-sensitive rhabdomeric photoreceptors: the ciliary photoreceptors of human eyes are intrinsically insensitive to polarization, at least under normal viewing conditions. Radial diattenuation is thought to result from the ordered arrangement of macular pigment molecules in fibre-like neural components radially configured around the centre of the fovea. This is confined to a specialized part of the retina known as Henle’s layer. Polarization by scatter results from light interacting with membranes and particles within the optic media and neural retina. Each mechanism generates its own entoptic visual phenomenon. Under conditions of uniformly illuminated linearly polarized light radial diattenuation results in a faint yellow hourglass-like pattern in the centre of the visual field, known as Haidinger’s brushes. When dark adapted, exposure to a bright point source of rotating linearly polarized light causes intraocular scatter that can be perceived as two flares of light of the same colour as the source, known as Boehm’s brushes. Entoptic phenomena resulting from spatially modulated polarization fields have recently been investigated in detail and, like Haidinger’s brushes, are thought to result from intraretinal radial diattenuation. Other less well-defined polarization entopic phenomena, caused by structured light stimuli modulated by photon orbital angular momentum, have yet to be characterized and differ in behaviour from both Haidinger’s and Boehm’s phenomena. The adaptive advantage of human polarization sensitivity is unknown, though it can be used to assist in skylight navigation. Clinical applications of polarization sensitivity have included detecting early signs of retinal disease, evaluation and correction of amblyopia (lazy eye) and assessment of macular pigment density. The earlier literature about human polarization sensitivity has been reviewed by Horváth and Varjú (Polarized light in animal vision—polarization patterns in nature. Springer, Heidelberg, Berlin, New York, pp 355–361, 2004) and McGregor et al. (Polarized light and polarization vision in animal sciences. Springer, Heidelberg, Berlin, New York, pp 303–315, 2014).