Haidinger's Brush - Physiological Causes of Haidinger's Brush

Physiological Causes of Haidinger's Brush

Haidinger's brush is caused by the blue cones, which contain one of the four visual pigments used by human vision, xanthophylls (also known as lutein). Since xanthophyll is anisotropic (rod shaped), radiation stimulates both electronic and molecular vibrational states, mainly along the molecule direction. That means that xanthophyll is potentially sensitive to the light's polarization. As xanthophyll is responsible for the absorption in the blue range of the light spectrum, the human eye is sensitive to polarization of light in this wavelength.

Haidinger's brush is usually attributed to the dichroism of the pigment of the macula. In this Fresnel–Arago laws effect, the unguided oblique rays in the cylindrical geometry of the foveal blue cones, along with their distribution, produce an extrinsic dichroism. The brush's size is consistent with the size of the macula. The macula's dichroism is thought to arise from some of its pigment molecules being arranged circularly. The small proportion of circularly arranged molecules accounts for the faintness of the phenomenon. Xanthophyll pigments tend to be parallel to visive nerves that, because the fovea is not flat, are almost orthogonal to the fovea in its central part while being nearly parallel in its outer region. As a result, two different areas of the fovea can be sensitive to two different degrees of polarization.