Editing Haidinger's brush (section)

==Physiological causes==
Haidinger's brush is usually attributed to the [[dichroism]] of the [[xanthophyll]] [[pigment]] found in the [[macula lutea]]. As described by the [[Fresnel equations|Fresnel laws]], the behavior and distribution of oblique rays in the cylindrical geometry of the foveal blue cones produce an [[Intrinsic and extrinsic properties|extrinsic]] dichroism. The size of the brush is consistent with the size of the macula.

It is thought that the macula's dichroism  arises 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 centralis|fovea]] is not flat), are almost [[orthogonal]] to the fovea in its central part but nearly parallel in its outer region. As a result, two different areas of the fovea can be sensitive to two different degrees of polarization.<ref>{{cite journal|last=Le Floch|first=A|author2=Ropars G |author3=Enoch J |author4=Lakshminarayanan V |year=2010|title=The polarization sense in human vision.|journal=Vision Res.|volume=50|issue=20|pages=2048–2054|doi=10.1016/j.visres.2010.07.007|pmid=20638403|doi-access=free}}{{open access}}</ref>