Here is a most interesting development that fits in with the idea that we are still ignorant of many details of the nervous system. Humans have the ability to perceive polarized light. We are not aware of this sense and do not use it, but it is there. Actually it has been known for sometime but not generally.
The paper by S. Temple (citation below) describes research which started with polarized light sight in sea animals, then its mechanism in humans, and finally it appears to offer a method of diagnosing AMD before it affects vision.
In the paper there are directions for seeing the polarization. I have to say I was skeptical and quite surprised when the faint yellow bowtie appeared and rocked back and forth as I tilted my head. When I stopped moving my head the bowtie disappeared. “We detect the orientation of polarized light using ‘Haidinger’s brushes’, an entoptic visual phenomenon described by Wilhelm Karl von Haidinger in 1844. He reported that when viewing a polarized light field, with no spatial variation in intensity or colour, it was possible for someone with normal sight to perceive a faint pattern of yellow and blue bowtie-like shapes that intersect at the viewer’s point of fixation. Haidinger’s brushes can be observed by looking at a region of blue sky approximately 90° from the sun, particularly around sunset or sunrise, or by looking at a region of white on a liquid crystal display (LCD). The effect vanishes within about 5 s, but can be maintained and/or increased in salience by rotating the eye around the primary visual axis relative to the light field, e.g. tilting one’s head side to side.” Entoptic means that the phenomenon has an origin within the eye rather than the outside world.
The bowties are created by two structures in the eye. The cornea has layers of collagen molecules arranged to create birefringence. This can be thought of as slow and fast orientations depending on the polarization angle of light rays. This interacts with carotenoid pigments in the macula or fovea which are also arranged in a particular way to form an interference filter (dichroic filter). The center of the lens behind the cornea, the center of the macula and the object of visual attention are in a straight line. Therefore the orientation of the collagen, carotenoid pigments and the direction of the light are always the same.
By studying the Haidinger’s brushes in an individual it is possible to examine aspects of the structure of the macula and the cornea.
Here is the abstract:
“Like many animals, humans are sensitive to the polarization of light. We can detect the angle of polarization using an entoptic phenomenon called Haidinger’s brushes, which is mediated by dichroic carotenoids in the macula lutea. While previous studies have characterized the spectral sensitivity of Haidinger’s brushes, other aspects remain unexplored. We developed a novel methodology for presenting gratings in polarization-only contrast at varying degrees of polarization in order to measure the lower limits of human polarized light detection. Participants were, on average, able to perform the task down to a threshold of 56%, with some able to go as low as 23%. This makes humans the most sensitive vertebrate tested to date. Additionally, we quantified a nonlinear relationship between presented and perceived polarization angle when an observer is presented with a rotatable polarized light field. This result confirms a previous theoretical prediction of how uniaxial corneal birefringence impacts the perception of Haidinger’s brushes. The rotational dynamics of Haidinger’s brushes were then used to calculate corneal retardance. We suggest that psychophysical experiments, based upon the perception of polarized light, are amenable to the production of affordable technologies for self-assessment and longitudinal monitoring of visual dysfunctions such as age-related macular degeneration.”
Citation: Temple SE, McGregor JE, Miles C, Graham L, Miller J, Buck J, Scott-Samuel NE, Roberts NW. 2015 Perceiving polarization with the naked eye: characterization of human polarization sensitivity. Proc. R. Soc. B 282: 20150338. http://dx.doi.org/10.1098/rspb.2015.0338