Blue Arc Phenomenon
The Blue Arc Phenomenon is an entoptic retinal effect that occurs when you look at a single light stimulus in the dark - for example a red light-emitting diode in an otherwise dark environment. If you move your eyes slightly next to the point of light (or fix a point if the light stimulus only occurs briefly), you can see blue arcs to the left and right of this stimulus. These arches, which differ in height and width depending on the fixation point, occur monocularly, that is, when viewed with only the right eye, only the arches to the right of the point can be seen and vice versa. Regardless of the color of the light stimulus, the arches are always described as bluish. Light stimuli with longer wavelengths are better suited to evoke the arcs than those with shorter wavelengths, so mainly red light is used in experiments on this phenomenon. The exact origin of this effect is still unclear.
The Blue Arc Phenomenon was first described by JE Purkinje in 1825 and referred to as the elliptical light streaks. Purkinje discovered the phenomenon when he wanted to start a fire with the help of a sponge and looked at the embers in the dark. After a few attempts, it was found that the "dull, light-bluish" arcs are visible if you do not look directly at the embers, but a little to the side (on the right with the right eye and vice versa). He also observed that the shape and size of the ellipses change when you change the point of view relative to the point of light, so that you can see the arcs both very flat elliptical and almost circular.
Other important observations that Purkinje made are the fact that the arches go from the "axis point of the retina to the point of entry of the optic nerve", that it becomes more difficult to see the arches if one remains in the dark for a long time and that too strong or too weak light make it impossible to see the streaks of light. As can be seen in Purkinje's drawings, he also perceived a circular halo around the stimulus, which no longer appears in the literature of later authors.
80 years later, Hans Gertz (1905) wrote an article in which he described the Blue Arc Phenomenon as a novel visual phenomenon that can be seen when looking at a vertical light bar (ideally in red) with a fixation point of 0.33 ° to 6 ° temporally forward of the light stimulus. Gertz wrote that the arches clearly correspond to the optic nerve tracts that precisely describe such elliptical paths to the blind spot in the retina. According to his hypothesis, the arcs are created because the light stimulus excites photoreceptors on the retina, this excitation is conducted on the elliptical optic nerve tracts in the direction of the blind spot and this leads to a kind of electrical crosstalk on the receptors located under the nerve tracts. Thus the arches that we see are our perception of the stream of action of the optic nerve fibers. This theory could not be refuted until today and is considered the most plausible explanation of the phenomenon, although it is not yet clear which cells of the retina are responsible for the development.
According to William Amberson (1924), the mistake that the Blue Arc Phenomenon is still unknown was made not only by Hans Gertz, but also by a number of other authors - even today it is still difficult to get good literature on this effect, as most of the articles about it are very old and the phenomenon is anything but well known. According to Amberson, the name "Blue Arcs", which is still used today, comes from Christine Ladd-Franklin .
In the article by Amberson (1924) it was also shown that the papillo-foveal line forms the symmetry axis of the blue arcs, which also corresponds to the pathways of the nerve fibers in the eye. One of his test subjects, who had a scotoma between the fovea and the blind spot, could also see the arches, but these were interrupted by the scotoma. According to Pasquale (2002), the Blue Arc Phenomenon is a suitable test for detecting visual field defects in glaucoma patients - it may also be useful for other purposes that have to do with the nerve conduction in the eye. Amberson also postulated that people in whom parts of the nerve fibers between the fovea and the blind spot are myelinated cannot perceive the arches in these places either - he was even able to show this, but only on the basis of a test person. Presumably, according to Amberson, the photoreceptors themselves are not excited by the action current of the nerve fibers, since the receptors are relatively far away from the fibers, but rather cells that are closer, such as the ganglion cells.
- WR Amberson: Secondary Excitation in the Retina . In: American Journal of Physiology , 69, 1924, pp. 354-370.
- H. Gertz: About entoptic perception of the action current of the retinal fibers . In: Zentralblatt für Physiologie , 19, 1905, pp. 229–232.
- LR Pasquale: Blue Arc Entoptic Phenomenon for Detecting Glaucomatous Visual Field Loss . In: Asian Journal of Ophthalmology , 4, 2, 2002, pp. 11-12.
- JE Purkinje: Observations and experiments on the physiology of the senses , 2. Reimer, Berlin 1825, pp. 74-78.
- G. Scheibelhofer: The Blue Arc Phenomenon: Investigation of the incidence in color-savvy and color-blind people . Diploma thesis: Karl-Franzens-University Graz., 2007.