Visual Memory Research
The Genesis of Mnemonic Visual Systems 
Recurrent Dynamics in Excitable Media

Topic E13: Purple: A Non-Spectral Color

The literature from 1823 refers to "entoptic violet and yellow-green flecks" seen with intermittent light.  Gebhard in 1943 mentions a "ubiquitous violet cloud floating about" during intermittent illumination, this hue was also reported by Brown and Gebhard in 1947, by Smythies in 1950, and by Shurcliff in 1959.  In addition to a "reddish-blue" entity, Shurcliff noted a greenish-yellow "antipathic" phenomena, whose kinetics differed from those of the purple mode Welpe, in 1978 studied a "violet effect … originating in the retina and visible from 28-43 Hz peaking at 40 Hz +/- 2 Hz."  This is also the maximal oscillatory activity of second- order retinal neurons identified by Smith in 2001.  The phenomenology of this entoptic purple ring is detailed in the following Topic E 14 which addresses the relationship with photic stimuli, its peak with neuro-vascular coupling at 40 Hz, and with enhanced ocular perfusion. 

"The linkage at ~40 Hz between maximal neuro-retinal activity and ocular perfusion levels now appears incontrovertible." Riva C., 2005

This entoptic purple phenomenon evidently is universally experienced, as indicated by the 95 participants in Shurcliff’s 1959 spectral-studies, and by the 35 in Welpe’s 1978 frequency- modulated study.  Shurcliff found no clear relation between the wave-length of the stimulus and the apparent hues of his induced phenomena; Welpe stated the color was most intense "between  2 8- 43 Hz. and that the color... the violet effect …was independent of the color of the stimulus." Shurcliff suggested that the mechanisms related to "anomalous ratios of radiation- activated switches in parallel … representing cone-functions."    

This  opponent  phenomenon  is  identified  also  in the  global  Helmholtz  Travelling Wave, HTW.  where  centrifugal  waves , the  purple hue  ( R+B)  bands  alternate with the yellow/green .Y/G  bands   (Yellow  hue  =  Red +Green ) 

Purple is non-spectral; its opponent is yellow-green.                                                                                                                                                                                                      

These  sections  E 13-16  review  the  triggers , domains, boundaries and local textures (granularity) of the phenomenal colors,  the mechanics of bi-stable hues,  and  the highly significant role of a  pre-adaptation by the ocular blood flow in response  to light " Daytime light exposure dynamically enhances brain responses."  Maquet P., 2006