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

Topic E14: A Purple/Green Entopic Manifestation

A Purple/Green Entoptic Manifestation
(The subjective findings date from 1964; with insights by 2004)

This unsought purple  phenomenon was found maximally evident  at  an  imposed  40 Hz. This was then verified by some fifty participants, including college students, the residents of a retirement community, and children who  acclaimed "'the purple donut !.''     Replicated more recently (2001−2008), and using the identical xenon lamp (the surviving old Strobotac 1531), the earlier data have withstood the test of time, now over four decades. The visual onsets can vary from a nominal "zero" to almost one minute. The chromatic annulus appears most immediate and salient at an imposed 40 Hz.   It emerges to view between the 15-degree and 5-degree isopters, a region where the majority of retinal ganglionic -cells synapse with bipolar and horizontal cells.  

Fig i.  The foveal zone, an oval of 5-degree subtense, generally remains clear of the colors which flood the parafoveal field. The outer margins of the annulus seen at 40 Hz may fluctuate briskly as yellow/white/black globules or lobules.


Eccentric viewing of the light source converts the annular configuration to a crescent, its opening in the direction of the global vector, as diagrammed below in four oculomotor-versions.  (These figures are not to scale.)

The 4 or 5 bands of alternating 'pixels' appear to travel for about 7 degree, vectored peripherally from the open cusp, and being re-directed as foveal re-fixations of the light source, rotate the orientation of the crescent.
Illusory motility of these chromatic particles is attributed to firing of adjacent voltage sensitive receptors; these chains are sequentially activated by local-ionic perturbations of their fields.

The  Spectral and Frequency Studies.  The entoptic purple phenomenon was experienced by the 95 participants in Shurcliff’s 1959 spectral-studies, and by the 35 in Welpe’s 1978 frequency-modulated study.  Shurcliff had found no clear relation between the wave-length of the stimulus and the apparent hues of the phenomena; Welpe stated the color was most intense  "between  28- 43 Hz. and that the color ... the violet effect …was independent of the color of the stimulus."   
This present report examines two other parameters:  Adaptation to steady-light  prior to the testing, and the onset-delay during  the exposure to the on-line test-flicker.
Onsets delayed over 40 seconds were commonly recorded with dark-adapted eyes.  In any one individual, these varied from a nominal "zero" up to forty seconds of imposed  flicker,  predictably  dependent upon  current  physiological  status.  Onset is hastened  by prior  physical activity  and by  light adaptation,  and is  delayed  several fold or may be absent,  reflecting  passivity  or  somnolence, aging or pathology.   Limited testing in a clinical setting distinguished low-tension glaucoma from ocular- hypertension.        
Increased metabolic activity, in the pigment epithelium, the photoreceptors and neuro-retina, demands increased ocular blood flow, notably in the choroidal circulation.  The following protocols address physiological variables obtained with healthy eyes in 1964, and in 2000.   
*  Pupil-status is controlled or monitored during the adapting  and during the testing                                                                                                                                                                                                       
*  Levels of personal physical activity and vigilance recorded
*  Pre-adaptation noted to light or to dark, with natural pupils
*  Fast tracking, with  onsets  measured  only at  the standard 40Hz.
* Parameters of the angular dimensions of the central hypo-chromatic ellipse 

PROCESS:   A stroboscopic Xenon lamp is viewed at arm's length; a diffuser is placed on the instrument, or is worn as spectacles, or held as a shield.  When gaze is directed centrally, the ring is seen.  With off-centered visual alignment, a crescent appears, and when further offset, a disc of diminishing dimensions appears. (See figure 2 above.)                            

Intra-subject testing with flicker shows that for a given individual, the immediately prior photic-adaptation, plus the current ambient light and/or flicker, together determine a predictable phenomenal onset.

In a general population, the finding of prolonged delayed onset or a complete failure to manifest the flicker-effect may indicate ocular pathology, somnolence, poor health, vasculopathies or aging.  The source of this Purple-green Entoptic Phenomenon, its topography and the mechanisms which enable its subjective perception are of interest.  This phenomenon also offers possible clinical applications for an in-expensive, non-invasive measure of acute changes in ocular perfusion in response to imposed visual tasks, to luminance or following acute therapeutic interventions.  Limited testing in a clinical setting distinguished low-tension glaucoma from ocular-hypertension.

"Widespread Choroidal Insufficiency in Primary Open-angle Glaucoma," Yin ZQ, 1997.
"Relationship between Ocular Perfusion Pressure and Retrobulbar Blood Flow in Patients with Glaucoma with Progressive Damage," Flammer J., 2001.