[net.general] Are you seeing pink walls?

wise@cal-unix.UUCP (12/09/83)

An interesting item appeared in the January, 1984 issue of the magazine
"Family Circle" on page 110.  The item was attributed to "The New England
Journal of Medicine", Vol 309, page 315.  The title in "Family Circle" was
"Computer Aftereffect - Not to Worry".  The text of the item follows.
While it specifically mentions personal computers, it seems clear that
it applies to any green on white VDU.

    A vivid visual illusion produced by working with a personal computer
    could be worrisome, but need not be.  Northwestern University
    researchers, who experienced the illusion, relate what happened to
    them:  After a session at the machine, which has a video display
    presenting luminous green characters on a dark backround, ordinary
    white letters and lines on contrasting backround appeared pink, and a
    pink fringe could be seen on sheets of white paper or white walls.
    Report Dr.  Mark J.  Greenwald and colleagues: "The pink aftereffect,
    which may persist for a day or longer, is a normal nervous system
    adaptation response.  While the response is harmless and never
    permanent, it could alarm anyone who hasn't heard of it, and
    physicians who are unaware of it could mistake it for a hysterical
    symptom or manifestation of neurological disease."

So now you know why the clothes you put on right after a session at the
terminal don't match.

I wonder if the reason that the color is pink is because green is yellow
and blue combined, i.e. white minus red.  Anyone out there know?  I assume
that this sort of problem has been encountered previously in some other
situation.


Rick Wise
seismo!rlgvax!cal-unix!wise

akp@isrnix.UUCP (12/15/83)

#R:cal-unix:-15300:isrnix:6500006:000:1119
isrnix!akp    Dec 14 04:45:00 1983

/./
Okay... Psych 101 was not so long ago for me, so here goes:
Your eye works by inhibiting itself. If a cone is hit by light, it "fires"
more rapidly, sending a "light" message to the Optic Nerve (indirectly).
As a side-effect, the firing is building up some waste material, which
in turn inhibits the cone which is firing AND THOSE AROUND IT. If you look
at the sun (on the right kind of day), you'll start seeing a bluish disk
blocking it; that's the result of the yellow-sensing cones being inhibited;
you sense more blue.  Same thing goes for green screens: the green receptors
are busy inhibiting each other, and when you look away it takes a bit for
them to recover. During that time, the red sensors are still as active as
normal, and the green ones are inhibited, so you see "extra" red on everything
(relatively speaking), especially light-colored (white) things.

	The above is a simplification. The "yellow" sensors are actually
a combination of the blue and the green, and all that inhibiting is more
complex than that, really.  But you get the idea.		-- Allan Pratt
					...decvax!ihnp4!iuvax!isrnix!akp

rpw3@fortune.UUCP (12/16/83)

#R:cal-unix:-15300:fortune:14500012:000:375
fortune!rpw3    Dec 15 22:16:00 1983

The complementary colors stuff is all o.k., except...

If you look at the "Land effect" reports, it is not clear that there
are specifically red, green, blue, or any other color cones. SOMETHING
is going on, and the inhibitory neuro-physiology sounds right, but
just because we chose to build T.V.'s with triple dots, don't assume
that's the way the eye works.

-Rob Warnock

norskog@fortune.UUCP (12/16/83)

#R:cal-unix:-15300:fortune:14500013:000:289
fortune!norskog    Dec 16 11:47:00 1983

The inhibition mechanism serves as a "contrast gain".  Has this technique
ever been used in the low-level analog parts of a digital camera system?


				Lance Norskog
				Fortune Systems
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