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From: mmt@dciem.UUCP (Martin Taylor)
Newsgroups: net.ai
Subject: Re: Visual After-effects
Message-ID: <240@dciem.UUCP>
Date: Thu, 9-Jun-83 16:42:42 EDT
Article-I.D.: dciem.240
Posted: Thu Jun  9 16:42:42 1983
Date-Received: Thu, 9-Jun-83 17:33:12 EDT
References: ncsu.2199
Lines: 28

Actually, the blind-spot game of removing people's heads has a
long history. King Charles II of England used to amuse himself by
seeing how his courtiers would look without their heads. And it is
true that any regular pattern behind will be filled in across either
the normal blind spot or blind spots due to retinal problems.

As for the effect in which objects tend to disappear if stared at,
this is normally studied with special devices attached to the eyeball
(on a contact lens) to ensure that the visual world remains stationary
on the eye. Objects rapidly vanish under these conditions, but reappear
in fragmentary form from time to time. Very slight shifts of viewpoint
tend to make the objects come back, which is probably the reason attending
to a detail "behind" the object makes it return. It is easier to
make things with blurred or diffuse edges go away than things with
sharp edges (so I imagine people with poor eyesight can do it easier
than people with good vision).

The effect of changing letter size after watching for game objects
that change size is another example of the same kind of thing as
the railroad track after-movement effect. It's probably a different
visual channel (we have separate channels for size changes and for
movement) but the principle is the same. Some people claim that the
effect is due to fatigue of the system sensitive to movement in
one direction, leaving the balancing components sensitive to movement
in the other direction to control what is seen when the stimulation
is neutral. (i.e. the other direction is more sensitive after one
is fatigued). I'm not convinced by this explanation. Things are
probably more complicated than that.