Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!dciem!mmt 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.