Does such reversibility 
require special explanation or 
does it suffice to say (as I 
already have) that, because the 
stimulus can (more or less) 
equally represent these 
different objects, it is perfectly 
understandable that perception 
will shift from time to time? 
Most psychologists would say 
that reversal requires 
explanation because a 
"decision" has been made by 
the perceptual system as to 
what the stimulus represents; 
thus some event must have 
occurred to change the 
perception at a particular 
moment. The only explanation 
of reversal that has been given 
any currency in the 
psychological literature for the 
last half century is the 
satiation theory, or fatigue 
theory. According to this 
theory, advanced most 
forcefully and explicitly by the 
Gestalt psychologist Wolfgang 
Köhler, each perceptual 
organization is determined by a 
separate neural event in the 
brain. If one ongoing neural 
event becomes satiated or 
fatigued (much the way neural 
discharge is known to become 
fatigued and resistant to 
further discharge in other 
realms, such as in color 
vision), we can assume that the 
brain resists its further 
occurrence. When the 
resistance reaches the point of 
completely blocking that 
neural event, the stage is set 
for a switch to the other neural 
event, the one to which the 
stimulus can equally well lead. 
When that switch occurs, it is 
experienced consciously as a 
change in the percept.