Often, peripheral 
explanations are the first to be 
proposed. In my opinion, at 
least, they are usually wrong 
and they die hard. Because 
such an explanation will 
generally be simpler than a 
central one, it is good science 
to begin an investigation by 
testing it. But it is bad science 
to cling to that explanation in 
the face of strong evidence to 
the contrary. Explanations in 
terms of eye movements have 
always been popular. Thus, in 
the domain of motion 
perception, such phenomena as 
apparent motion, induced 
movement, and the autokinetic 
effect (see Chapter 7) have all 
been held to be caused by eye 
movements at one time or 
another. Max Wertheimer, in 
his famous research on 
apparent movement that 
launched the Gestalt approach, 
found it necessary to perform a 
special experiment to 
demonstrate that such motion 
could not be explained by eye 
movements. He arranged for 
one object to appear to move 
leftward simultaneously with 
another object appearing to 
move rightward. The eyes 
cannot move in opposite 
directions at the same time. Eye 
movements have been invoked 
as the explanation of the origin 
of form perception, of depth 
perception based on retinal 
disparity, and of the perceived 
direction of all regions in the 
field, an example of which is 
the uprightness of vision 
despite the inverted 
orientation of the retinal 
image.

    As a contemporary example of 
this controversy, consider a 
phenomenon not yet 
mentioned. If a figure is moved 
behind a narrow, stationary 
slit in an opaque surface so that 
all but the segment behind the 
slit is occluded at any given 
moment, one still tends to 
perceive the entire figure. 
Helmholtz and others studied 
this effect in the last century, 
and Theodore Parks of the 
University of California at 
Davis rediscovered it in this 
century. It is now referred to as 
anorthoscopic perception—–an 
abnormal way of presenting 
something. How is such 
perception possible? Isn’t an 
extended, simultaneous retinal 
image of a figure necessary for 
the perception of its shape?