The effect of attention and 
perceptual organization on the 
Müller-Lyer illusion: if one 
attends to the red lines, the 
space on the left appears to be 
larger than the one on the 
right; if one attends to the blue 
lines, the opposite is the case.

    The figure to the left 
illustrates the assimilation 
process in another version of 
the Müller-Lyer figure. If one 
concentrates on the red lines, 
the space on the left looks 
longer than the one on the 
right. If one directs attention 
to the blue lines, the space on 
the left appears to be shorter 
than the one on the right. 
Attention here affects 
perceptual organization, and 
this in turn creates the crucial 
impression of connection 
between the spaces and the end 
lines. This figure is also 
evidence against other theories 
of the Müller-Lyer illusion, 
especially those that explain it 
exclusively in terms of such 
factors as eye movements, 
blurred images, or lateral 
inhibition. The fact is that the 
stimulus input from each half 
of this figure is identical.