When linear perspective is 
isolated, the equal horizontal 
bars look different in length 
but the vertical ones do not.

    Conversely, if one isolates 
only linear perspective, as in 
the illustration to the left and 
that shown previously or in the 
original version of the Ponzo 
illusion, an effect opposite to 
that in the foreshortening 
display occurs, as we have 
seen: Equal horizontal test 
lines generate an illusion 
(depth and contrast-
assimilation), whereas equal 
vertical lines yield only a 
miniscule effect (depth but no 
contrast or assimilation). 
Therefore, the evidence seems 
to support the hypothesis that 
the joint action--or 
interaction--of perspective and 
contrast yields a strong illusion 
of extent, whereas either of 
these factors alone yields only 
a slight illusory effect. 
Precisely why joint action of 
these two factors is so effective 
is not clear. Gillam’s 
explanation of the effects just 
considered is that the inducing 
perspective patterns create a 
scale of size, but that the scale 
only runs in one direction in 
these different figures, 
vertically in the 
foreshortening display and 
horizontally in the linear-
perspective display. Therefore, 
size illusions only occur for 
vertical lines in the first case 
and for horizontal lines in the 
second case.