Colin Blakemore and his 
associates at Cambridge 
University have attempted to 
test the angular-displacement 
theory by requiring observers to 
set a comparison line parallel 
to one side of an acute-angle 
pattern. They assumed that an 
overestimation of the angle can 
be demonstrated by measuring 
the perceived orientation of its 
sides. When the observers were 
asked to set the comparison 
line parallel to one side of the 
angle, they in fact placed it in 
a different orientation. The 
effect was appreciably greater 
for small angles (those of 
around 10 degrees) than it was 
for large angles.

    What is the mechanism that 
explains the angular-
displacement effect? The 
currently favored theory is 
based on Hubel and Wiesel’s 
discovery of cells in the visual 
cortex that "detect" contour 
orientation on the retina. A 
given contour orientation on 
the retina—–say, a vertical line
—–triggers activity in a whole 
population of cortical 
orientation detectors in the 
brain, but more so in those 
most attuned to that retinal 
orientation (e.g., vertical) and 
less so in those tuned to 
different retinal orientations 
(e.g., plus or minus a few 
degrees from the vertical). The 
presence of an adjacent 
contour of different 
orientation (e.g., 5 degrees 
clockwise) would have the 
effect of inhibiting some of 
these units from firing. The 
peak of activity shifts away 
from the one that would occur 
without the presence of the 
second contour (e.g., the peak 
would now be at 3 degrees 
counterclockwise). This is an 
example of the broader category 
of neural functioning known as 
lateral inhibition, which was 
discussed in Chapter 2. In that 
earlier example, the inhibition 
concerned intensity (or 
luminance) based on the rate of 
firing of neurons, where 
activity in one fiber had the 
effect of reducing the rate in an 
adjacent fiber. Here, it is the 
contour orientation rather 
than intensity that is relevant. 
This physiological theory, 
then, attributes illusions to the 
"hardware" of the visual 
nervous system, a very 
different explanation from 
others we have been 
considering.