YOUR SENSE OF THE WORLD AND MOVEMENT WITHIN ITtriangular chunks taken out of them. This assumption also leads to an
illusory perception that the occluding triangle is brighter than the white
background.
Another well-known visual trick
is the Ebbinghaus illusion. This
shows how context affects our
perception. In this case, a central
circle surrounded by larger circles
appears smaller than a central
circle surrounded by tiny circles,
even though both the central circles
are really the same size. There is
an added twist to this illusion.
In an influential study, involving
cut-out discs, Salvatore Aglioti and
colleagues showed that, although
the two inner circles appear to be of different size, when we reach for
them with forefinger and thumb, we actually form the same size grip
for both. Aglioti’s team interpreted this as further evidence for the dual
pathway account of visual processing. In this case, it seems that the more
conscious “what” pathway is tricked by the illusion, whereas the more
automatic “where” pathway, used for reaching, is somewhat immune.
As well as being useful research tools for studying perception, visual
illusions have started to be exploited for practical benefit. A great real-life
example is found on a dangerous stretch of Lake Shore Drive in Chicago.
The city authorities have altered the spacing of white lines on the road
so that they become progressively closer together on the approach to a
risky bend at Oak Street. The lines create the sensation of speeding up,
thus prompting drivers to slow down before the bend.
Other applications are at a more experimental stage. David Elliott and
colleagues published a study in 2009 in which they used an illusion to
make a step look higher than it really was. Recordings from an eight-
camera motion-capture system showed that students gave a step with
vertical lines on its forward face approximately 5mm greater clearance
than a step decorated with horizontal lines. The students also estimated
that the step with vertical lines was taller than the step with horizontal
lines. The increased clearance doesn’t sound like much, but it could
be enough to prevent people tripping – an accident that can be fatal to
elderly people. Two thousand elderly people die in the UK every year
following a fall, with the majority of these falls happening on stairs.