THE ROUGH GUIDE TO PSYCHOLOGYexample of this is provided by the “hollow-mask” illusion, of which
there are many examples on the Internet (see tinyurl.com/laxzx). Our
expectation that a face will protrude outwards is so strong that we usually
perceive hollowed-out faces as if they are convex rather than concave.
The message from all this is that while you experience the world as
seamless, immediate and complete, the truth is that it’s delayed, selective,
filtered and constructed. That our mental movie of the world feels so
convincing, so smooth and fluid is a testament to the engineering of the
brain. Exactly how it all comes together so successfully is an enduring
mystery that psychologists are still busy attempting to solve.
Vision
For many people, sight feels like our dominant sense. This is reflected in
the fact that around fifty percent of the brain is involved to some degree
in vision. Sight is also the sense that has been studied most extensively
by psychologists.
Seeing begins as light lands on your retina, stimulating the rods and
cones – your photo-receptors – which are arranged in a cup shape across
the back of each eye. The image that arrives here has been focused
and reversed (left to right, and up and down) by the cornea, the trans-
parent film at the front of the eye, which acts like a lens. The rods and
cones translate the light signal into an electrical wave of activity that
travels down the optic nerve towards the brain’s main relay centre – the
thalamus – from where it is routed to the visual cortex at the back of