So you think you can
see?

The notice on the door invited
volunteers to take part in a short, simple, psychological experiment.  Just go  inside, it said, and sign up at
the reception desk.

Encouraged by the reward of a book
token, a steady stream of students entered the campus building and told the
person at the desk they wanted to volunteer. Each one was handed a  consent form
to fill in. When they had completed it, the receptionist asked them to wait a
moment while they filed the form in an area behind a screen. Then the
receptionist returned and directed them  to another room where they were
greeted by a psychologist.

At this point the volunteers
discovered an unsettling fact. The experiment was already over.  It had taken place in the minute or so
during which they signed up. And the results of it gave three out of four of
them reason – literally - to doubt their own eyes.

What had happened was this: when
the student approached the reception desk they were greeted by a man  with blonde
hair , wearing a yellow shirt.  This
person  handed
out the consent form, waited while the student filled it in, then took it back
and disappeared around the back of the screen, ostensibly to file it. In fact,
while hidden from view, the blonde man swapped places with a colleague –
a man with dark hair and wearing a blue shirt.  This other person then returned to the
student and   directed them to
the other room. Astonishingly, most of the would-be volunteers failed to notice
the switch. Only when the psychologist showed them a video of the entire event
did they believe what had happened.

That experiment, carried out at
Harvard University in the late 90s, was one of the first to demonstrate a
phenomenon known as “change blindness” – our ability to miss
massive changes that occur in front of our eyes,  provided the “before” and
“after” scenes are separated by a short
gap or visual interruption.  As well
as failing to notice
when one person takes over from another, experiments have shown
that  vast chunks of a visual landscape  can be removed between glimpses without
most of us noticing. In one series of experiments none of the subjects noticed
when a large building, smack in the middle of the picture, shrunk by a quarter
between glimpses. None saw  that a mountain range  disappeared. And more than 90% failed to
spot the sudden extinction of 30 puffins on an otherwise uninhabited ice floe.

“Inattention
blindness” is a similar glitch in the brain’s perception system.
But here the thing that stops us  seeing the obvious is distraction
rather than interruption. One famous demonstration involves a short clip of
people playing basketball..  Before  it starts viewers are asked to
watch the film very carefully and to count the number of passes made by the
players. At the end of the clip most people are able to say exactly how many
times the ball moved from one player to another . Most
of them, however, entirely fail to see that a person in a gorilla suit walks
slowly  into view from one side,
weaves  through the players, stops
right in front of the camera, beats its chest,  then walks slowly off.

Inattention
and change blindness demonstrate that what we see is not at all what we think
we see. We seem to observe the world in all its richness, but in fact we only
register consciously a tiny handful of elements – those that catch our
attention. We are good at spotting changes if they happen while we are looking  because
change involves movement and movement grabs  attention. But we cannot spot changes if
there is a gap between the before and after because  to do so  we would have to hold a complete memory
of , ,the “before”
scene to compare with the
“after”one. And  our memories are only as complete
as our perceptions.

Magicians have always exploited  change
and inattention blindness. When a sleight-of-hand artist makes a big deal of
directing your attention to what is happening in his left hand you can be sure
that the real business is happening in his right. But inattention blindness is
not just an amusing
curiosity.  Our
tendency to neglect things that we are not deliberately attending to  has real life
implications. Drivers, and pilots, for example, frequently make dangerous
errors, not because they are not attending but because their attention has got
locked on to  the
wrong thing.  In one case, for
example, a plane crashed because the pilot and co-pilot were focussing so hard
on a dodgy instrument that they failed to notice the ground rushing up to meet
them. And the attention-grabbing effect of talking on a mobile phone  while driving
is reckoned to increase a driver’s risk of an accident fourfold.

Attention is largely an
unconscious faculty – most of the time  it is “grabbed” by
events rather than deliberately directed, so it is very difficult to avoid
zooming in on some things to the detriment of others. Our best defence may be
simply to remember that at any time we are only seeing a tiny bit of the
picture. The whole thing may look very different indeed.)