THE ROUGH GUIDE TO PSYCHOLOGYNot paying attention
This idea of attention as a limited resource is demonstrated by a well-
known phenomenon in psychology: the attentional blink. If you ask a
person to look out for a letter, say “X”, embedded in a stream of succes-
sively presented numbers, you’ll find that their ability to detect any
other stimuli immediately after that “X” will be severely compromised.
Related to this is a phenomenon known as change blindness or “inat-
tentional blindness”. The fact that we can only fully attend to so much
at once means that a surprising amount in a scene can change without
us realizing it. There are several videos on the Internet that dramati-
cally demonstrate just how much we can miss of what’s going on. Two
of the best are the “colour changing card trick” (tinyurl.com/39qlbl)
by psychologist Richard Wiseman, and the “gorilla in our midst”
test (tinyurl.com/2d29jw3), based on one of the most famous experi-
ments in modern psychology, conducted by Christopher Chabris and
Daniel Simons. Watch the gorilla test before reading on, if you want to
Weber-Fechner Law
The founding psychologists of the nineteenth century showed how
it is possible to apply the objectivity of science to the subjectivity
of sensory perception. The Weber-Fechner Law, named after the
nineteenth-century German physiologist Ernst Weber and his compa-
triot, physicist and psychologist Gustav Fechner, provides an apt
example. It describes how much of an increase in a sensory stimulus is
needed for us to detect a change. It turns out that the size of change
needed is not an absolute amount. Rather, the amount needed for a
perceptible change varies relative to the size of the initial stimulus –
usually an increase of about three percent.
If you consider this for a moment, you’ll see that it tallies with your
everyday experience. Imagine that you’re hauling a huge suitcase to
the airport. Adding a book to the front-zip compartment won’t make a
discernible change to the weight, even though you’d easily notice the
weight of the book if it were the only thing you were holding.
Psychologists have had fun over the years comparing the Weber-
Fechner law across the senses – it turns out, for example, that we’re
more sensitive to changes in brightness than loudness. Other studies
have compared the law across a life span, showing that the amount of
percentage change needed to provoke a change in perception actually
increases as we get older – as we gradually lose our sensitivity.