YOUR SENSE OF THE WORLD AND MOVEMENT WITHIN ITthe moment that they had made the decision to move. His surprising
discovery was that preparatory activity in the brain actually preceded by
about half a second the moment when participants said they had made
their decision. This result suggests that the feeling of ownership we have
over our voluntary movements is something of an illusion, apparently
undermining the idea that we have free will.
Some commentators have criticised Libet’s methods. For example,
at the instant a decision is made, the time perceived on the clock will
actually be the time from several moments earlier, because of the delays
inherent in neural transmission. Of course this would actually lead to
an underestimation of the extent to which preparatory brain activity
had preceded the conscious will to move. Another criticism is whether
we’re actually capable of detecting when we’ve made a conscious
decision. In a 2008 paper, William Banks and Eve Isham at Pomona
College in Canada claimed to show that people infer the timing of their
conscious decision indirectly, using feedback from the body movement
in question – a strategy the researchers were able to exploit with delayed
video feedback, thus leading their participants to make skewed claims
about their decision times.
These criticisms notwithstanding, another study using brain imaging,
published in 2008, appeared to replicate and extend Libet’s results. Chun
Siong Soon and colleagues scanned the brains of participants while
they decided to move either their right or left index-finger. Around ten
seconds prior to the instant that the participants said they had made
their conscious decision, Soon’s team observed patterns of brain activity
in two areas which not only revealed that a movement was about to be
made, but also revealed whether the movement would be with the left
or right hand.
Mixed messages
Most of us experience our five senses as if they’re completely separate, but
for a minority of people it’s as if their neural wires have got crossed. These
individuals have a heritable condition called synaesthesia, which means
they experience a mixing of two or more of their senses. One of the most
common forms is so-called “grapheme-colour synaesthesia”, in which
letters and numbers reliably trigger the sensation of certain colours.
There are many other forms of synaesthesia: lexical-gustatory synaes-
thetes, for example, experience a particular taste whenever they hear
certain words (there’s a report of one man who had to dream up