YOUR BRAINmagnetic resonance imaging (fMRI) and positron emission tomography
(PET) researchers can now pinpoint changing patterns of brain activity
while participants, healthy or otherwise, lie in a scanner performing
different tasks.
Psychologists of the past couldn’t possibly have dreamed of tools
like this. Thousands of brain imaging studies are now performed every
year, many of them published in the world’s most respected scientific
journals. Their findings also draw attention from the popular press,
usually prompting dubious headlines such as “scientists locate brain’s
love centre”, alongside eye-catching images of colourful blobs on
the brain.
So what does fMRI really measure, and what do those blobs mean?
When a region of your brain is more active, it uses up more oxygen. In
response, the brain sends along more oxygenated blood and it is the
relative concentration of oxygen-rich and oxygen-light blood that is
measured by fMRI – what scientists call the Blood Oxygenation Level
Dependent Contrast or BOLD response. Similarly, PET tracks blood
flow changes via the injection of a radioactive substance that collects in
greater concentrations where the brain is more active. In other words,
brain imaging does measure changing activity levels, but it is an indi-
rect, imperfect measure. The scanner isn’t actually recording the firing
of individual brain cells.
When we read about people’s brains being scanned while they are
shopping or falling in love, we must remember too that in reality they
were strapped prostrate inside a noisy (in the case of fMRI) metal tube,
their head held still with cushioned clamps, while they viewed shopping
or romantic images through goggles or an intricate system of mirrors.
Around twenty percent of participants bail out of these experiments
because they find the conditions too claustrophobic.
Despite the promise of brain imaging technology and the undeniable
contribution it has already made to our understanding of brain func-
tion, there has, in recent years, been something of a backlash. Critics
have begun to doubt the value of all these imaging experiments and
to reminisce with fondness about the old-school days of creative, care-
fully controlled behavioural experiments. What, after all, they ask, does
it mean to localize a particular function to a precise part of the brain?
Some critics have even branded the brain imaging project as nothing
more than a form of modern-day phrenology – the nineteenth-century
“science” that linked personality traits to the shape of the skull (see box
overleaf ). To paraphrase the scepticism of philosopher Jerry Fodor: we