Brain imaging studies are becoming increasingly prolific in the news and on websites, blogs
and journals. Some of these have good internal and external validity, methodology and peer
reviews, however a substantial amount do not. The study of brain imaging had its inception
back in 1895 on the 8th November when Wilhelm Röntgen discovered Xrays, and since this
time methods and analysis for investigating functional and anatomical aspects of the brain
have ever since improved and are still doing so today. These days when a ‘brain scan’ is
mentioned it is usually referring to Functional magnetic resonance imaging (FMRI) which
claims to be a direct measure of brain activity. This is false. FMRI is indeed more informative
than an Xray but it is not a direct measure of brain activity as credible news sites like to
claim. I shall refrain from name dropping here, but if you come across any such articles you
will know. This article is to tell you what to look out for when reading about brain imaging
studies, and will hopefully make an entertaining read too!
In reality FMRI is a measure of oxygenated blood; more precisely a measure of
oxyhaemoglobin (oxygenated haemoglobin). The premise is that higher amounts of oxygen
are required in the neurons that are doing the work (cognition requires certain neurons to
perfuse so they need more oxygen). This neuronal work increases the concentration of
oxyhaemoglobin at these sites and decreases deoxyhaemoglobin, and it is this that indicates
localised activity in FMRI. As blood supply is limited, an average adult has 8 – 10 pints of
blood in their system, so a decrease of oxyhaemoglobin indicates reduction of neuronal
activity. All sounds good so far but, and there are some big ‘buts’ coming up.
FMRI signal does not distinguish between actual activity and inhibitory activity in the brain.
This ambiguity exists because these types of responses generate increased levels of
oxyhaemoglobin, and there is currently no way to tell them apart. So what might be described
as a critical location of activation is actually the exact opposite. On top of this, when blood
leaves the brain it tends to flow through the larger veins as opposed to smaller ones, this then
affects the FMRI signal by indicating brain activity even though this is many neurons away
from the actual location of activity.
If you have ever wondered as to why or how the brain uses approximately 20% of our energy
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resources, even when we don’t have deadlines to meet! This is because the brain demands a
lot more energy (in the form of oxygenated blood including glucose) than what it actually
consumes (so why do our brains get tired…?) In regards to brain scanning this means that» >