Notes

You’re probably pretty good at subtracting numbers. But
have you ever tried subtracting brains? No wonder the boy
looks confused (cartoon). Subtracting brain images in 2- and
3 - dimensions turns out to be critical for the data analysis.
Most fMRI studies involve measuring the BOLD signal while
people are engaged in carefully controlled tasks. During a
scan, subjects lie within the bore of a magnet, and their
behavioural responses to stimuli are monitored. A wide range
of stimuli can be presented, either visually, projected onto a
screen for the subject to view, or in the auditory domain via
headphones. It is possible to examine covert phenomena
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Images of blood vessels in the brain. Changes in blood flow
can be detected and serve as an index of neural activity.
With computer technology, the images obtained by PET and
MRI scanners show exactly where the changes in blood flow
occur within the brain.
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such as perception, learning, remembering, thinking or
planning. Often two very similar tasks are designed with one
to be done immediately after the other. The idea is that the
first task should involve the brain process an
experimenter is interested in whereas the other should not.
The succession of brain images obtained are then subtracted
from each other to yield a pixellated 2D image of what
changes in activity are specifically associated with
performing the critical brain process. These images are
stacked up by the computer to yield an effective subtraction
of the image in 3 dimensions (see cartoon previous page).
Recent developments mean that even very brief thoughts or
brain events (as little as one or two seconds in duration) can
be measured. This is known as event-related fMRI.
Sophisticated methods of data analysis are used to test
whether changes in the signal during performance of a task
are statistically reliable. One widely-used analysis package
that has standardized the processing of imaging data is
called statistical parametric mapping (SPM). SPM maps are
often given colours, with a fiery yellow used for the ‘hottest’
areas of activity through to blue and black for ‘cooler’ areas.
Brain imaging scientists speak of areas ‘lighting up’ when
certain functions are carried out. If a person watches a
constantly changing checkerboard pattern, substantial
activation is observed in the primary visual cortex. The use
of moving and coloured colour patterns and other clever
stimuli designed to activate different areas of the visual
system has given us a great deal of new information about
the organisation of the human visual system. Similar
studies have been conducted for other sensory modalities.
This localisational way of thinking has also helped to identify
the brain areas involved in distinct components of reading –
such as transforming visual words into a phonological code,
the grouping of phonemes into whole words, the process of
extracting the meaning of words, and so on. Learning tasks
have also been studied, including work dissociating the brain
areas involved in anticipating and perceiving pain.
However, as research has proceeded, various surprises have
emerged. One early example was the unexpected failure to