In article <416fb952 at dnews.tpgi.com.au>,
"John Hasenkam" <johnh at faraway.> wrote:
> Thanks for the correction Matthew, like the original poster I was under the
> impression that fMRI was limited. Perhaps you can help me with another
> concern in relation to fMRI. I have noted in a number of articles that
> images produced for a given task can vary quite considerably.
I am by no means an fMRI expert, but the fundamental limitation of the
technique is that it relies on changes in MRI density caused by changes
in local blood oxygenation (often abbreviated BOLD fMRI - Blood Oxygen
Level Dependent fMRI). The technique, for those unfamiliar, involves
making a scan of the brain while the subject does nothing, and then
having the subject perform some kind of task or pay attention to a
stimulus, and making the scan again. The "resting" scan forms a
reference level, and at every point in the head, the "active" scan is
compared to the reference. If the "active" scan density is different at
some points, it can be concluded that the local levels of blood
oxygenation have changed, and the point is coloured according to how far
off-baseline it is (calculated statistically based on the overall amount
of change seen in the scan).
This leads to several serious limitations of fMRI, which are
particularly problematic when someone tries to treat an fMRI image as a
picture of "nervous system activity":
1. fMRI is a map of changes in blood flow: nothing more, nothing less.
It is conjectured, but not definitively proven, that active tissue in
the CNS can modulate local blood flow to produce changes in density on
an MRI scan. fMRI advocates would say that the blood flow changes are
produced by whatever processes contribute to the mental activity being
performed, but that's rather a circular argument. There's no proven
causal chain between, say, a neuron firing more actively and a change in
the amount of oxygenated blood flowing through the nearest capillary.
2. As far as I know, the mechanism by which blood flow is altered is not
understood. It is likely to be strongly mediated by astrocytes, whose
processes wrap brain capillaries in continuous sheaths, and hence would
be controlled by glial cells, rather than neurons. Hence it seems
likely that the needs of glia are more relevant to fMRI than the
activity of neurons directly.
3. There is no absolute scale for what constitutes "active" levels of
difference on the fMRI image. To a large extent they are set ad-hoc
based on the levels observed in individuals. Hence it is difficult to
compare between individuals and between studies. Normalised
co-ordinates such as the Talairach system can help adjust for the
structural differences in individual brains, but there's no way to
adjust for the real differences in regional function between
individuals, which have been demonstrated anatomically and
physiologically but don't show up on MRI.
4. Nobody has any idea if blood flow is modulated by the *type* of
processing in a given region - aside from the ignorant people who
believe we only use 10% of our brains, most neuroscientists would agree
that even during "resting", there is sustained and large scale activity
in the nervous system, and we are looking for fluctuations on a much
smaller scale, which may or may not be relevant to what the region is
doing. For instance, active suppression of activity in a region is
almost certainly more metabolically expensive than normal processing, so
a strongly suppressed area would show up as more "active" on an fMRI
scan. Histological stains which show markers of high metabolic need are
usually labelling inhibitory neurons, which tells us who the major
"blood consumers" are in the brain.
5. Similarly, it is possible that changes in patterns of activity,
rather than the overall amount of activity, could be far more relevant
in processing tasks and stimuli. The idea of synchronisation has been
widely proposed as a way that processes could be "bound" together in
consciousness, but that only involves altering the timing of spikes, not
the overall number. Presumably that wouldn't show up on fMRI at all,
since the tissue's metabolic demands would be unchanged.
6. An allied argument is that changes in blood flow are a cumulative
result of sustained activity, such that only the longest-duration
changes are going to show up as changes to blood flow. The blood flow
changes necessarily lag whatever processes cause them, and that may take
place over a much longer time scale than the processing itself takes.
The longest-duration processes are not necessarily the most relevant to
the ability under study.
Finally, the major problem with fMRI is that it produces pretty images,
at great expense, but really tells us very little about how the brain
does what it does. Ask yourself whether our knowledge of face
recognition and how the brain does it is enhanced by knowing it's done
on the fusiform gyrus, instead of knowing it happens in the head
"somewhere". Its illusion of specificity, combined with a lack of any
real insight to processing, produces the dangerous illusion of
understanding phenomena which in reality have only been *described*
slightly more accurately.
These and other reservations have led to fMRI being characterised by
some hard-core physiologists of my acquaintance as "phrenology with
coloured lights." A harsh assessment, but one which I have found quite
hard to escape after hearing it.
Of course there are insights to be gained from fMRI applied with regard
for its limitations, but there is a growing body of "soft" science which
appears to regard it as a kind of x-ray vision where you can watch
people thinking. Sorry, not even close!
> This concern was heightened recently when I read
> an unpublished paper citing that replicability in PET and fMRI is difficult
> to achieve. The author asserted that replicability was all but absent. When
> I found his web page he had posted various editor responses and none of
> these indicated a challenge to his paper, rather that he needed to do more
> work, or it was not appropriate for that particular journal. The only reason
> I read this paper is because I had noted this lack of replicability
> mentioned in quite a few other papers, though only in passing.
I'd love to read the paper you mention - these kind of views are not
very fashionable at the moment, and it would be great to hear what
others say.
> Your opinion on these matters would be appreciated,
Please bear in mind it's only an opinion, many others know way more than
I do about it, and I would appreciate corrections, refutations and
amplifications from others.
Sorry to go on at such length - all this has been brewing at the back of
my mind for several months. How do I know it was at the BACK of my
mind? Well, there's this type of brain scanning called fMRI, and ...
Cheers,
Matthew.
> ps, hope you get some rain down there soon, here in SE QLD dry as dust ... .
Hope so for all of us, but if the CO2 continues to rise it might be an
academic question.
