This is extracted from an un-published essay I wrote in 1995 named A manifesto for research in cognitive neuropsychiatry, with additional commentary from 1996.
I have corrected a few phrases and added a few sub-heading, but made no substantive changes.
19 years later, I can only observe "I was right".
*
A
Critique of Functional Brain Imaging, 1995
by Bruce G Charlton
School of Psychology, Newcastle University, England
Structure-Function correlations
One of
the most fruitful constraints in biology has been the assumption of
correlations between structure and function. This assumption is valid only when
structure is examined at the appropriate level; and may be obscured by
inter-subject differences in developmental history, age, coincident disease
etc. Furthermore, the necessary scale of analysis to reveal an association is
not always obvious.
Nonetheless,
a useful degree of functional localization has been described for many
neuroanatomical scales in the brain ranging from the basic divisions between
primitive fore-, mid-, and hind-brain to the tendency of individual cells to
respond to specific classes of visual information (Zeki, 1993).
The
existence of structure-function correlations is not surprising given an
understanding of natural selection. Newly-evolved functions usually require
appropriately-modified structures: so that a cognitive adaptation usually needs
a specifically constructed neural circuitry to perform novel computations.
And
because natural selection must produce a reproductive advantage for each
evolutionary change (Dawkins, 1987) , this limits evolution to an incremental,
stepwise modification of the parameters of existing neural circuitry.
Redesigning the brain from scratch is not an option, and ‘rewiring’ of the
brain is therefore piecemeal and additive, rather than radical and
substitutive.
Hence
the neocortex has evolved many additional stages of complexity to its hierarchy
of processing throughout evolutionary history. New structural/ functional
specializations have typically been accommodated by lateral expansion of the
surface of the cortex (Barton & Dunbar, 1997). Each distinct intermediate
cognitive process tends to be functionally localized to a cortical area; the
integrity of which area is necessary
for performance of a particular class of inferential reasoning tasks; and
disruption to which area is sufficient
to impair performance of this class of tasks.
The proper purpose of functional brain imaging
The
purpose of imaging in cognitive research is therefore to fractionate function
by displaying the location and sequence of activation in he brain structures
which are necessary for performance of specific stages in cognitive processing
(Jacobs & Carr, 1995). A method is therefore required that discriminates neural activity at an adequately
precise level of both spatial and temporal resolution (Kosslyn, 1994).
Such an
imaging technology could potentially demonstrate both the modularity of
intermediate level processes, and the number of steps of cognitive processing
within each of these pathways.
Functional
imaging might then constrain theories of cognitive functioning, yielding a
template of the number of modules, the number of discrete processing steps
within modules and the nature of the temporal inter-relationship between these
processes.
Brain
imaging would, in a sense, provide a flow diagram comprising ‘boxes’ with ‘arrows’
between them: it would remain for cognitive psychology to fill the boxes by
defining the nature of processing at each locus.
Inadequate spatial and temporal resolution of functional brain imaging
It is
uncertain exactly what level of spatial resolution is required of an imaging
technique to accomplish such fractionation, because the necessary size of
neural structures needed for intermediate processing is not yet properly
established (probably, the more complex the computational task, the larger the
necessary size of neural circuitry).
However,
many specialized cortical areas are some millimetres in diameter, so a
resolution of at least tenths of a
millimetre would seem to be necessary.
And temporal recording of neural activity would
need millisecond resolution in order to discriminate the steps of
neural activation involved in cognitive processing.
Unfortunately,
none of the currently available
techniques for functional imaging of the brain are able to provide the minimum
necessary structural and functional discrimination
of neural activation.
Magneto-encephalography
(MEG) can so far attain this temporal resolution only for restricted cortical
regions, and in situations where (probably) thousands of orientated neurons are
operating synchronously (Naatanen et al, 1994).
Positron
Emission Tomography (PET) has low spatial resolution and very low temporal
resolution, and visualizes blood flow changes – which are a crude, and probably
unreliable, proxy measure of neural activation (Posner & Raichle, 1994).
Functional
Magnetic Resonance Imaging (fMRI) probably has adequate spatial resolution, but
cannot form images rapidly enough to resolve cognitive processes; and also uses
proxy measures of neural activity (Kosslyn, 1994).
PET and
fMRI should, therefore, be considered as yielding proxy measures of brain anatomy averaged over time, rather than
revealing sequences of neural activity corresponding to cognitive function.
Electroencephalography
(EEG), by contrast, can demonstrate brain activation events in real-time; but
even in its modern ‘high resolution’ form, the technique provides only a very
blurred, two-dimensional structural image with a centimetre level of spatial
resolution (Gevins at al, 1995).
Functional Brain Imaging - an enumeration of misleading artefacts
The
present practice of disregarding the systematic limitations of existing
functional imaging, and using techniques such as PET to explore cognitive
functioning, is a dubious practice.
The
intrinsic tendency of low-resolution techniques of imaging to mislead is
exacerbated by the practice of constructing images using ‘subtraction’
techniques, averaging of repeated trials, and the pooling of patients in (often
diagnostic) group studies (Posner & Raichle, 1994). Presumably,
investigators believe (but without evidence or testing the assumption) that
such procedures simply remove ‘noise’ and thereby enhance precision and
sensitivity.
However,
given the complexity and dynamic, non-repeating nature of brain states, it is
almost certain that investigators are pooling systematically-different
instances. A comparison of single case, real time data from MEG with averaged
images from groups reveals highly significant averaging effects with time and
across groups (personal communication, Andreas A Ioannides, 6. 12. 95).
Despite
the proliferation of high status publications in the field of functional brain imaging; it therefore
remains entirely possible that the results reported so far represent little
more than an enumeration of artefacts.
*
- Excerpt from note added in 1996:
[Functional
brain] imaging has not ‘yet’ contributed anything of significance to the
understanding of human psychology – despite the billions of pounds pent on it
and the numbers of papers published into journals.
And I mean nothing at all.
Under the hype, the published work is
merely fourth-rate, stamp-collecting, pre-science; pumped-up to high prestige by
the cost of the technology and the attractiveness of its pictures.
- Further note added today, 18 years later:
Nothing
to add to, or subtract from, the above. Functional brain imaging was a
successful scam for extracting funding; a quarter century of which contributed nothing substantive to
science, and grossly misallocated prestige away from real science and into an
assortment of deluded and dishonest careerist confidence tricksters.
References
Barton R&
Dunbar R. (1997). Evolution of the social brain. In RW Byrne, A Whiten
(Editors) Machiavellian Intelligence II. Cambridge University Press: Cambridge,
UK.
Dawkins
R. (1986) The Blind Watchmaker. Longmans: London.
Gevins
A et al. (1995). Mapping brain function with modern high-resolution
electroencephalography. Trends in Neurosciences (TINS). 18: 429-436.
Kosslyn
SM. (1994). Image and Brain. MIT Press: Cambridge, MA, USA.
Naatanen
R et al. (1994). Magnetoencephalography in studies of human cognitive brain
function. Trends in Neurosciences (TINS). 17: 389-395.
Posner
MI. (1994). Images of Mind. Scientific American Library: New York.
Zeki S.
(1993). A Vision of the Brain. Blackwell: Oxford.
