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10 GENERAL PROBLEMS OF (AND PERSPECTIVES FOR) FMRI OF MOTOR FUNCTION
motor control in particular. Sensorimotor function is associated with a distributed
neural substrate, and the fact that fMRI readily covers the entire brain is helpful in
this respect. Highly focalized research techniques are hypothesis-driven, at least in
terms of the location they target, and this impairs the potential for new discoveries.
In that sense, even if fuzzy, a picture of activity in the entire brain may help to
generate and then test novel hypotheses, apart from offering plausibility controls for
ongoing studies. Furthermore, the fact that the contrast agent exploited in the BOLD
contrast is endogenous and thus permanently present permits a true neurophysiological recording that can go beyond evoked responses.
However, the spatiotemporal response function, i.e., the dispersion of the fMRI
response in time and space, poses the most relevant limitation for this type of
recording. In other words, fMRI hits hard biological limits, not technical ones, even
though technical difficulties are abundant and not yet always fully mastered. Studies
in the visual system with fMRI have established that dedicated acquisition and
analysis techniques can resolve much smaller functional cortical units than in the
currently available motor studies discussed here.166 This increase in spatial resolution
is of interest because, in contrast to earlier neuroimaging techniques, fMRI experiments readily generate highly significant findings in single subjects.
Many of the topics discussed in the previous sections illustrated that one of the
major shortcomings of functional neuroimaging studies still lies in the uncertainties
of anatomical labeling. Each brain is different, but previous neuroimaging techniques
required normalizing the data into a common standard stereotactic space so as to
perform averaging of voxel-based signals from roughly homologous brain areas
across subjects. These group analyses then had sufficient statistical power and the
advantage of ensuring some degree of generality in terms of volume coverage and
intersubject variability. Yet, the price paid for this procedure was at the level of
anatomical analysis. Even if a spatial normalizing technique incorporates nonlinear
algorithms that warp one gyrification pattern rather well into another, the correspondence of actual brain areas becomes blurred by these procedures, and accordingly
probabilistic atlases are the closest one can get to reality in this setting. In the
previous sections, it has become obvious that such maps can indeed be helpful in
tentatively assigning fMRI responses to certain areas, but often enough, even probabilistic statements leave painful uncertainties as to which areas we are obtaining
But what defines an area as charted in an atlas? The set of neuroanatomical
criteria range from cyto- and myeloarchitectonic features to densities and laminar
distributions of receptors and other neurochemical markers.167 In the case of M1,
recent detailed analyses have demonstrated a considerable degree of variability both
between different brains and within individual brains, i.e., between hemispheres.168
Moreover, similar methods have rather recently unveiled the fact that, regarding
Brodmann area 4, we are actually dealing with two architectonically distinct areas
instead of one.169 If form follows function, we must also assume different response
properties of these two areas, and we have discussed some of the evidence from
functional neuroimaging that this may indeed be the case. However, these conclusions were based on relating functional findings from one or several brains to a
database formed from many other and thus different brains. The desideratum at this
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stage is to map areas in individual subjects, namely those subjects in whom we can
also obtain physiological observations, thus avoiding the limitations introduced by
intersubject variability. Whether this will be achieved by morphological or functional
criteria is not yet clear, but in any case this will need to be done in a noninvasive
fashion and will thus require imaging techniques.170,171 The exquisite sensitivity of
magnetic resonance to a whole range of biophysical parameters suggests that it will
take center stage in this promising effort.
Andreas Kleinschmidt is funded by the Volkswagen Foundation. We thank Ulf
Ziemann for helpful comments on the manuscript.
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in the Motor Cortex
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