fMRI for mapping the plastic somatotopy of primary somatosensory cortex - Development and clinical applications

Functional magnetic resonance imaging (fMRI) is a widely used tool for
studying brain function in vivo. The technique is based on acquiring brain
images sensitive to the physiological response following neural activation,
and hence, allows brain activity to be examined and documented.
In this thesis, methods for fMRI mapping of the primary somatosensory
cortex (S1) are optimised and subsequently applied in studies where a
plastic reorganisation of S1 is hypothesised.
Initially, the impact of spatial resolution and smoothing on fMRI data of
detailed S1 activation was investigated using a theoretical model of fMRI
performance. The impact of these parameters was also examined in healthy
volunteers where different fingers were mapped in S1. This was
accomplished using computer controlled and reproducible tactile
stimulation. It was found that both the optimal spatial resolution and
preferred level of smoothing were intimately coupled to the experiment’s
contrast-to-noise.
These results were utilised for monitoring sensory activation of S1 in three
cohorts where cortical reorganisation was anticipated: (i) In healthy
volunteers where the volar part of the forearm was anaesthetised, (ii) in
hand amputees and (iii) in subjects suffering from long-term exposure to
vibrating tools. In all these groups, evidence of plastic changes in the
sensorimotor system were found. This suggests that plastic processes could
be an underlying mechanism for the symptoms experienced in patients
following nerve injury and neuropathy.
Finally, alternative methods for mapping functional networks of the
sensorimotor cortex during rest were explored. We found that the resulting
networks were comparable to activation maps during a finger-tapping task,
although only partly overlapping. Such network maps could potentially add
to our understanding of brain plasticity in this region of the brain.
In conclusion, this work has improved the feasibility of monitoring plastic
reorganisation in S1. This may contribute to the process of rehabilitation in
patients suffering from sensory disorders following nerve injury and
neuropathy.