rTMS elicits tactile discrimination improvement and parallel plastic reorganization in human SI

rTMS is widely used to investigate different aspects of cortical excitability and inhibition, which play an important role in controlling plasticity. These experiments imply that cortical processing can be altered by repetitive current pulses, but little is known about perceptual consequences of high-frequency (5Hz) rTMS in human subjects. Here we demonstrate, using psychophysical testing and parallel functional magnetic resonance imaging (fMRI), that subliminal rTMS applied with a figure-eight coil positioned over the digit representation of left somatosensory cortex (SI) evokes plastic changes on the stimulated, ipsilateral cortical hemisphere. Two rTMS sessions (25 trains with 50 single pulses, for 10min) with a rest period of 1 hour between sessions resulted in selective and reversible reorganization of cortical finger areas in SI. As an indirect marker of cortical reorganization we measured tactile spatial two-point discrimination of the right index (IF) and ring finger (d4) in a AFC task before and after rTMS. The left IF served as control. rTMS revealed a significant lowering of thresholds for the right IF and d4, which was reversible within 135min for the IF and within 90min for d4. The left IF was not affected by rTMS. A combined assessment of discrimination thresholds and fMRI recordings revealed an enlargement of activation pattern and an enhancement of the amplitude of the blood-oxygen-level-dependent (BOLD) signal only in the right, rTMS-treated SI., which was linearly correlated with the individual gain of discrimination improvement after rTMS. As an additional control we found that sham-stimulation over SI revealed no changes in tactile performance for the right and left IF and no changes of the corresponding cortical representations. The local specificity of the rTMS effects was additionally tested, where we applied rTMS over the lower leg representation, but measured discrimination thresholds for the IFs. We found no significant changes in tactile discrimination thresholds and cortical digit representations measured with fMRI. These findings support the view that primary sensory areas play a crucial role in perception. The results indicate that a stimulation protocol resembling those used in LTP studies, applied from outside directly to selected brain regions can induce meaningful cortical reorganizations paralleled by improvement of discrimination thresholds.