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Abstract

The mammalian neocortex has an amazing capacity for reorganization. This is the basis for lifelong adaptation to
changes in sensory environments and behavioural demands. Hebbian mechanisms seem to play a key role in useand
input-dependent cortical plasticity (Clark et al. 1988; Wang et al. 1995). We used functional MRI together
with a spatial discrimination task to investigate in detail the effects of synchronous and asynchronous tactile
coactivation on cortical organisation in the human primary somatosensory cortex (SI) and its behavioural
consequences. Coactivation, which is the Hebbian-like associative pairing of tactile stimulation, was applied for
three hours to the distal phalanges of index, middle and ring fingers of the right hand either synchronously or
asynchronously. Strengthening previous findings in rats we show that somatosensory cortical representations for
synchronously coactivated fingers move closer together. On the other hand, cortical representations for
asynchronously coactivated fingers become segregated and move further apart (Zepka et al. 2000). Behaviourally,
this coincides with a reduced number of mislocalisations between fingers that have been coactivated
asynchronously and an increased number of mislocalisations between fingers that have been coactivated
synchronously. These results were reversible within one week after stimulation. Thus, not only synchronous but
also asynchronous coupling of passive tactile stimulation is able to induce short-term cortical reorganisation that
is associated with functionally relevant changes in the perceptual and behavioural capacities of an individual
without active training or attention directed to the stimuli.