Synchronous and asynchronous tactile coactivation differentially affect human somatosensory cortical organisation and tactile discrimination performance

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