Cellular mechanisms underlying synaptic plasticity are in line with the Hebbian concept. In contrast, data linking Hebbian learning to altered perception are rare. Combining functional magnetic resonance imaging with psychophysical tests, we studied cortical reorganization in primary and secondary somatosensory cortex (SI and SII) and the resulting changes of tactile perception before and after tactile coactivation, a simple type of Hebbian learning. Coactivation on the right index finger (IF) for 3 hr lowered its spatial discrimination threshold. In parallel, blood-oxygen level-dependent (BOLD) signals from the right IF representation in SI and SII enlarged. The individual threshold reduction was linearly correlated with the enlargement in SI, implying a close relation between altered discrimination and cortical reorganization. Controls consisting of a single-site stimulation did not affect thresholds and cortical maps. Accordingly, changes within distributed cortical networks based on Hebbian mechanisms alter the individual percept.

We report experiments combining assessment of spatial tactile discrimination behavior and measurements of somatosensoryevoked potentials in human subjects before and after short-term plastic changes to demonstrate a causal link between the degree of altered performance and reorganization. Plastic changes were induced by a Hebbian coactivation protocol of simultaneous pairing of tactile stimuli. As a result of coactivation, spatial discrimination thresholds were lowered; however, the amount of discrimination improvement was variable across subjects. Analysis of somatosensory-evoked potentials revealed a significant, but also variable shift in the localization of the N20-dipole of the index finger that was coactivated. The Euclidean distance between the dipole pre-and post-coactivation was significantly larger on the coactivated side (mean 9.13 ؎ 3.4 mm) than on the control side (mean 4.90 ؎ 2.7 mm, P ‫؍‬ 0.008). Changes of polar angles indicated a lateral and inferior shift on the postcentral gyrus of the left hemisphere representing the coactivated index finger. To explore how far the variability of improvement was reflected in the degree of reorganization, we correlated the perceptual changes with the N20-dipole shifts. We found that the changes in discrimination abilities could be predicted from the changes in dipole localization. Little gain in spatial discrimination was associated with small changes in dipole shifts. In contrast, subjects who showed a large cortical reorganization also had lowest thresholds. All changes were highly selective as no transfer to the index finger of the opposite, non-coactivated hand was found. Our results indicate that human spatial discrimination performance is subject to improvement on a short time scale by a Hebbian stimulation protocol without invoking training, attention, or reinforcement. Plastic processes related to the improvement were localized in primary somatosensory cortex and were scaled with the degree of the individual perceptual improvement. N oninvasive imaging techniques used to explore cortical reorganization in human subjects revealed that improvement of behavioral performance following extensive use or training is paralleled by substantial changes of cortical representations (1-7). These findings confirmed the relevance of cortical plasticity for everyday life; however, it remains open how far differences in the magnitude of reorganizational changes can explain individual differences in learning-induced changes of performance. Specifically, there is a controversy in how far the variability of improvement is reflected in the degree of reorganization. Here, we report experiments combining simultaneous assessment of spatial tactile discrimination behavior and measurements of somatosensory-evoked potentials (SSEPs) before and after short-term plastic changes to demonstrate a close link between altered performance and reorganization in primary somatosensory cortex.To induce cortical reorganization without invoking training or cognitive factors such as attention ...

The effect of the glutamate antagonist riluzole on excitatory and inhibitory phenomena in the human motor system was studied by transcranial magnetic stimulation (TMS) and peripheral electrical nerve stimulation. The motor threshold, the intracortical inhibition and intracortical facilitation as assessed by paired TMS, the cortical and peripheral silent periods, F wave amplitudes and F wave latencies were measured. Riluzole suppressed the intracortical facilitation whereas other parameters remained unchanged, indicating that the neurotransmitter glutamate is mainly involved in facilitatory mechanisms in the motor system.