No abstract

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.

The pharmacological basis of perceptual learning and associated cortical reorganizations remains elusive. We induced perceptual learning by Hebbian coactivation of the skin of the tip of the right index finger in humans. Under placebo, tactile two-point discrimination was improved on the coactivated but not on the left index finger. This augmentation was blocked by an N-methyl-D-aspartate-receptor blocker, but doubled by amphetamine. No drug effects were found on the left index finger. The individual amount of cortical reorganization as assessed by mapping of somatosensory evoked potentials was linearly correlated with the pharmacological modulation of discrimination thresholds, implying that perceptual learning and associated cortical changes are controlled by basic mechanisms known to mediate and modulate synaptic plasticity.