Functional Imaging of Perceptual Learning in Human Primary and Secondary Somatosensory Cortex

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.

Cortical topography of intracortical inhibition influences the speed of decision making

“…S1). Moreover, in recent experiments on changes in tactile acuity after coactivating fingers of the right, dominant hand, measurable changes of acuity of the left, nonstimulated hand had never been observed (22,23,(25)(26)(27)(28)(29), which argues for a substantial locality of coactivation-induced changes and supports the view that after repeated testing, RT effects are due to practice.…”

“…To provide further evidence for the Hebbian nature of coactivation, we used a modified version of the coactivation protocol consisting of a single, small stimulation site instead of one large area. This protocol lacking coactivation evoked no effects, neither perceptually nor in cortical maps (28). The lack of effects implies that spatial summation requirements indicative of cooperative processes need to be fulfilled to drive behavioral changes.…”

“…Coactivation is a form of repetitive stimulation, which follows the idea of Hebbian learning: Synchronous neural activity, necessary to drive plastic changes, is evoked by tactile "co"-activation of the skin. Several studies have shown that after a few hours of coactivation, tactile discrimination abilities were improved (22,23,25,(27)(28)(29). To provide further evidence for the Hebbian nature of coactivation, we used a modified version of the coactivation protocol consisting of a single, small stimulation site instead of one large area.…”

“…Coactivation led to a decrease of cortical inhibition (29), which has been assumed to cause an expansion of the size of d3 representation (28). We therefore implemented the observation about reduced inhibition in the model by decreasing the involvement of inhibitory neurons coding for the coactivated right d3 in inhibitory interaction.…”

“…Coactivation is a form of repetitive sensory stimulation based on the induction of presumably Hebbian learning mechanisms (24-26). Coactivation has been shown to expand cortical finger representation (27,28) and to reduce intracortical inhibition, as indicated by reduced paired-pulse inhibition (29). Here we found that after coactivating the middle finger, its RTs were significantly shortened, resulting in the near disappearance of the inverted U-shaped distribution.…”