Figure 1. Experimental paradigm and main electrophysiological measures.

(a) The relationships between pre-stimulus alpha oscillations, stimulus-evoked responses, and perceived intensity of somatosensory stimuli were examined in a continuous sequence of median nerve stimuli of two intensities with inter-stimulus intervals of ISI = 1513 ± 50 ms. After every stimulus, participants were to rate the perceived intensity as either ‘strong’ or ‘weak’ as fast as possible by button press. The raster plots represent the data of an exemplary subject with the rows corresponding to single trials. Displayed from left to right: Average pre-stimulus alpha amplitude, intensity of the presented stimuli (red = strong; blue = weak intensity), short-latency somatosensory evoked potentials (SEPs), and the perceived intensity as reported by the participants (red = strong; blue = weak intensity). Alpha activity and the SEP were both retrieved from the same tangentially oriented canonical correlation analysis (CCA) component (displayed in panels b–e) and hence reflect activity of the same neuronal sources. (b) Grand average of the SEP (N = 32) in sensor space (electrodes F4, CP4, and P4) and for the tangential CCA component as derived from the single-trial extraction approach using CCA. (c) Activation pattern of the tangential CCA component displaying a tangential dipole contralateral to stimulation site over the central sulcus which is typical for the N20-P35 complex of the SEP. Averaged across participants (N = 32). (d) Neuronal sources (absolute values) underlying the activation pattern of the tangential CCA component, reconstructed using eLoreta inverse modeling. Averaged across participants (N = 32). (e) Same as d but applying an amplitude threshold of 95% in order to indicate the strongest generators of neural activity (displayed on a smoothed cortex surface).