Now you'll feel it, now you won't: Pre-stimulus mu and beta amplitude predict conscious perception of somatosensory stimuli
A recent electrophysiological study revealed that parieto-occipital alpha-power correlates with conscious perception of near-threshold (50% detection rate) visual stimuli (Babiloni et al., 2005). In the somatosensory system, conscious perception of a clearly supra-threshold target stimulus applied at the left index finger can be suppressed in a high proportion of trials when followed by a higher-intensity mask stimulus at the right index finger. Previously, we showed that this somatosensory “extinction” in healthy subjects is reflected in decreased post-stimulus mid-latency ERP components (Schubert et al., 2006). Here, we analysed how pre-stimulus EEG rhythms correlate with these different perceptive states (conscious perception vs. extinction) of clearly supra-threshold somatosensory stimuli. We conducted a time–frequency EEG analysis for perceived and extinguished stimuli in the time interval of 500 ms immediately preceding stimulus onset. For perceived compared to extinguished target stimuli, the power of the pre-stimulus rolandic mu-rhythm, i.e., the alpha-band (8–12 Hz) and lower beta–band (12–18 Hz), decreased significantly (p<0.001). The effect was dominant over right sensorimotor regions, contralateral to the target stimulus. The results suggest that a modulation of low-frequency bands (<20 Hz) is related to conscious perception of also supra-threshold somatosensory stimuli and may imply a general mechanism for conscious perception. We propose that pre-stimulus suppression of the neuronal “baseline” oscillatory activity in the somatosensory cortex (murhythm) facilitates conscious somatosensory perception. Top-down control from higher-order attentional systems may produce a transient suppression of rhythmic activity in sensory receiving areas and provide a selective preparedness for processing of incoming stimuli.