Conscious perception of a weak somatosensory „target“ stimulus can be suppressed in a significant number of trials by a rapidly succeeding higher-intensity „mask“ stimulus applied to the contralateral finger at an ISI of 50–100 ms. A recent fMRI analysis of such somatosensory backward-masking (Blankenburg et al. NeuroImage 2003; 19: 1432) revealed comparable activations for perceived and extinguished targets in the contralateral primary somatosensory cortex. When the target was consciously perceived, there was additional bilateral BOLD activity in the inferior parietal cortices and frontal insulae. In the present study, we used high-density electroencephalography (EEG) (n=12) in a backward-masking paradigm analogous to the previous fMRI study in order to investigate correlates of selective attention and conscious perception in the somatosensory system. We hypothesized that stimulating both index fingers with an ISI of adequate duration to elicit potentials in contralateral S1 following the first stimulus is not sufficient to create perceptual awareness. In addition, components correlated with selective attention between 100 and 500 ms over posterior parietal and frontal areas should be modulated by the level of perception. Electrical current pulses were applied to both index fingers with asymmetrical intensities (left target < right mask) at an ISI of 70 ms (left before right stimulus). Subjects indicated perception of stimuli by pressing a button with their right foot. Subsequent ERP analysis revealed that early components (P45, N80) peaking over the contralateral S1 did not differ significantly between levels of perception. Significant differences arose from amplitudes correlated with selective attention (P100, N140, N200) and stimulus evaluation (P300) on contralateral parietal, central and bilateral frontal electrode positions between perceived/ extinguished targets. In congruence with our fMRI results, these data support the notion that early activation of modality-specific areas is a necessary but not sufficient condition for conscious perception of somatosensory stimuli. Rather, attentional engagement recruiting the parietal and frontal regions appears to be essential for conscious stimulus perception.
