Now you feel itFnow you don’t: ERP correlates of somatosensory awareness RUTH SCHUBERT, a FELIX BLANKENBURG, a STEVEN LEMM, b,c ARNO VILLRINGER, a and GABRIEL CURIO b a Department of Neurology, Charite´ –University Medicine, Campus Mitte, Berlin, Germany b Neurophysics Group, Charite´ –University Medicine, Campus Benjamin Franklin, Berlin, Germany c Fraunhofer–Institut FIRST, Kekule´ strasse 7, Berlin, Germany Abstract We investigated correlates of somatosensory awareness for supratheshold stimuli using event-related potentials in a masking paradigm: Conscious perception of a weak, but suprathreshold ‘‘target’’ stimulus was suppressed in a significant number of trials when followed by a higher-intensity ‘‘mask’’ stimulus. ERPs were compared for trials with perceived versus unperceived target stimuli. Early ERPs (P60, N80), generated in the contralateral S1, were found independent of stimulus perception. In contrast, for consciously perceived target stimuli, amplitude enhancements were observed for the P100 and N140. Thus, early activation of S1 is not sufficient to warrant conscious stimulus perception. Conscious stimulus processing differs significantly from unconscious processing starting around 100 ms after stimulus presentation when the signal is processed in parietal and frontal cortices, brain regions crucial for stimulus access into conscious perception. Descriptors: Perceptual awareness, Somatosensory event-related potentials, Masking, Primary somatosensory cortex, Frontoparietal network, Spatial attention Investigating neural correlates of perceptual awareness is of grow- ing interest to cognitive neurosciences (Dehaene & Naccache, 2001). In particular, the amount of unconscious cerebral process- ing occurring before a sensation enters awareness and its temporal dynamics has been subject of recent electrophysiological studies. In comparison to the extensive research on visual awareness (Crick & Koch, 1995; Kanwisher, 2001; Koivisto, Revonsuo, & Lehtonen, in press), fewer studies exist in the field of somatosen- sory awareness (Libet, Alberts, Wright, & Feinstein, 1967; Meador, Ray, Echauz, Loring, & Vachtsevanos, 2002; Palva, Linkenkaer-Hansen, Naatanen, & Palva, 2005). In their pioneering work, Libet et al. (1967) reported evoked potentials recorded subdurally over the primary somatosensory cortex following somatosensory stimulation below the percep- tual threshold. They concluded that ‘‘the primary evoked re- sponse was not sufficient for sensation’’ (p. 1597) This was confirmed by the studies of Ray (Ray et al., 1999) and Meador et al. (2002), who found similar early evoked potentials ( 100 ms) from intracranial recordings for perceived and un- perceived near-threshold stimuli. In a recent study using single cell recordings in trained monkeys which had to respond to per- ceived near-threshold somatosensory stimuli, there was no dif- ference in the activity of S1 neurons between perceived and missed stimuli (de Lafuente & Romo, 2005). In more general terms, one might wonder whether the failure to elicit stimulus awareness by an activation of (S1) is specific for threshold stimuli, for example, due to a minor S1 activation, which might impede further processing beyond S1. Contradic- tory to the above mentioned findings of a similar activation of S1 for perceived and unperceived stimuli are the results from a study by Palva et al. (2005). They electrically stimulated the index fin- ger of subjects with a weak (detection rate 5 50%) current pulse. Subjects had to respond by a thumb twitch whenever they per- ceived the stimulus. Stimuli not followed by a response were classified as unperceived. During the stimulation, the authors recorded cortical activity using magnetoencephalography. They found an enhancement of early S1 activity for perceived stimuli as compared to unperceived stimuli. Notably, in paradigms using a very low intensity stimulation, the subjects’ prestimulus at- tentional state could fluctuate during the recording session and might be responsible for effects on early evoked activity as well as the failure of perception. Thus, this paradigm cannot unequiv- ocally assess the effect of awareness on S1. To further characterize the role of stimulus intensity in relation to cortical activation and sensory awareness, a This study was supported by the Graduiertenkolleg 238 ‘‘Damage cascades in neurological disordersFstudies with imaging techniques’’ (R.S.) and SFB 618, project B4 (S.L.), of the Deutsche Forschungs- gemeinschaft (DFG) and the Bundesministerium fu¨ r Bildung und Forsc- hung (BMBF) in the Berlin NeuroImaging Center (AV) and Bernstein Center for Computational Neuroscience Berlin, project C4 (G.C.). The authors thank Claudia Preuschhof for comments on earlier drafts of this article. Address reprint requests to: Ruth Schubert, Berlin NeuroImaging Center, Charite´ Campus Mitte, Schumannstr. 20/21, 10117 Berlin, Deutschland. E-mail: ruth.schubert@charite.de. Psychophysiology, 43 (2006), 31–40. Blackwell Publishing Inc. Printed in the USA. Copyright r 2006 Society for Psychophysiological Research DOI: 10.1111/j.1469-8986.2006.00379.x 31