Neuroscience Letters 391 (2005) 68–70 Paradoxical scalp lateralization of the P100 cognitive somatic potential in humans: A magnetic field study Claude Tomberg a,∗ , Hal Weinberg b , Jiri Vrba c , Teresa Tcheung c a Medical Faculty CP 630, Brain Research Unit, Brussels University, 808 route de Lennik, B-1070 Brussels, Belgium b Office of Research Ethics, Simon Fraser University, Burnaby, BC, Canada V5A 156 c VSM MedTech Ltd, Coquitlam, BC, Canada V3K 7B2 Received 2 June 2005; received in revised form 14 July 2005; accepted 17 August 2005 Abstract In somatic selective attention, electrical brain mapping disclosed a P100 cognitive electrogenesis with a scalp field paradoxically lateralized ipsilaterally to the target finger stimulus. We used 64 sensors magnetoencephalography (MEG) and source localization software in six normal humans to identify the P100 neural generator. Calculated dipole sources for P100 were iteratively compared with the recorded MEG data. The equivalent dipole at somatic P100 latency was located in the parietal lobe contralateral to the finger stimulus and had an oblique positive gradient pointing towards the ipsilateral side, thus explaining the paradoxical positivity in electrical brain mapping. It is suggested that the somatic P100 is generated in parietal area 7b which indeed appears to be specialized for cognitive processing within the somatic modality rather than for multimodal association. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Human cognitive brain potentials; MEG dipole; Somatic P100; Selective attention; Brain mapping Neuroscience research is developing a major interest for the neural correlates of conscious behavior. Physiological meth- ods have now identified several cognitive brain neurogeneses in intact humans performing simple perceptual tasks [12]. For example, in somatic selective attention the subject receives a random series of mild electrical stimuli to distinct fingers and is asked to identify each “target” finger stimuli while ignoring intermixed “nontargets” stimuli to other fingers. These stimuli elicit an afferent volley reaching in about 20 ms the primary cor- tical area 3b, just behind the central fissure. The volley elicits a N20 evoked potential component [5] which is not modified by attention or neglect [7] similarly to the primary component to visual stimuli [12]. For attended targets N20 is followed by cog- nitive electrogeneses whereby the sensory input is consciously identified by the subject [7]. Fig. 1 shows P40 generated in pari- etal area 2 [25], followed by parietal P100 and prefrontal N140 [24] after which P300 (P3b) terminates cognitive operations on that input [7,26]. Electrical Brain mapping with 28 scalp electrodes revealed a paradoxical lateralization of P100 to the side ipsilateral to ∗ Corresponding author. Tel.: +32 477 31 48 69; fax: +32 2 538 29 05. E-mail address: ctomberg@ulb.ac.be (C. Tomberg). the target finger (Fig. 1B). This was surprising because sensory inputs from one hand are conveyed to the contralateral postcen- tral cortex [10,13–16]. In order to clarify the actual cognitive processing chain leading from cortical input to output behavior we used magneto-encephalography (MEG) and source localiza- tion software to identify the P100 generator. Experiments were carried out non-invasively in 23 normal adult humans (14 males) in good health, free from neurologi- cal disease and non-addicted to drugs, tobacco or alcohol. Six of these subjects were tested separetely with MEG recording. They had given informed consent. Subjects were selected for lack of excess alpha in the EEG and ability to fully relax with minimum muscle and eyeblinks. They all achieved consistent discrimi- nation of targets from non-targets. Finger stimuli were 0.2 ms electric pulses delivered through ring electrodes at an intensity two times the subjective threshold (defined by the methods of limits). Right index finger stimuli (p = 0.2) were targets to be identified (mental count or key press). Right fifth finger stimuli (p = 0.8) were to be ignored. Stimuli were randomly intermixed in series at 1.2 s intervals. MEG sampling rate was 1.2 kHz with bandpass 0–500 Hz. The seated subjects had their head inserted into a whole cortex MEG sensor system (CTF Inc.) with an array of 64 axial gradiometers in liquid helium, housed in a head-shaped 0304-3940/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2005.08.035