Neuroscience Letters 399 (2006) 215–219 Asymmetrical hemispheric EEG activation evoked by stimulus position during the Simon task Chiara Spironelli a , Mariaelena Tagliabue a , Alessandro Angrilli a,b,* a Department of General Psychology, University of Padova, Via Venezia 8, 35131 Padova, Italy b CNR Institute of Neuroscience, Center on Aging, Padova, Italy Received 12 October 2005; received in revised form 16 December 2005; accepted 2 February 2006 Abstract The Simon effect has been previously shown to be asymmetric at both the behavioral and electrophysiological levels. The present investigation was aimed to clarify whether, during a Simon task, hemispheric asymmetry is also observed in the early phases of stimulus processing. In a group of healthy subjects performing the Simon task, we analyzed scalp potentials evoked by the first lateralized cue (left or right), instead of the classical readiness potential preceding the motor response. ERP results showed a significant left cortical activation to stimuli presented in the right visual field at the 140–160ms time window. Instead, left stimuli elicited a significant activation of the right versus left hemisphere starting at the next 160–180 ms time interval. We linked this asymmetry to that observed in behavioral data: the Simon effect recorded with left stimuli is smaller than the Simon effect recorded with right stimuli. Results confirm the hypothesis that in right handed subjects, left hemisphere is specialized for motor response selection and is able to process right stimuli faster than the right hemisphere does for left stimuli. © 2006 Elsevier Ireland Ltd. All rights reserved. Keywords: Evoked potentials; Attention; Lateralization; Simon task; Hemispheric asymmetry; Handedness In a typical Simon task, participants are asked to press one of two keys in response to a non-spatial attribute (e.g. color) of a later- alized stimulus. Even though stimulus position is not relevant to perform a response selection, reaction times (RTs) are typically faster, and responses are more accurate, when stimulus side and response side do correspond (corresponding S–R conditions) than when they do not correspond (non-corresponding S–R conditions). The RT difference between non-corresponding and corresponding conditions is referred as to the Simon effect (see [6] for a review). A widely accepted explanation of this effect is that in the Simon task, a spatial code is automatically generated for the task-irrelevant stimulus location attribute. The orienting of the attention toward the stimulus position provides the spatial cue which interferes with response selection [6,14]. In line with the premotor theory of attention, in S–R tasks, a motor program is automatically generated every time there is an attentional shift. The motor program contains the final target coordinates of the shift, i.e. the stimulus position [9], thus it might be predicted that a neural correlate/substrate encoding the attention-driven spatial * Corresponding author. Tel.: +39 049 8276692; fax: +39 049 8276600. E-mail address: alessandro.angrilli@unipd.it (A. Angrilli). coordinates is activated in cerebral areas probably devoted to action and motor planning. Indeed, there is converging behav- ioral and psychophysiological evidence that the stimulus spatial code automatically activates the corresponding motor response spatial code [3,4,17]. Another widely accepted assumption is that the Simon effect is a phenomenon related to response selec- tion rather than to stimulus selection [6], and in line with this, neuroimaging evidences have demonstrated a left hemisphere specialization for response selection [2,5,10]. Particularly, the left parietal cortex seems to play a dominant role in the prepa- ration of selected movements, irrespective of the hand involved [11]. The Simon effect, like other response interference phe- nomena (e.g. the Stroop effect), is thought to be very robust and relatively resistant to the influence of practice or strategies. However, it is not a symmetrical effect, as asymmetries in the Simon effect have been shown occasionally in the behavioral measures of the first pioneer studies (e.g. [12,13,16]). Neverthe- less, since in these studies asymmetries were not the main aim of the experiment, such asymmetries have not been discussed. A similar asymmetry has been found also at the cortical level in a previous experiment from our laboratory, in which the influ- ence of learning in spatially compatible tasks on the Simon effect were stronger for right than on left responses [1]. We suppose 0304-3940/$ – see front matter © 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2006.02.013