Original Research The effects of cross-hemispheric dorsolateral prefrontal cortex transcranial direct current stimulation (tDCS) on task switching Jorge Leite a, * , Sandra Carvalho a , Felipe Fregni b , Paulo S. Boggio c , Óscar F. Gonçalves a a Neuropsychophysiology Laboratory, CIPsi, School of Psychology (EPsi), University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal b Neuromodulation Laboratory, Spaulding Rehabilitation Hospital, Harvard Medical School, Boston, MA, USA c Social and Cognitive Neuroscience Laboratory and Developmental Disorders Program, Center for Health and Biological Sciences, Mackenzie Presbyterian University, Sao Paulo, Brazil article info Article history: Received 11 February 2012 Received in revised form 21 September 2012 Accepted 14 October 2012 Available online xxx Keywords: Task switching PFC Cross-hemispheric tDCS abstract Background: Task switching, defined as the ability to flexibly switch between tasks in the face of goal shifting, is a central mechanism in cognitive control. Task switching is thought to involve both prefrontal cortex (PFC) and parietal regions. Our previous work has shown that it is possible to modulate set shifting tasks using 1 mA tDCS on both the left dorsolateral prefrontal cortex and the left primary motor area. However, it remains unclear whether the effects of PFC tDCS on task switching are hemisphere-dependent. Objectives: We aimed to test the effects of three types of cross-hemispheric tDCS over the PFC (left anodeeright cathode [LA-RC], left cathodeeright anode [LC-RA] and sham stimulation) on participants’ performance (reaction time) and accuracy (correct responses) in two task-switching paradigms (i.e., letter/digit naming and voweleconsonant/parity tasks). Methods: Sixteen participants received cross-hemispheric tDCS over the PFC in two task-switching paradigms. Results: The results show that cross-hemispheric tDCS over the PFC modulates task-switching ability in both paradigms. Our results were task and hemisphere-specific, such that in the letter/digit naming task, LA-RC tDCS increased switching performance, whereas LC-RA tDCS improved accuracy. On the other hand, in the voweleconsonant/parity task, LA-RC improved accuracy, and decreased switching performance. Conclusions: Our findings confirm the notion that involvement of the PFC on task switching depends critically on laterality, implying the existence of different roles for the left hemisphere and the right hemisphere in task switching. Ó 2012 Elsevier Inc. All rights reserved. Introduction The ability to flexibly switch between tasks when changes in the goal state occur [1] is considered to be a central mechanism in cognitive control and behavior flexibility. Task switching (see [2] for review) has been found to be dependent on a broader frontoparietal brain network [3] with higher parietal activation, particularly in situations with high shift costs [4]. Several neuroimaging studies have also associated prefrontal cortex (PFC) regions with task switching, particularly in situations that require some type of rule shifting (see [5] for a detailed explanation). Frontal regions are thought to be involved in condi- tions requiring the inhibition of a previously acquired prepotent response (i.e., top-down conflict reduction), with the parietal cortex playing a role in the reconfiguration of the stimuluseresponse (S-R) mappings [6]. Patients with a lesioned PFC frequently show impairments in task-switching ability [7]. Taken together, experimental and clinical evidence has shown that the endogenous preparation and the exogenous executive adjustments required in task switching depend on both PFC and parietal regions [8]. Despite this evidence, little is known about the differential contributions of each hemisphere, particularly in the PFC. Our previous work [9] has shown that it is possible to modulate set shifting tasks using “offline” (i.e., the stimulation occurs immediately before the task performance) 1 mA transcranial direct current stimulation (tDCS) on both the dorsolateral prefrontal cortex (DLPFC) and the primary motor area (M1). This study only tested unilateral PFC (left hemisphere) tDCS. However, data on Author’s notes: Special authorship statusdJorge Leite and Sandra Carvalho share co-first authorship. This work was supported by the Portuguese Foundation for Science and Tech- nology with individual grants (SFRH/BD/41484/2007 and SFRH/BD/64355/2009). Conflict of interest: The authors do not have any conflicts of interest, financial or otherwise, to disclose. * Corresponding author. Tel.: þ351 253604220, fax: þ351 253604224. E-mail address: jorgel@psi.uminho.pt (J. Leite). Contents lists available at SciVerse ScienceDirect Brain Stimulation journal homepage: www.brainstimjrnl.com 1935-861X/$ e see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.brs.2012.10.006 Brain Stimulation xxx (2012) 1e8