COGNITIVE NEUROSCIENCE NEUROREPORT 0959-4965 & Lippincott Williams & Wilkins Vol 11 No 9 26 June 2000 1849 New insights into the Stroop effect: a spatio- temporal analysis of electric brain activity Asaid Khateb, 1,2,CA Christoph M. Michel, 2,3 Alan J. Pegna, 1,2 Theodor Landis 4 and Jean-Marie Annoni 1 1 Neuropsychology Unit, 2 Functional Brain Mapping Laboratory, and 4 Department of Neurology, University Hospital, 24 rue Micheli-du-Crest, CH-1211 Geneva 14; 3 Plurifaculty Program of Cognitive Neuroscience, University of Geneva, Geneva, Switzerland CA Corresponding Author Received 7 March 2000; accepted 31 March 2000 Recent clinical and imaging studies suggest the involvement of anterior brain regions in the Stroop effect without providing consensus on the hemisphere being involved. Here, we investigated the dynamics of brain activation during a modi®ed Stroop task using behavioural, event-related potential map series, and source localization analysis. Behavioural analysis showed an increased RT in the interference (IC) as compared to the neutral (NC) and congruence conditions (CC). Map series analysis in these conditions displayed a similar sequence of 10 stable segments. From these, only segment S6, occurring at 300 ms and displaying a dominant right anterior activation, was of increased duration in IC. Furthermore, in IC only, RT was shown to correlate with S6 duration. These results are discussed in terms of increased duration of an attentional process needed to solve the con¯ict. NeuroReport 11:1849± 1855 & 2000 Lippincott Williams & Wilkins. Key words: Brain mapping; Event-related potentials; Selective attention; Source localization; Stroop task; Temporal segmentation INTRODUCTION The Stroop colour-naming task is a classic paradigm [1] used in psychology to test concepts such as interference and automaticity. In this task, subjects who are asked to report the colour in which a word is displayed, are in¯uenced by word meaning even though it is irrelevant to the task. Reaction times and error rate increase in the incongruent condition or the so-called interference condi- tion as, for example, when the word `red' is displayed in the colour blue as opposed to other congruent (the word `blue' displayed in blue) or neutral stimuli (series of Xs or infrequent words such as `helot' displayed in blue, see [2± 5]). Numerous variations of the Stroop task were used and con®rmed the robustness of the interference effect [4,6±8]. Both clinical and functional imaging studies strongly suggest the involvement of frontal brain regions, in parti- cular the anterior cingulate cortex in the Stroop task, supporting the importance of this area in cognitive inter- ference, attention, and response selection [8]. Lesions of left frontal areas have been showed to produce large inter- ference effect in the Stroop task [9]. However, recent stud- ies report that patients with right hemisphere (RH) lesions are more sensitive to the interference condition than pa- tients with left hemisphere (LH) lesions [10,11]. Further- more, while some functional imaging studies report the speci®c activation of the left inferior frontal gyrus [12] in the interference condition, other studies demonstrate the involvement of mainly the right anterior cingulate cortex [3,13]. Since the robust effect of interference is an increase in reaction time, methods that allow to correlate the dura- tion of activity in certain brain regions with this increased time to solve the task might give more direct answers on the speci®city of the involved cortical areas. Event-related brain potential (ERP) recordings combined with recent methods for source localization in the brain offer this possibility by directly assessing brain activity related to the mental chronometry. In previous ERP studies using the Stroop tasks, it has been shown that the inter- ference condition modulated brain activity mainly in the time window between 300 and 600 ms post-stimulus [4,5,7,14]. However, in all of these studies, using either few channels [4,7,14] or multi-channels recordings [5], no attempt was undertaken to determine the neural basis of the Stroop effect. Our aim in this study was to investigate the dynamics of brain activation during a modi®ed (rela- tively simple) Stroop task using ERP map series and source localisation analysis. Based on previous studies using this analysis method [15±18], we expected to ®nd time periods of brain electric ®elds that are speci®c for the interference condition and that source localisation procedures would indicate the brain regions responsible for this activity. MATERIALS AND METHODS Subjects: Ten right-handed (mean  s.d. laterality quoti- ent of 80  27) native French speaking subjects (®ve females and ®ve males, mean age 22.25 years) participated in this experiment. All had normal or corrected-to-normal vision. No history of neurological diseases was noted in any individual and all were medication free. Before the experiment, all subjects gave their written consent.