subjects show reliable ERPs to faces and noise, which differ reli- ably across subjects, for reasons yet to be discovered (Rousselet et al., 2010; Gaspar et al., in press Reliability of ERP and single-trial analyses). Individual differences in early visual processing have been largely ignored in the face literature, and implicitly treated as measurement errors that can be filtered out by averaging data across subjects. Although understanding the average brain is a worthy goal, only the single-trial approach, in conjunction with parametric designs, will allow us to understand brain mecha- nisms and the information content of brain states (Schyns et al., 2009; Schyns, 2010). In the single-trial framework, timing is essential. Indeed, how fast the visual system can discriminate among object categories provides strong constraints on possi- ble computational implementations ( Thorpe and Fabre-Thorpe, 2001; Rousselet et al., 2004; Thorpe, 2009). In particular, the timing of task modulations might help us tease apart periods of mostly bottom-up, stimulus driven activity, from time-windows engaging flexible neuronal populations that might be tuned to certain tasks. Thus, task modulations are key to understand brain mechanisms (Schyns, 1998; Pernet et al., 2007; Schyns et al., 2009). INTRODUCTION Following the first reports of larger scalp responses to faces com- pared to objects (Bötzel and Grüsser, 1989; Jeffreys, 1989; Jeffreys and Tukmachi, 1992; Jeffreys et al., 1992; Seeck and Grüsser, 1992), there have been hundreds of studies on the early event-related potentials (ERPs) to faces and objects. The vast majority of these studies used (i) averaged ERP, (ii) group statistics, and (iii) categori- cal designs. Their findings can be summarized shortly: sometime between 100 and 200 ms after stimulus onset, ERPs to different object categories tend to differ from each other, and faces are most of the time associated with larger N170 peaks than other object categories (Rossion and Jacques, 2008). Recently, several research groups have started to study these early preferential responses to faces in individual subjects (Schyns et al., 2003, 2007; Philiastides and Sajda, 2006; Philiastides et al., 2006; Rousselet et al., 2007a, 2008a,b, 2009, 2010; Smith et al., 2007; Liu et al., 2009; Ratcliff et al., 2009; van Rijsbergen and Schyns, 2009). Individual subjects’ ERPs, show, not surprisingly, system- atic differences between faces and objects consistent with group effects reported so far (Rousselet et al., 2008a). These studies have also revealed inter-subject differences: despite coarse agreement between group and individual subject statistical analyses, individual Modeling single-trial ERP reveals modulation of bottom-up face visual processing by top-down task constraints (in some subjects) Guillaume A. Rousselet 1 *, Carl M. Gaspar 1 , Kacper P. Wieczorek 1 and Cyril R. Pernet 2 1 Centre for Cognitive Neuroimaging, Institute of Neuroscience and Psychology, University of Glasgow, Glasgow, UK 2 Brain Research Imaging Centre, SINAPSE Collaboration, University of Edinburgh, Edinburgh, UK We studied how task constraints modulate the relationship between single-trial event-related potentials (ERPs) and image noise. Thirteen subjects performed two interleaved tasks: on different blocks, they saw the same stimuli, but they discriminated either between two faces or between two colors. Stimuli were two pictures of red or green faces that contained from 10 to 80% of phase noise, with 10% increments. Behavioral accuracy followed a noise dependent sigmoid in the identity task but was high and independent of noise level in the color task. EEG data recorded concurrently were analyzed using a single-trial ANCOVA: we assessed how changes in task constraints modulated ERP noise sensitivity while regressing out the main ERP differences due to identity, color, and task. Single-trial ERP sensitivity to image phase noise started at about 95–110 ms post-stimulus onset. Group analyses showed a significant reduction in noise sensitivity in the color task compared to the identity task from about 140 ms to 300 ms post-stimulus onset. However, statistical analyses in every subject revealed different results: significant task modulation occurred in 8/13 subjects, one showing an increase and seven showing a decrease in noise sensitivity in the color task. Onsets and durations of effects also differed between group and single-trial analyses: at any time point only a maximum of four subjects (31%) showed results consistent with group analyses. We provide detailed results for all 13 subjects, including a shift function analysis that revealed asymmetric task modulations of single-trial ERP distributions. We conclude that, during face processing, bottom-up sensitivity to phase noise can be modulated by top-down task constraints, in a broad window around the P2, at least in some subjects. Keywords: ERP, GLM, single-trial analyses, bootstrap, N170, individual differences, faces, noise Edited by: Paul Sajda, Columbia University, USA Reviewed by: Marios G. Philiastides, University of Nottingham, UK Michael Tangermann, Technischen Universität, Berlin *Correspondence: Guillaume A. Rousselet, Centre for Cognitive Neuroimaging, Institute of Neuroscience and Psychology, College of Medical, Veterinary and Life Sciences, 58 Hillhead street, Glasgow, G12 8QB, UK. e-mail: guillaume.rousselet@glasgow. ac.uk www.frontiersin.org June 2011 | Volume 2 | Article 137 | 1 ORIGINAL RESEARCH ARTICLE published: 23 June 2011 doi: 10.3389/fpsyg.2011.00137