Controlling for interstimulus perceptual variance abolishes N170 face selectivity Guillaume Thierry 1 , Clara D Martin 1,2 , Paul Downing 1 & Alan J Pegna 3 Establishing when and how the human brain differentiates between object categories is key to understanding visual cognition. Event-related potential (ERP) investigations have led to the consensus that faces selectively elicit a negative wave peaking 170 ms after presentation, the ‘N170’. In such experiments, however, faces are nearly always presented from a full front view, whereas other stimuli are more perceptually variable, leading to uncontrolled interstimulus perceptual variance (ISPV). Here, we compared ERPs elicited by faces, cars and butterflies while—for the first time—controlling ISPV (low or high). Surprisingly, the N170 was sensitive, not to object category, but to ISPV. In addition, we found category effects independent of ISPV 70 ms earlier than has been generally reported. These results demonstrate early ERP category effects in the visual domain, call into question the face selectivity of the N170 and establish ISPV as a critical factor to control in experiments relying on multitrial averaging. In the study of human cognition, one major endeavor is to identify the functional dissociations that reveal how the brain organizes conceptual knowledge. For instance, cognitive neuroscientists have attempted to characterize the neural substrates involved in categorizing objects in the environment. In pursuit of this goal, numerous ERP and, more recently, magnetoencephalography (MEG) studies have investigated the time course of category effects in visual object recognition. These have led to the observation that a specific electrical response systema- tically occurs for faces at a latency of 170 ms after stimulus onset, which is characterized by a vertex positive and bilateral temporal negative deflection 1–3 . A consensus has now been reached regarding the face- specificity of the N170 (refs. 4–6) and its magnetic equivalent, the M170 (refs. 7,8), as no other stimulus category is reported to elicit negativities as pronounced as faces at 170 ms or to share the same scalp topography 5 . The sensitivity of the N170 to faces, however, is not necessarily interpreted as evidence for a face-specific modular system in the human brain, as modulation of the N170 by expertise is found for objects other than faces 9,10 . Furthermore, it has been argued that low-level visual characteristics of the stimuli could account at least in part for some of the differences observed between categories of objects 11 . Because neuroimaging studies of visual face and object recognition generally use full front views of faces contrasted with pictures of other objects presented in a variety of sizes and spatial layouts, category contrasts typically involve implicit comparisons of low and high ISPV conditions (Fig. 1). Unfortunately, as the amount of ISPV involved is virtually never evaluated or reported (see ref. 5 for an exception), it is impossible to know whether differences between experimental condi- tions arise from categorical differences or whether they are merely driven by ISPV. We therefore formulated the hypothesis that the face selectivity of the N170 might be an artifact driven by ISPV differences. To test this hypothesis, we manipulated ISPV as an experimental factor independently of object category: we compared the ERPs elicited by pictures of faces and cars, half of which were highly variable in position, orientation and size, and half of which were systematically centered, full front, and resized to fit in a predefined template (Fig. 2). If the N170 is genuinely face selective, its amplitude should be relatively insensitive to ISPV, but maximally sensitive to object category (Experiment 1). We then tested whether the effects of ISPV found in Experiment 1 could be due to differences in symmetry between high and low ISPV conditions by comparing ERPs elicited by side views of faces and butterflies, half of which had low ISPVand half of which had high ISPV (Experiment 2, Fig. 2). Another aspect of Experiment 2 is that the high ISPV stimuli were generated by manipulating the size, eccentricity and height/width ratio of the stimuli from the low ISPV condition. This enabled us to control for stimulus variance in the stimuli while holding object similarity from a psychological standpoint relatively constant. Finally, we tested whether the effects seen in Experiments 1 and 2 could be due to faces attracting attention more than other objects. We compared the ERPs elicited by pictures of blue and green overlapped faces and cars that were controlled for IPSV (Fig. 3), and instructed participants to detect repetition of images in one color only (Experiment 3). Here we expected differential attentional effects on P1 and N170 amplitude elicited by faces and potentially confounding ISPV modulations in Experiment 1 and 2 to be revealed. RESULTS Controlling ISPV cancels N170 face-selectivity In Experiment 1, visual ERPs recorded in 28 participants displayed a characteristic P1-N1-P2 complex in all experimental conditions Received 21 December 2006; accepted 6 February 2007; published online 4 March 2007; doi:10.1038/nn1864 1 School of Psychology, Brigantia Building, Penrallt Road, University of Wales, Bangor, LL57 2AS, UK. 2 Laboratoire Langage Cerveau et Cognition, Institut des Sciences Cognitives, Universite ´ de Lyon, CNRS, 67, boulevard Pinel, 69675 Bron cedex, France. 3 Laboratory of Experimental Neuropsychology, Neuropsychology Unit, Neurology Clinic, Geneva University Hospital, 24, Rue Micheli du Crest, CH-1211 Geneva 14, Switzerland. Correspondence should be addressed to G.T. (g.thierry@bangor.ac.uk). NATURE NEUROSCIENCE VOLUME 10 [ NUMBER 4 [ APRIL 2007 505 ARTICLES © 2007 Nature Publishing Group http://www.nature.com/natureneuroscience