Controlling for interstimulus perceptual variance abolishes N170 face selectivity

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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.

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Figure 1: Visualizing interstimulus perceptual variance.
Figure 2: Visualization of ISPV in Experiment 1 (left) and 2 (right).
Figure 3: Design of Experiment 3.
Figure 4: Event-related potential results from Experiment 1.
Figure 5: Topographic segmentation analysis and amount of variance explained by maps P1f (faces) and P1c (cars) in the P1 range in Experiment 1.
Figure 6: Event-related potential results from Experiment 2.
Figure 7: Event-related potential results from Experiment 3.


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The authors thank M. Roberts, S. Boehm, D. Linden, S. Tipper, K. Djali, E. Stevens, M. Thoré, C. Willmes and P. Zeller for assistance and comments. Some of the face images used in this work have been provided by the Computer Vision Laboratory, University of Ljubljana, Slovenia. Data were analyzed using “Cartool” (, by D. Brunet, Center for Biomedical Imaging of Geneva and Lausanne, Switzerland). G.T. is funded by the Biotechnology and Biological Science Research Council UK (S18007) and the Economic and Social Research Council (RES-000-23-0095). C.D.M. is funded by the Fyssen Foundation. A.J.P. is funded in part by the Swiss National Foundation for Scientific Research (grant 320000-109928). P.D. is funded by the Biotechnology and Biological Science Research Council UK.

Author information

G.T. conceived Experiments 1 and 3, analyzed ERP data of Experiment 1, supervised overall data analysis and wrote the manuscript. C.D.M. conceived Experiment 2 and analyzed ERP data of Experiment 2 and 3. P.D. performed pixel-wise correlation analyses, provided expertise on neuroanatomical interpretations and wrote parts of the manuscript. A.J.P. performed the segmentation analyses and wrote parts of the manuscript.

Correspondence to Guillaume Thierry.

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