title: The face-responsive M170 is modulated by sensor selection: An example of circularity in the analysis of MEG-data.
abstract: Magnetoencephalography (MEG) experiments create large data sets obtained from many sensors at different locations. Therefore, a process of sensor-selection prior to hypothesis-driven data analysis is common, e.g., when studying the face-selective M170 component occurring at temporal sites around 170ms post-stimulus. However, the strategies to identify sensors of interest vary across investigations, and frequently the contrast used for sensor-selection is not independent from the contrast between experimental conditions.
  
  
  
  We re-analyzed data from a previously published MEG-experiment where participants viewed faces of either a loved person or two friends. We included different strategies for identifying face-responsive sensors based on all or each one of the face-categories before comparing M170 amplitudes across conditions.
  
  
  
  When sensor-selection was based on signal strength elicited by one experimental condition alone, a comparison across face-categories revealed significantly increased M170 amplitudes for the respective face category. Conducting the same analysis following sensor-selection based on averaged activity across all face-categories did not yield different M170 amplitudes.
  
  
  
  Whereas this pitfall of selective analysis has been studied and discussed in detail for fMRI methods there is no comparable re-analysis of real EEG/MEG-data.
  
  
  
  Our results demonstrate that selection-bias is as relevant for EEG/MEG analysis as for fMRI methods. Sensor-selection must be independent from the contrast analyzed with statistical comparisons, because otherwise a distorted or 'circular' analysis might result.
authors.affiliation: Department of Neurology, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Hindenburgdamm 30, 12203 Berlin, Germany; Institute of Sexology and Sexual Medicine, Charité Center for Health and Human Sciences, Charité-Universitätsmedizin Berlin, Luisenstr. 57, 10117 Berlin, Germany. Electronic address: hannes.tiedt@charite.de.
authors.forename: Hannes O
authors.initials: HO
authors.lastname: Tiedt
authors.affiliation: Department of Neurology, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Hindenburgdamm 30, 12203 Berlin, Germany. Electronic address: andreas.lueschow@charite.de.
authors.forename: Andreas
authors.initials: A
authors.lastname: Lueschow
authors.affiliation: Institute of Sexology and Sexual Medicine, Charité Center for Health and Human Sciences, Charité-Universitätsmedizin Berlin, Luisenstr. 57, 10117 Berlin, Germany. Electronic address: alfred.pauls@charite.de.
authors.forename: Alfred
authors.initials: A
authors.lastname: Pauls
authors.affiliation: Department of Neurology, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Hindenburgdamm 30, 12203 Berlin, Germany. Electronic address: joachim.weber@charite.de.
authors.forename: Joachim E
authors.initials: JE
authors.lastname: Weber
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005145:Face; D000066499:Facial Recognition / Q000502:physiology*; D005260:Female; D006801:Humans; D015225:Magnetoencephalography / Q000295:instrumentation* / Q000379:methods*; D008297:Male; D009483:Neuropsychological Tests; D010775:Photic Stimulation; D012815:Signal Processing, Computer-Assisted; D055815:Young Adult
keywords: Circular analysis; Circularity; Face recognition; M170; Magnetoencephalography; Selection bias