title: Separable effects of inversion and contrast-reversal on face detection thresholds and response functions: a sweep VEP study.
abstract: The human brain rapidly detects faces in the visual environment. We recently presented a sweep visual evoked potential approach to objectively define face detection thresholds as well as suprathreshold response functions (Ales, Farzin, Rossion, & Norcia, 2012). Here we determined these parameters are affected by orientation (upright vs. inverted) and contrast polarity (positive vs. negative), two manipulations that disproportionately disrupt the perception of faces relative to other object categories. Face stimuli parametrically increased in visibility through phase-descrambling while alternating with scrambled images at a fixed presentation rate of 3 Hz (6 images/s). The power spectrum and mean luminance of all stimuli were equalized. As a face gradually emerged during a stimulation sequence, EEG responses at 3 Hz appeared at ≈35% phase coherence over right occipito-temporal channels, replicating previous observations. With inversion and contrast-reversal, the 3-Hz amplitude decreased by ≈20%-50% and the face detection threshold increased by ≈30%-60% coherence. Furthermore, while the 3-Hz response emerged abruptly and saturated quickly for normal faces, suggesting a categorical neural response, the response profile for inverted and negative polarity faces was shallower and more linear, indicating gradual and continuously increasing activation of the underlying neural population. These findings demonstrate that inversion and contrast-reversal increase the threshold and modulate the suprathreshold response function of face detection.
authors.affiliation: Psychological Sciences Research Institute and Institute of Neuroscience, University of Louvain, Belgium.
authors.forename: Joan
authors.initials: J
authors.lastname: Liu-Shuang
authors.affiliation: School of Psychology and Neuroscience, University of St Andrews, St Andrews, UK.
authors.forename: Justin
authors.initials: J
authors.lastname: Ales
authors.affiliation: Psychological Sciences Research Institute and Institute of Neuroscience, University of Louvain, Belgium.
authors.forename: Bruno
authors.initials: B
authors.lastname: Rossion
authors.affiliation: Department of Psychology, Stanford University, Stanford, CA, USA.
authors.forename: Anthony M
authors.initials: AM
authors.lastname: Norcia
mesh_terms: D000328:Adult; D001921:Brain / Q000502:physiology; D015350:Contrast Sensitivity / Q000502:physiology*; D004569:Electroencephalography; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005145:Face; D005260:Female; D006801:Humans; D008297:Male; D009949:Orientation / Q000502:physiology; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010775:Photic Stimulation / Q000379:methods; D012684:Sensory Thresholds; D055815:Young Adult
keywords: contrast polarity reversal; face detection; face inversion; parametric image variation; sweep SSVEP