Author:

Keywords:

11 Medical and Health Sciences, 17 Psychology and Cognitive Sciences, Experimental Psychology

Abstract:

In humans, non phase-locked (NPL) brain oscillations in the gamma band (30 - 80 Hz) have been related to the integration of widely distributed cortical representations and are thought to participate in the top-down modulations of visual processes, and more generally in the constructive aspects of perception (Engel et al., 2001). In the present study, we employed the technique of Superstitious Perceptions (Gosselin & Schyns, 2003) to better understand the neurophysiological determinants of the superstitious perception of a face. Using high-density EEG recording, sixteen observers were asked to detect a face while stimulated with only white noise (400 different templates of 32 × 43 pixels, to mirror the aspect ratio of a face). Superstitious perceptions occur whenever the observer believes that a face is present in the white noise. This paradigm is better suited to isolate top-down components of visual processing because white noise is only very weakly correlated with a face signal. The observers detected a face on 36% of the noise templates, on average. When observers reported seeing a face, EEG recordings at posterior-visual scalp regions revealed a NPL increase of gamma (∼40 Hz) oscillation amplitudes centred around 200 ms post-stimulus onset. A classification image rendered the face information that best predicts the superstitious perceptions, and the gamma activity. This image reveals two eyes, bilaterally organized around the X axis, at about 2/3 height on the Y axis. This study shows that NPL gamma oscillations in the human visual system subtend the type of top-down activity required for superstitious visual perceptions.