Figure 6.

Face-detection sweep VEP (adapted from Ales et al., 2012). (a) Six images are presented per second, with every other image being (partially) intact and the others scrambled. Over a 20-s trial, a new level of phase randomization is presented over a series of 20 equal steps of phase coherence. The power spectrum is constant during the stimulation. Populations of neurons coding for faces independent of low-level information should respond exactly 3 times/s, the rate at which partially intact faces are presented. (b) Grand-averaged (N = 10) EEG spectrum elicited by the stimulation depicted in (a). This stimulation leads to a response dominated by 6 Hz (2f) at medial occipital sites (left panel). A 3-Hz (1f) response dominates the recording over right occipitotemporal sites, reflecting face perception (right panel). (c) SSVEP voltage versus stimulus coherence (0% to 100%) at 3 Hz (filled squares). The 3-Hz signal is compared to the activity in the neighboring frequency bins of the spectrum (e.g., 2.5 and 3.5 Hz), which serve as a noise baseline during the stimulus sequence that evolves from a fully phase-scrambled face to a clearly visible face stimulus. Face detection emerges in the EEG at about 35% of phase coherence, before the behavioral report of face detection (arrow). The topographical map is extracted at the level of the blue dotted line (about 35% of phase coherence), indicating the emergence of face detection over right occipitotemporal electrode sites.