Inversion and contrast-reversal effects on face processing assessed by MEG.

The processing of upright, inverted and contrast-reversed faces was investigated using MEG. Peak and global field power analyses revealed that the M100, M170 and M220 components were delayed for inverted and contrast-reversed compared to normal upright faces but no amplitude modulations were found. Source analyses using an event-related SAM beamformer technique revealed bilateral occipital sources for the M100 and M220 components. For the M170, two distinct sources simultaneously active were found, a bilateral and posterior source (M170A) and a right lateralized ventral and more anterior source (M170B) around the fusiform gyrus. None of the sources varied in location or intensity between face types. However, although different from the M100, the location of the M170A was not significantly different from that of the M220, suggesting the latter could be a reactivation of the former. Confirming previous ERP results on the processing of inverted faces, the present study extends the findings to contrast-reversed face stimuli and suggests that deviations from the standard upright face format do not activate extra areas but simply result in the delayed activation of the sources generating the M100, M170 and M220 components. The data confirm the sensitivity of the M100 to face manipulations and further suggest that the M170 is generated by two distinct sources, one of which situated in occipital extrastriate areas (M170A) could be reactivated around 220 ms to generate the M220 component.