Turning the Face Inversion Effect on Its Head: Violated Expectations of Orientation, Lighting, and Gravity Enhance N170 Amplitudes

Face Inversion Effects (FIEs) – differences in response to upside down faces compared to upright faces – occur for both behavioural and electrophysiological responses when people view face stimuli. In EEG, the inversion of a face is often reported to evoke an enhanced amplitude and delayed latency of the N170 event-related potential. This response has historically been attributed to the indexing of specialised face processing mechanisms within the brain. However, inspection of the literature revealed that while the N170 is consistently delayed to photographed, schematic, Mooney and line drawn face stimuli, only naturally photographed faces enhance the amplitude upon inversion. This raises the possibility that the increased N170 amplitudes to inverted faces may have other origins than the inversion of the face’s structural components. In line with previous research establishing the N170 as a prediction error signal, we hypothesise that the unique N170 amplitude response to inverted photographed faces stems from multiple expectation violations, over and above structural inversion. For instance, rotating an image of a face upside down not only violates the expectation that faces appear upright, but also lifelong priors that illumination comes from above and gravity pulls from below. To test this hypothesis, we recorded EEG whilst participants viewed face stimuli (upright versus inverted), where the faces were illuminated from above versus below, and where the models were photographed upright versus hanging upside down. The N170 amplitudes were found to be modulated by a complex interaction between orientation, lighting and gravity factors, with the amplitudes largest when faces consistently violated all three expectations and smallest when all these factors concurred with expectations. These results confirm our hypothesis that FIEs on N170 amplitudes are driven by a violation of the viewer’s expectations across several parameters that characterise faces, rather than a disruption in the configurational disposition of its features.