Why is the N170 enhanced for inverted faces? An ERP competition experiment.

The amplitude of the ERP component N170 is larger in response to inverted than to upright faces. The current study tested two main hypotheses that have been suggested to explain this effect: according to the first hypothesis inverted faces lead to augmentation of activity of the same neural populations used for the processing of upright faces due to the increased processing difficulty that these stimuli impose; according to the second hypothesis the processing of inverted faces involves recruitment of additional neural mechanisms to those used for upright face processing. We employed an ERP competition paradigm in which the effect of a context stimulus on the ERP to a simultaneously presented target stimulus is measured. In Experiment 1, an upright target face was paired with an upright face, inverted face or a non-face context stimulus. ERPs time-locked to the presentation of the target showed reduced N170 amplitude when the context was an upright face more than in non-face context trials, replicating the ERP competition effect for faces. Interestingly, in contrast to the hypothesis that inverted faces recruit mechanisms used for upright faces to a greater extent, competition effects were similar in the context of inverted and upright faces. In Experiment 2, the target stimuli were inverted faces. This time, competition effect was larger in inverted than in upright face context. Taken together, these findings correspond with the hypothesis that inversion of the face leads to recruitment of additional mechanisms as early as at the N170 time-window.