Are inverted faces processed at a later stage?

Abstract

Previous work by Tjan, Lestou, and Kourtzi (2006) suggests that the relation between brain activity of a cortical area and stimulus signal-to-noise ratio is a signature of the degree of feature invariance and feature complexity processed by the cortical area. In a cascade of decision stages, activity associated with later stages will produce a steeper response function for a given range of signal-to-noise ratios. We extend this work into the electrophysiological domain by recording EEG responses to upright and inverted faces presented in various levels of amplitude-matched noise. We plot the activity of N170 component against the signal-to-noise ratio for upright and inverted faces and find that the slope of the function corresponding to inverted faces is steeper than that of upright faces. This suggests that the N170 that corresponds to inverted faces represents activity in later processing stages. This is consistent with a model in which upright faces are processed by an earlier stage, perhaps corresponding to configural or holistic processing, which may explain why inverted faces have a larger and later N170 than upright faces. We explore earlier time points and additional channels to characterize the development of this set of relations.

B. S. Tjan. V. Kourtzi, and Z. (2006). Uncertainty and invariance in the human visual cortex. Journal of Neurophysiology. 96, 1556-1568.