The Thatcher illusion reveals orientation dependence in brain regions involved in processing facial expressions

Abstract

Although the processing of facial identity is known to be sensitive to the orientation of the face, it is less clear whether orientation sensitivity extends to the processing of facial expressions. To address this issue, we used functional MRI (fMRI) to measure the neural response to the Thatcher illusion. This illusion involves a local inversion of the eyes and mouth in a smiling face-when the face is upright, the inverted features make it appear grotesque, but when the face is inverted, the inversion is no longer apparent. Using an fMRI-adaptation paradigm, we found a release from adaptation in the superior temporal sulcus-a region directly linked to the processing of facial expressions-when the images were upright and they changed from a normal to a Thatcherized configuration. However, this release from adaptation was not evident when the faces were inverted. These results show that regions involved in processing facial expressions display a pronounced orientation sensitivity.

Keywords: cognitive neuroscience; face perception; facial expressions; facial features; neuroimaging.

Fig. 1

The six identities used in the experiment. Identities were shown in both Thatcherized (top row) and normal (bottom row) configurations. Thatcherization consists of turning the eyes and the mouth upside down relative to the rest of the face. On each trial, stimuli were presented either in an inverted orientation (shown here) or in an upright orientation.

Fig. 2

Stimulus conditions (a, b) and results from the behavioral experiment (c, d). On each trial, a pair of faces with either (a) the same identity or (b) different identities were presented in the upright or inverted orientation. Faces could both be normal (i.e., non-Thatcherized; top row), both be Thatcherized (middle row), or one could be normal and one Thatcherized (bottom row). Participants were asked to report whether the images were completely identical or different in any way. The graphs show the mean percentage of correct responses as a function of face orientation and stimulus condition, separately for (c) same-identity trials and (d) different-identity trials. The border indicates stimuli for which performance was below chance (< 50% correct). Error bars represent standard errors of the mean. The asterisk indicates a significant difference between conditions (**p < .001).

Fig. 3

Functional MRI results. The brain images (a) show the locations of the three face-selective regions defined by an independent localizer scan: the fusiform face area, occipital face area, and superior temporal sulcus. The graphs (b) show the mean percentage signal change as a function of face orientation and stimulus condition, separately for same-identity and different-identity trials. Results are presented for each of the three regions identified in the localizer scan. Error bars represent standard errors of the mean. The asterisks indicate significant differences between conditions (*p < .05). R = right.