What the human brain likes about facial motion

Abstract

Facial motion carries essential information about other people's emotions and intentions. Most previous studies have suggested that facial motion is mainly processed in the superior temporal sulcus (STS), but several recent studies have also shown involvement of ventral temporal face-sensitive regions. Up to now, it is not known whether the increased response to facial motion is due to an increased amount of static information in the stimulus, to the deformation of the face over time, or to increased attentional demands. We presented nonrigidly moving faces and control stimuli to participants performing a demanding task unrelated to the face stimuli. We manipulated the amount of static information by using movies with different frame rates. The fluidity of the motion was manipulated by presenting movies with frames either in the order in which they were recorded or in scrambled order. Results confirm higher activation for moving compared with static faces in STS and under certain conditions in ventral temporal face-sensitive regions. Activation was maximal at a frame rate of 12.5 Hz and smaller for scrambled movies. These results indicate that both the amount of static information and the fluid facial motion per se are important factors for the processing of dynamic faces.

Figure 1.

Examples of the stimuli used in the fMRI experiment: moving face stimuli with frame rates of 25, 12.5, and 5 Hz, all presented either with ordered or scrambled frame order, a static face, and a static phase-scrambled face (from top to bottom). Each stimulus was presented for 1040 ms. Images depict the start of the presentation of a frame; stimuli were shown continuously without a blank interval between frames.

Figure 2.

Participants' ratings on the fluidity (left) and meaning (right) of the face motion as a function of the frame rate of the video stimuli (3, 4, 5, 6, 7, 9, 12.5, and 25 Hz) and the frame order (ordered or scrambled). Error bars represent standard errors of the mean across participants.

Figure 3.

Results of the whole-brain fMRI group analysis: results of 1-tailed t-tests projected on the surface of an inflated standard structural scan. (A) Brain regions showing greater activation in response to the recorded movies of facial motion than in response to static faces; the contrast used was 25 Hz ordered frames > static face. (B) Brain regions showing activation proportional to the frame rate, irrespective of frame order. (C) Brain regions with greater activation in response to stimuli with ordered frames than to stimuli with scrambled frames, irrespective of frame rate. O, stimuli with ordered frames; S, stimuli with scrambled frame order; STS, activation near superior temporal sulcus; STS ant, anterior part of the STS; hMT+/V5, human motion complex; IFG, inferior frontal gyrus; Mid occip, middle occipital gyrus.

Figure 4.

Percent signal change from fixation in individually defined ROIs. For abbreviations, see main text. Error bars represent standard errors of the mean across participants.

Figure 5.

Stimulus frame rates evoking maximum activation in the different ROIs, assessed by the peak of a Gaussian function fitted to the data of each ROI and each participant, separately for the conditions with ordered frames and those with scrambled frame order. Error bars represent standard errors of the mean across participants.