The naked truth: the face and body sensitive N170 response is enhanced for nude bodies

Abstract

Recent event-related potential studies have shown that the occipitotemporal N170 component--best known for its sensitivity to faces--is also sensitive to perception of human bodies. Considering that in the timescale of evolution clothing is a relatively new invention that hides the bodily features relevant for sexual selection and arousal, we investigated whether the early N170 brain response would be enhanced to nude over clothed bodies. In two experiments, we measured N170 responses to nude bodies, bodies wearing swimsuits, clothed bodies, faces, and control stimuli (cars). We found that the N170 amplitude was larger to opposite and same-sex nude vs. clothed bodies. Moreover, the N170 amplitude increased linearly as the amount of clothing decreased from full clothing via swimsuits to nude bodies. Strikingly, the N170 response to nude bodies was even greater than that to faces, and the N170 amplitude to bodies was independent of whether the face of the bodies was visible or not. All human stimuli evoked greater N170 responses than did the control stimulus. Autonomic measurements and self-evaluations showed that nude bodies were affectively more arousing compared to the other stimulus categories. We conclude that the early visual processing of human bodies is sensitive to the visibility of the sex-related features of human bodies and that the visual processing of other people's nude bodies is enhanced in the brain. This enhancement is likely to reflect affective arousal elicited by nude bodies. Such facilitated visual processing of other people's nude bodies is possibly beneficial in identifying potential mating partners and competitors, and for triggering sexual behavior.

Figure 1. Illustration of the stimuli used in Experiments 1 and 2.

In Experiment 1, pictures of nude bodies (a), bodies wearing swimsuits (b), faces (d), and cars (e) were used. In Experiment 2, pictures of fully clothed bodies (c) were presented instead of bodies wearing swimsuits. In Experiment 1, the heads of the bodies were masked by pixellation. In Experiment 2, the bodies were presented with both intact and masked heads, and the face and car stimuli were presented in intact and masked forms. In both experiments, the face and body pictures were color photographs of human models. The images shown here do not depict the actual stimuli but are intended only as examples.

Figure 2. Brain responses to different stimuli in Experiment 1.

(a) ERPs from occipitotemporal channels T5 (left) and T6 (right) to cars, faces, nude bodies, and bodies in swimsuits. The responses are averaged across stimulus sex. (b) Scalp topographies of the mean voltage amplitudes for different stimulus types in five consecutive 20-ms time windows starting at 100 ms post-stimulus.

Figure 3. Histograms (mean + s.e.m.) showing the amplitude of the N170 response to different stimuli in Experiment 1.

Amplitudes are averaged across recording channel and stimulus sex. (**P<.01; ***P<.001).

Figure 4. Brain responses to different stimuli in Experiment 2.

(a) ERPs from occipitotemporal channels T5 (left) and T6 (right) to cars, faces, nude bodies, and clothed bodies. The responses are averaged across stimulus and participant's sex. (b) Scalp topographies of the mean voltage amplitudes for different stimulus types in five consecutive 20-ms time windows starting at 100 ms post-stimulus. For nude and clothed bodies, the data are from the trials with masked head.

Figure 5. Histograms (mean + s.e.m.) showing the amplitude of the N170 responses in Experiment 2.

Amplitudes are shown to intact and masked versions of different stimuli, averaged across the recording channel, stimulus sex, and participant's sex.

Figure 6. The N170 amplitude modulation as a function of the degree of clothing.

The graph illustrates the N170 amplitude difference (mean + s.e.m.) calculated between each human stimulus category and cars in Experiments 1 and 2. In Experiment 2, data from male participants only (n = 15) were included. The regression line is based on the mean amplitude differences (for faces and nude bodies, averaged across data from Experiments 1 and 2).

Figure 7. Arousal and valence responses.

(a) Skin conductance responses (mean + s.e.m.) to pictures of faces, nude bodies with masked heads, clothed bodies with masked heads, and cars. (*P<.05; ***P<.001). (b) Subjective arousal ratings (mean + s.e.m.) to different stimulus categories. (c) Subjective valence ratings (mean + s.e.m.) to different stimulus categories.