Research on the neural mechanisms of face identity constitutes a fruitful method to explore the affective contributions to face processing. Here, we investigated central and peripheral electrophysiological indices associated with the perception of loved faces. Subjects viewed black-and-white photographs of faces that belonged to one of five categories: loved ones, famous people, unknown people, babies, and neutral faces from the Eckman and Friesen system. Subcategories of loved faces included romantic partner, parents, siblings, second-degree relatives, and friends. Pictures were presented in two separate blocks, differing in viewing time (0.5s vs. 4s), inter-stimulus interval (1.2s vs. 18s), and number of face presentations (200 vs. 50). Heart rate, skin conductance, electromyography of the zygomatic muscle, and event-related potentials (ERPs) were obtained while participants passively viewed the pictures. Subjective picture ratings of valence, arousal, and dominance were obtained at the end of the experiment. Both central and peripheral electrophysiological measures differentiated faces of loved ones from all other categories by eliciting higher heart rate, skin conductance, and zygomatic activity, as well as larger amplitudes of the late ERP components P3 and LPP. Loved faces also resulted in higher valence and arousal, but lower dominance ratings. Additional differences were found among subcategories of loved faces. Faces of romantic partners elicited higher physiological (skin conductance and zygomatic activity) and subjective (emotional arousal) responses than parents, siblings, or friends, suggesting that looking at the image of someone we love evokes strong positive affect and emotional/cognitive arousal that go beyond a feeling of familiarity or simple recognition.
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