Head Motion Elicited by Viewing Affective Pictures as Measured by a New LED-Based Technique.

The complex sensory input and motor reflexes that keep body posture and head position aligned are influenced by emotional reactions evoked by visual or auditory stimulation. Several theoretical approaches have emphasized the relevance of motor reactions in emotional response. Emotions are considered as a tendency or predisposition to act that depends on two motivational systems in the brain - the appetitive system, related to approach behaviours, and the defensive system, related to withdrawal or fight-or-flight behaviours. Few studies on emotion have been conducted employing kinematic methods, however. Motion analysis of the head may be a promising method for studying the impact of viewing affective pictures on emotional response. For this purpose, we presented unpleasant, neutral and pleasant affective pictures. Participants were instructed to view the pictures and to remain still. Two light-emitting diodes (LEDs) were attached to the foreheads of participants, and a Wii Remote controller, positioned 25 cm away, detected the position of the LEDs in the medial-lateral and anterior-posterior axes. We found more sway in response to unpleasant pictures. In addition, unpleasant pictures also provoked faster movements than both neutral and pleasant pictures. This response to unpleasant pictures, in contrast to pleasant ones, might reflect the readiness or predisposition to act. Our data also revealed that men moved faster than women, which is in accordance with previous findings related to gender differences.