Facial expressions of pain modulate observer's long-latency responses in superior temporal sulcus

The strength of brain responses to others' pain has been shown to depend on the intensity of the observed pain. To investigate the temporal profile of such modulation, we recorded neuromagnetic brain responses of healthy subjects to facial expressions of pain. The subjects observed grayscale photos of the faces of genuine chronic pain patients when the patients were suffering from their ordinary pain (Chronic) and when the patients' pain was transiently intensified (Provoked). The cortical activation sequence during observation of the facial expressions of pain advanced from occipital to temporo-occipital areas, and it differed between Provoked and Chronic pain expressions in the right middle superior temporal sulcus (STS) at 300-500 ms: the responses were about a third stronger for Provoked than Chronic pain faces. Furthermore, the responses to Provoked pain faces were about 40% stronger in the right than the left STS, and they decreased from the first to the second measurement session by one-fourth, whereas no similar decrease in responses was found for Chronic pain faces. Thus, the STS responses to the pain expressions were modulated by the intensity of the observed pain and by stimulus repetition; the location and latency of the responses suggest close similarities between processing of pain and other affective facial expressions.