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Expectations and motor reactions related to pain are mainly acquired through personal experiences. Contingent negative variation (CNV) has been shown to be an informative electrophysiological measure of this pain anticipation. Expectations can also arise while observing others in painful conditions. However, it still remains unclear what are the neural correlates of this phenomenon and how the observation of others in pain can subsequently change our personal pain perception as well as our motor reaction to pain. Using CNV as a measure of expectation, this study aims to assess whether expectations formed through observation change the observer's own experience of pain and reaction to pain. A new cooperative task was designed where one participant, the model, received an electrical stimulation while another, the observer, watched the experiment and both were asked to stop the stimulation as fast as possible. Crucially, in a successive session, participants inverted their roles so that models became observers and vice versa. CNV was recorded in both participants simultaneously by means of two synchronized electroencephalograms. Results showed that CNV area did not differ between models and observers and reaction times were significantly faster in observers compared to models. Moreover, observers' pain perception was correlated to models' pain perception as well as to observers' empathy scores. These data show how expectations, perceptions as well as reactions related to pain are crucially affected not only by observation but by personal attitudes toward others and all these changes can be clearly described through CNV.

Background Expectation is an important mechanism underlying placebo response. Here, we analysed expectation of placebo hypoalgesia and nocebo hyperalgesia by using, for the first time, the contingent negative variation (CNV), also known as expectancy wave. Methods Subjects were presented a green or red cue followed by a train of either non painful or painful electrical stimuli, and expected hypoalgesia after the green and hyperalgesia after the red cue. In experiment 1, expectation was reinforced using a conditioning procedure whereby the green and red cues were paired with non painful and painful stimuli, respectively (acquisition). In a second session (test) the intensity of the stimuli was kept constant, regardless of cue. In experiment 2 no conditioning was performed and participants expected an altered pain perception indicated by the visual cues. CNV mean amplitude, time necessary to stop the train of stimuli (reaction time) and pain ratings were measured. Results A difference in pain perception occurred when electrical stimuli followed the presentation of the green cue compared to the red in the test session, whereas reaction times showed no changes. The same difference occurred in the early CNV component, related to cognitive stimulus anticipation, whereas the late CNV component, related to motor preparation, did not change. Moreover, these differences in pain perception and CNV amplitude were less robust in the experiment 2. Conclusion Placebo hypoalgesia and nocebo hyperalgesia differently affect sensory (pain perception) and motor components (pain avoidance) of pain. Furthermore, CNV is an electrophysiological objective measure capable of dissecting these components. Significance Dissection of placebo hypoalgesia, differentiating the sensory component (pain perception) from the motor component (pain avoidance). Study of these components using the contingent negative variation (CNV) as an electrophysiological objective measure.