Is a neutral face really evaluated as being emotionally neutral?

Most of the functional neuroimaging studies on emotion have used neutral faces as a baseline condition. The aim of the present study was to explore whether prototypical neutral faces are evaluated as displaying neutral emotions. Twenty-one subjects performed the Extrinsic Affective Simon Task (EAST), a validated implicit task that measures the emotional evaluation of target stimuli. All stimuli consisted of two juxtaposed faces from standardized facial pictures. The attribute stimuli (positive vs. negative), which needed to be classified on the basis of extrinsic valence, were presented as black and white facial pictures. The target stimuli were color-filtered positive, negative, neutral, and positive/negative faces, and subjects were instructed to classify them on the basis of the filtered color (blue vs. green). The responses to the positive target faces were associated with the positive emotions and the responses to the negative target faces were associated with the negative emotions. For the neutral faces, the responses were similar to those of negative faces, while for the positive/negative stimuli, the responses were undifferentiated. These findings suggested that prototypical "neutral" faces may be evaluated as negative in some circumstances, which suggests that the inclusion of neutral faces as a baseline condition might introduce an experimental confound in functional neuroimaging studies.