Capacitive Measurement of Facial Activity Intensity

The measurement of the intensity of facial muscle activity can be used in several applications such as human–computer interaction and behavioural science. A new method for the intensity measurement is presented. It is based on a contactless, capacitive measurement of the movements that the facial activity produces. The muscles responsible for raising the eyebrows, lowering the eyebrows, raising the mouth corners, and pulling down the mouth corners were measured simultaneously with the capacitive method and electromyography (EMG) during controlled experiments. Each muscle was activated by 10 participants at three different intensity levels (low, medium, and high), 10 repetitions at each level. The capacitive intensity values were in good agreement with the ones registered with the EMG: average mean absolute errors were between 7–12% of the observed intensity range. However, compared to the EMG, the capacitive intensity values were noticed to have offsets that may be partly caused by the measurement itself and partly by the EMG reference. As a result, the measurement may require a calibration for more intensity values than just the maximum. In the case of the capacitive method it is also required to distinguish between the muscle activations originating from the same facial regions to determine which activation is taking place. This was done with an almost perfect performance by using hierarchical clustering to cluster the intensity values.