Efference copy is necessary for the attenuation of self-generated touch

A self-generated touch feels less intense than an external touch of the exact same intensity. According to a prevalent computational theory of motor control, this attenuation occurs because the brain uses internal forward models to predict the somatosensory consequences of our movements using a copy of the motor command, i.e., the efference copy. These tactile predictions are then used to suppress the perceived intensity of the actual tactile feedback. Despite being highly influential, the core assumption of theory has never been tested; that is, whether the efference copy is necessary for somatosensory attenuation. A possible alternative hypothesis is that a predictable contact of two of ones own body parts is sufficient. Using a psychophysical task, we quantified the attenuation of touch applied on the participants left index finger when the touch was triggered by the active or passive movement of the participants right index finger and when it was externally generated in the absence of any movement. We observed somatosensory attenuation only when the touch was triggered by the voluntary movement of the participants finger. In contrast, during the passive movement, the intensity of the touch was perceived to be as strong as when the touch was externally triggered. In both active and passive movement conditions, the participants showed the same discrimination capacity. Electromyographic recordings confirmed minimal activity of the right hand during the passive movement. Together, our results suggest that the efference copy is necessary for computing the somatosensory predictions that produce the attenuation of self-generated touch.