A system comprises a capacitive sensing unit for capturing an electrophysiological signal from a body part. The capacitive sensing unit includes a first electrode plate that forms a capacitor with the body part. Motion of the electrode plate with respect to the body part may be detected by a motion sensitive unit mechanically coupled to the capacitive sensing unit. The motion sensitive unit detects motion by self-mixing interferometry. Then, a processing unit rejects the electrophysiological signal if a great displacement of the electrode plate is detected.