A system comprises wearable apparatus (300, fig.3) for detecting facial muscle 107 activity wherein the wearable apparatus comprises a plurality of optical flow sensors 100 configured to capture time-varying data for detecting facial skin 104 movement. Preferably, the optical flow sensor comprises a light source 205, a light sensor 206, and a processor 207 for processing the output of the light sensor so as to determine relative movement between the sensor 100 and the skin 104. The light source is arranged to illuminate the patch of skin whose movement is to be detected. The light source may be an LED or laser. The light sensor may be a Charge Coupled Device (CCD) or a Complementary MetalOxideSemiconductor (CMOS) sensor. The processor may be configured to track multiple features so as to determine a vector field, whereby a vector descriptive of the net skin movement may be derived from the vector field. In Figure 3 a set of optical flow sensors (301-304) are provided at the frame (307) of a pair of glasses (300).
