A sensor unit (F) has at least a first and second sensor (10, 12) arranged along a sensor axis (14). A head adapter (16) provides support to mount at least one selected device about a user's cranium (18). A securing module (20) is attached to the sensor unit for mounting the sensor unit (F) to the head adapter (16). The sensor unit (F) is to be mounted above an ocular axis (22) formed between a pair of eyes (24) of the user (U) when the sensor unit (F) is attached to the head adapter element (16). When the sensor unit (F) is secured to the user (U) with the head adapter element (16), the sensor axis (14) is essentially perpendicular to the user's ocular axis (22).