An electrode cap for obtaining EPM signals from a head of a subject, the cap comprising: (i) an insulating layer (33) for positioning adjacent to the head of the subject; (ii) an electrically conductive layer (29) comprising ionic conduction means, situated above the insulating layer so that in use, it is separated from the head of the subject by the insulating layer; (iii) a plurality of measurement signal electrodes (13) extending through the electrically conductive layer and the insulating layer for contacting the head of the subject, the measurement signal electrodes being electrically insulated from the electrically conductive layer; and (iv) at least one reference node (53, 55) electrically connected to the electrically conductive layer.