A device comprises a coupling configured to couple signals to and from a communications path including at least a part of a human or animal body; a data transmitter coupled to the coupling and configured to transmit, from time to time, a data signal of at least a predetermined temporal duration via the communications path; and a data receiver coupled to the coupling and configured to detect the presence of a signal on the communications path at sets of one or more successive detection instances disposed between successive transmissions of the data signal by the data transmitter, the data receiver being configured so that the successive detection instances of a set are temporally separated by no more than the predetermined temporal duration; the device being configured to initiate a processing operation in response to a detection by the data receiver of the presence of a signal on the communications path.