In a planar-array cell structure, an area required by interconnectors is reduced and a fuel cell is made further compact. The access is configured to maintain the low equipment costs and widespread equipment availability of standards-based communication systems. The output frequency of the ALO is controlled by a control signal superimposed on the single cable that is output by the receiver. By alternating stimulation and recording at gradually increasing levels, one can determine the minimum input that creates a neural response, thereby identifying the threshold stimulation level. The frequency response of the receiver is determined from the first combined frequency response and the second combined frequency response.