A new method for dealing with the stimulus artefact in electrically evoked compound action potential measurements

OBJECTIVE: Residual charge on recording electrodes leads to elevated potentials after the end of the stimulus, which can easily overload the electrically evoked compound action potential (eCAP) recording systems (neural response imaging or neural response telemetry). A new method for dealing with this problem was tested in a series of animal experiments. MATERIAL AND METHODS: We developed an amplifier with a compensation circuit that reduces the effect of the residual charge by electrical subtraction at the input. Using this amplifier we compared different artefact rejection protocols simultaneously in chronically implanted guinea pigs. A new, systematic nomenclature for the various forward masking schemes, based on the number of frames involved, is proposed. RESULTS: Proper adjustment of the compensation circuit reduces the overload time from > 200 micros to < 30 micros, but the compensation signals influence the final output signal considerably. To eliminate this deliberately introduced, reproducible artefact, an additional artefact rejection scheme is necessary. With alternating polarity (AP) and forward masking paradigms we could reliably record the N1 peak. Forward masking responses reveal shorter latencies for cathodic-first biphasic stimuli than for anodic-first pulses. The average of these two closely resembles the response obtained with the AP paradigm. CONCLUSIONS: It is worthwhile implementing the electrical compensation method proposed herein in clinical neural response imaging or neural response telemetry systems, as it represents a more robust way of assessing the eCAP.