Spatial spread of neural excitation: comparison of compound action potential and forward-masking data in cochlear implant recipients

The feasibility of using the electrically evoked compound action potential (ECAP), measured with the NRT system (Neural Response Telemetry) and the NRT software (version 2.04). to quantity the longitudinal spread of neural excitation was examined in four subjects fitted with the Nucleus CI24M cochlear implant. The ECAP and psychophysical forward-masking profiles were measured using stimulation on each of three electrodes in basal, middle and apical positions. Spatial spread profiles derived front the ECAP measure produced broader functions titan those derived front the psychophysical measure. These results. together with investigation of the change of ECAP spread pattern it with stimulation current, suggest that functions derived front this 'simple' method were more influenced by the spread of electric field from excited neuron to electrode array than by breadth of the neural excitation pattern. The recently released NRT version 3.0 permits the masker and probe Pulses to be delivered to separate electrodes. thus removing a fundamental limitation of version 2.04. Results flout a pilot study, in which this capability was exploited suggest that the Spatial functions front this 'advanced' method may provide a better correlation with results front psychophysical forward masking.