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 quantify 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 from the ECAP measure produced broader functions than those derived from the psychophysical measure. These results, together with investigation of the change of ECAP spread pattern with stimulation current, suggest that functions derived from 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 from a pilot study, in which this capability was exploited, suggest that the spatial functions from this 'advanced' method may provide a better correlation with results from psychophysical forward masking. Sumario Se examino´ la factibilidad de utilizar el potencial de accio´n compuesto evocado ele´ctricamente (ECAP), medido con el sistema de NRT (telemetri´a de respuesta neural) y el software NRT (versio´n 2.04), para cuantificar la propagacio´n longitudinal de la excitacio´n neural en cuatro sujetos con implante Nucleus C124M. Se midio´ el ECAP y los perfiles psicofisiolo´gicos de enmascaramiento antero´grado utilizando estimulacio´n separada en tres electrodos: basal, medio y apical. Los perfiles de propagacio´n espacial derivados de la medicio´n ECAP produjeron funciones ma´s amplias que las derivadas de las mediciones psicofisiolo´gicas. Estos resultados, junto con las investigaciones del cambio en el patro´n de expansio´n del ECAP con la estimulacio´n actual, sugieren que las funciones derivadas de este "sencillo" me´todo estuvieron ma´s influidas por la expansio´n del campo ele´ctrico por neuronas excitadas hacia el electrodo que por la amplitud del patro´n de la excitacio´n neural. La ma´s reciente versio´n 3.0 de NRT permite que los pulsos del enmascarador y la sonda sean enviados a diferentes electrodos y con ello se supera la limitacio´n fundamental de la versio´n 2.04. Los resultados