Conference on Implantable Auditory Prostheses location:Lake Tahoe (CA) date:12-17 July 2009
Commercially available cochlear implants (CIs) stimulate the auditory nerve (AN) using symmetric biphasic current (BP) pulses. However, recent data have shown that the use of asymmetric pulse shapes could be beneficial in terms of reducing power consumption, increasing dynamic range and limiting channel interactions. In these charge-balanced stimuli, the effectiveness of one phase (one polarity) is reduced by making it longer and lower in amplitude than the other. For the design of novel CI speech processing strategies using asymmetric pulses, it is particularly relevant to know which of the two polarities (negative/cathodic or positive/anodic) is most effective. Animal research and theoretical modeling suggest that the cathodic phase is the most “effective” one in terms of producing most of the excitation in the auditory nerve. However, recent behavioral (psychophysical) and electro-physiological data obtained from CI users in our lab suggest that the anodic phase is more effective than the cathodic one (Macherey et al., 2008).
The present study tested the generality of the finding that the anodic phase is most effective to different sites along the cochlea. Polarity effects were examined by means of the Electrically Evoked Compound Action Potential (ECAP) obtained by using the masker-probe paradigm, in which a large ECAP is obtained only when both the masker and the probe produce a large neural response. Monopolar stimulation was applied to the apical, middle and basal electrodes of the Advanced Bionics device. The results of all experiments were similar for all electrodes. Experiment 1 used a standard biphasic cathodic-1st masker and showed that the ECAP latency was shorter for anodic-1st than for cathodic-1st probe, respectively, on all electrodes. Experiment 2 used anodic and cathodic maskers with standard biphasic probes. ECAP responses were only obtained when the masker was anodic. Experiment 3 used a standard biphasic cathodic-1st masker and showed that latency was shorter for the anodic-1st than for cathodic-1st probe and it increased when the phase duration of the probe was increased. Experiment 4 used a masker and probe with the same polarity and showed that ECAP responses were only obtained only when the masker and the probe were anodic.