Channel interactions in patients using the Ineraid multichannel cochlear implant

Electrode interactions were investigated on two totally deaf patients fitted with the Ineraid multichannel cochlear implant. Currents were applied to the most apical electrode (the 'perturbation' electrode) and their effects on psychophysical thresholds on the other electrodes (the 'test' electrodes) of the intracochlear array were studied. Two experimental protocols were used. In experiment I, we used a detection protocol to study how the perception of signals presented on each test electrode was affected by subthreshold, simultaneous or non-simultaneous stimulation of the perturbation electrode. Strong electrode interactions were observed with simultaneous stimulation and monotonically decreased as a function of electrode separation. Electrode interactions were weak with non-simultaneous stimulation. In experiment II, we used a discrimination protocol to study how the perception of signals presented on the test electrode was affected by suprathreshold, non-simultaneous stimulation of the perturbation electrode. Subjects could discriminate stimulation of 'perturbation+test' versus 'perturbation alone' when the level of stimulation on the test electrode was near threshold. These results demonstrate that strong electrode interactions in the Ineraid multichannel cochlear implant system are generated by electrical field summation due to simultaneous stimulation of different electrodes, and that one can reduce electrode interactions by sequential activation of the electrodes. These observations might help to understand basic phenomena underlying recent significant improvements in speech recognition scores when switching from simultaneous to interleaved pulsatile stimulation in patients wearing the same cochlear implant system (Wilson et al., 1991).