Effects of electrode configuration on threshold functions for electrical stimulation of the cochlea

Psychophysical detection threshold vs frequency functions for sinusoidal electrical stimulation of the deafened cochlea were measured in 18 nonhuman primate subjects. Functions for monopolar or widely-spaced ( > 2.5 mm) bipolar stimulation were lower and usually had steeper slopes than those for more narrowly-spaced ( < 2.0 mm) bipolar stimulation. In 56% of the cases the difference between thresholds for narrowly-spaced bipolar of monopolar stimulation was greater for low frequency stimuli (63 or 100 Hz) than for high frequency stimuli (800 or 1,000 Hz) by 5 dB or more. Two cases were compared in more detail using pulsatile stimuli. For sinusoidal stimuli, one of these cases showed a moderate frequency dependent effect of electrode configuration and the other did not. The case with the frequency dependent effect of electrode configuration for sinusoids also showed a phase-duration dependent effect of electrode configuration for detection of single biphasic pulses: strength-duration curves (detection threshold in decibels vs pulse duration in ms/phase) were steeper for monopolar stimulation than for narrowly-spaced (0.7 mm) bipolar stimulation. This effect was not seen in the case that showed little or no frequency dependence in the effect of electrode configuration for sinusoidal stimuli. Slopes of threshold vs pulse rate functions where pulse duration was held constant at 2 ms/phase were not affected by electrode configuration in either subject.