Monopolar Intracochlear Pulse Trains Selectively Activate the Inferior Colliculus

Abstract

Previous cochlear implant studies using isolated electrical stimulus pulses in animal models have reported that intracochlear monopolar stimulus configurations elicit broad extents of neuronal activation within the central auditory system-much broader than the activation patterns produced by bipolar electrode pairs or acoustic tones. However, psychophysical and speech reception studies that use sustained pulse trains do not show clear performance differences for monopolar versus bipolar configurations. To test whether monopolar intracochlear stimulation can produce selective activation of the inferior colliculus, we measured activation widths along the tonotopic axis of the inferior colliculus for acoustic tones and 1,000-pulse/s electrical pulse trains in guinea pigs and cats. Electrical pulse trains were presented using an array of 6-12 stimulating electrodes distributed longitudinally on a space-filling silicone carrier positioned in the scala tympani of the cochlea. We found that for monopolar, bipolar, and acoustic stimuli, activation widths were significantly narrower for sustained responses than for the transient response to the stimulus onset. Furthermore, monopolar and bipolar stimuli elicited similar activation widths when compared at stimulus levels that produced similar peak spike rates. Surprisingly, we found that in guinea pigs, monopolar and bipolar stimuli produced narrower sustained activation than 60 dB sound pressure level acoustic tones when compared at stimulus levels that produced similar peak spike rates. Therefore, we conclude that intracochlear electrical stimulation using monopolar pulse trains can produce activation patterns that are at least as selective as bipolar or acoustic stimulation.

Simulating the dual-peak excitation pattern produced by bipolar stimulation of a cochlear implant: Effects on speech intelligibility

“…They observed that the patterns produced by both stimuli at levels within a 5-dB range above their respective thresholds had comparable peak amplitudes and spreads of excitation. Similarly, Schoenecker et al (2012) equated their MP and BP stimuli so that they produced the same peak spike rate in inferior colliculus neurons and found similar tonotopic spreads of excitation for both configurations.…”

Early UCSF contributions to the development of multiple-channel cochlear implants

“…It might be noted that later studies conducted at UCSF in a guinea pig model using the same electrode mapping strategy but applying a scala tympani-filling silicon rubber insert carrying the electrodes, again to 'ideally' locate stimulus contacts under the bony modiolus, recorded discrete monopolar stimulation-evoked response tuning that was equivalent to that achieved with bipolar stimulation (Schoenecker et al, 2012). The origin of these sharp differences with our studies in the 1970s and early 1980s is unclear.…”

“…Don Eddington had already noted that interleaved pulsing could be used to minimize it. In studies largely led by Mark White, we had directly documented interference parametrically in both animal and human models (Merzenich et al, 1974b;Shannon, 1983a;White, 1978;Schoenecker et al, 2012;White, 1984;White et al, 1984)dand well understood the conditions for minimizing or avoiding it. Those and other observations led to our development, in 1984-86, of an initial model of an interleaved pulse-processor (IPP) CI model, designed to drive an 8-channel device, operating at rates up to about 600 Hz.…”

Interpulse interval discrimination within and across channels: Comparison of monopolar and tripolar mode of stimulation

“…Smith and Delgutte (2007) reported essentially no difference in spatial selectivity between MP and BP configurations at the level of the inferior colliculus in cats when levels were lower than 5 dB above absolute threshold of neural activation. Also, Schoenecker et al (2012) reported that suprathreshold MP pulse trains can produce activation patterns that are at least as selective as BP pulse trains at the level of the inferior colliculus in cats and guinea pigs. While these two research studies serve to temper the belief that differences between electrode configurations are clearly demonstrated in animal studies, it is difficult to understand why such differences, when reported, are not easily demonstrable in humans.…”