Effects of Pulse Shape and Polarity on Sensitivity to Cochlear Implant Stimulation: A Chronic Study in Guinea Pigs

Abstract

Most cochlear implants (CIs) stimulate the auditory nerve with trains of symmetric biphasic pulses consisting of two phases of opposite polarity. Animal and human studies have shown that both polarities can elicit neural responses. In human CI listeners, studies have shown that at suprathreshold levels, the anodic phase is more effective than the cathodic phase. In contrast, animal studies usually show the opposite trend. Although the reason for this discrepancy remains unclear, computational modelling results have proposed that the degeneration of the peripheral processes of the neurons could lead to a higher efficiency of anodic stimulation. We tested this hypothesis in ten guinea pigs who were deafened with an injection of sysomycin and implanted with a single ball electrode inserted in the first turn of the cochlea. Animals were tested at regular intervals between 1 week after deafening and up to 1 year for some of them. Our hypothesis was that if the effect of polarity is determined by the presence or absence of peripheral processes, the difference in polarity efficiency should change over time because of a progressive neural degeneration. Stimuli consisted of charge-balanced symmetric and asymmetric pulses allowing us to observe the response to each polarity individually. For all stimuli, the inferior colliculus evoked potential was measured. Results show that the cathodic phase was more effective than the anodic phase and that this remained so even several months after deafening. This suggests that neural degeneration cannot entirely account for the higher efficiency of anodic stimulation observed in human CI listeners.

The Effect of Interphase Gap on Neural Response of the Electrically Stimulated Cochlear Nerve in Children With Cochlear Nerve Deficiency and Children With Normal-Sized Cochlear Nerves

“…One obvious factor is the difference in experimental procedures and study subjects tested in these studies. Anatomical characteristics of the cochlea and neurophysiological responsiveness of the CN in human listeners differ from those of guinea pigs (e.g., Tuncel et al, 2005;Macherey & Cazals, 2016). Other differences include etiology and duration of deafness, rate of neural degeneration, duration of CI use, and CI electrode design and placement.…”

“…Finally, we speculate that the permeability of ion channels of CN fibers reduces in children with CND. It is also possible that the vulnerability of the propagating spike caused by neural degeneration (Tasaki, 1955;Cragg & Thomas, 1964;McDonald & Sears, 1970;Rasminsky & Sears, 1972;Pender & Sears, 1984) is higher in children with CND than in children with normal-sized CN. Both factors lead to a prolonged "vulnerable period".…”

ECAP growth function to increasing pulse amplitude or pulse duration demonstrates large inter-animal variability that is reflected in auditory cortex of the guinea pig

“…Here, the current spread associated with monopolar stimulation was not assessed with techniques such as the spread of excitation ([ 3 ], [ 41 ], [ 42 ], [ 43 ]) because such techniques cannot be used when a small number of electrodes are inserted in the scala tympani. Lastly, “anodic first” stimulations were used to match human studies, which show that humans are more sensitive to the anodic phase of the pulse ([ 44 ], [ 45 ], [ 46 ], [ 47 ], [ 48 ]), even if it has been reported in animals, that responses are more sensitive to the cathodic phase of the stimulation [ 49 ]. We considered that this should not have a major impact on the comparison between pulse amplitude and pulse duration strategies, and this choice should allow us to extrapolate our results to human studies.…”

Lautheitsoptimierte Messung von Summenaktionspotentialen bei Cochlea Implantat Trägern

“…Mögliche Erklärungsansätze finden sich in den Phänomenen des nichtlinearen Zusammenhangs zwischen Stimulusparametern und Perzeption, des Effektes kapazitiver Lasten im Gewebe sowie der unterschiedlichen Wirkung anodischer und kathodischer Stromphasen. Diese Phänomene sind noch nicht voll-ständig verstanden [30]. Die Klärung der kausalen Zusammenhänge für den in Tab.…”