Anodic stimulation is more effective in human implant users but less effective in animals

Evidence

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. — Dijk, Pim van, Physiology, Psychoacoustics and…, 2268

Animal recordings performed at the level of the auditory nerve usually show longer latencies and lower thresholds for cathodic than for anodic stimulation (e.g., Miller et al. 1999 ). This observation is consistent with theoretical models of extracellular electrical stimulation of the nerve showing that a cathodic current produces a larger depolarization and elicits action potentials more peripherally than an anodic current of the same level (Rattay 1989 ). In past years, studies performed with human CI listeners have shown different results. While at hearing threshold, no consistent difference between polarities has been observed, a large number of studies have reported that anodic stimulation is more effective than cathodic stimulation at suprathreshold levels (Macherey et al. 2006 , 2008 ; Undurraga et al. 2013 ; Carlyon et al. 2013 ). — Dijk, Pim van, Physiology, Psychoacoustics and…, 2273

Categorization

• QUESTION: The mechanism behind the human-animal polarity discrepancy is unresolved; periprocess degeneration was proposed as an explanation but this study’s chronic design failed to confirm it.
• XREF: Connects to the reader’s core interest in mechanisms of electrical vs acoustic hearing; a species-difference in polarity sensitivity that animal models fail to reproduce is methodologically striking.
• corroborated: 2 independent passages support this claim — Dijk, Pim van, Physiology, Psychoacoustics and…, 2268; Dijk, Pim van, Physiology, Psychoacoustics and…, 2273