Journal Club: Positive-first or negative-first: stimulus type for cochlear implants and its effectiveness in healthy and injured cochlea.

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Today's journal article

Konerding W, Arenberg J, Kral A, Baumhoff P. Cochlear health alters the polarity effect and spike-initiation sites in guinea pigs. 

Why I picked this article

Cochlear implants are medical devices for those with severe-to-profound hearing loss. An implanted electrode directly stimulates the auditory nerve, restoring perception of sound. Cochlear implants deliver tiny electrical pulses to activate the auditory nerve. Cochlear implants deliver tiny electrical pulses to activate the auditory nerve. As they do, perceived loudness from a cochlear implant depends on the amount of electrical pulse delivered. 

Manufacturers have developed cochlear implants to maximise the dynamic range of loudness without compromising safety. Most cochlear implants are monopolar, where a pulse is sent from electrodes in the cochlea and flows towards the reference electrode outside the cochlea. Cochlear implants send two pulses of opposite direction, either as anodic-leading pulses (anodic-first = the first half of the biphasic pulse is positive) or cathodic-leading pulses (cathodic-first = the first half is negative).

Clinically, many listeners report that anodic-leading pulses feel louder than cathodic-leading pulses. One idea is that cathodic pulses trigger spikes in the peripheral dendrites of spiral ganglion neurons, while anodic pulses trigger spikes more centrally along the axon. If the peripheral dendrites are damaged, anodic-leading pulses might work better. 

This study uses an animal model to test the above idea by artificially inducing lesions and testing which polarity of the cochlear implants works the best in the model. 

Some of the research findings

Animal model: 
  • Species: guinea pig; cochlear implant adapted for guinea pig anatomy.
    • Acute lesion group: mechanical lesion of the spiral ganglion region, approximately 400 μm in diameter (n = 18 cochleae). 
    • A needle electrode (TECA elite needle electrode 75 mm × 26 G, Natus Neurology, Galway, Ireland) with a sharp, tapered tip was inserted into the area where spiral ganglion neurons were to injure neurons. 
    • Chronic degeneration group: same lesion with a 9-day degeneration period before testing (n = 13 cochleae) to mimic human-like neural loss. 
  • Controls: intact ears (n = 20 cochleae).

Figure 2. Showing cochlear implant electrodes (1-6) and the injury site (lesion, red dot). Konerding et al. 2025. 

Electrically evoked compound action potential (eCAP): 
  • eCAP records response from auditory neurons. 
  • The eCAP recording showed a larger response to cathodic stimulation than to anodic stimulation. 
  • Across all six electrode contact positions (1-6), cathodic stimulation had higher amplitude, dynamic range, and longer latencies than the anodic stimulation. 
  • There was no effect from the lesion type, whether chronic or acute. 
  • However, after re-examining if the lesion had affected the dendrite lesion (of the spiral ganglion neurons or the body of the spiral ganglion neurons, and focused on the cell body injury, chronic cell body (somata) degeneration led to an elevated threshold for cathodic-leading stimulation, while an acute damage at the cell body led to prolonged latencies to anodic-leading stimulation.
Cathodic stimulation was better for healthy auditory neurons. However, depending on the injury type, where excitation for auditory neurons starts may be different, contributing to different sensitivities to cathodic or anodic stimulation. 

Haruna's takeaway

The implication suggested by researchers of this study is that, the loss of effectiveness of cathodic-leading stimulation with damage to auditory nerve is something we should consider. 

This is a really interesting article, as I have not thought about the polarity of the electrodes or the injury type affecting the excitability of spiral ganglion neurons. The guinea pig cochlea is, of course, much smaller in terms of the cochlear diameter compared to the human cochlea. With the human cochlea, everything will be slightly larger and spread out, e.g. the distance between the cell body and any potential injury sites for dendrite/axon of auditory nerves will be further apart. I wonder if the difference between cathodic and anodic current may be more profound in a larger cochlea? 

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This is Haruna's 50/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology. ***50/100 blog milestone!***