Magnetic stimulation precisely activates the mouse cochlea

For us to perceive any stimulus from the outside environment, whether the colors of a rainbow or the chords of a piano, the sensory stimuli of light or sound waves need to be converted into electrical impulses. Sound waves picked up by our ears are converted to neural impulses by the movement of tiny hair cells within the inner ear and sent to the brain, which interprets them as the sounds we experience. These hair cells are located within a snail-shaped tube in the inner ear called the cochlea. 

People with sensorineural hearing loss have a loss of hair cells in the cochlea. They are helped by cochlear implants (CIs), surgically implanted devices that bypass the hair cells and convert sounds to electrical signals that stimulate the auditory nerve (1). While cochlear implants generally allow for speech comprehension in quiet environments, they falter in performance when background noise levels are high. People with cochlear implants also have difficulty appreciating music (2). A new study published in the journal eLife  reported that magnetically stimulating the mouse cochlea using micro-coils generates more spatially precise activation of the auditory pathways than electrical stimulation (2). 

For hearing to be precise, it is important that spectral resolution, the ability to distinguish between sounds that are similar in frequency, is high. The greater the number of channels of activation within the auditory system, the greater the precision.

“The technique of activating a small and confined area is very crucial for CIs to produce a more precise and diverse sounding noise. This would also lead to better clinical outputs,” said Jae-Ik Lee, a neurosurgery researcher at Harvard Medical School, who led the study.  

To investigate the effect of magnetic stimulation on the mouse cochlea, the authors used mice that were deafened by the antibiotic gentamycin, which damages hair cells. The authors then used simple bends in microwires, which effectively activate neurons when an electric current is passed through them, to magnetically stimulate the cochlea (3).

Auditory signals from the cochlea pass through a region of the midbrain called the inferior colliculus before reaching the auditory cortex in the brain. Because the neurons in the cochlea hide inside its bony walls, it is not possible to record auditory signals from them directly. In addition to being more accessible surgically than the cochlea, the inferior colliculus is tonotopically organized, meaning that neurons are arranged in space based on the frequencies to which they best respond. The authors made use of this tonotopic organization of the inferior colliculus to ascertain how precisely the magnetic stimulation activated the auditory system.

“Higher frequency signals pass through deep inside the inferior colliculus and the lower frequency signals pass through the superficial layer. So, if we measure the response using a multielectrode array (MEA) inserted vertically through the inferior colliculus, we can see which range of frequency was activated,” said Lee. 

The responses recorded by the MEA showed that “the spread of activation from the magnetic stimulation was much more restricted than that from electrical stimulation,” said John Middlebrooks, a neuroscientist at the University of California Irvine, who was not involved in this study, in an email.

The channels of magnetic activation of the cochlea were 60 percent narrower than those generated by electrical stimulation. The dynamic range (DR), the range of sound levels picked up by the coils, was also more than three times larger than that of electrodes, indicating that magnetic stimulation leads to more precise hearing outcomes

While these results are encouraging, elements of the design and performance of the micro-coils will have to be optimized before moving to trials in humans. “There's a strong, good device out there that works well for a lot of people. So, we have a higher bar to prove efficacy with this new technology,” said Shelley Fried, a neurosurgeon at Harvard Medical School and coauthor of the study. 

Middlebrooks agrees. “The high current demand of the magnetic cochlear stimulation raises some serious concerns. First is heating of the magnetic loop and its wires. The long-term effects of even 1°C of localized cochlear heating are not known,” he said. 

The high amplitudes of current flowing through the coil also raises concerns about power consumption and the battery life of potential CIs that might use micro-coil based magnetic stimulation. The authors suggest that making minor modifications to the design of the micro-coil, including switching to higher conductivity materials and incorporating magnetic cores into the coil could significantly reduce power consumption. 

“With present CI devices, the battery does not need to be changed very often, so we can't put in a device that needs a battery change every week or every month. There are a bunch of challenges, but I think the real goal is to assess the potential of this technology in our next round of experiments. Do we really get these narrower spectral channels? I think if that's the case, we'll be able to solve many of these engineering problems down the road,” said Fried.  

References

1. Winn, M. B., Won, J. H. & Moon, I. J. Assessment of spectral and temporal resolution in cochlear implant users using psychoacoustic discrimination and speech cue categorization. Ear & Hearing  37, (2016). 
2. Lee, J.-I. et al. Magnetic stimulation allows focal activation of the mouse cochlea. eLife 11, (2022). 
3. Ryu, S. B. et al. Spatially confined responses of mouse visual cortex to intracortical magnetic stimulation from micro-coils. Journal of Neural Engineering 17, 056036 (2020).
4. Obien, M. E., Deligkaris, K., Bullmann, T., Bakkum, D. J. & Frey, U. Revealing neuronal function through Microelectrode Array Recordings. Frontiers in Neuroscience  8, (2015).