Cochlear implants have restored hearing to thousands of deaf people, but what about when deafness is caused by a damaged cochlea or nonfunctional auditory nerve? A possible solution is to bypass the cochlea and stimulate the brain directly. Scientists are developing a new technology that uses laser light instead of electricity to stimulate brain cells to restore hearing.
In fact, we hear with the brain. When the ear is not functional, it is possible to restore hearing by using a hearing aid-like device to stimulate the brain directly with pulses of electricity delivered through electrodes implanted in the brain. The electrical pulses mimic the firing pattern of the auditory nerve that is normally generated by different frequencies of sound stimulating the ear. The results are quite limited, however. Patients with auditory brainstem implants can hear sound, but complex sounds like speech can’t be understood clearly.
The problem is that neural circuitry in the auditory region of the brain is extremely intricate and microscopic. Electric shocks spread too broadly through the tissue to stimulate the appropriate neurons individually. Even the finest electrodes are enormous compared to the microscopic size of neurons. Different frequencies of sound are decoded in an organized manner in the brain, much like a keyboard spans from left to right from low to high frequencies. Electrical stimulation applied through electrodes is like trying to play Beethoven’s “Für Elise” with clenched fists rather than finger tips. Not ideal. To create high fidelity hearing, the right neurons in the brain must be stimulated in the right way. Also, bear in mind that unlike piano keys, neurons are not all like. They are as different as the variety of electronic components in a complex computer circuit.
Optogenetics is a technique in which neurons are genetically engineered to fire an electrical impulse when illuminated by light. Protein channels engineered into the neuronal membrane, called opsins, open when they are flashed with the correct color of light. This discharges the voltage across the neuronal membrane, making the neuron fire a neural impulse. Genetic engineering permits the opsins to be inserted selectively into the desired type of neurons. After inserting opsin proteins into the neurons in the auditory brain region of mice, researchers in the laboratory of Daniel Lee, an MD in the Department of Otology and Laryngology at Harvard Medical School, then pulsed light from a laser beam through a fiber optic cable that was inserted into the mouse’s brain. Taking advantage of the fact that a spot of laser light can be focused into an extremely small spot, scanned with precision to any location desired, and the opsins can be inserted into only the desired type of neurons, the researchers found that this method is a far better way to restore hearing than using electrical stimulation of the brain.
There are considerable hurdles to overcome before this technique of using laser light to restore hearing can be used in people. The opsin genes are inserted into neurons by using viruses that contain the genetic code needed to synthesize opsins in only a specific type of neuron. It will be a while before such a method could be perfected and used in people. However, Ariel Hight, who presented this new research at the Association for Research in Otolaryngology meeting in San Diego, told me that he expects to see this method of “hearing with light” perfected in his lifetime. Hight also happens to hear using cochlear implants in both ears, so he knows his area of research like few others could.
Ariel Hight, Shreya Narasimhan, Xiankai Meng, Albert Edge, M. Christian Brown, Daniel Lee (2016) Optogenetic stimulation of mouse cochlear nucleus in an auditory brainstem implant model using transgenic lines for cell-specific expression of opsins. Poster 253 39th Association for Research in Otolaryngology meeting, San Diego.
Shreya Narasimhan, Ariel Hight, Amelie Guex, Stehanie Lacour, M. Christian Brown, Daniel Lee (2016) Optogenetic stimulation in a mouse ABI model reveals superior access to the tonotopic aaxis compared to electrical stimulation. Poster 254, 39th Association for Research in Otolaryngology meeting, San Diego.
Guo, W., et al., (2015) Hearing the light: neural and perceptual ecoding of optogenetic stimulation in the central auditory pathway. Scientific Reports, 5:10319 DOI: 10.1038/srep10319
Photo from A.E. Hight, et al., (2015) Superior temporal resolution of Chronos versus channelrhodopsin-2 in an optogenetic model of the auditory brainstem implant. Hearing Research, 322: 235-241.