The auditory system of newborn mammals is still developing when a baby mouse – or a baby human – is born. Mice are born deaf and don't start hearing until the twelfth day of life. In a study published Monday in PNAS, Stanford researchers report that new auditory neurons are not only being developed in the first month of life, but continue to develop in juvenile mice.
Whether these results can be applied to our understanding of the hearing process in humans is still speculative at this point, says the senior author of the paper, Stefan Heller, MD, PhD. But they could helpful in understanding how hearing develops in children. Heller wrote in an e-mail:
We all know that the human brain is highly plastic in the first years of life. Babies, toddlers and preschool children learn with a speed that is unprecedentedly efficient and plasticity is certainly highly involved when children acquire for example the ability to speak.
Hearing and the fine tuning of sound perception plays a major role in this. A child that is born deaf will have an incredibly difficult time to learn speech unless the auditory system is stimulated, for example, with a cochlear implant.
A cochlear implant is an electronic device that bypasses damaged portions of the ear and directly stimulates the auditory nerve. Signals generated by the implant are sent by way of the auditory nerve to the brain, which recognizes the signals as sound. Hearing through a cochlear implant is different from normal hearing and takes time to learn or relearn. But it's been shown, that the earlier children with hearing loss receive the implants the better they do at learning language. Heller continues:
We now know that children who receive cochlear implants early in life do very well and on average better than children who receive an implant later in life. The plasticity and the ability to generate new neurons in the auditory brain stem could be one of the underlying reasons of this effect.
I must say that we need to be careful not to be overly speculative because making the leap from a mouse to human in this specific case is difficult. But I am excited about the finding because it provides a logical explanation for auditory pathway plasticity and it extends previous concepts that dealt with neuron survival in the auditory brain stem.
Previously: Stanford chair of otolaryngology discusses future regenerative therapies for hearing loss, Stefan Heller discusses stem cell research on Science Friday and Growing new inner-ear cells: a step toward a cure for deafness