The idea that the brain is behind this kind of hearing loss opens up a whole new way of treating it. Instead of cranking up the volume with a hearing aid, why not train the brain to process the ear’s signals better? A number of computer programs now promise to enhance the brain’s auditory machinery from the comfort of your home. Curious and a little sceptical, I had to try it.
To understand how such an idea might work, let’s start with what happens to our hearing as we age. We are born with about 11,000 hair cells in each ear. These delicate cells convert sound waves to electrical signals, but they are easily damaged by ageing and excessive noise. Hearing aids and cochlear implants are designed to treat this kind of impairment, and they can be very successful.
But in recent years, Liberman and his colleagues have turned their attention to the next step in sound processing – the auditory nerve fibres that pass the hair cells’ signals to the brain. Their work has revealed that loud noises can cause a shocking amount of damage to these nerves (see “Silent epidemic“). More specifically, it’s the connections, or synapses, between the hair cells and the auditory nerves that are destroyed, a problem known as synaptopathy. The hair cells themselves remain.
Synaptopathy doesn’t show up in audiograms because these only test the effect of hair-cell loss. “The whole domain of hearing assessment has systematically ignored the fact that the brain is a participant,” says Michael Merzenich, a neuroscientist and creator of a brain-training program called BrainHQ. Perversely, the nerve fibres that are most damaged by noise are those we rely on to understand speech in a noisy environment – up to 90 per cent of these synapses can be damaged, yet you can still perform normally in a hearing test.