When good ears go bad

I’ve been attending the 2013 Aquatic Noise conference this week and it is no surprise to me that “out the gate” we were treated to some really important information – although given the aquatic theme of the conference it was a surprise that this information came from a terrestrial hearing specialist. We heard from Dr. Charles Liberman who has been exploring the physiology of the human auditory system whose findings will undoubtedly have far reaching impacts on how we look at damaging noise exposures – both for us humans as well as for marine animals.

The human ear is a spectacular organ that takes molecule-scale pressure fluctuations in our physical environment and converts them to electro-chemical impulses that we perceive as our acoustical world – from the delicate snap of a twig in a dark forest, to the lush acoustical fabric of a jazz quartet, to the strained voice of a friend in the chaos of a noisy restaurant.

The fulcrum of this sound-to-nervous system transaction occurs through the cochlea – an organ about the size of a pea that sorts out all of our frequency and phase relations though an elegant system of membranes and hair cells. One set of these hair cells – the “inner hair cells” are where hearing leaves the realm of gross physics and enters the realm of neurology. Each one of these ~3,500 inner hair cells is caressed by 30 dendrites which are all bundled into the ‘auditory vestibular nerve’ as they head off to our brain.

Cochlea Anatomy, Illustration

Cochlea Anatomy, Illustration: American Physiological Society

The legacy thinking on the “redundancy” of these dendrites was that they were an accommodation for age-related attrition, or a hedge against the damage caused by the occasional exposures to some extra-high level noise that might cause permanent damage to our hearing sensitivity (called “permanent threshold shift” or PTS). This legacy thinking has also included the phrase “temporary threshold shift” or TTS – the “rock concert effect” that has our ears ringing with a “temporary” loss of hearing sensitivity after being exposed to loud noise.

What Dr. Liberman revealed to us today was that while the “threshold shift” may seem temporary, a more subtle damage occurs and that after a TTS event some of the dendrites can die off from the inner hair cell – thinning out the neural connection to our brain. And while some dendrites remain to report acoustical stimulation back to our brain, the inner hair cells lose their ability to report complex stimulation because some of the neural pathways are lost.

This would explain that while my hearing tests fairly well in an audiology lab using pure tones I can still have a hard time understanding what people are saying in crowded restaurants with noisy backgrounds.

This casts a whole different light on “permissible exposure levels” and mitigation thresholds set in ocean noise regulatory settings. I have always argued that testing animals with pure tones in lab settings is a poor proxy for wild animal hearing in natural settings. Dr. Liberman’s work seems to substantiate this idea. By permitting noise exposures that cause “temporary threshold shift” in marine mammals we may be compromising their ability to discriminate the complex sounds of their habitats.