This is a continuation of last week’s discussion of a genetically mediated biochemical process that may cause mice to be less susceptible to hearing loss. In that blog, I had lauded the research, but had criticized the sensationalism of its coverage. I had stated that there were other well-known features of the cochlea that can be protective. One has been referred to as “auditory toughening.”
Auditory toughening, as the name suggests, is the apparent “toughening” against both temporary (TTS) and permanent (PTS) threshold shift, i.e., hearing loss, from subsequent noise exposure. That is, for a wide range of mammals, pre-exposure to a non-damaging level of noise (e.g., below 85 dBA) can affect the subsequent shift in sensitivity to pure tones when the mammal is subjected to a potentially damaging noise level. It’s as if the auditory system was “toughened” up by the pre-exposure to non-damaging levels of noise. This phenomenon was first noted in 1963 by Miller, Watson, and Covell (Miller, J.D., Watson, C.S., & Covell, W.P., “Deafening effects of noise on the cat”. Acta Otolaryngologica, Suppl. 176, 1-91.), who studied threshold shifts in cats. This phenomenon has been observed for permanent and temporary threshold shifts in mammals and in humans (obviously TTS studies).
Two important experiments were carried out in the early 1990s. One was by Miyakita, Hellstrom Frimansson, & Axelsson (1992) and the other by Subramaniam, Henderson, & Spongr (1991). In the Miyakita et al. experiment, chinchillas were exposed to a 500-Hz pre-exposure narrow band noise that was not damaging. They were then exposed to a potentially damaging level of 500-Hz narrow band noise and the result is that the pre-exposure was protective (i.e., there was toughening). In contrast, the Subramaniam et al. (1991) study used a pre-exposure of a 500-Hz narrow band of noise and then the chinchillas were exposed to a potentially damaging level of 4000-Hz narrow band noise. In this experiment, there was an exacerbation of the pure tone hearing loss. The pre-exposure was anything but protective.
It seemed that in these studies, pre-exposure to the same stimulus that will later be used to create a temporary or permanent hearing loss can be protective, and that pre-exposure to a different stimulus that will later be used to create a hearing loss works the other way.
We do not fully understand the nature of this potentially protective mechanism (in the case of similar spectra) and exacerbation (in the case of different spectra) and here it is, more than 20 years later. One suggestion is that the phenomenon of auditory toughening may be related to various proteins in the outer hair cells, … or not. This reminds us that the cochlea is an extremely complex structure and we are only scratching the surface of how it functions.
And let’s not forget that these experiments were only about changes in pure tone thresholds before and after exposures. Pure tone thresholds are rather blunt measures of cochlear sensitivity. The work of Sharon Kujawa, Charlie Liberman, Jos Eggermont, and others over the years have clearly demonstrated that despite the pure tone thresholds returning to a normal level after a period of time, there can still be significant inner hair cells pathology of the synapse and the dendrite, as well the spiral ganglia.
I would dissuade anyone from trying an experiment on themselves of listening to a quieter version of a rock group’s music before going to a concert and then expecting some “toughening” protection to be apparent during the actual concert. The idea of auditory toughening, at least for now, should be left in the laboratory and not be an element in anyone’s hearing loss protection toolbox.