This is a continuation of something that is possibly similar to the phenomenon mentioned in part 1 of this blog but I am not sure. In part 1 of this blog series it was shown that hearing loss over time was asymptotic, meaning that over the years, even though hearing loss continued to increase, the increase was less and less. Special care was urged to educate the musician as early as possible in their careers in order to prevent hearing loss from occurring. Alternatively, if a musician is seen for the first time and they have been at it for 20 years, one can calmly state that this same amount of measured hearing decrease will probably not occur in the next 20 years.
I suspect that this is part of a larger phenomenon called “auditory toughening”. I think that it’s worth reviewing what little we do know about this phenomenon.
Auditory toughening, as the name suggests, is that the measured temporary threshold shift (TTS) or permanent threshold shift (PTS) can be less than expected if the test animals are subjected to a non-damaging noise prior to the higher level, potentially damaging exposure. That is, if an animal is exposed to low level stimuli (below the damaging level), then this, in some sense toughens the cochlea, such that higher levels that are known to create measureable threshold changes, do not result in as great a PTS or TTS, as would be found if the low level “toughening” had not occurred first.
The phrase “auditory toughening” is indeed just a metaphor- it’s not as if callouses have been formed in the cochlea but it’s interesting to see retired workers who say, “At first it was bad, but I got used to it and toughed it out”. Between what is known about asymptotic hearing loss and auditory toughening, perhaps workers do “get used to it”.
There are three seminal studies in this area- all about 25 years old now. Although this was first noted in 1963 by Miller and his colleagues working with cats, the work in the early 1990s delineates the features of auditory toughening.
Miyakita, Hellstrom, Frimansson, and Axelsson, in 1992, noted that auditory toughening created PTS in a wide range of mammals and also TTS in teenagers.
The SUNY-at Buffalo folks, under the direction of Dr. Donald Henderson, got in to high gear. Pierre Campo came to Buffalo, New York to work with Dr. Henderson from France. ( A bit of trivia here- Dr. Henderson played professional football in Canada. Good thing for us that he was cut by the British Columbia Lions and traded, or perhaps he would never have earned his PhD).
I met Pierre Campo at the World Congress of Audiology in Brisbane, Australia in 2014 and at first didn’t put two-and-two together. Pierre Campo is known primarily for his excellent work on noise/chemical interaction. Because of this I was more “cool” that when I met Ulf Rosenhall (see part 1 of this blog series), so the security guards did not need to be called. Besides, Pierre Campo is bigger than I am.
Campo, Subramaniam, and Henderson (1991), exposed chinchillas to a non-damaging level of narrow band noise centered at 500 Hz for 10 days. And even after 5 days of complete silence, a smaller PTS was found in chinchillas that were not toughened (by as much as 15 dB). It should be pointed out that the higher level exposure that caused the PTS was also a narrow band of noise centered at 500 Hz. That is, “pre-toughening” of the cochlea was established by exposing the chinchillas to the SAME stimulus that was later used to create a PTS.
A bit later, in the same laboratory, Subramaniam, Henderson, and Spongr (1991) then examined what would happen to chinchillas who were pre-exposed or “toughened” to a narrow band of noise centered at 500 Hz, and then exposed to a (different frequency) higher level of narrow band noise centered at 4000 Hz. In this case, the reverse was found- the toughening actually INCREASED the PTS over that of the non-toughened, or control group. That is, “pre-toughening” of the cochlea worked in the opposite direction by exposing the chinchillas to a DIFFERENT (higher frequency) stimulus that was later used to create a PTS. The chinchillas would have been better off if they were never “pre-exposed” to the 500 Hz stimulus in this experiment.
Two things are obvious here:
- the group of researchers at SUNY-at Buffalo were either amazing researchers, or they were very lucky in running these two experiments side by side. It would have been erroneously “obvious” to many of us that auditory toughening would have the same properties if pre-toughened, and who would have thought that using different stimuli would give such opposing results. I suspect that the SUNY- at Buffalo group were just amazing researchers who didn’t make false “assumptions”.
- Auditory toughening is not something that we can use in our toolbox of strategies to counsel a client about future hearing loss prevention.
There has not been much research in this area in the last 25 years, but auditory toughening is yet one more element that makes prediction of hearing loss from noise (or music) exposure very difficult. This may or may not be the same phenomenon that results in asymptotic hearing loss, although I am leaning towards making that “assumption”… I think…