Question
I have read that temporary threshold shifts (TTS) are a common occurrence with hearing aid users, not just those exposed to acoustic trauma. Of course, if this true, user volume controls would be needed far more in hearing aids than is widely believed in the hearing aid industry. Can you provide some additional information and/or references on this topic?
Answer
The question of whether human hearing thresholds are static or dynamic should be of utmost interest to an industry that strives to accommodate the sensory deficit of hearing with amplification.
On the matter of auditory fatigue, most of the literature focuses on the effects of the noise trauma aspect of temporary threshold shifts (TTS). However, we know that there exists also time-sensitive TTS in normal hearing, though slight in comparison. The gross physiological model of that might be comparable to such phenomena as the fact that we are all about a 1/2"-1'' shorter at night than we are in the morning when we rise after a good night's sleep. Also, our sense of vision is not as acute at the end of a day as it is at the beginning.
Indeed, many hearing impaired patients with severe loudness growth abnormalities report that when they take their hearing instruments off at night, leaving the VC in its position, that upon waking and placing their instruments back onto to their ears, that the amplification is far louder than is comfortable. In those cases, we may safely assume that we are looking at several difference limens (DLs).
But the model that supports these empirical observations, and more precisely the underlying theories of psychoacoustics, go back at least to Georg von Bekesy (1960/1989) in his Experiments of Hearing when he was devising measures for his "Just-noticeable Differences of Amplitude and Frequency". For as he delved into the minutiae of human hearing at the single decibel level, he kept running in the pesky little problem of changing hearing thresholds---discovering along the way that human hearing thresholds are not as constant as originally supposed.
Stephens and Davis (1938/1983) even earlier ran into a similar dilemma when describing "Auditory Masking, Fatigue and Persistence" and "Microphonics Action of the Cochlea". Though their text is long out of print, you may find access to it at asa.aip.org/books/hearing.html. Another reference (Boston University, 2001), on the neurophysiology of the fatigue aspect may be found at www.bu.edu/neuro/departments/dpt_be.html. Finally, Roland (2004), in his paper titled "Inner Ear, Noise-Induced Hearing Loss" does an outstanding job of stepping us through the TTS question, and ably differentiates between acoustic trauma and normal aging effects.
If you've not already read my synopsis on the importance of regarding the changeable and dynamic qualities of threshold shifts in hearing aid users and the need for them to be able to adjust hearing aid volume during the course of the day, I refer you to "User Volume Controls: Fit(tings) to be tied!" (Chartrand, 2003).
References
Boston University, (2001). Neuroscience Within: The Biomedical Engineering Department. Retrieved on September 20, 2005, from www.bu.edu/neuro/departments/dpt_be.html.
Chartrand, M.S. (2003). User Volume Controls: Fit(tings) to be tied! Retrieved on September 20, 2005, from www.audiologyonline.com/articles/pf_arc_disp.asp?article_id=410.
Roland, P. (2004, October 28). Inner Ear, Noise-Induced Hearing Loss. Retrieved on September 20, 2005, from www.emedicine.com/ent/topic723.htm.
Stevens, S.S., and Hallowell, D. (1938/1983). Hearing, Its Psychology and Physiology. Harvard University Press, asa.aip.org/books/hearing.html.
von Bekesy, G. (1960, 1989). Experiments of Hearing. Out of print, accessed through asa.aip.org/books/experiments.html.
Max Stanley Chartrand, DigiCare Hearing Research & Rehabilitation, has served in various capacities within the hearing health industry for almost 3 decades, and today serves as Managing Director for DigiCare Hearing Research & Rehabilitation, Colorado City, CO. He lectures and publishes widely on hearing health topics, and is also profoundly deaf and a cochlear implant user. He may be contacted at www.digicare.org/.