I found this particularly interesting from the study:
"Those subjects with elevated EHF thresholds also had broader psychophysical tuning curves at 2 kHz. This suggests that hearing loss in the EHF region removes or at least reduces information vital to speech recognition tasks and introduces distortion in the mid- frequency region deemed crucial for speech intelligibility."
Is this suggesting that losses in the extended high frequency caused the brain to change the tuning of the OHC in the cochlea at lower-frequency ranges to compensate for the losses? As if to imply the brain is trying to "stretch" the remaining hair cells to compensate for losses?
In doing so, I could see how distortions and hyperacusis might be present as the brain is over-utilizing surviving damaged structures.
This is a good question, so I decided to investigate a bit more.
I think the first thing to note is that there is a difference between psychophysical tuning curves and frequency threshold curves - the reason for making this distinction will become more obvious down below. The difference, as far as I understand it, is that psychophysical tuning curves are
estimated through behavioural responses in subjects, whereas frequency threshold curves, at least to my understanding, are
measured through obtaining the different action potentials in OHC neurons at given thresholds and frequencies, typically by placing an electrode on the neuron. It's important to note that the latter can only be typically done ex vivo (perhaps with the exception of a zebrafish, as its OHCs are on its skin), hence why we resort to psychophysical tuning curves for live subjects. But the pertinent point is that psychophysical tuning curves may not always be representative of what's happening at a peripheral level in the cochlea, because our measurements have to go through an extra point in the signal chain: the brain.
Setting the above to the side for a moment, it's not immediately obvious to me how the OHC neuron can "stretch" its responsiveness across different frequencies, because this would mean changing the OHC action potential response at those given frequencies, which is a very physiological/mechanical feature as far as I understand it. Having said this, we know the brain can do some amazing things, and one of the quotes further down below would suggest your thesis is possible.
Before I come on to that, I should say that it wouldn't surprise me if action potentials/synaptic capacitance changed merely in response to exposure from noise, either due to inflammation or some other peripheral mechanism. In fact, the
Baldri et al. study, which frequency paraphased, does seem to allude to this:
"impaired peripheral mechanisms ... are thought to influence the characteristics of the auditory filter estimated psychophysically".
This would seem to be a more obvious pathway for how an OHC neuron can change/stretch its tuning curve, because the neuron is now receiving information from an OHC that has changed on a structural level.
What would seem more obvious to me is that the brain could, in response to EHF loss, adjust its own interpretation of peripheral information once received at a central level. With this in mind, the
Badri et al. study gave the following as an explanation for the broader psychophysical tuning curve response at 2 kHz. The interesting thing to note is that it alludes to both possibilities: the brain "stretching" the OHC response, as you put it, but also the brain changing the peripheral information at a central level:
"Another possibility is that the broader filters in this population result from an impairment in central processes themselves or their influences on the periphery. The idea that impairments in central processes may contribute to altered tuning, even when the periphery appears to be normal, gains some support from a report by Sanes and Constantine-Paton (1985). They showed that frequency tuning curves in the inferior colliculus of mice who were raised in an environment of continuous clicks were wider than those of controls, especially on the high side, even though the two groups had identical cochlear thresholds."
What implications this all may have for those of us with hyperacusis is something I still need to get my head around.