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I am wondering whether increasing the volume would result in (a) a neuron that was already firing to now increase its spike rate, (b) a different group of neurons to add their activity to the population total, (c) a different group of neurons, coding for the new volume, to become active while the first group of neurons silences, or (d) some combination of the above. In particular, I am curious about the subcortical portion of the auditory pathway.

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    $\begingroup$ Fantastic question. I think the answer is going to lie in the association cortices, since this is as much a tactile and proprioceptive issue as it is an auditory one. That's just a gut feeling, but there must be literature on this somewhere. $\endgroup$ – Chuck Sherrington Nov 18 '14 at 21:27
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    $\begingroup$ @ChuckSherrington, I am actually curious because I recently read about sound level adaptation in the auditory nerve, inferior colliculus and medial geniculate body. Sound level adaptation is the following: when a sound is played equiprobably at different volumes, neural activity scales with the volume. But if one of the volumes is more frequent than others, neural activity to that volume will dampen. Now I'm wondering whether a whole new set of neurons is recruited with a change in volume, or not. Given that it was found on the auditory nerve (albeit weakly), probably not. But still... $\endgroup$ – Ana Nov 18 '14 at 21:55
  • $\begingroup$ There are going to be a lot of projections in the other direction from A1 to the MGN as well, which would be fascinating to study in that regard. I wish I had a ready answer for you, but we'll see if anyone can come up with one. $\endgroup$ – Chuck Sherrington Nov 18 '14 at 22:00
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    $\begingroup$ @ChuckSherrington - yeah, I've been thinking about that as well. Feedback connections in a predictive coding type framework would explain sound level adaptation well - if louder tones recruit a new group of neurons. It it's the same neuronal population, I have something interesting to muse about :) $\endgroup$ – Ana Nov 18 '14 at 22:04
  • $\begingroup$ Audio signals are encoded (in part) by hair cells and the hair cells have a tethered lever under tension. One of the ways desensitization occurs is via adjustments to the tension of the lever. $\endgroup$ – Keegan Keplinger Nov 28 '14 at 22:03
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There are quite a few stations between cochlea and the brain and I will focus on the auditory nerve. That said, your theories (a) and (b) are both correct, and therefore (d) applies as well.

(a) Neurons in the auditory nerve increase their firing rate when sound level is increased (Heil et al, 2011). This can be regarded as the primary mechanism for encoding sound level.

(b) Increased sound levels are accompanied by a larger area in the cochlea being activated due to the low-frequency tail (Kiang & Moxon, 1974). Hence, more neurons start firing at higher sound levels when the tone frequency is the same. Although this may add in the perceived sound level, it is likely of less importance than (a), as it is basically a reflection of the mechanical properties of the basilar membrane in the cochlea and primarily reduces frequency resolution.

(c) There are no neurons ever identified that are dedicated to sound level encoding. Instead, auditory nerve fibers encode sound frequency (according to the place-frequency map of the cochlea) and they encode sound intensity via their firing rate.

(d) Hence, since both (a) and (b) are correct, your hypothesis (d) holds, as it is a combination of the above.

One can safely assume these intensity-coding mechanisms hold up in the responses of neurons in the next station - the cochlear nucleus. However, the higher up you go in the auditory system, the less likely a 1:1 relationship as found in the far periphery applies.

References
- Heil et al., J Neurosci 2011; 31(43): 15424-37
- Kiang & Moxon, JASA 1954; 55(3): 620-30

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    $\begingroup$ Wonderful! It's so amazing to be able to come here and get an answer that exactly matches the nitty gritty little question I was interested in. Thank you. $\endgroup$ – Ana Nov 24 '14 at 21:18
  • $\begingroup$ When I click on the Kiang reference, a google search window appears?! $\endgroup$ – Ana Nov 27 '14 at 16:46
  • $\begingroup$ @Ana - My pleasure to help and my apologies for the link; I inserted the proper link now $\endgroup$ – AliceD Nov 28 '14 at 2:30
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    $\begingroup$ Thank you! (comments must be at least 15 characters in length...adding length) $\endgroup$ – Ana Nov 28 '14 at 13:47
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Siebert (1968) modelled level discrimination based on the information in the firing rate of auditory nerve fibers. The model does a reasonable job over a narrow range of conditions, but misses a large number of effects. Since Siebert's original effort, a number of more advance models have been developed. A more recent model by Colburn et. al (2003) highlights just how complicated sound level encoding is at the level of the auditory nerve:

It is shown that the rate information provided by individual AN fibers is more constrained by increases in variance with increases in rate than by saturation. As noted in previous studies, there is sufficient average-rate information within a narrow-CF region to account for robust behavioral performance over a wide dynamic range; however, there is no model based on a simple limitation or use of AN information consistent with parametric variations in performance. This issue is explored in the current study through analysis of performance based on different aspects of AN patterns. For example, we show that performance predicted from use of all rate information degrades significantly as level increases above low–medium levels, inconsistent with Weber’s Law. At low frequencies, synchrony information extends the range over which behavioral performance can be explained by 10–15 dB, but only at low levels. In contrast to rate and synchrony, nonlinear-phase cues are shown to provide robust information at medium and high levels in near-CF fibers for low-frequency stimuli. The level dependence of the discharge rate and phase properties of AN fibers are influenced by the compressive nonlinearity of the inner ear.

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Siebert, W. M. (1968). Stimulus transformations in the peripheral auditory system. Recognizing patterns, 104-133.

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