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Assume a pure tone (single frequency) is listen, lets say 2 kHz. If I understand correctly the temporal theory (aka timing theory), in a cochlea neuron the action potentials create a signal that will have one spike for each single tone cycle with the exception of some gaps due to the minimal interval between neuron triggers.

Volley theory extends previous model to a set of neurons. The union of triggers in all neurons of the set will have at least one spike per cycle (with few exceptions). In other words, the base frequency of the signal composed as the union of triggers of all neurons in the set will be the same frequency than the one of the incoming sound.

My question is, if in this model the trigger intervals is used to encode the sound frequency, and taken into account that usual models for neuron activity only take into account intervals between action potentials (all-or-none), how sound amplitude (power) of the sound is translated to the nerve signals. That is, the differences in the nerve activity caused by two tones of same frequency but different amplitudes/powers, assuming frequency/volley model.

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It's complicated.

The simplest explanation is that sound amplitude/level/loudness is encoded by the number of participating units, including off-best frequency cells that get recruited when amplitudes are high (the concept of 'tuning width' is helpful to understand). Loud sounds will recruit cells of a wider best-frequency range with high coincidence, for example (see Carney, 1994).

In some experimental paradigms, this hypothesis holds; in others, it does not. It is most likely that the perception of loudness is quite context-dependent and also depends on background, the presence of energy in other frequencies, and adaptation.

Similar to how absolute brightness is not all that perceptually useful (and therefore humans display incredible invariance over very wide brightness ranges), loudness is probably perceptually most useful in terms of relative loudness: sounds that are getting louder may be approaching, for example.

References

Carney, L. H. (1994). Spatiotemporal encoding of sound level: Models for normal encoding and recruitment of loudness. Hearing research, 76(1-2), 31-44.

Heinz, M. G., Issa, J. B., & Young, E. D. (2005). Auditory-nerve rate responses are inconsistent with common hypotheses for the neural correlates of loudness recruitment. Journal of the Association for Research in Otolaryngology, 6(2), 91-105.

Neuhoff, J. G., McBeath, M. K., & Wanzie, W. C. (1999). Dynamic frequency change influences loudness perception: A central, analytic process. Journal of Experimental Psychology: Human Perception and Performance, 25(4), 1050.

Relkin, E. M., & Doucet, J. R. (1997). Is loudness simply proportional to the auditory nerve spike count?. The Journal of the Acoustical Society of America, 101(5), 2735-2740.

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  • $\begingroup$ "Best frequency" is not a term related to the place theory? It applies also to volley one? $\endgroup$
    – pasaba por aqui
    Aug 27, 2018 at 19:24
  • $\begingroup$ @pasabaporaqui They aren't really competing theories anymore, rather the one that predominates depends on the frequency range you are considering. Certainly people can still argue which is important for some of the intermediate frequencies, but that's more a niche argument. But in principle, louder sounds will induce synchronous volleys over a wider 'place' on the cochlea. Not even an extreme proponent of volley theory would try to argue that the cochlea is not tuned in space. $\endgroup$
    – Bryan Krause ♦
    Aug 27, 2018 at 19:31

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