Where in the cochlea are frequencies below 200 Hz detected?

Question

I understand that different frequencies are detected in different positions along the cochlea.

I'm also aware that the range of human hearing is roughly between 20 Hz and 20 kHz.

However, looking at the below diagram, the lowest frequency, at the end (apex) of the cochlea, 200 Hz is the lowest frequency.

Hence the question: wwhere or how are the lower frequencies (between 20 Hz and 200 Hz) detected?

Answer 1

The answer is two fold, each related to the two ways pitch is encoded in the inner ear. These two mechanisms are place coding and rate coding.

1. Regarding place-pitch; given that there are hair cells tuned to frequencies lower than 200 Hz, it's not surprising that your linked image is incorrect. Below are 2 (random) figures drawn from the internet showing the human tonotopic map that do show the lower frequencies:

2. Regarding rate-pitch: at low frequencies, say from 1 kHz and below, the auditory nerve is more and more able to phase lock onto the stimulus. Because of the refractory period of auditory nerve fibers of a few ms or so, this process becomes more pronounced at frequencies below, say, 500 Hz. In guinea pigs phase locking (e.g., Stronks et al., 2010) becomes apparent at higher frequencies than in humans. This rate pitch coding still is most pronounced at the place where the frequency of the tone stimulus matches the characteristic frequency in the cochlea, yet because of the high frequency tail (see the other answer from Bryan Krause), it may occur at other places as well at high stimulus levels. Phase locking is thought to contribute to pitch perception, although it's a matter of debate.

In cochlear implants, for instance, rate coding could be easily implemented, yet in most systems speech coding is based only on frequency mapping based on place-coding by placing electrodes at the corresponding frequency bands in the cochlea. Cochlear implants, in a nutshell, bypass the degenerate hair cells in severe-to-profoundly deaf ears by directly stimulating the auditory nerve. They consist of an implanted array of electrodes in the cochlea (for place coding), and an external speech processor converts speech into a number of frequency bands equal to the number of electrodes to be able to place-code different frequencies along the tonotopic cochlear map.

<sub>Reference
- Stronks et al., Hear Res (2010); 259(1–2): 64-74</sub>

^{Fig. 1. Human tonotopic map. source: Cochlea.eu}

^{Fig. 2. Human tonotopic map. source: Abdulla, Audio Watermarking for Copyrights Protection, 2007}

Answer 2

A figure like this is likely representing the best frequency for hair cells. In actuality, cells respond to a broad range of frequencies. Perceptual specificity comes thanks to lateral inhibition. Tuning curves look something like this:

image from: http://www2.tulane.edu/~h0Ward/BrLg/AuditoryTransduction.html

The y-axis here is in units of threshold, so a small number means the cell is detecting quieter sounds at that frequency. For example, cell D here has a best frequency around 300 Hz, but was still somewhat sensitive to the lowest frequencies presented (looks like about 180 Hz). You won't see many experimental tuning curves that go below that because a typical speaker doesn't go down that low.

If someone says humans can detect sounds down to 20 Hz, that doesn't necessarily mean any inner hair cell has a best frequency at 20 Hz; if it did, it would probably be somewhat sensitive to some even lower frequencies, too, and you'd have to report a different number. If humans can hear 20 Hz sounds, that just means that for some cell, 20 Hz is on the far left end of its tuning curve, such that a loud enough sound at 20 Hz can be perceived.

You can assume the cells most sensitive to the lowest frequencies are at the apex.