How do we come to hear?

I was browsing through my reference library the other day and came across two references for how we come to hear. The references are below. I had earmarked them for my previous book, The Tinkerer’s Accomplice. Ultimately, I didn’t use them there, but they illustrate a fundamental point I tried to develop in Purpose and Desire: that life is fundamentally a cognitive phenomenon, not a genetic phenomenon.

The two papers are concerned with a mysterious set of cells found in the inner ear of fetuses: the Kölliker’s organ. Kölliker’s organ disappears at birth, and its function has long been a mystery. The mystery is now clarified: it helps the fetal cochlea to learn to hear without sound.

Before I say more, I want to clarify what it means “to hear.” Hearing is not a mere mechanical process, fascinating though the mechanics might be. Rather, it is a means of building a cognitive world around sound. Hearing involves not only the organs that physically shape and mold sound—the outer ear, the eardrum, the exquisitely contrived bones of the middle ear, and the cochlea.

Hearing is also transforming energy in sound into information that the brain (the mind, really) can interpret, that it can use to build the cognitive world of sound.

In the case of the ear, this means having some way to encode sound frequency (which he hear as pitch) into a pattern of nerve impulses. This is done in the inner ear (the cochlea), where sound frequency is mapped along the length of the cochlea. I don’t want to get too deep into the weeds here, but different sets of sensory cells respond to different sound frequencies, so that different frequencies translate into different patterns of excitation of the signals sensory cells send to the brain.

To “hear”, this means a very complicated and specific set of connections between cochlea and brain must exist. So complicated are these connections that the temptation is to refer to the auditory system as being “wired” in a particular way, to fall back on the metaphor of the auditory system as a computer. Indeed, the performance of the auditory system is nothing short of wondrous, to the point of stunning. But hiding behind all this awesomeness is a question: how did it come to be that way?

One common model involves a kind of stepwise bootstrapping. The sensory cells of the ear put out many possible connections to the cells of the auditory context, which are then “pruned” as the ear is trained to discriminate sounds. Key to this training is the transmission of sound information from the cochlear cells to the brain. As these sounds “train” the auditory system, the connections become refined and sorted into the highly ordered “wiring” of the cochlea to the brain.

This kind of training is a common feature of many sensory systems. The visual system is similarly trained, as I outlined in The Tinkerer’s Accomplice.

But here’s the dilemma for the auditory system. It “wires itself” with no sound to train it! At some point in fetal life, the ears can discriminate sound frequencies. But by then, the auditory system is wired and ready to go. There’s another problem. Sounds that can actually make it to the fetus are very narrowly constrained—low frequencies, highly muffled, and so forth—compared to the fetal cochlea’s ability hear them. I’ve added below a third reference on how and what human fetuses can hear while they are in the womb

So, the cochlea presents a fundamental problem in design. The cochlea is exquisitely designed. But its design cannot fall back on the bootstrapping model. It comes out pre-built and ready to hear. Where did its design come from? The temptation is to fall back on genetic determinism—the exquisite design comes from generations of selection on “exquisite design” genes, or on a crude form of intelligent design—the exquisite design comes from a designer.

Here is where the Kölliker’s organ comes in. In the young fetus, the cells of the Kölliker’s organ do the training, but without sound! They stimulate the sensory cells of the cochlea in patterns that mimic what sound would do. This helps build the cochlea to the necessary specifications, so that it is ready to go immediately once sounds become available, that is at birth. This is one reason why hearing of newborn infants is more acute than their vision.

Thus, it is one set of cognitive agents—the cells of Kölliker’s organ—that imposes its cognitive map onto another set of cognitive agents—the sensory cells of the ear. It is a system “designed” by cognition.

Forsythe, I. D. (2007). A fantasia on Kölliker’s organ. Nature
450: 43-44.

Tritsch, N. X., E. Yi, et al. (2007). The organ of spontaneous activity in the developing auditory system. Nature
450: 50-55.

Gerhardt, K. J. and R. M. Abrams (2000). The Fetus. Journal of Perinatology
20: S20-S29.

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