The Prescience of Riemann's Scientific Method

Riemann wrote a refutation of Helmholtz' reductionist and mechanistic theory of hearing and the functioning of the ear in a short, unfinished paper entitled "The Mechanism of the Ear." What Riemann succinctly points out is that in scientific investigation we must not deviate from focusing on the problem of what hearing actually accomplishes. Thus the wild suppositions of Helmholtz that the mechanisms of the ear somehow break down primary tones into components that are processed by various structures and sent to the brain are rejected. Today we have ascertained that the uncovering the physiology of hearing must resolve the issue of amplification of sound on the order of 4 x 10 ³. Currently there is no known mechanism that can account for this gain. But per Riemann the method of asking the question of how this is accomplished is the right way toward its resolution. Therefore the following investigation and hypothesis is to be commended.

Tuesday, September 14, 2010

The Puzzle of Sound Amplification in the Inner Ear

Scientists have long puzzled over the inner ear's ability to amplify sound. Now they think they know how the ear does it

One of the extraordinary features of the mammalian sound detection system is the range over which it works. This extends from 11 KHz in birds to 200 KHz in marine mammals.

This is only possible because the inner ear amplifies sounds by a factor of up to 4000. That's a huge amount of gain. So much, in fact, that it's hard to square with conventional thinking about mechanical amplification. So there is much head scratching among biologists over how the ear achieves this amplification.

Part of the puzzle is that the amplification is not entirely passive. The inner ear is essentially a fluid-filled tube, divided along its length by a thin elastic membrane. This membrane is covered in hair cells, which come in two types.

The so-called inner hair cells convert pressure waves within the fluid into electrical signals the brain can interpret. However, the outer hair cells act like mechanical amplifiers. When struck by a pressure wave, the cells themselves begin to vibrate at the same frequency, thereby boosting the wave as it passes.

The trouble is that measurements using outer hair cells indicate that they amplify pressure waves by a factor of about 10, a gain that falls far short of what's required.

Today, however, Tobias Reichenbach and James Hudspeth at The Rockefeller University in New York city say they've worked out what else is going on to boost the signal.

Sound enters the inner ear as a pressure wave which travels through the fluid filled chamber, causing the membrane that divides it along its length to vibrate, like a sheet of rubber. Since the hair cells sit on this membrane they also move.

Reichenbach and Hudspeth calculate that the vibration of the outer hair cells not only amplifies the pressure wave, but also increases the displacement of the membrane, like a child bouncing on a trampoline.

When these effects combine, they result in a positive feedback effect, which creates a huge gain. This easily explains the 4000x amplification. In fact, the team says the gain can be even larger: "The overall cochlear gain, the product of these two components, can exceed 10,000," say Reichenbach and Hudspeth.

All that remains is for the experimentalists to devise a way of showing that this is actually the process that achieves the gain, admittedly not the easiest of tasks. But one that could soon lead to this conundrum being settled once and for all.