Saturday, March 18, 2006

Ear's spiral responds to bass

Ear's spiral responds to bass: New theory explains why our hearing machinery is coiled up, Nature News, 13 March 2006, Philip Ball Why is our cochlea, the key organ of hearing, curled into a spiral? It has been often thought to be a space-saving measure. But researchers in the United States have shown that the spiral could be vital for increasing our ear's sensitivity to sound, particularly at low frequencies. Daphne Manoussaki of Vanderbilt University in Nashville, Tennessee, and her colleagues believe that the snail-shell curve of the cochlea focuses sound waves at the spiral's outer edge, making it easier for vibration-sensitive cells to detect them. If the researchers are right, then the ear is more sophisticated than we thought. "It would show we need to take a step back from the cell biology and see how the cochlea works as an integrated system", says Karl Grosh of the University of Michigan in Ann Arbor, who studies the ear's structure. The findings also suggest that artificial cochlear implants for the hard of hearing could be improved. Grosh, who is working on such microscopic devices, says that the work will encourage him to think about mimicking the coiled structure, which was thought previously to have no real function. The cochlea is a fluid-filled, coiled tube, about one cubic centimetre in volume, that narrows towards one end. Sitting in the inner ear, it separates the different frequencies of a sound, picking up sound waves from 20 to 20,000 hertz. Different frequencies peak at different positions along the tube: high frequencies near the spiral's broad mouth, and low frequencies further up the tube. The tube also carries nerve cells that fire in response to vibrations in the watery cochlear fluid. The separation of frequencies happens just as effectively in a straight tube, and so until now it hadn't been clear that the cochlea's coiling did anything other than keep it compact. Some designs for an artificial cochlea have represented it as a straight, tapering channel. But Manoussaki and her colleagues have calculated the way that sound waves travel in a cochlear tube with a realistic coiled shape. They find that the wave energy is not evenly distributed throughout the tube, but becomes concentrated along the outer wall, the more so the further up the duct the wave travels. The researchers say this is similar to the way sound in a cylindrical space such as St Paul's cathedral in London, UK, gets concentrated around the walls; known as the 'whispering gallery' effect, it allows a listener at one part of the wall to hear a murmuring speaker on the other side of the cathedral. Turn up the volume The concentration of energy in one part of the tube could help the membrane cells to detect sound, if they are clustered in that region. Thus the cochlea may be more sensitive further up the tube, where lower frequencies are detected. The researchers estimate that this amplification means that sound at the inner tip of the spiral is boosted by 20 decibels relative to sound at the outer face: the difference between the volume of a normal conversation and that of a vacuum cleaner. A boost of 20 decibels would be significant in an artificial cochlea, says Grosh: "we'd love that." He says that it would be relatively easy to make miniaturized channels in the shape of a coil. ... [Note the teleological sub-headline: "New theory explains why our hearing machinery is coiled up"! But since "The separation of frequencies happens just as effectively in a straight tube" it would be hard for even Dawkins to think of the selective advantage an originally straight tube beginning to coil up when it would take a lot more coils (perhaps the whole coiled structure) to better hear bass sounds. Yet that is what happened, as the late Gordon Rattray Taylor (an agnostic engineer), in one of my favourite quotes, points out (note the "progressive series of changes which certainly look more like the refinement of a plan than the result of a series of happy accidents":

"With all this, of course, went improvements in the brain, most notably the power to compare the times at which signals from one source reach each ear, thus providing a method of estimating the direction in which the source lies. Thus, in the course of evolution, there were six major developments, two of which occurred in the fishes, two in the amphibia and two in mammals. Such, at least, is the account given by people like Willem van Bergeijk, of Bell Telephone Laboratories who is the acknowledged authority. But the eminent morphologist J.W. Torrey is not convinced. 'The evolutionary origin of the inner ear is entirely unknown,' he insists. In contrast with the case of the eye, where undifferentiated cells were specialised into the required forms, here existing structures have been profoundly modified and even shifted to another position in a progressive series of changes which certainly look more like the refinement of a plan than the result of a series of happy accidents. But the insoluble problem is how and why did a balance organ become an organ of hearing? As van Bergeijk pointedly asks: 'What prompts the fish to begin developing a sensory apparatus that will respond to a stimulus about the very existence of which the fish knows nothing?' Van Bergeijk believes that the original balance organ would never have evolved mechanisms for hearing but for the emergence of the swim bladder. The original purpose of this organ is to enable the fish to adjust its density to the density of the ambient water and so control the depth at which it swims. Since the bladder is sensitive to changes in external pressure, it vibrates in harmony with pressure changes in the water. In time these vibrations came to excite the ear. Hearing as distinct from the mere detection of pressure waves, was born. After describing the last part of this process, the adaptation of the bones linking the jaw to the skull into a chain of ossicles linking the eardrum to the inner ear, Ernst Mayr sweepingly remarks: 'Not all the steps in this process are yet entirely apparent, but I think little doubt is left as to the principle involved.' If by 'principle' one means merely progressive remodelling, the statement is a truism. But if 'principle' means that chance selection brought about these elaborate changes, then there must be very great doubt indeed. Like de Beer, Mayr does not seem to appreciate the elementary point that demonstrating the occurrence of a sequence of events does not explain why they happened. But what kind of mutations could bring about the major changes I have described? Could cause a tube to roll up into a helix? Could cause other tubes to form semi-circular canals accurately set at right angles to each other. Could grade sensory hairs according to length? Could cause the convenient deposit of a crystal in the one place it will register gravity? Even more amazingly, some fishes do not trouble to secrete a crystal but incorporate a bit of sand or stone. What kind of mutation could achieve this - when and only when a natural crystal is not formed? The purpose is fulfilled, the means are unimportant. It just doesn't make sense." (Taylor G.R., "The Great Evolution Mystery," Abacus: London, 1983, pp.105-106. My emphasis)

My answer to Taylor's, "But what kind of mutations could bring about the major changes described?" is, intelligently directed mutations! I will add both this article and the above quote to a new section of my "Problems of Evolution" book outline, PE 13. 2.3. "Animals Organs Ear."

PS: Those who are interested in the issue of whether the Bible in 1 Kings 7:23 (and in the parallel passage 2 Chronicles 4:2) claimed that pi = 3, might check out the later note in my post, The life of pi.

Stephen E. Jones, BSc (Biol).
"Problems of Evolution"

"Like many revolutionaries, Darwin embarked upon this revolutionary enterprise in the most innocent and reasonable spirit. He started out by granting the hypothetical nature of his theory and went on to defend the use of hypotheses in science, such hypotheses being justified if they explained a sufficiently large number of facts. His own theory, he continued, was `rendered in some degree probable' by one set of facts and could be tested and confirmed by another-among which he included the geological succession of organic beings. It was because it `explained' both these bodies of facts that it was removed from the status of a mere hypothesis and elevated to the rank of a well-grounded theory.' [Darwin C.R., "The Variation of Animals and Plants under Domestication," John Murray: London, 1868, Vol. 1, pp.8-9] This procedure, by which one of the major difficulties of the theory was made to bear witness in its favor, can only be accounted for by a confusion in the meaning of `explain'-between the sense in which facts are `explained' by a theory and the sense in which difficulties may be `explained away.' It is the difference between compliant facts which lend themselves to the theory and refractory ones which do not and can only be brought into submission by a more or less plausible excuse. By confounding the two, both orders of explanation, both orders of fact, were entered on the same side of the ledger, the credit side. Thus the `difficulties' he had so candidly confessed to were converted into assets." (Himmelfarb G., "Darwin and the Darwinian Revolution," [1959], Elephant Paperbacks: Chicago IL, 1996, reprint, p.334)

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