DISCUSSION SYMPOSIUM ON HEARING

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  • DISCUSSION

    SYMPOSIUM ON HEARING

    B Atrtrstrs G.

    Buffalo, New York

    Audition is a bio-physical phenomenon and should therefore be discussed frcm a biological as well as a physical viewpoint. The biologist is particularly interested in the correlation of structure with function. Close study of such correlations, however, often leads to the conclusion that "things are as they are" and sometimes a qualifying clause is added to make the explanation sound more plausible "things are as they are because there is no good reason why they should be otherwise." It is not my purpose to congratulate the essayists on clarity of exposition because clarity of exposition may be nothing more than salesmanship of ideas. I do not believe that the precious pearls of scientific research necessarily come already bored to string on some theory or hypothesis. Accordingly I am taking the liberty of presenting a dissenting opinion quite frankly and without prejudice.

    One ofthe chief objections to the theories ofhearing is that an attempt is being made to visualize how an auditory apparatus might operate and in so doing the physical possibilities have been somewhat over- emphasized and the biological probabilities more or less neglected. May I point out wherein visualization of the available apparatus has seemingly led us astray before applying the actual biological tests of audition to the several theories proposed.

    All theories of hearing agree in two major essentials. First; the drum membrane and ossicular chain afford the most efficient route for the

    transmission of air vibrations to the cochlea for analysis. And second; while the auditory epithelium lies functionally intermediate between the sound stimulus applied and the impulses sent into the central nervous system, this epithelium shows no structural adaptation which may be associated with pitch analysis, let alone any structural differ- ences which may be correlated with discrete frequency response.

    The discovery that the ear ossicles form a bent-lever system connect- ing the drum membrane with the oval window immediately lent itself to the interpretation that the apparatus was more efficient at low fre- quencies. This harmonized with the findings in the frank conduction deafness of the otologist. However recent investigations appear to show that the bent-lever form of sound transmission system is not

    348

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  • 1930] AUGUSTUS G. POHLMAN 3[9

    associated with the classical description of decrease in amplitude and increase in force of the drum membrane vibrations in their propogation to the labyrinth. Audiometric methods seem to indicate that the sound transmission apparatus is at least equally efficient at all ordinary audio- frequencies, and if anything perhaps somewhat more efficient at the high end of the sound spectrum. In a word, the entire mechanics of the middle ear apparatus is susceptible to an entirely different func- tional interpretation than the commonly accepted one. Physically there is little doubt but that the middle ear apparatus overcomes in part at least the enormous impedance differences found in air and in water. Biologically, it is true that acuity of hearing is lowered when the sound transmission system is crippled.

    It is a fact that the auditory cells show no structural adaptation which may be correlated with pitch analysis. It is also a fact that the cells show no differences which may be related to the analysis of high frequencies at the base and of low frequencies at the apex of the ribbon- like end organ. The failure to see adaptation however does not rule out the possibility of an intrinsic response which is not visible. We ex- pect to see everything in this day and age. We surely would not examine an eye with a stethescope to determine the errors in refraction. And by the same token a physicist might not make careful examinations of ground sections of a copper conductor to discover why certain currents tend to hug the periphery nor would he smell at a carbon button trans- mitter to find out whether it was alive or dead.

    Physiologists tell us of the great difficulty in the translation of the operation of one sense organ into terms of another sense organ. It is therefore biologically possible for high frequencies to be'analyzed at the lower end of the cochlea for the same reason that one tastes sweets

    on the front of the tongue or why the hair stops short, sometimes too short, at the forehead and gradually drifts out on the back of the neck. The fact that a photographic plate cannot perform the physical im- possibility of correcting for the fuzziness of focus does not prove that a living retina cannot and does not do it. It is quite notorious that an eye sees far too well for its optical system and it is possible that the ear hears far too well for its seemingly crude construction. But see something we must and if the adaptation is not a visible intrinsic one, then logically it must be a visible extrinsic factor. Let us look over the adventitious scenery which may therefore explain pitch analysis.

    The obvious failure of the Helmholtz basilar membrane conception :to-meet 'the requirement of a ten octave analyzing harp has been com-

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  • 350 JOURNAL OF THE ACOUSTICAL SOCIETY [A,a,

    pensated for in a number of ways. Probably the most satisfactory ad- justment is the one presented by Dr. Fletcher. Here a selective weight- ing factor has been added in the two columns of liquid applied to the basilar membrane and the inter-action of the two windows. High fre- quencies are therefore analyzed at the lower end of the cochlea and lower frequencies requiring greater weighting at the distal end. I do not doubt that the explanation is quite satisfactory from the stand- point of the physics, and therefore, as a man of much faith I apply biological tests to discover whether or not it actually does explain the facts. If the selective weighing through inter-action of the two windows determines true pitch hearing, then anything which interferes with the interaction of the two windows will produce a false pitch discrimination. Let us see what the experimental evidence may be in the only form available for such study--the living human being. Barany loaded the membrane of the round window with mercury and Oppikofer placed a pellet of wet cotton against it in two cases where the area was exposed. The three cases reported an increase in the acuity of hearing but no change in pitch. Kranz and I had the opportunity of examining a case before and after a bilateral operation which was undertaken to improve the hearing. The drum membrane, and two outer ossicles were removed and the tendon of the muscle attaching to the Stapes was severed. The acuity curve before and after the operation were identical. The woman was naturally depressed on the account of the failure. A pellet of glyc- erine cotton was applied to the Stapes, which was moveable, and a great enhancement in air acuity obtained although no change in the pitch discrimination. Now how did we know the stapes bone was moveable and how did we know that the cotton prosthesis was applied to it? We know this because the increased acuity occurred only when a certain stop was touched and when this spot was touched the woman became dizzy and showed an eye imbalance which is quite characteristic in sudden changes in labyrinthine pressure. It is very common to find effusion in the middle ear which may mount above the level of the round window and et Politzer found in cases with unilateral effusion there was no change in the pitch discrimination on the two sides. I have personally done pressure experiments on over three hundred normal and deafened people and the only change reported has been one of lowered acuity. Cases of bony fixation of the stapes are quite common and in spite of the marked interference with the inter-action of the two windows false hearing is exceedingly uncommon. Further in the deafness which comes with age, there is progressive loss of sensitivity beginning

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  • 1930] AUGUSTUS G. POHLMAN 351

    at the high frequency end. This sensitivity loss is accounted for by O. Mayer by a progressive thickening and stiffening of the basilar mem- brane but such cases do not show false pitch discrimination. The experi- mental evidence on the living human being appears to indicate that interference with the interaction of the two windows has an effect on

    the intensity of the sound heard but not on the pitch. Accordingly a biologist would favor the interpretation of this interaction of the two windows as one having to do with intensity even though it would be be necessary to modify Dr. Fletcher's proposal by putting the two windows into full phase rather than half phase relations.

    The conception of the action of the basilar membrane as a sound analyzer was originally suggested because the auditory cells showed no visible structural adaptation to their function. Just so the theory ad- vocated by Dr. Shambaugh arose because many investigators felt that the basilar membrane theory could not obtain. It is true that the tec- torial membrane is better developed at the distal end of the cochlea but the dimensional differences throughout its length are no greater than those of the basilar membrane. The tectorial membrane is a semi-gela- tinous and almost structureless membrane with a specific gravity about that of the liquid in which it is immersed. It therefore appears poorly adapted from a physical standpoint to behave in a manner essentially different from that of the liquid contents of the cochlea. Morphologi- cally, as Dr. Shambaugh has already pointed out, it is closely related to similar structures found in the balancing organ from which the audi- tory apparatus was derived. This satisfies the biologist on the basis that "things are as they are." Dr. Shambaugh has also mentioned that it may be necessary to abandon an extrinsic resonation theory to which I agree.

    We may turn our attention, briefly, to the non-specific activation theories and take for an example the sound pattern theory of Ewald. However if we are to agree to a non-specificity in the auditory cells themselves and a non-specific response in the basilar membrane, then we must complicate the picture by adding a specificity in the nerve impulses which is contrary to all physiology of the nervous system. Nor will the physiologists agree to Dr. Troland's suggestion that co6r- dination of the action currents in the nerve fibers will compensate for the failure of a single nerve fiber to transmit high frequency pulsations demanded in his interpretation. Still further if we are to agree in a cor- tical analysis of frequency, we must explain away the unique case re- ported by Dandy where removal of one cerebral hemisphere had practically no effect on the pitch discrimination of the ear of the oppo-

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  • 352 JOURNAL OF THE ACOUSTICAL SOCIETY [A,t,

    site side. Perhaps it will be well to settle the physiology of the definite physical factors of pitch and intensity before attempting to interpret the subjective additions demanded by the psychologists.

    Personally I am experimenting on deafened people in Buffalo in the hope that certain facts may be developed in relation to the intensity of sounds heard. Very few deafened people show any serious disability in so far as pitch discrimination is concerned and a very large percent of deafened people do show evidence of possessing internal ears in which both pitch range and threshold are quite norma.

    Theories of hearing have been proposed in many forms. In closing I take the liberty of presenting an explanation of audition in poetic form by Fletcher; not Harvey Fletcher, 1929 but Phineas Fletcher 1633. I am presenting it because few have ever heard of the "Purple Island" and the two short verses are in my opinion excellent.

    "As when a stone, troubling the quiet waters, Prints in the angry stream, a wrinkle round Which soon another and another scatters

    Till all the lake with circles now is crown'd

    All so the aire struck with some violence nigh, Begets a world of circles in the skie; All which infected move with sounding qualitie. These at Auditus' palace soon arriving, Enter the gate and strike the warning drumme; To these three instruments fit motion giving, Which every voice discern; then that third room Sharpens each sound and quick conveys it thence; Till by the flying poast 'tis hurried hence, And in an instant brought unto the judging sense."

    Canto v 47 and 48

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