The middle ear of the Tokay Gecko

J. comp. Physiol. 81,239--250 (1972) 9 by Springer-Verlag 1972

The Middle Ear of the Tokay Gecko* Geoffrey A. M a n l e y Department of Biology, McGill University, Montreal Received September 4, 1972

Summary. 1. Amplitudes of middle ear structure were measured in Gekko gecko, using the MSssb&uer effect. 2. The amplitude of the columella/extraeolumell~ system is more or less constant up to 1 kHz and then falls off. 3. A frequency-dependent lever ratio exists between the inferior process on the drum and the eolumella. 4. Below 2 kHz the drum locations tested all had higher amplitudes than the inferior process. 5. The vibration pattern of the drum is simple below 3 kHz, but breaks into at least two peaks above this frequency. 6. The drum vibrates in phase at low frequencies, but at higher frequencies larger phase variations exist. 7. All the above factors contribute to highly efficient impedance matching at low frequencies, but poor matching above about 4 kHz. 8. The eardrum intensity response is linear at the intensities used. Introduction Behavioral, physiological a n d b i o p h y s i c a l e x p e r i m e n t s h a v e p r o d u c e d d a t a which c o n s i s t e n t l y i n d i c a t e s t h a t t h e t e r r e s t r i a l n o n - m a m m a l i a n ear is v e r y l i m i t e d in its h i g h - f r e q u e n c y responsiveness. The evidence a v a i l a b l e is of t h r e e kinds. F i r s t l y , b e h a v i o r a l a u d i o g r a m s h a v e been d e r i v e d for some birds (e. g., Dooling a n d Mulligan, 1970; Trainer, 1946) a n d a r e p t i l e (Patterson, 1966). Secondly, n e u r o p h y s i o l o g i c a l investigations h a v e r e v e a l e d l i m i t e d f r e q u e n c y response ranges for single a u d i t o r y neurons in a m p h i b i a (e.g., C a p r a n i c a a n d F r i s h k o p f , 1966; F r i s h k o p f a n d Goldstein, 1963; Lift, 1969), reptiles (Campbell, 1969; J o h n s t o n e a n d J o h n s t o n e , 1969; G. Manley, 1970a, b, 1971, 1972; J. l~anley, 1971; * Supported by Grant A6368 from the Canadian l~ational Research Council, and a grant to ]3. M. Johnstone from the Australian Research Grants Committee. This work was carried out while the author was a Queen Elizabeth I I Fellow at the University of Western Australia. I thank ]3. M. Johnstone for generously putting his equipment at my disposal for these experiments, and Roberta Webster and Debbie Nolte for expert technical assistance. 17 J. comp. Physiol., Yol. 81

240

G.A. Manley:

Saga and Campbell, 1967) and birds (e. g., Konlshi, 1970). Lastly, some experiments have shown that the middle ear of non-mammals transmits only lower frequencies to the inner ear (Loftus-ttills and Johnstone, 1970; Saunders a n d J o h n s t o n e , 1972; W e r n e r a n d Wever, 1972; W e v e r a n d Werner, 1970). I n a d d i t i o n to the above, m u c h d a t a is available from cochlear microphonic experiments. However, the lack of a definite threshold using this m e t h o d makes this d a t a v i r t u a l l y impossible to i n t e r p r e t i n t e r m s of s e n s i t i v i t y range. The l i m i t a t i o n i n high-frequency response has been shown to have a n i n n e r - e a r c o m p o n e n t as well as a middle-ear c o m p o n e n t (Manley, 1971 ; 1972). Manley (1972) a n d W e r n e r a n d W e v e r (1972) d e m o n s t r a t e d a high f r e q u e n c y cut-off i n Gekko gecko i n d e p e n d e n t of the middle ear, b u t showed also t h a t the middle ear efficiency falls r a p i d l y above a few kHz. T h e s t u d y reported here was u n d e r t a k e n to define i n more precise terms the properties of t h e middle ear components.

Materials and Methods Adult Tokay Geckos were anaesthetised with Ethyl earbamate (urethane) and part of the soft tissue of the external auditory meatus (which normally covers at least half of the drum area) was trimmed away to exteriorize the eardrum. Thefrequeney response of the various parts of the middle ear was examined utilizing the MSssbauer effect. The equipment and techniques (developed in this case by B. M. Johnstone and his associates) have been fully described elsewhere (Johnstone and Taylor, 1970; Johnstone, Taylor and Boyle, 1970). In brief, the MSssbauer system is an extremely sensitive velocity-measuring device. Gamma radiation from, in this case, 57C0 atoms diffused into a Palladium crystal lattice undergoes a doppler shift in frequency when the radiation source is moving. This doppler shift is large enough at small velocities (less than 1 ram/see) to allow the 14 KeV rays to pass through a thin stainless-steel foil absorber which would normally absorb most of these rays from a stationary source (with this source-absorber combination 100% absorption is not at zero velocity). A counter tube placed behind the absorber thus registers more radiation from the source when the source has negative or positive relative velocities. Calibration of individual source-absorber combinations permits conversion of the count-rate differences between stationary and moving source to source velocity. IG-mwing the frequency of oscillation of the source, the amplitude of movement is easily calculated. As the source is very small (10 it • 100 tz • 200 ~z) if'does not significantly load the moving object. Placement of the source under an operating microscope was easy in most situations and tiny amounts of high-vacuum grease were used to temporarily hold the source in place. Thus, the 2ViSssbauersystem is very suitable for study of the response of middle ear structures to various frequencies. In these experiments, seven of the eight source locations used were on the outer surface of the eardrum, the eighth was on the ventral surface of the columella somewhat nearer the inner drum surface than the footplate in the oval window. The seven eardrum locations are illustrated in Fig. 1, with measurements. Placement errors are difficult to estimate, but would be

241

The Middle E a r of the Tokay Gecko

\

/ 9

~

DORSAL

,T: ;( \ "

[~1"2~e"~1"2---'~1"2~ 6,3

" 1

///

i

"T .8

.:-'~ "".

L 1:

~:1

".C-OL "~". ..-" " . . . . " ".,

"

I

:

~ PROCESS"/

PLANE OF HEAD 8.4 Fig. 1. Diagram of right eardrum of G. gecl~oas seen from the outside, illustrating appearance, dimensions and locations of measurement sites, drawn to scale. Dimensions in millimeters. " C O L " is the approximate position opposite the columella insertion. The tip of t h e inferior process of the extracolumella is n o t central in the d r u m

no larger t h a n 200 ~. One of these seven locations was opposite the tip of the cartilaginous inferior process of the extracolumella (location D), and the others at various points on the free d r u m surface. I n this way, the amplitudes of different parts of the drum a t various frequencies were obtained. Sound from a Geloso high-power driver was delivered to the drum through a closed tube which was sealed over the meatus with high vae~mm silicone grease. Sound intensity was monitored constantiy through a calibrated probe-tube system and 1/2 inch condenser microphone. The waveform of the differentiated o u t p u t of the microphone amplifier was also monitored to ensure t h a t only undistorted waveforms were used. The sound intensities necessary to give a n acceptable count-rate difference (5-25 % above sound-off count rate) varied with frequency a n d source location b u t ranged from 75 to 110 dB re 2 X10 -~ dyne em -2. Frequency was varied from 0.1 to 10 kHz a n d counts accumulated through about ten repetitions of a seven-second sound on, seven-second sound off cycle. Source velocity was calculated from t h e count difference and then this value was transformed to amplitude. The computer program was designed to give all values corrected to a s t a n d a r d 100 dB sound intensity. 17,

242

G.A. Manley:

In a few cases, phase differences were measured. In order to do this, the counts over each cycle of the source frequency were divided into the 64 channels of a multi-channel scaler. The scaler was locked to the phase of the input sound. The scaler accumulated waveform shows maxima and minima corresponding to the points on the waveform where the source had high and low velocities during each cycle. The outputs are compared for different source locations, and relative phase at the same frequency can be obtained. The phase information so derived is accurate to within a few degrees, and has a 360 ~ ambiguity. Assuming the drum to be elliptical, the mean area was calculated as 42 mm ~. The dimensions used in this calculation are, however, not easy to measure, as the edge of the eardrum grades into true "skin" and no clear delimitation is visible. The columella Iootplate area was 0.96 mm ~. Both of these values are in close agreement with the data of Werner and Wever (1972) who describe in detail the anatomy of the middle ear of this species. All experiments were run at room temperature (23-25 ~C) and no attempt was made to studythe effects of temperature.Temperature has been shown to have no noticeable effect on the middle ear response of a bat (Manley et at., 1972).

Results

a) ExtracolumeUa and Columella Amplitude T h e d r u m l o c a t i o n o p p o s i t e t h e t i p of t h e inferior process is t h e " u s u a l " p o s i t i o n for m e a s u r i n g e a r d r u m a m p l i t u d e . Mean a m p l i t u d e a t this l o c a t i o n over t h e f r e q u e n c y r a n g e m e a s u r e d is shown in Fig. 2 A . A b o v e 1 k H z , t h e a m p l i t u d e falls off f a i r l y r a p i d l y , t h e slope being m a x i m u m b e t w e e n 2 k H z a n d 4 k t t z . Also shown on Fig. 2 A is t h e m e a n columella a m p l i t u d e . T h e a m p l i t u d e s are consistently smaller t h a n t h o s e a t t h e t i p of t h e inferior process a n d i n d i c a t e t h e existence of a lever a c t i o n in t h e m i d d l e ear. The d B difference b e t w e e n t h e two is not, however, c o n s t a n t (Fig. 2 B ) i n d i c a t i n g t h a t t h e lever a c t i o n is f r e q u e n c y d e p e n d e n t . T h e differences are q u i t e large below 1 k H z (14 d B ~--5 • a m p l i t u d e of m o v e m e n t ) b u t decrease t o a m i n i m u m of 6 d B ( ~ 2 • ) a t 4 k H z . B e y o n d this frequency, t h e r e is a s h a r p increase t o over 20 d B a t 10 k H z . S u b s e q u e n t i n v e s t i g a t i o n has shown t h a t a b o v e 4 k H z , t h e relat i v e a m p l i t u d e difference is n o t due t o t h e lever s y s t e m b u t t o p o o r t r a n s m i s s i o n of e n e r g y along t h e inferior process.

b) Relative Amplitudes o/Drum Locations I t was f o u n d t h a t t h e a m p l i t u d e s of all d r u m locations exceeded t h e a m p l i t u d e of t h e t i p of t h e inferior process (location D) u p to a b o u t 2 k H z . A b o v e this frequency, t h e inferior process a m p l i t u d e b e g a n to exceed t h a t of some o t h e r points. A b o v e 4 k H z , t h e a m p l i t u d e in t h e v i c i n i t y of l o c a t i o n D is u s u a l l y as large as or larger t h a n all o t h e r points e x c e p t C2, a n d t h a t of C2 g r a d u a l l y diminishes r e l a t i v e to D u n t i l a t 10 k H z t h e m a x i m u m of t h e whole d r u m is c e n t e r e d n e a r D. Re-

243

The l~Iiddle Ear of the Tokay Gecko

B

L. . . . . . .

~

-5

"--.,,~..,~ r

k

o

.1

1 .01

ix II

\

\ I

.oo

Jt FREQUENCY, kHz

Fig. 2. A, Amplitude of location D (tip of inferior process) at 100 dB SPL (continuous line) and of the columella (filled circles, dashed line) at 100 dB, in microns.

Refers to scale on left (from Manley, G. A., 1972; used by permission). B, Difference between amplitude of inferior process and columella expressed as dB of columella below inferior process. Refers to scale on right

membering t h a t beyond 1 k H z the absolute amplitude of D falls, it is clear t h a t relatively little energy is transmitted to the extraeolumella from the drum above the mid-frequencies. The amplitude differences of some locations, in dB relative to D, are shown in Fig. 3.

c) Amplitude Contours o[ the Drum Iso-amphtude contours were drawn for the whole drum in order to gain some idea of the general vibration patterns at different frequencies. The simplest possible pattern for each frequency was derived from noting the amplitudes at all locations and b y assuming t h a t a contour

244

G. A. Manley: The Middle Ear of the Tokay Gecko + 14

+12

~oo...'

+10'

".

"

+8'

? ..... /

+6 .-,-""''

.o" .......

,,q "'q, ....... ,r

............~"

~. ! "

9"

i ;

It.. "

.~

"~t

-2, -4 9-

.I

.2's

.i

i

L5

~

vt~

10

FREQUENCY kHz

Fig. 3. Relative amplitudes of some drum locations expressed as dB re amplitude of location D. The three curves show relative amplitudes of location A, mean of four measurements (continuous line, open diamonds), Co, mean of four measurements (dashed line, filled triangles) and E, one measurement (open circles, dotted line)

whose amplitude is m i d w a y between those of the nearest two points would be at half the linear distance between them. The relationship is no d o u b t n o t linear, b u t then we do not have a sufficient knowledge of d r u m mechanics to assume a more sophisticated relationship. Also, if the fiber distribution patterns on the d r u m resemble those in the guinea pig (Kawabata and Ishii, 1971) this relationship would be different for different locations and directions. However, the patterns are only rough and would not be greatly affected b y these assumptions. Of course, it was also assumed t h a t the edge of the d r u m has zero amplitude. The position

Fig. 4. Iso-amplitude contours of the eardrum at twelve frequencies (kHz). Small numbers give the amplitude of each contour in microns. Dots represent measurement locations. For explanation of contour derivation, see text. Each contour joins points having the same vibration amplitude at 100 dB

~

J

1

246

G.A. Manley:

" C O L " is directly above the columella insertion and the amplitudes here were assumed to be those of the columella. If the extracohimella is easily compressed at these frequencies then these values should be slightly higher. The vibration patterns shown in Fig. 4 are relatively simple ones up to about 3 kHz and, as noted above, the maximum amplitude is away from the inferior process. The drum does not vibrate like a stiff plate, but amplitudes on either side of the inferior process are higher than those of the process itself. The sides of the drum are not identical in their dimensions and the response exhibits some asymmetry. The wider side has higher amplitudes at low frequency, and vice versa. Between 4 kHz and 8 kHz, the pattern breaks up giving two maxima. Of course, there m a y be other undetected peaks. These areas will interfere with one another if they do not vibrate in phase. I t should be noted from Fig. 2A that the amplitude of location D falls most rapidly between 2 kHz and 4 kHz, when the dual mode is being initiated. At 10 kHz a simple pattern is re-established. As can be readily seen from the 10 kHz contours, the slope along the inferior process is steep at this frequency, indicating poor transmission along the inferior process. Also, the contours no longer curve around the inferior process as, for example, at 1 kHz indicating that the contribution of the drum membrane to the amplitude of the inferior process is much less.

d) Relative Phase Measurements Relative phase was measured at five frequencies between locations D and C2. At low frequencies C2 leads (~-) or lags (--) location D by only small amounts (0.5 kHz, -~ 15 ~ ; 1 kHz, --16 ~; 2 kHz, ~- 8 ~ indicating that the drum is vibrating more or less in phase. Above this frequency, the two points become more out of phase (4 kHz, ~-50 ~ until at 10 kHz they are nearly completely out of phase (10 kHz, ~-144 ~ This phase data correlates well with the amplitude data reported above, and indicates that the area of the drum which contributes energy to the extracolumella is large at low frequencies but becomes smaller above 3-4 kHz. This, of course, strongly affects the impedance matching efficiency of the middle ear.

e) Linearity The scaling of all amplitude measurements to a standard 100 dB is based on the assumption that the response of the eardrum is linear over the range of intensities used. In order to check this experimentally, tests were run at three frequencies and over as wide a range of inten-

247

The Middle Ear of the Tokay Gecko

i1

F=5OOHz

'%"-'--'

F:I.SkHz ~2J

2

78

80

82

84

86

88

90

1

2-'

92

94

F:3kHz S4

86

88

90

92

94

96

98

dB

Fig. 5. Diagram illustrating linearity of intensity response of the location D at three frequencies. Bottom scale for each frequency represents the intensities of a series of amplitude measurements. Each result was then scaled to 100 dB. Differences from the amplitude obtained by scaling from the "normal" test intensity are represented as dB differences, left scales (from Manlcy, G.A., 1972; used by permission)

sities as possible for l o c a t i o n D. All results were t h e n scaled to 100 d B a n d a n y differences f r o m t h e v a l u e o b t a i n e d a t t h e i n t e n s i t y n o r m a l l y u s e d a t t h a t f r e q u e n c y e x p r e s s e d in t e r m s of + or - - dB. T h e results are p l o t t e d in Fig. 5. W i t h i n t h e l i m i t s of a c c u r a c y of s e t t i n g t h e s o u n d i n t e n s i t y ( : L 2 d B ) a n d of t h e MSssbauer s y s t e m , t h e d r u m response shows no consistent t r e n d s a n d can be r e g a r d e d as linear.

Discussion The d a t a r e p o r t e d here confirm a n d e x t e n d t h e findings of W e r n e r a n d W e v e r (1972) b y a q u i t e different technique. T h e y showed t h a t in this species t h e m i d d l e ear t r a n s f o r m e r " f a i l s a b o v e 4 k H z " . Their evidence was b a s e d on c o m p a r i n g t h e s e n s i t i v i t y of t h e e a r ( e s t i m a t e d

248

G.A. Manley:

by recording cochlear mierophonics at a standard level) when sound was fed to the eardrum as normal, with the sensitivity when sound was fed directly to the oval window. The latter response occurs, of course, without the benefit of the impedance-matching middle ear. They found that the sensitivity loss was large in the range 0.5 k H z - - 1 kHz (35 dB-57 dB) but this fell to a loss of only 10 dB above 4 kItz. Similarly, the data reported here indicate that at low frequencies the drum vibrates in phase and is thus well coupled to the extracolumella. At higher frequencies, not only does the single peak break up but also the new peaks become increasingly out of phase. In addition to the above, the experiments reported here produced clear evidence of the existence of a frequencydependent ratio. Due to the difficulty of their technique, Werner and Wever found no lever ratio. The ratio is high at low frequencies and, combined with efficient drum coupling indicates the middle ear is highly efficient at low frequencies. The lever ratio effectively disappears at high frequencies, due to large amplitude losses down the inferior process itself. These findings explain why Werner and Wever, after removing the middle ear, found only small losses of sensitivity at the higher frequencies. I t would be difficult to relate dB losses on removing the middle ear very specifically to different factors, even knowing the amplitudes of various middle ear components. This is especially so in view of the fact that the vibration patterns change subtly from one frequency to the next. However, it is clear that the eardrum does not vibrate like a stiff plate at low frequencies, although it does vibrate in phase. The "stiff plate" mode of vibration was observed by yon B~k6sy (1941) in human cadaver eardrums. Khanna and Tonndorf (1972) have recently shown by time-averaged holography that the cat tympanic membrane does not show a stiff-plate mode of vibration. This implies, as the amplitudes to the sides of the inferior process are high, that the Helmholtz curved-membrane lever could and probably does operate in these eardrums (see Khanna and Tonndorf, 1972 for a discussion of this theory). In view of the above considerations, and the changing patterns of vibration on the drum it is no longer helpful to consider that at any frequency, 2/3 of the drum area is the "effective" area (Werner and Wever, 1972). This is obviously a gross oversimplification. The type of lever system shown here necessitates a flexible junction between the columella and inferior process and the function is well served by non-ossification of the extracolumella. The fact that the extracolumella is cartilage, however, allows the steep slope along the inferior process seen at high frequencies and the consequent lack of efficiency of the lever system in this range. A further series of experiments has been undertaken to determine the cause of this high-frequency loss of efficiency.

The Middle Ear of the Tokay Gecko

249

The increasing inefficiency of this m i d d l e ear a t high frequencies occurs, however, in a f r e q u e n c y r a n g e where t h e inner e a r s e n s i t i v i t y is also i n d e p e n d e n t l y r a p i d l y decreasing (Manley, 1972). T h e inefficiency is t h u s of little consequence to this a n i m a l , t h e i n n e r ear a n d m i d d l e e a r are well m a t c h e d . A t low frequencies, t h e m i d d l e e a r is a n excellent i m p e d a n c e - m a t c h i n g device. I t has been n o t e d b y G a u d i n (1968) t h a t t h e e x t r a c o l u m e l l a in birds is p a r t l y ossified a n d this p r o b a b l y c o n t r i b u t e s to t h e b e t t e r h i g h - f r e q u e n c y response of birds (Mauley, 1971).

References B~k~sy, G. yon: ~ber die Mcssung der Schwingungsamplitnde der Geh6rknSchelchen mittels einer kapazitiven Sonde. Akust. Z. 6, 1-16 (1941). Campbell, H. W. : The effects of temperature on the auditory sensitivity of lizards. Physiol. Zool. 42, 183-210 (1969). Capranica, R. R., Frishkopf, L. S. : Responses of auditory units in the medulla of the cricket frog. J. acoust. Soe. Amer. 49, 1263A (1966). Dooling, R. L., Mulligan, J. A. : Audibility curve of the common canary. J. acoust. Soc. Amer. 47, 67 (1970). Frishkopf, L. S., Goldstein, M. H. : Responses to acoustic stimuli from single units in the eighth nerve of the bullfrog. J. aeoust. Soc. Amer. 35, 1219-1228 (1963). Gaudin, E. P. : On the middle ear of birds. Acta otolaryng. (Stockh.) 65, 316-326 (1968). Johnstone, J, R., Johnstone, B. M. : Unit responses from the lizard auditory nerve. Exp. Neurol. 24, 528-537 (1969). Johnstone, B.M., Taylor, K. J.: Mechanical aspects of cochlear iunction. In: Frequency analysis and periodicity detection in hearing (eds. R. Plomp and G. F. Smoorenburg), p. 81-93. Leiden: A. W. Sijthoff 1970. Johnstone, B. M., Taylor, K. J., Boyle, A. J. : Mechanics of the guinea pig cochlea. J. aeoust. Soc. Amer. 47, 504-509 (1970). Kawabata, I., Ishii, H. : Fiber arrangement in the tympanic membrane. Acta otolaryng. (Stockh.) 72, 243-254 (1971). Khanna, S. M., Tonndorf, J. : Tympanic membrane vibrations in cats studied by time-averaged holography. J. acoust. Soc. Amer. 51, 1904-1920 (1972). Konishi, M. : Comparative neurophysiological studies of hearing and vocalizations in songbirds. Z. vergl. Physiol. 66, 257-272 (1970). Lift, H.: Responses from single auditory units in the eighth nerve of the leopard frog. J. acoust. Soe. Amer. 45, 512-513 (1969). Loftus-Hills, J. J., Johnstone, B. M. : AuditolW function, communication, and the brain-evoked response in anuran amphibians. J. aeoust. Soc. Amer. 47, 11311138 (1970). Manley, G. A. : Frequency sensitivity of auditory neurons in the caiman cochlear nucleus. Z. vergl. Physiol. 66, 251-256 (1970a). Manley, G. A.: Comparative s~udies of auditory physiology in reptiles. Z. vergl. Physiol. 67, 363-381 (1970b). Manley, G. A. : Some aspects of the evolution of hearing in vertebrates. Nature (Lond.) 230, 506-509 (1971). Manley, G.A.: Frequency response of the ear of the Tokay Gecko. J. exp. Zool. 181, 159-168 (1972).

250

G. A. Manley: The Middle Ear of the Tokay Gecko

Manley, G. A., Irvine, D. R. F., Johnstone, B. M. : Frequency response of bat tympanic membrane. Nature (Lond.} 237, 112-113 (1972). Manley, J. A. : Single unit studies in the midbrain auditory area in Caiman. Z. vergl. Physiol. 71, 255-261 (1971). Patterson, W. C.: Hearing in the turtle. J. Aud. Res. 6, 453-464 (1966). Saunders, J. C., Johnstone, B. M. : A comparative analysis of middle ear function in non-mammalian vertebrates. Acta otolaryng. (Stockh.) 73, 353-361 (1972) Suga, N., Campbell, H. W.: Frequency sensitivity of single auditory neurons in the gecko, Coleonyx variegatus. Science 157, 88-90 (1967). Trainer, J. E. : The auditory acuity of certain birds. Ph.D. Thesis, Cornell University. (1946) Werner, u L., Wever, E. G.: The function of the middle ear in lizards: Gekl~o gecko and Eublephari8 macularius (Gekkonoidea). J. exp. Zool. 179, 1-16 (1972). Wever, E. G., Werner, Y. L.: The function of the middle ear in lizards: Crotaphytu8 collari8 (Iguanidae). J. exp. Zool. 175, 327-342 (1970). Geoffrey Manley Biology Department MeGill University P. O. Box 6070 Montreal 101, Quebec, Canada