Directionality of the lizard ear
1 Center for Sound Communication, SDU Odense University, Campusvej 55,
DK-5230 Odense M, Denmark
2 Lehrstuhl für Zoologie, Technische Universität München,
Lichtenbergstrasse 4, 85747 Garching, Germany
* Author for correspondence (e-mail: JCD{at}biology.sdu.dk)
Accepted 26 January 2005
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Summary |
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The present work shows that acoustical coupling of the two eardrums in lizards produces the largest directionality of any terrestrial vertebrate ear studied. Laser vibrometric studies of tympanic motion show pronounced directionality within a 1.8-2.4 kHz frequency band around the best frequency of hearing, caused by the interference of ipsi- and contralateral inputs. The results correspond qualitatively to the response of a simple middle ear model, assuming coupling of the tympana through a central cavity. Furthermore, observed directional responses are markedly asymmetrical, with a steep gradient of up to 50-fold (34 dB) response differences between ipsi- and contralateral frontal angles. Therefore, the directionality is easily exploitable by simple binaural subtraction in the brain. Lizard ears are the clearest vertebrate examples of directionality generated by tympanic coupling.
Key words: hearing, auditory, eardrum, tympanum, reptile, lizard, frog, bird
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Introduction |
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This pressure-difference receiver principle was first described for insect
ears by Autrum (1940). Thus,
the driving force for tympanum vibrations, i.e. the pressure difference
between its two sides, depends on the amplitude and phase of the direct and
the indirect sound components, which again depend on sound direction and
frequency. In such a pressure-difference receiver, binaural interaction
already takes place at the tympanum, and the directionality should be strongly
frequency dependent. Hence, at low frequencies the sound pressure will be
nearly equal on the two sides of the tympanum. At high frequencies, the phase
shift resulting from arrival-time differences exceeds a cycle and the
resulting pressure difference will be a complicated, non-monotonic function of
sound direction.
The directionality of the ear is also highly dependent on the interaural
attenuation in the frequency range of interest
(Klump, 2000). For example, in
the barn owl, it has been shown that at high frequencies, the acoustical
transmission of the interaural canal is highly attenuated and the ears are
essentially unconnected at behaviorally important frequencies (above 5 kHz),
whereas interaural transmission is efficient at lower frequencies
(Moiseff and Konishi, 1981
).
In other birds, however, strong directional effects resulting from interaural
coupling have been demonstrated. In the quail, pronounced directionality of
the auditory periphery results from strong interaural coupling (less than 5 dB
interaural canal transmission attenuation at frequencies below 5 kHz;
Coles et al., 1980
;
Hill et al., 1980
).
Directivity (i.e. the physical directional characteristics) patterns based on
cochlear microphonic measurements were cardioid at lower frequencies and
figure-of-eight shaped at high frequencies, and the patterns were changed when
one eardrum was blocked. The directionality approaches 25 dB, but the
resulting patterns of directionality are complicated and so strongly frequency
dependent that the functional implications for sound localization, including
for further neural processing, are very unclear. In frogs, where the tympana
are coupled through large Eustachian tubes and the mouth cavity, the
directivity patterns of eardrum vibrations are ovoidal with a maximal
directional difference of 6-10 dB
(Jørgensen, 1991
;
Jørgensen et al.,
1991
). Maximal sensitivity and directivity are found at around 2
kHz, depending on the size and on the species of frog
(Christensen-Dalsgaard,
2005
).
Lizards in general have very sensitive ears with delicate eardrums, no
external ear (although a short external ear canal is present in some) and,
like frogs and birds, a single auditory ossicle, the columella, that is
coupled to the eardrum through a cartilaginous extracolumella
(Manley, 1990). Their best
frequencies of hearing range from 1 to 3 kHz, and the eardrum vibrations show
band-pass characteristics (Saunders et
al., 2000
). The high-frequency sensitivity is influenced by the
mechanics of the auditory ossicle and the extracolumella
(Manley, 1990
;
Werner et al., 1998
). At the
best frequencies of hearing in lizards, the wavelengths of sound are much
larger than the head dimensions, so ILD cues due to sound diffraction will be
small. The Eustachian tubes are very large, however, so the eardrums are
connected directly to the mouth cavity. Based on these anatomical features, it
has previously been speculated that there would be considerable acoustical
interaction between the eardrums, which would lead to strong directionality
(Wever, 1978
). The
directionality would be frequency dependent and dependent on the properties of
the acoustical elements, however, and given the large variation in directivity
in other animals with coupled ears (frogs and birds), direct measurement is
needed to demonstrate whether the acoustical coupling leads to any useful
directionality. We present here such direct measurements of the directionality
of eardrum vibrations in four lizard species, showing a large eardrum
directivity that in all species is dependent on acoustical interaction between
the two tympana. We have found that the interaural coupling in lizards
produces the largest directivity of any tetrapod studied so far and that a
simple model of middle ear acoustics can account for most of the
directionality.
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Materials and methods |
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Sound was emitted in turn from one of 12 loudspeakers (JBL 1G, Northridge, CA, USA) that were placed at 30° intervals around and approximately 50 cm from the animal's head. The sound was generated using Tucker-Davis (TDT, Alachua, FL, USA) system 2 hardware. The stimuli used were frequency sweeps, at levels of 80-90 dB SPL, flat from 0.2 to 8 kHz and 175 ms in duration. The signal sent to the loudspeakers was deconvoluted by the transfer function of each loudspeaker (measured using a 0.5'' microphone B&K, Copenhagen, Denmark, at the center of the set-up before placing the animal) by dividing the spectrum of the sweep with the transfer function of the speaker. The sweeps were directed in turn to each of the speakers using a customised switching device. The sound at the animal's eardrum was measured with a B&K 4182 probe microphone, digitised (22 kHz sample rate, 8192 samples) using the TDT AD-converter (AD2) and stored in a PC. Stimulation and recording was controlled by custom software (DragonQuest, Odense, Denmark).
Vibration of the tympanic membrane, the skin of the head and the body wall overlying the lung was measured using a Dantec laser doppler vibrometer. Tiny, highly reflecting white flakes (`Tippex') were placed on the tympanum at the tip of the columella, on the nearby skin of the head and over the ipsilateral lung. The analog laser signal was digitised using the TDT AD-converter (AD2). Sound and laser recordings were averaged over 10 presentations. In some of the animals, for comparative measurements, the contralateral eardrum was temporarily occluded by a dome of Vaseline that did not touch the eardrum. The data were analysed using custom software.
Data analysis
The laser spectra were corrected for small directional variations in the
sound spectrum due to sound diffraction by subtracting the spectrum recorded
by the probe microphone from the laser spectrum. Since the speakers were
equalized and centred at the start of the experiment, sound diffraction could
be measured by comparing the probe spectrum recorded with ipsilateral and
contralateral stimulation. Eardrum directivity was displayed either as
conventional line plots or cylinder surface plots. Cylinder surface plots are
interpolated contour plots of amplitude with direction (x, 12
directions) and frequency (y, 500 frequency bands) as independent
variables. Each horizontal line corresponds to a polar plot, and each vertical
line corresponds to an amplitude spectrum of eardrum motion stimulated by
sound from a certain direction. All plots were generated by SigmaPlot, version
8.0. The directional bandwidth was measured from the spectra as the frequency
band where the response to ipsilateral and contralateral stimulation differed
by more than 3 dB.
To study the possible neural processing of such input signals to the ears,
we used the interaural vibration amplitude difference (IVAD) function
(Jørgensen et al.,
1991) to predict the output of a simplistic model neuron
that is excited by the ipsilateral ear and inhibited by the contralateral ear
(an EI neuron; Goldberg and Brown,
1969
). The function computes the vibration amplitude difference
(in dB) between the input from the ipsi-and contralateral ear. The
directionalities of the two ears are assumed to be mirror reflections along
the frontal-caudal axis.
The model data presented were based on a lumped-element electrical analog
of a lizard middle ear (see diagram in fig. 7 in
Fletcher, 1992). The model has
two sound inputs (P1 and P2) that
differ in phase by:
![]() | (1) |
where is the angular frequency, c the velocity of sound,
d the interaural distance and
the sound incidence angle
relative to the body axis. The parameters used were based on measurements from
Mabuya: interaural distance 13 mm, cavity volume V=0.7
cm3, tympanum area 20 mm2 and tympanum thickness 8
µm. Estimated parameters were tympanum resonance frequency 2800 Hz, mass of
tympanum (loaded by middle ear) 0.5 mg, tympanum quality factor Q 1.2.
From these parameters, the tympanic impedance Zt and
the cavity impedance ZV can be calculated (see
Fletcher, 1992 for details) as:
![]() | (2) |
and
![]() | (3) |
These values deviate by up to a factor 30 from the impedances calculated
for the frog tympanum by Aertsen et al.
(1986). A part of the
discrepancy is probably due to the different structure of the lizard tympanum
(smaller mass), but the calculations in Aertsen et al.
(1986
) were also based on the
acoustical measurements, whereas the parameters used here are estimated from
the structural characteristics of the ear.
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Results |
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Linearity
Calculations of transfer functions between sound and eardrum vibrations
presuppose that the eardrum vibrations are linear at the amplitude ranges
used. We therefore investigated the linearity of the eardrum vibrations. As an
example, Fig. 1A shows the
response of the agamid lizard Leiolepis stimulated at three levels.
When the sound spectrum is subtracted, the spectra have nearly identical
shapes, and the levels are also similar, indicating a linear response. This
was also characteristic of the data from Mabuya and
Ctenosaura. In contrast, the response of Gekko is very
different. Here, the vibration amplitude (when corrected for sound level)
decreases at high stimulus levels, and the vibration spectrum also has a
different shape at low and high stimulus levels
(Fig. 1B),indicative of a
non-linear response at high stimulus levels. Consequently, for Gekko
only data from low-level stimulation are used.
|
Directionality
Fig. 2 shows eardrum
vibration amplitude spectra for the four species. In all animals, the
vibration spectrum has band-pass characteristics with peak response to
ipsilateral stimulation around 3 kHz in the smaller species (Mabuya,
Fig. 2A; Leiolepis,
Fig. 2B) and 1.8 kHz in the
larger species (Ctenosaura, Fig.
2C; Gekko, Fig.
2D). Peak vibration amplitudes are up to 4.9 mm s-1 at
94 dB SPL (1 Pa). Data are summarized in
Table 1. There are large and
consistent differences (up to 28 dB) between the responses to ipsi- and
contralateral stimulation. The response to contralateral stimulation shows a
reduction in vibration amplitude compared to the response to ipsilateral
stimulation, and one or more pronounced dips in the frequency spectrum.
Generally, at all frequencies responses from all contralateral angles are
lower than or approximately equal to ipsilateral responses. The directional
bandwidth (for definition, see Materials and methods) was measured from the
spectra and ranges from 1.79 kHz to 2.42 kHz (see
Table 1).
|
In Fig. 3 the directional responses of the eardrum are displayed as cylinder surface plots (for details, see Materials and methods) to facilitate comparison between the species. The plots are similar in the four species and show large ipsi-contralateral differences. The response is markedly asymmetrical with a steep gradient across the midline (0°) and, due to this asymmetry, the difference is maximal between 60° ipsi- and contralateral angles (up to 34 dB). When the contralateral eardrum was occluded by Vaseline, the directivity changed to an omnidirectional pattern (Fig. 3E,F). The maximal response amplitude of the ipsilateral eardrum is, however, comparable before and after occlusion.
|
The diffraction of sound around the body of the lizard produces possible additional directional cues. Fig. 4 shows diffraction data from the four species. Diffraction measured as ipsilateral-contralateral sound pressure difference increases with frequency. A 2 dB difference is found at approximately 2 kHz in the smaller lizards (Leiolepis, Mabuya) and at approximately 1 kHz in the larger lizards (Gekko, Ctenosaura).
|
A common type of binaural interaction in neurons of the auditory brain is
that the input from one ear is excitatory and the input from the other ear
inhibitory (EI neuron; Goldberg and Brown,
1969). As a simple model, the resulting directivity of an EI
neuron can be found by subtracting the directivity of one ear by its mirror
reflection along the midline of the animal. The result of this operation is
the interaural vibration amplitude difference (IVAD) plot shown for the four
species in Fig. 5. Generally,
the asymmetrical directivity is sharpened (differences across the
frontal-caudal axis are doubled) by the subtraction, increasing the steepness
of the gradient across the frontal midline to more than 50 dB by this simple
operation.
|
It is instructive to compare the directivity of the lizards with the directivity of another animal with acoustically coupled ears. Fig. 6A shows results from measurements of eardrum vibrations in a grass frog Rana temporaria. The frog tympanum also has a band-pass characteristic, with a more limited frequency response than the lizards. Also, the spectrum clearly shows two peaks around 670 Hz and 1570 Hz. The directionality is largest between the two peaks. The directional bandwidth (as defined in Materials and methods) is 550 Hz, and the maximal interaural difference (IVAD plot, Fig. 6B) is 14 dB.
|
The occlusion experiments (Fig.
3E,F) indicate that the directionality is generated by acoustical
coupling of the two eardrums. The eardrums are effectively connected by a
common air space, since the Eustachian tubes connecting the middle ear and
mouth cavities in lizards are very wide. To investigate the effects of
acoustical coupling, we used a simple acoustical model of the ear
(Fig. 7). The model (Fletcher
and Thwaites, 1974; Fletcher,
1992) is based on an electrical analog of the ear
(Fig. 7B), and the parameters
used are estimated from measurements (for the parameter values, see Materials
and methods). The model generates a directional response in the frequency
range of 1-2.5 kHz (Fig. 7D),
with reduced vibration spectrum for contralateral stimulation and a pronounced
dip in the contralateral frequency response
(Fig. 7C). Note, however, that
this simple two-input model will necessarily produce a response that is
symmetrical around the interaural axis.
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Discussion |
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The consequence of the strong frontal asymmetry of the lizard ear is that
binaural comparison will intensify the directionality. As shown in
Fig. 5, when the directional
response of one ear is subtracted from its mirror image, the directionality is
greatly enhanced, generating a very steep gradient along the frontal angles in
the frequency range from 1.5 to 3 kHz. This operation can be seen as a
simplified model of the output of a central binaural neuron that is excited by
inputs from one ear and inhibited by input from the other ear (that is, an EI
neuron; Goldberg and Brown,
1969). The steep gradient is especially useful, since a very
simple behavioral rule, i.e. turning towards the side of excitatory input to
the binaural neuron, would direct the lizard's head to the sound source with
considerable precision. Since the neural input will also reflect sound
diffraction effects around the animal, the data in
Fig. 5 are not corrected for
sound diffraction. We only show amplitude data in the plot, but the
concomitant phase changes in the tympanic responses would result in an
increased time difference between ipsi- and contralateral sound directions (up
to 180° ipsilateral lead, i.e. 250 µs at 2 kHz), which could be
important for inhibitory interactions that sharpen the neural
directionality.
The difference between the lizards and other tetrapods that have
acoustically connected eardrums (i.e. birds and frogs) is that the interaural
attenuation is probably much smaller in lizards than in birds
(Klump, 2000) and that, as
stated above, the directionality of the frog ear
(Fig. 6) is complicated by the
fact that sound also enters the middle ear cavity via the floor of
the (closed) mouth and via the lungs
(Jørgensen, 1991
;
Jørgensen et al., 1991
;
Narins et al., 1988
;
Christensen-Dalsgaard, 2005
).
The mouth floor in lizards is probably relatively impermeable to sound and,
while sound is also received via the lungs in the lizards
(Hetherington, 2001
), in the
species investigated here the effects were very small and confined to a narrow
frequency range around 1 kHz (data not shown). Thus, lizards are the clearest
tetrapod example of pressure-difference receiver ears.
The neural processing of directional information in lizards is almost
unknown (Manley, 1981;
Szpir et al., 1990
), but it is
nevertheless likely that EI neurons are present in the auditory brain, since
such binaural neurons are found in both the mammalian
(Goldberg and Brown, 1969
;
Irvine, 1992
) and anuran
(Feng and Capranica, 1976
) CNS.
The only available data on directional sensitivity of neurons in the auditory
pathway of lizards (single-cell recordings from torus semicircularis in the
midbrain of Gekko gecko; Manley,
1981
) show pronounced differences between ipsi- and contralateral
stimulation that probably reflect both acoustical interaction between the ears
and neural interaction in the auditory pathway of the brain. Similarly,
behavioral data on sound localization in lizards are very scarce, limited to
one study showing that Mediterranean house geckos Hemidactylus
tursicus intercept calling crickets and approach loudspeakers
transmitting cricket calls (Sakaluk and
Bellwood, 1984
). The paucity of data reflects the fact that it is
very difficult to condition lizards to respond to sound
(Manley, 2000
), and therefore
the use of the acute directional hearing in lizards is presently unknown. The
present data do, however, strongly suggest that acute directional hearing is
likely to be a much more generally useful and advantageous feature of lizard
ears.
One interesting additional feature of the data may reveal the reason why,
in all auditory papillae of lizards, the hair cells are divided into two
areas. There is one area of hair cells that is evolutionarily stable, almost
certainly plesiomorphic (primitive) and in which the cells respond best to
frequencies below roughly 1 kHz. There is in addition one or, in some groups,
two, areas of hair cells that respond to frequencies above 1 kHz. This is true
of all groups, in spite of the independent evolution of the particular
configurations shown by the different lizard families and the high anatomical
variability of the higher-frequency hair-cell areas
(Manley, 2002). With the
present data, interaural differences were negligible at frequencies below 1
kHz, but were large at frequencies processed in the variable high-frequency
hair-cell areas. This suggests that the family-specific patterns of the
higher-frequency hair-cell areas could have been due to the presence in the
auditory nerve of information relevant to sound localization.
It should be noted, however, that pressure gradient receivers, often regarded as specializations of small animals for directional hearing, may more likely reflect the plesiomorphic state of the auditory system in the terrestrial vertebrates, since an ancestral ear having tympana formed from skin covering skull fenestrations that opened into the mouth cavity would essentially show the response of the model in Fig. 7. Any enclosure of the middle ear and tympanum in a middle ear cavity leading to a pressure-sensitive (e.g. mammalian-type) ear would be a derived condition, maybe caused by an increase in the size of the brain and by the benefits of isolating the middle ear from the respiratory and food-intake pathways.
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Acknowledgments |
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