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5/13/2017
Directional Hearing & LDV
Daniel Robert
Abstract 1 "The ears of a fly: Innovative biomechanics for directional hearing"
In most animals, the localization of sound constitutes a basic processing task of
the auditory system. The directional detection of an incident sound wave relies on
interaural amplitude and time differences and, in some cases, also on the
spectral content of the signal. In small animals, the auditory receptors are forcibly
set close together, a design constraint that imposes a very short interaural
distance and that results in interaural time and amplitude cues (ITD and IID) that
can become vanishingly small. Yet, some small animals are endowed with
directional hearing, and are therefore very attractive study subjects because they
constitute, in some sense, original biological solutions to some acute
miniaturization problems.
One such small animal is a parasitoid fly that acoustically locates and attacks
singing field crickets (1). This behavior is mediated by minute auditory organs
that are separated from each other by only 520 µm (2). In such case, the largest
ITDs and IIDs in the incident sound field (for 90° azimuth) are no larger than 2 µs
and 1 dB, respectively.
The mechanical response of the tympanal membranes was measured by laser
Doppler vibrometry (LDV). The mechanical response of the tympanal membranes
has a pronounced directional sensitivity, in which ITD and IID in the mechanical
response are significantly larger than those available in the acoustic field. At 5
kHz (6.8cm wavelength), the membrane closest to the sound source vibrates
significantly more (3-6 dB), and earlier (50 µs), than the contralateral tympanal
membrane. Deflection shape analysis at different frequencies reveals that the
mechanical dynamics of both tympana is responsible for the observed
asymmetrical response (Fig.1). Anatomically, these ears are particular in that
they are physically connected by a cuticular bridge, the relative rigidity of which
constitutes the key to the observed directional mechanical response (3). Hence,
mechanical preprocessing increases minimal acoustic cues into more substantial
mechanical cues that can be processed by the nervous system.
Ref. 3D-Video
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Fig.1. Mechanical response of the tympanal
complex. The upper panel shows maximum outward
deflection of the ipsilateral tympanal membrane
(ipsi) at 5 kHz, while the lower panel depicts
maximum inward deflection. Microscanning LDV
was used to monitor vibration velocities at 364
tympanal locations. Coherence between acoustic
input and mechanical response exceeded 0.95. The
acoustic stimulus consisted in band limited random
noise (1 to 30 kHz; 94 dB SPL) delivered at 90°
azimuth. Supported by the Swiss National Science
Foundation
References: 1. Cade WH (1975) Science 190: 13121313, 2. Robert D, Amoroso J, Hoy RR (1992)
Science 258: 1135-1137. 3. Robert D, Miles RN,
Hoy RR (1996) J. comp. Physiol. A 179: 29-44,
Miles RN, Robert D, Hoy RR J (1995) Acoust Soc
Am 98: 3059-3070.
Abstract 2 - "Insect ears and principles of acoustic detection"
In technology, as in biology, the process of acoustic sensing involves, as a first
step, the conversion of acoustic energy into mechanical energy. In insects, such
conversion takes place in well-developed, yet small, auditory organs. In the
course of evolution, insects have much diversified and such diversity is also
reflected in structures and functions of their hearing organs (1). Ears can thus be
found just about anywhere on the general insect body plan; on the legs,
abdomen, thorax, chest, wings, mouthparts. Insects are however good at doing
things small. Their ears can be thus regarded as miniaturized acoustic sensors
that have, in an evolutionary sense, been proven sufficiently refined and efficient
by the processes of natural and sexual selection. From this evolutionary vantage
point, the resources embedded in the considerable diversity of insect auditory
systems promise to be of significant inspirational value. Our investigations thus
focus on questioning what are the information processing capacities of very small
auditory organs, and what can be learned from it. The analysis has concentrated
on peripheral auditory mechanisms in flies and mosquitoes.
One of the outcomes of this research has been to explore the possibility of
transferring the some of the observed sound processing mechanisms to
microphone technology. Using MEMs technology, early prototypes provide
evidence that sensors inspired from the flies auditory organs are, despite their
small size, directionally sensitive at about 5 kHz. Further optimization promises
to provide the basis for subminiature microphones endowed with directionality in
the range of human speech.
[Work supported by the Swiss National Science Foundation].
References: 1. Hoy RR, Robert D 1996 Tympanal hearing in insects. Ann Rev Entomol
41:433-450
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Organs of sound reception in invertebrates > Types of insect auditory structures >
Tympanal organs
The tympanal organ of insects consists of a group of scolophores associated
with a thin, horny (chitinous) membrane at the surface of the body, one on
each side. Usually the scolophores are attached at one end by a spinous process
to the tympanic membrane (eardrum); the other ends rest on an immobile
part of the body structure. When the membrane moves back and forth in
response to the alternating pressures of sound waves, the nerve fibre from the
ganglion cell of the scolophore transmits impulses to the central nervous
system. Because the tympanic membrane is activated by the pressure of sound
waves, this is a pressure type of ear.
Simple tympanal organs, such as those found in moths, contain only two or four
elements, or scolophores. In cicadas, on the other hand, these organs are
highly developed; they include a sensory body (a number of scolophores in a
capsule) that may contain as many as 1,500 elements.
With 80 to 100 scolophores, the grasshopper ear, which has been studied more
thoroughly than any other insect ear, is structurally between that of moths and
cicadas. Ordinarily, the tympanic membrane is hidden beneath the base of the
insect's wing cover. A bundle of auditory nerve fibres runs from one side of the
sensory body, which lies on the inner surface of the membrane, and joins other
nerve fibres of the region to form a large nerve extending to a ganglion (nerve
centre) in the thorax.
Ref. 3D-Video
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