Hearing

Question 1

what is hearing?

Answer 1

Perception of airbourne vibrations

Question 2

what functions do hearing organs serve?

Answer 2

1. coupling of sound to organism
2. conversion of sound into mechanical energy
3. conversion into useful nerve signals.

Question 3

What dimensions do ears face trade offs between?

Answer 3

freq range
freq resolution (different freqs merge)
Temporal resolution

Question 4

in what 2 basic ways do ears adapt?

Answer 4

Lose sensitivity at the end of the day

Increase volume after a while (adapt to amount of energy like eyes do in a dark room)

Question 5

what is the minimum detectable threshold of mechanical displacement
What is the human threshold for hearing.

Answer 5

100 picometre to 1 nanometre.minimum energy detectable at threshold is 42J, 1000x more sensitive than sight.
threshold for human hearing is 1000Hz (0dB).
human sensitivity has decreased over 1000 years or so due to ecotoxicity and earphones.

Question 6

what is a pure tone, and how can it be produced?

Answer 6

a sinosoidal wave
by a rotating circle which moves a piston inflating and deflating balloon, causing air around it to be repeatedly compressed and dispersed.

Question 7

what 3 ways do sound waves interact with the environment?

Answer 7

1. reflection - echo off a heavy object, when the object is larger than the WL
2. Transmission - waves pass over a small object
3. Diffraction, sound shadow created over a big object. Object sort of equal to WL.

usually a very complex soundfield, with physically much difference, but our ears manage to convey a continuous sound.

Question 8

2 examples of how some sounds are favorable in diff environments.

Answer 8

Elephants use infra sound freq which we cant hear, favourable in savannah.
Cluttered structure in forests favours mid range freqs.

Question 9

how does temp affect sound?

Answer 9

temperature affects sound conduction capacity.
sound waves move faster in warm temps, eg birds calling near ground have soundwaves refracted upwards, and there are sound shadows near the ground.
Whales call at particular depths to allow sound to travel long distance.

Question 10

give an example of acoustic competition

Answer 10

When both mygalopsis and hemisaga are present, hemisaga sings in the day and mygalopsis at night. interpreted as partitioning calling time.
however when mygalopsis removed, hemisaga calls all the time, suggesting mygalopsis inhibits hemisaga, and there is acoustic competition.

Question 11

give an example of animal adapting to ambient conditions

Answer 11

Lardner and bin Lakim, 2002.
Tree frog - Metaphrynella sundara, actively tune the pitch of their calls to resonate inside hollow tree cavities. when it resonates, male tree frogs sound closer, more attractive and louder. also expend more energy when resonating, in increasing tempo to take advantege of the acoustics.

Question 12

what change does doubling the distance from a sound source cause?

Answer 12

drop in 6 dB off sound pressure.

Question 13

What two types of hearing organs are there?

Answer 13

1. Pressure receiver - membrane over cavity, many vertebrates and insects.
2. Particle velocity receivers - hair like structure w sensory cells at base. directional, in many antennae, likely most common type.
   eg in Chironomid flies, base of antenna is pedicel, contains 16000 mechanosensory cells. Huge metabolic investment.

Question 14

what 3 factors allow directional hearing?

Answer 14

interaural time, amplitude and spectral content differences.
As the angle of incidence increases, the ITD increases. this feeds info to interneurones which respond to specific time delays btw sides of the animal.

Question 15

what is reduced as the size of the animal reduces?

Answer 15

ITD and IID - interaural time and intensity differences.

smaller animals have more problems encoding temporal cues in the nervous system.

Question 16

describe the cricket tympanal organs

Answer 16

On forelegs, with tracheal tubes running between legs and thorax, through which sound waves travel to other side.
Guard hairs protect the membrane from being punctured.
Pressure receivers convert acoustical energy into mechanical.
Open pressure receivers

Question 17

What 3 different pressure receivers are there?

Answer 17

1. Closed/independent - only detects pressure from external side.
2. open/pressure difference receivers - linked, detects difference in pressure from each side. allows sound from inside the ear to act on the tympanum. can compare pressure on contra and ipsilateral sides, which compensate each other, to find direction.
3. Mechanically coupled pressure receivers - earsclose together, with a pivot between two tympanum. increased IID and IAD, in very small animals eg flies.

Question 18

Describe the reproduction strategy of Ormia ochracea fly - phonotactic parasitoid.

Answer 18

Finds male cricket in which to rear its larvae, at night using acoustics. deposits 2-3 larvae on cricket, and 7-10 nearby to be picked up by others. Burrow into cricket to feed and emerge after 7-10 days.
larvae spare the wing muscle so the cricket can still sing for 5 more days and still reproduce.
How the fly finds the cricket has challenged the theory of directional hearing.

Question 19

describe the hearing organs of Ormia ochracea

Answer 19

prosternal tympanal membrane (only found in parasitoids, in 50 of 1000s species of fly).
females hearing range overlaps with cricket song on the maximal slope (when sensitivity to changes in sound intensity is highest). tympana and nerve coding are well adapted to detecting cricket calls.

Question 20

Describe a test to see ormia ochracea strategy for finding host.

Answer 20

Mueller and Robert, 2001.
cameras track flies movements when placed on a podium and played cricket noises from a loudspeaker. Used info to create a virtual reality experiment.
typical trajectory is efficient and accurate.
experienced fly tested again, with loadspeaker turned off partway through flight. Fly lands near the target but less accurately when switched off earlier. if there are 2 sound sources, one turned off, fly will switch to other target. perhaps why crickets chorus in groups, to spread parasitic load.

Question 21

How do fly hearing organs act as a mechanical processor?

Answer 21

a microphone next to each ear - aITD was 1.45 microseconds.
Muse laser doppler vibrometry to measure vibrations of membranes - mITD was 50 microseconds, much larger than aITD. therefore ears acting as a mechanical processor, generating the delay between ears and diff in vibration amplitude.

Question 22

How can you demonstrate that a fly can sense changes in directionality?

Answer 22

place on levitating styrofoam balls, sound played and walks.
accurately changes direction as sound source changes location, even on a scale on 1 degree. aka hyperacuity

Question 23

what is a method for detecting the frequency of sound?

Answer 23

Laser Doppler Virbometry
doppler shift of reflected beam compared to reference beam.
Fluctuations in freq are proportional to the vibration velocity of the object reflecting the light.
can use this method to see how the eardrum reflects.

Question 24

What is time stamping?

Answer 24

a method of neurones responding to a stimulus.
w/o time stamping, there is a spike for each stimulus occurring.
Time stamping used for high resolution temporal coding, neurone reacts only to onset of stimulus, accurately and consistently. shuts off as soon as stimulus stops. increased delay but reduced variation.

Question 25

How is amplitude coded for, in flies?

Answer 25

The lower the amplitude, the later the spike. so an ipsilateral neurone will experience the stimulus earlier than contralateral.
delay between 2 sides is in milliseconds.

Question 26

How does the fly coupled tympanal membrane work?

what 3 modes does it have?

Answer 26

Has inter-tympanal bridge between coupled receptors.

1. at freq below 4kHz, both sides move in unison, no directionality.
2. intermediate frequencies, system rocks about the fulcrum. time delays on order of 0.5 vibration.
3. when amplitude and direction not the same, hinge allows flexibility. one side is silenced by resonant behaviour of two arms of tympanal ridge.

Question 27

in Katydid (crickets) ears, how is sound energy coupled and amplified on the tympanum?

Answer 27

Strangely similar to mammalian ear but also in the cricket leg.
tympanum vibrates, and a small section vibrated inversely, over a pivot which transfers the same amount of energy, called an impedance transformer (in mammals this is the ossicles.). presence of impedance converter shows tympanum only functions to convert energy to freq analyser. is changes large displacement with small force (tympanum) to small displacement with a large force.
vibrations transmitted through the freq analyser - insect auditory vesicle (mammals - cochlea). codes for low freq at one end and high at the other.

Question 28

How does the human organ of corti work?

Answer 28

vibrations across cochlear partition where organ of corti is. basilar membrane is below OoC and vibrates the hair cells in the OoC. if oscillations of hair cells at a certain freq add up with good timing with those of basilar membrane, there is amplification of the resonance. process is a feedback amplifier and known as the cochlear amplifier.