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==== Infrasound sensitivity ====
The auditory sensitivity thresholds have been measured behaviorally for one individual young female Indian elephant. The [[Classical conditioning|conditioning]] test for sensitivity requires the elephant to respond to a stimulus by pressing a button with its trunk, which results in a sugar water reward if the elephant correctly identified the appropriate stimulus occurrence.<ref name="Heffner & Heffner 1980">{{cite journal |last=Heffner |first=H. |author2=R. Heffner |title=Hearing in the elephant (Elephas maximus) |journal=Science |year=1980 |volume=208 |pages=518–520 |doi=10.1126/science.7367876 |pmid=7367876 |issue=4443 |bibcode=1980Sci...208..518H}}</ref> To determine auditory sensitivity thresholds, a certain frequency of sound is presented at various intensities to see at which intensity the stimulus ceases to evoke a response. The auditory sensitivity curve of this particular elephant began at 16 Hz with a threshold of 65 dB. A shallow slope decreased to the best response at 1 kHz with a threshold of 8 dB, followed by a steep threshold increase above 4 kHz. According to the 60 dB cut-offcutoff, the upper limit was 10.5 kHz with absolutely no detectable response at 14 kHz.<ref name="Heffner & Heffner 1980" /> The upper limit for humans is considered to be 18 kHz. The upper and lower limits of elephant hearing are the lowest measured for any animals aside from the pigeon.<ref name="Heffner & Heffner 1980" /> By contrast, the average best frequency for animal hearing is 9.8 kHz, the average upper limit is 55 kHz.<ref name="Heffner & Heffner 1980" />
The ability to differentiate frequencies of two successive tones was also tested for this elephant using a similar conditioning paradigm. The elephant's responses were somewhat erratic, which is typical for mammals in this test.<ref name="Heffner & Heffner 1980" /> Nevertheless, the ability to discriminate sounds was best at frequencies below 1 kHz particularly at measurements of 500 Hz and 250 Hz.<ref name="Heffner & Heffner 1980" />
==== Infrasound sensitivity ====
In experiments using heart- rate conditioning, pigeons have been found to be able to detect sounds in the infrasonic range at frequencies as low as 0.5 Hz. For frequencies below 10 Hz, the pigeon threshold is at about 55 dB which is at least 50 dB more sensitive than humans.<ref name="Kreithen & Quine 1979" /> Pigeons are able to discriminate small frequency differences in sounds at between 1 Hz and 20 Hz, with sensitivity ranging from a 1% shift at 20 Hz to a 7% shift at 1 Hz.<ref name="Quine 1981" /> Sensitivities are measured through a heart- rate conditioning test. In this test, an anesthetized bird is presented with a single sound or a sequence of sounds, followed by an electric shock. The bird's heart- rate will increase in anticipation of a shock. Therefore, a measure of the heart- rate can determine whether the bird is able to distinguish between stimuli that would be followed by a shock from stimuli that would not.<ref name="Kreithen & Quine 1979" /><ref name="Quine 1981" /><ref>{{cite journal |last=Delius |first=J. D. |author2=R. M. Tarpy |title=Stimulus control of heart rate by auditory frequency and auditory pattern in pigeons |journal=Journal of the Experimental Analysis of Behavior |year=1974 |volume=21 |issue=2 |pages=297–306 |doi=10.1901/jeab.1974.21-297 |pmid=4815397 |pmc=1333197}}</ref> Similar methods have also been used to determine the pigeon's sensitivity to barometric pressure changes, polarized light, and UV light.<ref name="Kreithen & Quine 1979" /> These experiments were conducted in sound isolation chambers to avoid the influence of ambient noise. Infrasonic stimuli are hard to produce and are often transmitted through a filter that attenuates higher frequency components. Also, the tone burst stimuli used in these experiments were presented with stimulus onset and offsets ramped on and off gradually in order to prevent initial turn-on and turn-off transients.<ref name="Kreithen & Quine 1979" />
In order to use infrasound for navigation, it is necessary to be able to localize the source of the sounds. The known mechanisms for sound localizations make use of the time difference cues at the two ears. However, infrasound has such long wavelengths that these mechanisms would not be effective for an animal the size of a pigeon. An alternative method that has been hypothesized is through the use of the [[Doppler shift]].<ref name="Quine 1981" /> A Doppler shift occurs when there is relative motion between a sound source and a perceiver and slightly shifts the perceived frequency of the sound. When a flying bird is changing direction, the amplitude of the Doppler shift between it and an infrasonic source would change, enabling the bird to locate the source. This kind of mechanism would require the ability to detect very small changes in frequency. A pigeon typically flies at 20 km/h, so a turn could cause up to a 12% modulation of an infrasonic stimulus. According to response measurements, pigeons are able to distinguish frequency changes of 1-7{{nowrap|1–7{{hsp}}%}} in the infrasonic range, showing that the use of Doppler shifts for infrasound localization may be within the pigeon's perceptive capabilities.<ref name="Quine 1981" />
In early experiments with infrasound sensitivity in pigeons, surgical removal of the columella, a bone that links the [[tympanic membrane]] to the [[inner ear]], in each ear severely reduced the ability to respond to infrasound, increasing the sensitivity threshold by about 50 dB. Complete surgical removal of the entire [[cochlea]], lagena, and columellae completely abolished any response to infrasound.<ref name="Kreithen & Quine 1979" /> This shows that the receptors for infrasonic stimuli may be located in the inner ear.
Neural fibers that are sensitive to infrasonic stimuli have been identified in the pigeon and their characteristics have been studied. It turns out that, although these fibers also originate in the inner ear, they are quite different from normal acoustic fibers.
Infrasound sensitive fibers have very high rates of spontaneous discharge, with a mean of 115imp/s115{{nbs}}impulses per second, which is much higher than the spontaneous discharge of other auditory fibers.<ref name="Schermuly 1990a" /> Recordings show that discharge rates do not increase in response to infrasound stimuli but are modulated at levels comparable to the behavioral thresholds.<ref name="Schermuly 1990a" /> Modulation depth is dependent on stimulus frequency and intensity. The modulation is phase locked so that the discharge rate increases during one phase of the stimulus and decreases during the other, leaving the mean discharge rate constant.<ref name="Schermuly 1990a" /> Such pulse-frequency modulation allows the stimulus analysis to be independent of the peripheral tuning of the [[basilar membrane]] or the [[hair cells]], which is already poor at low auditory frequencies.<ref name="Schermuly 1990a" /> Unlike other acoustic fibers, infrasonic fibers do not show any indication of being tuned to a particular characteristic frequency.<ref name="Schermuly 1990a" />
By injecting fibers that were identified to be sensitive to infrasound with HRP (Horseradish Peroxidase), the ___location and morphology of the stained fibers can be observed in sections under a microscope. Infrasound sensitive fibers are found to be simple bipolar cells in the [[auditory ganglion]] with a diameter of {{nowrap|1.6-{{hsp}}–{{hsp}}2.2 μm}} at the axon and {{nowrap|0.9-{{hsp}}–{{hsp}}1.2 μm}} at the dendrites.<ref name="Schermuly 1990b">{{cite journal |last=Schermuly |first=L. |author2=R. Klinke |title=Origin of infrasound sensitive neurones in the papilla basilaris of the pigeon: an HRP study |journal=Hearing Research |year=1990 |volume=48 |issue=1–2 |pages=69–78 |doi=10.1016/0378-5955(90)90199-y|pmid=1701169 |s2cid=4761698 }}</ref> They originate in the apical end of the cochlea and they are located near fibers that transmit low frequency sounds in the acoustic range. Unlike the ordinary acoustic fibers which terminate on the neural limbus, the infrasonic ones terminate on cells on the free basilar membrane.<ref name="Schermuly 1990b" /> Furthermore, infrasonic fibers terminate on {{nowrap|2-{{hsp}}–{{hsp}}9}} hair cells while normal acoustic fibers connect to only one.<ref name="Schermuly 1990b" /> Such characteristics would make these fibers analogous to fibers connecting to the outer hair cells in mammals, except that mammalian outer hair cells are known to have efferent fibers only and no afferents.<ref name="Schermuly 1990b" /> These observations suggest that the infrasound sensitive fibers are in a class separate from ordinary acoustic fibers.
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