Perception of infrasound: Difference between revisions

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{{Ethology}}

'''[[Infrasound]]''' is an anthropocentric term that refers to sounds containing some or all energy at frequencies lower than the low frequency end of human hearing threshold at 20&nbsp;Hz. It is known, however, that humans can perceive sounds below this frequency at very high pressure levels.<ref>{{cite journal|last=Yeowart|first=N. S.|coauthorsauthor2=M. J. Evans|title=Thresholds of audibility for very low-frequency pure tones|journal=[[J. Acoust. Soc. Am.]]|year=1974|volume=55|pages=814–818|doi=10.1121/1.1914605}}</ref> Infrasound can come from many natural as well as man-made sources, including weather patterns, topographic features, ocean wave activity, thunderstorms, [[magnetic storms]], earthquakes, [[jet streams]], mountain ranges, and rocket launchings.<ref>{{cite journal|last=Cook|first=R. K.|title=Atmospheric sound propagation|journal=Atmospheric exploration by remote probes|year=1969|volume=2|pages=633–669}}</ref><ref>{{cite journal|last=Procunier|first=R. W.|title=Observations of acoustic aurora in the 1-16 Hz range|journal=Geophys. J. R. Astron. Soc.|year=1971|volume=26|pages=183–189}}</ref> Infrasounds are also present in the vocalizations of some animals. Low frequency sounds can travel for long distances with very little attenuation and can be detected hundreds of miles away from their sources.<ref name="Kreithen & Quine 1979">{{cite journal|last=Kreithen|first=M. L.|coauthorsauthor2=D. B. Quine|title=Infrasound detection by the homing pigeon: A behavioral audiogram|journal=Journal of Physiology A|year=1979|issue=129|pages=1–4}}</ref><ref name="Langbauer et al 1990">{{cite journal|last=Langbauer|first=W. R.|coauthors=K. B. Payne, R. A. Charif, E. M. Thomas|title=Responses of captive African elephants to playback of low-frequency calls|journal=Canadian Journal of Zoology|year=1990|issue=67|pages=2604–2607}}</ref>

== Mammals ==

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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.|coauthorsauthor2=R. Heffner|title=Hearing in the elephant (Elephas maximus)|journal=Science|year=1980|volume=208|pages=518–520|doi=10.1126/science.7367876}}</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&nbsp;Hz with a threshold of 65dB. A shallow slope decreased to the best response at 1&nbsp;kHz with a threshold of 8dB, followed by a steep threshold increase above 4&nbsp;kHz. According to the 60dB cut-off, the upper limit was 10.5&nbsp;kHz with absolutely no detectable response at 14&nbsp;kHz.<ref name="Heffner & Heffner 1980" /> The upper limit for humans is considered to be 18&nbsp;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&nbsp;kHz, the average upper limit is 55&nbsp;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&nbsp;kHz particularly at measurements of 500&nbsp;Hz and 250&nbsp;Hz.<ref name="Heffner & Heffner 1980" />

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== Birds ==

Although birds do not produce vocalizations in the infrasonic range, reactions to infrasonic stimuli have been observed in several species, such as the homing pigeon, the guinea fowl, and the Asian grouse.<ref>{{cite journal|last=Yodlowski|first=M. L.|coauthors=M. L. Kreithen, W. T. Keeton|title=Detection of atmospheric infrasound by pigeons|journal=Nature|year=1977|volume=265|pages=725–726|doi=10.1038/265725a0}}</ref><ref>{{cite journal|last=Theurich|first=M.|coauthors=G. Langner, H. Scheich|title=Infrasound re-sponses in the midbrain of the Guinea Fowl|journal=Neurosci Lett|year=1984|volume=49|pages=81–86|doi=10.1016/0304-3940(84)90140-x}}</ref><ref>{{cite journal|last=Moss|first=R.|coauthorsauthor2=I. Lockie|title=Infrasonic components in the song of the Capercaillie Tetrao urogallus|journal=Ibis|year=1979|volume=121|pages=95–97}}</ref> It is postulated that birds might use the detection of naturally occurring infrasound for long-range directional cues from distant landmarks, or for weather detection.<ref name="Quine 1981">{{cite journal|last=Quine|first=D. B.|title=Frequency shift discrimination: Can homing pigeons locate infrasounds by Doppler shifts?|journal=Journal of Comparative Physiology A|year=1981|volume=141|issue=2}}</ref>

=== Pigeons ===

Infrasound perception has been observed and quantified in the homing pigeon which has particularly good long distance navigation skills. The precise relevance of such signals for the pigeon is still unknown, but several uses for infrasound have been hypothesized, such as navigation and detection of air turbulences when flying and landing.<ref name="Kreithen & Quine 1979" /><ref>{{cite journal|last=Griffin|first=D. R.|title=The physiology and geophysics of bird navigation|journal=Q Rev Biol|year=1969|volume=44|pages=255–276|doi=10.1086/406142}}</ref><ref name="Schermuly 1990a">{{cite journal|last=Schermuly|first=L.|coauthorsauthor2=R. Klinke|title=Infrasound sensitive neurons in the pigeon cochlear ganglion|journal=Journal of Comparative Physiology A|year=1990|issue=166|pages=355–363}}</ref>

==== 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&nbsp;Hz. For frequencies below 10&nbsp;Hz, the pigeon threshold is at about 55dB which is at least 50dB more sensitive than humans.<ref name="Kreithen & Quine 1979" /> Pigeons are able to discriminate small frequency differences in sounds at between 1&nbsp;Hz and 20&nbsp;Hz, with sensitivity ranging from a 1% shift at 20&nbsp;Hz to a 7% shift at 1&nbsp;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.|coauthorsauthor2=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|pages=297–306|doi=10.1901/jeab.1974.21-297}}</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&nbsp;km/hr, 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% 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" />

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Infrasound sensitive fibers have very high rates of spontaneous discharge, with a mean of 115imp/s, 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 1.6-2.2&nbsp;µm at the axon and 0.9-1.2&nbsp;µm at the dendrites.<ref name="Schermuly 1990b">{{cite journal|last=Schermuly|first=L.|coauthorsauthor2=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|pages=69–78|doi=10.1016/0378-5955(90)90199-y}}</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 2-9 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.

== References ==