Perception of infrasound: Difference between revisions

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'''[[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.|coauthors=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}}</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.|coauthors=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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=== Elephants ===

Elephants are the terrestrial animal in which the production of infrasonic calls was discovered by Katy Payne.<ref>{{cite book|last=Payne|first=Katy|title=Silent Thunder: In the presence of Elephants|year=1998|publisher=Simon & Schuster|___location=New York}}</ref>. The use of low frequency sounds to communicate over long distances may explain certain elephant behaviors that have previously puzzled observers. Elephant groups that are separated by several kilometers have been observed to travel in parallel or to change the direction simultaneously and move directly towards each other in order to meet.<ref name="Langbauer et al 1991">{{cite journal|last=Langbauer|first=W. R.|coauthors=K. B. Payne, R. A. Charif, L. Rapaport, F. Osborn|title=African elephants respond to distant playbacks of low-frequency conspecific calls|journal=J. Exp. Biol.|year=1991|volume=157|pages=35–46}}</ref>. The time of [[estrus]] for females is asynchronous, lasts only for a few days, and occurs only every several years. Nevertheless, males, which usually wander apart from female groups, rapidly gather from many directions to compete for a receptive female.<ref name="Langbauer et al 1991" />. Since infrasound can travel for very long distances, it has been suggested that calls in the infrasonic range might be important for long distance communication for such coordinated behaviors among separated elephants.<ref name="Langbauer et al 1991" /><ref name="Payne et al 1986">{{cite journal|last=Payne|first=K. B.|coauthors=W. R. Langbauer, E. M. Thomas|title=Infrasonic calls of the Asian elephant (Elephas maximus|journal=Behav. Ecol. Sociobiol.|year=1986|volume=18|pages=297–301}}</ref> <ref name="Langbauer et al 1991" />.

==== Infrasound production and perception ====

Recordings and playback experiments support that elephants use the infrasonic components of their calls for communication. Infrasonic vocalizations have been recorded from captive elephants in many different situations. The structure of the calls varies greatly but most of them range in frequency from 14 to 24 &nbsp;Hz, with durations of 10-1510–15 seconds. When the nearest elephant is 5 m from the microphone, the recorded sound pressure levels can be 85 to 90 dB SPL.<ref name="Payne et al 1986" /> Some of these calls are completely inaudible to humans, while others have audible components that are probably due to higher frequency [[harmonics]] of below 20Hz20&nbsp;Hz fundamentals.<ref name="PayneLangbauer et al 19861990" /> <ref name="LangbauerPayne et al 19901986" />. Sometimes, vocalizations cause perceptible rumbles that are accompanied by a fluttering of the skin on the calling elephant’s forehead where the nasal passage enters the skull. This fluttering can also occur without causing any perceptible sound, suggesting the production of a purely infrasonic call.<ref name="Payne et al 1986" /> The mechanism of infrasonic call production in elephants has not been determined.

Playback experiments using prerecorded elephant vocalizations show that elephants can perceive infrasound and how they respond to these stimuli. In playback experiments, certain behaviors that occur commonly after vocalizations are scored before and after a call is played. These behaviors include lifting and stiffening of ears, vocalization, walking or running towards the concealed speaker, clustering in a tight group, and remaining motionless ("freezing"), with occasional scanning movements of the head.<ref name="Langbauer et al 1990" />. The occurrence of such behaviors consistently increases after the playing of a call, whether it is a full-bandwidth playback or a playback in which most of the energy above 25 &nbsp;Hz was filtered out. This filtering shows that the behaviorally significant information of the call is contained in the infrasonic range, and it also simulates the effect of frequency-dependent attenuation over distance as it might occur in the wild.<ref name="Langbauer et al 1990" /> Behavioral responses do not increase for pure tone stimuli that are similar to recorded infrasonic calls in frequency and intensity. This shows that the responses are specifically to signals that were meaningful to the elephants.<ref name="Langbauer et al 1990" />.

The use of prerecorded playbacks and behavioral scoring also shows that the infrasonic elephant calls are behaviorally significant over long distances. The degree of response behaviors performed by an elephant group, such as lifting of ears, walking towards the speakers, “freezing”, or scanning movements, was compared visually before and after the presentation of a stimulus, scoring a trial as a positive response if the amount of behaviors is greater after the stimulus. In one particular experiment performed on elephants living in the wild, the presentation of playbacks for 20-4020–40 seconds from loudspeakers at distances of 1.2km2&nbsp;km and 2km2&nbsp;km caused a significant increase in response behaviors.<ref name="Langbauer et al 1991" />. Since the playbacks were done at half the amplitude at which they were recorded, it is estimated that these calls would be perceptible by elephants at distances of at least 4km4&nbsp;km<ref name="Langbauer et al 1991" /> Even this may be an underestimate because animals do not respond every time they perceive a conspecific call, and they are probably less likely to respond to calls from further distances even if they do perceive them.<ref name="Langbauer et al 1991" />.

There are some confounding factors that might influence the results of this kind of experiment. Firstly, the animals might actually be more sensitive than the experiments would indicate owing to [[habituation]] of the animals to the playback stimuli after several trial repetitions. To avoid this, researchers present several different types of playbacks in random order. Another problem that might arise in interpreting field experiments done on groups of animals is that animals may be responding to signals from other elephants in the group rather than the playback stimulus. However, an assumption is made that at least one animal in the group did perceive and respond directly to the stimulus.<ref name="Langbauer et al 1990" /> <ref name="Langbauer et al 1991" />

==== Infrasound sensitivity ====

The auditory sensitivity thresholds have been measured behaviorally for one individual young female Indian elephant. The [[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.|coauthors=R. Heffner|title=Hearing in the elephant (Elephas maximus)|journal=Science|year=1980|volume=208|pages=518–520}}</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 16Hz16&nbsp;Hz with a threshold of 65dB. A shallow slope decreased to the best response at 1kHz1&nbsp;kHz with a threshold of 8dB, followed by a steep threshold increase above 4kHz4&nbsp;kHz. According to the 60dB cut-off, the upper limit was 10.5kHz5&nbsp;kHz with absolutely no detectable response at 14kHz14&nbsp;kHz.<ref name="Heffner & Heffner 1980" /> The upper limit for humans is considered to be 18kHz18&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 55kHz55&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 1kHz1&nbsp;kHz particularly at measurements of 500Hz500&nbsp;Hz and 250Hz250&nbsp;Hz.<ref name="Heffner & Heffner 1980" />.

Tests of the ability to localize sounds also showed the significance of low frequency sound perception in elephants. Localization was tested by observing the successful orienting towards the left or the right source loudspeakers when they were positioned at different angles from the elephant’s head. The elephant could localize sounds best at a frequency below 1kHz, with perfect identification of the left or right speaker at angles of 20 degrees or more, and chance level discriminations below 2 degrees<ref name="Heffner & Heffner 1980" />. Sound localization ability was measured to be best at 125Hz and 250Hz, intermediate at 500Hz, 1kHz, and 2kHz, and very poor at frequencies at 4kHz and above<ref name="Heffner & Heffner 1980" />. A possible reason for this is that elephants are very good at using [[interaural phase differences]] which are effective for localizing low frequency sounds, but not as good at using [[interaural intensity difference]]s which are better for higher frequency sounds. Because of the elephant head size and the large distance between their ears, interaural difference cues become confused when wavelengths are shorter, explaining why sound localization was very poor at frequencies above 4kHz.<ref name="Heffner & Heffner 1980" /> It was observed that the elephant spread the pinna of its ears only during the sound localization tasks, however the precise effect of this behavior is unknown<ref name="Heffner & Heffner 1980" />.

Tests of the ability to localize sounds also showed the significance of low frequency sound perception in elephants. Localization was tested by observing the successful orienting towards the left or the right source loudspeakers when they were positioned at different angles from the elephant’s head. The elephant could localize sounds best at a frequency below 1kHz1&nbsp;kHz, with perfect identification of the left or right speaker at angles of 20 degrees or more, and chance level discriminations below 2 degrees.<ref name="Heffner & Heffner 1980" />. Sound localization ability was measured to be best at 125Hz125&nbsp;Hz and 250Hz250&nbsp;Hz, intermediate at 500Hz500&nbsp;Hz, 1kHz1&nbsp;kHz, and 2kHz2&nbsp;kHz, and very poor at frequencies at 4kHz4&nbsp;kHz and above.<ref name="Heffner & Heffner 1980" />. A possible reason for this is that elephants are very good at using [[interaural phase differences]] which are effective for localizing low frequency sounds, but not as good at using [[interaural intensity difference]]s which are better for higher frequency sounds. Because of the elephant head size and the large distance between their ears, interaural difference cues become confused when wavelengths are shorter, explaining why sound localization was very poor at frequencies above 4kHz4&nbsp;kHz.<ref name="Heffner & Heffner 1980" /> It was observed that the elephant spread the pinna of its ears only during the sound localization tasks, however the precise effect of this behavior is unknown.<ref name="Heffner & Heffner 1980" />.

== 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}}</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}}</ref><ref>{{cite journal|last=Moss|first=R.|coauthors=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}}</ref> <ref name="Kreithen & Quine 1979" /> <ref name="Schermuly 1990a">{{cite journal|last=Schermuly|first=L.|coauthors=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 0.5Hz5&nbsp;Hz. For frequencies below 10Hz10&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 1Hz1&nbsp;Hz and 20 &nbsp;Hz, with sensitivity ranging from a 1% shift at 20Hz20&nbsp;Hz to a 7% shift at 1Hz1&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.|coauthors=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}}</ref> <ref name="Kreithen & Quine 1979" /> <ref name="Quine 1981" /> 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 20km20&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" />

In early experiments with infrasound sensitivity in pigeons, surgical removal of the calumella, 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 50dB. Complete surgical removal of the entire [[cochlea]], lagena, and calumellae completely abolishes any response to infrasound.<ref name="Kreithen & Quine 1979" /> This shows that the receptors for infrasonic stimuli may be located in the inner ear.

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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/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 1.6-2.2µm at the axon and 0.9-1.2µm at the dendrites<ref name="Schermuly 1990b">{{cite journal|last=Schermuly|first=L.|coauthors=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}}</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.

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.|coauthors=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}}</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 ==

{{Reflist}}

== Further Readingreading ==

*Cook, R.K. (1969) Atmospheric sound propagation. Atmospheric exploration by remote probes, Vol. 2, pp. 633-669&nbsp;633–669. Washington, D.C. Committee on Atmospheric Sciences, National Academy of Sciences, National Research Council

*Delius JD, Tarpy RM (1974) Stimulus control of heart rate by auditory frequency and auditory pattern in pigeons, Journal Of The Experimental Analysis Of Behavior 1974, 21, 297-306

*Griffin DR (1969) The physiology and geophysics of bird navigation. Q Rev Biol 44:255~76

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*Schermuly, L. and Klinke R. (1990). Infrasound sensitive neurons in the pigeon cochlear ganglion. Journal of Comparative Physiology A, 166:355-363

*Schermuly L. and R. Klinke. (1990) Origin of infrasound sensitive neurones in the papilla basilaris of the pigeon: an HRP study, Hearing Research, 48: 69-78

*Theurich M, Langner G, Scheich H (1984) Infrasound re-sponses in the midbrain of the Guinea Fowl. Neurosci Lett 49:81-86

*Yeowart NS, Evans MJ (1974) Thresholds of audibility for very low-frequency pure tones. J Acoust Soc Am 55:814-818

*Yodlowski ML, Kreithen ML, Keeton WT (1977) Detection of atmospheric infrasound by pigeons. Nature 265:725- 726