Perception of infrasound: Difference between revisions

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==== 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 10–15 seconds. When the nearest elephant is 5 m from the microphone, the recorded sound pressure levels can be 85 to 90&nbsp;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 20&nbsp;Hz fundamentals.<ref name="Langbauer et al 1990" /><ref name="Payne et al 1986" /> Sometimes, vocalizations cause perceptible rumbles that are accompanied by a fluttering of the skin on the calling elephant’selephant'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" />

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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&nbsp;Hz with a threshold of 65&nbsp;dB. A shallow slope decreased to the best response at 1&nbsp;kHz with a threshold of 8&nbsp;dB, followed by a steep threshold increase above 4&nbsp;kHz. According to the 60&nbsp;dB 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’selephant'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" />

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’selephant's head. The elephant could localize sounds best at a frequency below 1&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 125&nbsp;Hz and 250&nbsp;Hz, intermediate at 500&nbsp;Hz, 1&nbsp;kHz, and 2&nbsp;kHz, and very poor at frequencies at 4&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 4&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 ==

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==== 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 55&nbsp;dB which is at least 50&nbsp;dB 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’sbird'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’spigeon'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’spigeon'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 50&nbsp;dB. 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.