Perception of infrasound: Difference between revisions

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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'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&nbsp;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&nbsp;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.