The job of the ear is to turn sound waves in the air into electrical nerve impulses that travel to the brain. The ear thus occupies a key "interface" position between the physical and the psychological aspects of sound. The ear is an interesting organ on many levels. Because sound waves usually contain only minuscule amounts of physical energy, the ear has to be phenomenally sensitive, and it is a marvel of "biological engineering". But such a delicate structure is also fragile, and hearing impairments most often originate from damage to the ear, rather than to higher order processing stations in the brain. The anatomy and physiology of the ear is described in chapter 2 of "Auditory Neuroscience" . These web pages provide material to complement that chapter.
The above animation shows the motion of the basilar membrane in response to a frequency modulated tone. The green line shows the sound wave form, a tone that increases from 500 Hz to about 3500 Hz, and which rises accordingly in pitch. The blue line shows a simulation of the basilar membrane motion, which exhibits a characteristic standing wave. The standing wave moves from a more apical position (plotted to the right) to a more basal (left) position as the frequency (and the perceived pitch) rises.
This illustrates the physiological basis of "tonotopy", and it illustrates why place of cochlear stimulation is often thought to be directly related with perceived pitch. However, as is explored further in other parts of this web site and discussed in detail in chapter 3 of Auditory Neuroscience, it is important to realize that pitch perception is complex, and may have relatively little to do with tonotopic place coding in the ascending auditory pathway.
(To download a copy of this video in mpg format, suitable for embedding in teaching material, click the link below).
This animation shows a simulation of "travelling wave motion" in the basilar membrane in response to a sound composed of two frequencies (1000 and 2500 Hz). The sound waveform is shown in the top panel, the basilar membrane response is shown below. Since the frequency components of the input are separated by more than an octave, they are well separated by the mechanical filtering of the cochlea, producing clearly separated "travelling waves" for each frequency component.
Note that this simulation is based on a simple, linear gamma-tone filter model of the cochlea, as described in chapter 2 of "Auditory Neuroscience". (It was created using Matlab, together with the GammaTone Tool Kit). The amplitude of the basilar membrane motion is massively exaggerated. Also, real basilar membrane motion is quite a bit more complex due to the nonlinearities introduced by the motion of outer hair cells, which make the basilar membrane much more sensitive to very weak sounds and help our auditory system cope with an enormous range of possible sound amplitudes, but which can also lead to significant distortions.
Here another animation showing the mechanical response of the basilar membrane, but this time the incoming sound is not the sum of two sine waves, but a single "ideal impulse", or click.
The top trace shows the click stimulus. Think of the click as travelling through air, but also impinging on the cochlea at time zero. The bottom trace shows the basilar membrane, with distance from the basal end on the x-axis.
Clicks are broad band sounds, and you might therefore expect clicks to excite all parts of the basilar membrane, but they do not excite all parts at once. Each point on the basilar membrane has its own "impulse response", and each will resonate at its own characteristic frequency. Also note that the response of the high frequency part of the basilar membrane starts earlier and dies away quicker than that of the low frequency part. This allows the basilar membrane to carry out a multiresolution analysis, using long integration time windows to analyse the low frequency content of sounds, and shorter windows at the higher frequencies.
And another animation showing the mechanical response of the basilar membrane, but this time the incoming sound is a 500 Hz click train i.e. one click every 2 milliseconds. Such click trains sound like a "buzz" with a very clear pitch at the click rate.
The Fourier spectrum of such a regular click train is composed of regular spaced harmonics at multiples of 500 Hz.
The top trace again shows the click stimulus, while the bottom trace shows the basilar membrane, with distance from the basal end on the x-axis.
This animation may help illustrate the important concept of resolved and unresolved harmonics. Note that both the 500 Hz and the 1000 Hz points of the basilar membrane respond with regular vibrations, while the region in between (around ca 700 Hz) appears to be more or less at rest. Place along the basilar membrane is therefore said to "resolve" the first two harmonics (500 and 1000 Hz) of the click train stimulus, as each produces a distinct region of high amplitude vibration, clearly separated by regions of low vibration amplitude. However, the points on the basilar membrane tuned to higher harmonics (say 2000, 2500 or 3000 Hz...) do not show distinctly separate peaks in their vibration amplitude, i.e. these higher harmonics are "not resolved".
The video below, which I obtained from the website of Prof. Tom Yin , shows an oscilloscope trace of a recording from a single auditory nerve fibre in vivo. The video is somewhat of a "classic" (they don't make oscilloscopes like that any more) but auditory nerve fibers haven't changed, and the video remains instructive. The crackling that you here in the background, and the spikes that you see in the green trace on the oscilloscope screen, are nerve impulses fired by the auditory nerve fibre. They are recorded from a very fine recording electrode (a microwire or a glass pipette) inserted into the auditory nerve of an anestehtized experimental animal. (No animals were hurt during the making of this movie). The first thing to notice is that the nerve fibre fires spontaneously at random intervals "at rest", when no sounds are presented to the experimental animal. When a 400 Hz pure tone is played, the firing rate increases, and the firing pattern becomes a lot more regular, with inter-spike intervals of approximately 1/400 of a second (or 1/200 of a second) becoming much more prevalent, as the nerve fibre's firing patter becomes entrained to the period of the sound stimulus.
Since the amplitude, and hence the mechanical energy, of airborne sounds is tiny, the cochlea mechanically amplifies the incoming vibrations. The motors which supply this mechanical amplification are the outer hair cells. Like inner hair cells, they use stretch receptors associated with the stereocilia at their tips to sense vibrations and convert them to electrical currents. But only in outer hair cells are these currents used to control length changes which parallel, and reinforce, the incoming mechanical vibration. The video below, which was recorded in the laboratory of Prof. Jonathan Ashmore , shows an isolated guinea pig outer hair cell to which a whole cell patch electrode has been attached. Through the pipette, an alternating current signal is injected, and the resulting motor response is observed under a microscope. The alternating current signal is also played to a loudspeaker, so we can hear the signal that the outer hair cell receives.