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)