The small bones of the middle ear input a displacement signal to one side of the tube via a window. This signal drives a transverse wave in the basilar membrane, whose cutoff frequency decreases along its length. As a result, high frequencies cause maximum vibration at the window end, and low frequencies cause maximum vibration at the other. In the normal ear, action potentials are produced in an array of hair cells which reside on the basilar membrane. Ohm [2] and Helmholtz [3] proposed that pitch was encoded tonotopically, i.e. by the place along the basilar membrane of the nerve stimulated (place theory). Seebeck [4] argued that nerve pulses were produced by each vibration and that their rate determined the perceived pitch (rate theory). In the place theory, it is difficult to explain the observed fine resolution of frequency (~0.2%). The rate theory cannot readily explain the perception of tones with frequencies many times greater than the maximum firing rate of neurones. Despite many elegant acoustic experiments, the relative importance of rate and place are still debated because, in the normal ear, the rate of mechanical stimulation of the basilar membrane is strongly correlated with position. Cochlear implants allow the local electrical stimulation of different regions of the cochlea at different rates. A range of experiments have studied pitch using CIs: Simmons et al [5] reported pitch estimates from a single subject with low resolution in position. Pitch as a function of stimulation rate was reported by Pijl [6] and by Collins et al [7].

For the CI subjects, pitch also depends on place of stimulation, decreasing with distance from the round window. This can be compared with the tonotopic arrangement of the normal ear where a doubling in the frequency of the acoustic signal corresponded to a displacement of about 4 mm along the basilar membrane for frequencies above several hundred Hz, and smaller displacements for lower frequencies [10].

Because the pitch scales shown in Fig 2b are approximately logarithmically dependent on rate, we can calculate that a doubling in stimulation rate corresponds to a displacement of about 2 mm in this range. For the series of experiments reported in Fig 2a, the displacement corresponding to a doubling of stimulation rate depends on position and rate. It is about 4-6 mm at low rates and decreases for higher rates.