6.2 Number of neurons hypothesis
In addition to an increase in firing rate of neurons with differing dynamic ranges, the inclusion of discharges from many fibres whose CFs are different from those of the stimulus may also help to account for the wide dynamic range of the ear. You know from Section 3.3 that in response to a pure tone stimulus the basilar membrane vibrates maximally at a given point. You should also be aware, however, that a pure tone will also cause vibration at points on the membrane adjacent to that of maximum stimulation. These vibrations are then reflected in the responses of the hair cells. Thus a pattern of excitation is produced in the auditory nerve such that fibres with a CF close to the input frequency will fire more strongly than those fibres whose CF is very different from the signal. Figure 26 shows a pattern of neural excitation along the basilar membrane that may be produced by a pure tone of 80 dB SPL (solid line). Assume that the neurons most excited by this stimulus are firing at a maximum rate so that any increase in intensity of the stimulus causes no increase in the firing rate of these cells. However, the cells with a CF either higher or lower than the stimulus frequency are not firing at their maximum level, so increasing the stimulus level causes an increase in firing rate of these cells. The effect of this is to broaden the excitation pattern as shown by the dashed curve. Thus, intensity could be encoded by how broad the excitation pattern may be to a given stimulus.
Now read The transformation of sound stimuli into electrical signals by Robert Fettiplace attached below. This chapter reinforces some of the material you have studied so far on the transduction of sound stimuli and frequency coding.
Click View Document to open The transformations of sound stimuli into electrical signals by Robert Fettiplace