Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course


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.

Figure 26
Figure 26 The proposed mechanism for the coding of intensity. An increase in stimulus intensity results in an increased firing rate of neurons with characteristic frequencies below or above the stimulus frequency, but no change in the firing rate of the neurons most sensitive to the stimulus, since these are already saturated

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 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]   to open The transformations of sound stimuli into electrical signals by Robert Fettiplace


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371