5 Frequency coding in cochlear nerve fibres
5.1 Place code
We know that each hair cell occurs in a localised region of the cochlea, and that auditory nerve fibres contacting each hair cell fire action potentials in response to movement of the basilar membrane at that location. This means that the response of any given fibre should reflect the frequency selectivity of that location on the basilar membrane from which it comes. In other words, cochlear nerve fibres preserve the frequency selectivity found along the basilar membrane. Fibres on the outside of the auditory nerve bundle (those that innervate the basal hair cells) have high characteristic frequencies whereas those towards the middle of the nerve bundle (those that innervate the apex of the cochlea) have low characteristic frequencies. Thus, each place or location within the nerve responds ‘best’ to a particular frequency. The nerve fibres are spatially arranged to correspond directly to their basilar membrane origin. This arrangement is known as tonotopic organisation, which can be defined as the orderly spatial arrangement of neural elements corresponding to the separation of different frequencies. Functionally, tonotopic organisation allows the input frequency to be determined according to which nerve fibre discharges with the greatest relative discharge rate. This way of determining frequency is known as the place theory and gives rise to the place code. Tonotopic organisation is found at all higher levels of the auditory system up to and including the auditory cortex.
There are several ways in which the frequency selectivity of single fibres can be determined. One way is to present a single fibre with a wide range of stimuli of different frequencies but identical intensity. The function generated when responses to the stimuli are plotted against the frequency of each stimulus is called an iso-intensity contour. Figure 23 a shows a number of such contours for a single fibre in the auditory nerve. Each curve is generated using a different intensity level of the stimulus.
What do you notice about the different contours?
The higher the intensity of the stimulus, the broader the contour.
What does this indicate in terms of the frequency selectivity of the fibre?
It means that at lower intensities, the fibre responds maximally to a narrow range of frequencies but as the intensity of the signal increases, the range of frequencies to which the fibre responds increases, i.e. the fibre shows lower frequency selectivity at high intensities.
A second way of displaying the tuning characteristics of single auditory fibres is to generate a tuning curve. This is done the same way as tuning curves for hair cells are generated (Section 3.7). Figure 23b is a tuning curve for a hair cell from a guinea pig showing how the threshold intensity for a given fibre varies with stimulus frequency.
What is the characteristic frequency (CF) of this particular fibre?
About 18 kHz. The intensity of the stimulus needed to elicit a response is lower than for any other frequency (less than 40 dB SPL).
You can see that the high-frequency side of the curve is very steep whereas the low-frequency side is less steep and may have a long, low-frequency tail.
What does this indicate?
It means that nerve fibres are unlikely to respond to many frequencies higher than their characteristic frequency (CF) even when the intensity of the signal is very high. (At a frequency of about 23 kHz, the intensity of the signal needed to be about 70 dB to elicit a response.) For frequencies below CF the intensity of the signal needed to elicit a response is not quite so high and there is a broader range of frequencies to which the fibre will respond.