6 Intensity coding

6.1 Firing-rate hypothesis

Information about stimulus intensity is encoded in two ways: the firing rates of neurons and the number of active neurons.

Intensity is assumed to be encoded by an increase in discharge rate of action potentials within the auditory system. As the stimulus gets more intense, the basilar membrane vibrates at a greater amplitude causing the membrane potential of activated hair cells to be more depolarised and this causes the nerve fibres that synapse onto the hair cells to fire at a greater rate. However for single fibres, the discharge rate increases only for a relatively small range of level changes. Figure 25 shows the results of recordings from an auditory nerve fibre in response to a stimulus of increasing intensity.

Figure 25 Intensity response function for a single auditory nerve fibre. The stimulus is a pure tone at the characteristic frequency of the neuron. As the intensity of the stimulus increases, so does the number of action potentials (spikes) generated per second

For this fibre a change in response with intensity only occurs over a range of about 40 dB, after which it no longer responds to increases in sound level with an increased firing rate: the fibre is said to be saturated. The range of sound levels between threshold and the level at which saturation occurs is called the dynamic range.

Humans are sensitive to a 140 dB dynamic range. Since a single fibre's discharge rate will only increase for a relatively small range of level changes (usually less than 35 dB), single fibre responses alone cannot encode changes in signal intensity. However, if intensity is determined by an increase in discharge rate of a large number of fibres with different response thresholds, then a large dynamic range could be accommodated. The most sensitive nerve fibres have response thresholds of about 0 dB SPL and characteristically have high rates of spontaneous activity. They produce saturating responses for stimulation at moderate intensities, about 40 dB SPL. At the opposite extreme, some afferent fibres display less spontaneous activity and much higher thresholds and give graded responses to intensities of stimulation in excess of 100 dB SPL. Activity patterns of most fibres are between these two extremes. Thus combining information from low-, medium- and high-threshold fibres may serve as the code for sound level.