12.2 Interaural time delays: non-continuous sounds
The average distance between human ears is about 20 cm. Therefore, if a sudden noise comes at you from the right, perpendicular to your head, it will reach your right ear 0.6 ms before it reaches your left ear. For a sound coming from directly in front of you there will be no delay, and at angles between, the delay will be between 0 and 0.6 ms. Therefore there is a simple relationship between the location of the sound source and the interaural delay. It is this delay that enables us to localise the source of a sound in the horizontal plane. How does the auditory system encode information about interaural time delays? It is thought that the mechanism involves a series of delay lines and coincidence detectors, as illustrated in Figure 43.
We know from our discussion about the auditory pathway that the first place where information from both ears comes together is in the superior olivary nucleus. Briefly, when a sound arrives at one ear it is transduced by the hair cells, elicits firing in the auditory nerve and evokes spikes in the axons that project from the cochlear nuclei to the medial superior olive. The same sound will initiate a similar series of events when it reaches the opposite ear. In Figure 43a, the sound reaches the left ear and hence the left cochlear nucleus first resulting in the generation of action potentials that are relayed to cells in the superior olive. A fraction of a millisecond later, sound reaches the right ear, initiating activity in the right cochlear nucleus (Figure 43). By this time however, the impulses travelling from the left cochlear nucleus have travelled further along the axon (which is the delay line). Impulses from both ears reach coincidence at, in this case, olivary neuron 3, which then fires an action potential (Figure 43c).
According to this illustration, if the sound were directly in front of the person, which of the olivary neurons would fire?
Olivary neuron 2, because the time it would take the action potential to travel from the left and the right cochlear nuclei would be the same and coincidence would occur in the midline.
You can see from this example that the auditory system can extract information about the location of a sound source by attending to which neuron in the superior olivary nucleus fires in response to the sound. This is because neurons will only fire if there is some specific delay between the spikes arriving from the left and right cochlear nuclei, and different neurons fire in response to different delays.