Chapter 2 · Section 4
Hearing and the direction of sound
Hearing sorts sound by frequency in the cochlea – but where the sound comes from must be worked out by comparing the ears.
Sound is pressure variations in the air. When they reach the ear they set the eardrum vibrating, and the small bones of the middle ear – the ossicles – carry the motion on to the cochlea in the inner ear. There the motion propagates through the fluid and sets the basilar membrane in motion. The hair cells on the membrane convert the mechanical influence into changed receptor potentials, which in turn affect the action potentials in the auditory nerve's fibres.
Cochlea
The spiral-shaped cochlea is filled with fluid. The motion of the stapes sets the fluid in motion, and along the basilar membrane inside the cochlea the hair cells convert the mechanical influence into changed receptor potentials.
The basilar membrane responds differently along its length. Near the base, by the middle ear, it is narrow and stiff and vibrates most for high frequencies; towards the apex it becomes wider and more compliant and responds most to low ones. This ordered frequency map is called tonotopy and is preserved through large parts of the auditory system. But unlike the retina, where the position on the sensory surface corresponds to the position in the visual field, the place in the cochlea says nothing directly about where the sound comes from.
Base of the cochlea
The base is the first part of the spiral, nearest the middle ear. Inside the cochlea the basilar membrane is narrow and stiff here. High frequencies produce the greatest movement near the base.
To see how the frequency ordering lies along the membrane, we unroll the cochlea in a separate functional schematic – a different kind of picture from the model above.
The direction must therefore be computed. Nerve cells in the brainstem compare the signals from the two ears: a sound from the left reaches the left ear slightly earlier (interaural time difference) and can be stronger there, because the head shadows the far ear (interaural level difference). The shape of the outer ear also filters sound differently depending on direction, which helps us tell front from back and up from down.
From the brainstem, several parallel pathways run, with connections on both sides, via the inferior colliculus and the medial geniculate nucleus (MGN) in the thalamus to the auditory cortex in the temporal lobe. It is thus not a single obligatory chain, but a network in which different properties of the sound – frequency, timing, level and direction – are processed in parallel.
You can compare how receptors in vision and hearing convert their stimulation in the receptor comparison in section 2.2, and the descending control that reaches the cochlea through the brainstem is described under feedback in the same section.
Inner and outer hair cells: different contributions
Inner hair cells release glutamate onto the nerve fibres carrying most cochlear afferent signals. Outer hair cells help amplify and shape the mechanical motion. Descending olivocochlear pathways through the brainstem reach different targets: MOC contacts outer hair cells, while LOC influences auditory nerve endings near inner hair cells. There is no direct axon from auditory cortex to a hair cell.
This influence works alongside selection at several brain levels. Animal experiments show peripheral influence from cortex; how much voluntary attention changes human cochlear responses is a more specific question. Compare visual and auditory feedback in section 2.2 →