From wiggling to the wonderful variety of sounds

In the beginning, it's just a wiggle: sound waves cause the eardrum to vibrate. But how does this become the fascinating world of sounds, the birdsong in the morning, the delicate strumming of a violin? A lot of computing power is required at numerous points along the auditory pathway.

First, the sound is processed mechanically. The eardrum vibrates, and these movements are transmitted by the ossicles of the middle ear – the malleus, incus, and stapes – to a membrane called the oval window. Behind it lies the inner ear with the cochlea. There, the basilar membrane, a tissue structure that runs the entire length of the cochlea, picks up the vibration. This basilar membrane is narrow and stiff at the beginning but becomes wider and more flexible. In conjunction with the spiral-shaped and tapering anatomy of the cochlea, this ensures that each section of the basilar membrane is only set into vibration by a specific frequency range of sound. High-pitched sounds set the membrane at the beginning of the cochlea in motion, while low-pitched sounds only do so a few cochlear turns later.

The basilar membrane is covered by the so-called organ of Corti, which contains the inner hair cells that convert the analog sound-induced vibrations into nerve impulses. Only then does the auditory information become accessible for data processing in the brain. Essential for the conversion, or transduction, of physical stimuli into electrical impulses is a specific type of sensory cell in the organ of Corti: the auditory cells. These cells have around a hundred hair-like projections at their tips, the stereocilia,which is why they are also called hair cells. When an area of the basilar membrane vibrates, the hairs of the auditory cells at that point are stimulated. This is the crucial movement, because it opens special ion channels in the cell membrane, the transduction channels.

As soon as the transduction channels open, positively charged potassium ions flow into the interior of the hair cell, causing a change in charge. Each hair cell is connected via a synapse to a spiral ganglion cell, whose extensions form the auditory nerve, the nervus cochlearis. Only when this very specific hair cell is stimulated according to its frequency is the corresponding nerve cell in the spiral ganglion excited – and only then does it fire its action potentials.

This signal is transmitted to various areas in the brain stem: the fibers of the auditory nerve lead via the vestibulocochlear nerve to the two auditory nuclei, the ventral cochlear nucleus and the dorsal cochlear nucleus. These nuclei form a kind of distribution station from which numerous parallel signal pathways originate.

To keep things simple, we will follow just one, but very important, pathway on its way to the auditory cortex in the brain: the cells in the ventral cochlear nucleus send input to the so-called superior olive nucleus on both sides of the brain stem,which is actually a complex of several nuclei. The nerve network there reacts sensitively to time differences: if a sound reaches the left ear fractions of a second earlier than the right ear, the sound source is very likely to be to the left of the head. The upper olive is therefore involved in sound localization. From there, fibers also return to the inner ear. This feedback can influence the sensitivity of hearing.

Let us now continue to follow the auditory pathway towards the auditory cortex: from the olive complex, impulses travel via a lateral loop pathway (Latin: lemniscus lateralis) to a specific location in the midbrain. The “lower colliculi” located there, the colliculi inferiores, are important for attention processes. They also help to control the movement of the head towards a specific stimulus.

The colliculi inferiores send the auditory information to the thalamus, which is considered the “gateway to the cortex.” The medial geniculate nucleus (CGM) is responsible for auditory signals there. Since the thalamus receives input from both sides, each hemisphere of the brain receives information from both ears. The extensions of the neurons in the CGM form the auditory radiation, which transmits the information to the primary auditory cortex in the temporal lobe. Speech is then analyzed in the secondary areas of the auditory cortex (Wernicke's area). This area, also known as the auditory center, processes acoustic signals. Ultimately, it is mainly thanks to this area that we are able to consciously perceive the voice of a loved one or the rustling of leaves.

There are two pathways in the auditory system itself. The dorsal pathway to areas in the parietal lobe presumably processes spatial acoustic information. This pathway, sometimes referred to as the “where pathway,” probably comes into play when, for example, the hated sound of the alarm clock rings in the morning, which we reach for even when it is still dark in the room. The so-called “what pathway,” the ventral pathway from the auditory cortex to the superior temporal sulcus, is probably essential for identifying human speech, for example, among the multitude of acoustic stimuli.

Researchers such as neuroscientist Josef Rauschecker from Georgetown University in Washington, D.C. have found confirmation for this distinction in recent years. However, it is not entirely uncontroversial: “The hypothesis of these separate processing pathways is essentially nothing more than the adoption of corresponding hypotheses from the cortical visual system,” says Rudolf Rübsamen. “The extent to which this transfer to the auditory system has heuristic value is controversial among experts.”

Whatever the processing pathways may look like in detail, it is a long journey that auditory information takes from the ear to the brain. But for us, it is always worth it.

Further reading

• Rauschecker, J.: An expanded role for the dorsal auditory pathway in sensorimotor control and Integration. Hearing Research. 2011; 271:16 – 25 (zum Abstract).
• Leaver, A., Rauschecker, J.P.: Cortical representation of natural complex sounds: effects of acoustic features and auditory object category. Journal of Neuroscience. 2010; 30(22):7604 – 7612 (zum Abstract).

First published on July 27, 2012
Last updated on October 17, 2025