Auditory cortex

The auditory cortex is the region of the brain that processes sound. It sits in the temporal lobe, the part of the brain just above and behind each ear, on a ridge called the superior temporal gyrus (a gyrus is a fold on the brain's surface). Within it lies Heschl's gyrus, which contains the primary auditory cortex (often labelled A1), the first cortical area to receive incoming sound signals. Around A1 are secondary and association areas that build on that first stage to handle more complex listening, such as recognising speech, music, and familiar sounds.

A key feature of the auditory cortex is that it is organised by frequency, an arrangement called tonotopy: neighbouring patches of cortex respond to neighbouring pitches, so the area holds an orderly map running from low frequencies to high. Together, these areas let the brain interpret the qualities of a sound: its pitch (how high or low it is), its loudness, its timbre (the tone colour that makes a violin and a flute sound different on the same note), and where the sound is coming from. See Wikipedia: Auditory cortex for an overview, and the US National Institute on Deafness and Other Communication Disorders explainer NIDCD: How Do We Hear?, which describes how the auditory nerve carries the signal to the brain to be recognised as sound.

Sound reaches the auditory cortex at the end of a relay that starts in the ear. Sound waves enter the ear and reach the cochlea, the fluid-filled, snail-shaped structure of the inner ear, where tiny hair cells sort the sound by frequency and convert it into nerve signals; this is where the frequency map (tonotopy) begins. Those signals travel along the auditory nerve into the brainstem, the stalk at the base of the brain, where several relay stations begin to compare the input from the two ears and work out timing and loudness differences that help locate a sound. From there the signal passes through the thalamus, a central hub deep in the brain, by way of a relay called the medial geniculate nucleus, which forwards the information up to the auditory cortex.

At the cortex, the orderly frequency map is preserved, so neighbouring neurons still respond to neighbouring pitches. The primary auditory cortex (A1) handles the first stage of cortical processing, and the surrounding secondary and association areas combine that information into the experience of hearing: recognising a word, following a melody, or noticing where a sound came from in the room. See NCBI Bookshelf (StatPearls): Neuroanatomy, Auditory Pathway, which describes the ascending pathway from the cochlea through the brainstem and thalamus to the cortex and the tonotopic organisation that runs through it. For the wider library, see our glossary entry on psychoacoustics (how the brain turns these signals into perceived sound) and the sound healing pillar.