Primary auditory cortex · Grey Matter

The primary auditory cortex (A1) is the patch of the upper temporal lobe where sound first reaches the cortex, organised by pitch so that low and high frequencies land at opposite ends of a map.

Primary auditory cortex. The primary auditory cortex (A1) is the patch of the upper temporal lobe where sound first reaches the cortex, organised by pitch so that low and high frequencies land at opposite ends of a map.

It lies on the transverse temporal gyri (Heschl's gyri), on the upper surface of the temporal lobe inside the lateral fissure, roughly Brodmann areas 41 and 42. Its input comes from the medial geniculate nucleus of the thalamus, after several relays in the brainstem where the signals of the two ears are already compared, so each A1 hears both ears, with a preference for the opposite one. The frequency map (tonotopy) copies the cochlea, whose base responds to high tones and apex to low ones. Around A1, belt and parabelt areas extract the identity and location of sounds and feed speech into Wernicke's area on the left.

Hearing loss from one temporal lesion is mild because each side hears both ears. Bilateral damage can leave a person hearing sounds without recognising them, a rare condition called auditory agnosia (or, for speech only, pure word deafness).

Its map is shaped by early experience. In young animals, exposure to one tone during a short critical window enlarges that frequency's share of A1 for life.

Music that moves a listener starts here and reaches reward circuits in the basal ganglia; that route is one of the questions on this card.

Questions: How can someone hear sounds normally and still not recognise speech or a ringing phone? Hearing tests check whether a tone is detected, which needs only the ear and the early auditory pathway. Recognising a sound as a word, a voice or a phone needs the auditory association cortex of the temporal lobe, and damage there (usually on both sides, or on the left together with the fibres from the right) leaves sounds heard but meaningless, a rare condition called auditory agnosia. When only speech is affected it is called pure word deafness: the patient hears that someone is talking, can read and write, and cannot understand what is said. How does a piece of music produce chills of pleasure? The sound is analysed in the auditory cortex and the temporal lobe, which track melody and harmony and build expectations about what comes next. In 2011 Salimpoor and colleagues measured dopamine release with PET while listeners heard music that gave them chills: dopamine rose in the caudate during the build-up before the peak, and in the nucleus accumbens at the peak itself. Music thus engages the same reward circuit of the striatum as food or money, driven by expectation and its resolution, and the medial prefrontal cortex tracks music tied to personal memories. Where does sound reach the cortex, and how is it laid out there? Sound reaches the cortex on Heschl's gyri, on the upper surface of the temporal lobe hidden inside the lateral fissure, after relays in the brainstem and the medial geniculate nucleus of the thalamus. The primary auditory cortex there is laid out by frequency, low tones at one end and high tones at the other, copying the layout of the cochlea. Each side hears both ears, so damage to one auditory cortex barely affects hearing, while the belt areas around it go on to identify voices, words and melodies.