LESSON 46

The auditory cortex and language

Name: The auditory cortex and language
Uploaded: 2026-11-04T03:00:00+03:00
Duration: 10 min 41 s
Description: The last stop of the auditory pathway is the auditory cortex. In this lesson we look at the primary auditory cortex on Heschl's gyrus (Brodmann areas 41 and 42), Wernicke's area and the planum temporale, hemispheric specialisation, the corpus callosum and the right ear advantage in dichotic listening.

Year 1
Spring semester
Anatomy II (Neuroanatomy)
Central auditory and vestibular pathways
Duration: 10:41

The last stop of the auditory pathway is the auditory cortex. In this lesson we look at the primary auditory cortex on Heschl's gyrus (Brodmann areas 41 and 42), Wernicke's area and the planum temporale, hemispheric specialisation, the corpus callosum and the right ear advantage in dichotic listening.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn about the last stop of the auditory pathway, the auditory cortex, and the language areas.
We will also look at the different roles of the two hemispheres and the dichotic listening tests that measure them in the clinic.
In the previous lesson we saw that auditory information is relayed to the cortex from the medial geniculate body in the thalamus.
The auditory cortex lies on the upper surface of the temporal lobe.
This region is hidden inside a deep groove on the side of the brain, the lateral sulcus, so it cannot be seen from outside.
Here, a fold running across the surface of the temporal lobe stands out.
There is an auditory cortex in each hemisphere, and each one receives information from both ears.
This transverse fold is called Heschl's gyrus, or the transverse temporal gyrus.
It is named after the Austrian anatomist Heschl, who described it in the nineteenth century.
The primary auditory cortex lies on this gyrus and corresponds to area 41 in the Brodmann classification.
The primary auditory cortex carries a copy of the frequency map of the cochlea.
So the high and low frequency regions at the base and apex of the cochlea are also represented in orderly sequence in the cortex.
Heschl's gyrus varies from person to person; some people have two gyri in one hemisphere.
The auditory cortex is made up of areas arranged like nested rings.
The primary cortex at the center processes basic sound features such as frequency, intensity and timing.
The surrounding secondary areas, Brodmann area 42, respond to more complex sound patterns.
The association areas further out, Brodmann area 22, link sounds to speech and meaning.
So as information moves outward from the center, more abstract features of sound are processed.
Behind the auditory cortex, in the back part of the superior temporal gyrus, lies Wernicke's area.
In most people this area is part of the language network in the left hemisphere.
It is named after the German neurologist Carl Wernicke, who described it in 1874; it plays a key role in understanding speech.
With damage here, a person speaks fluently but their words lose meaning and they struggle to understand; this is Wernicke's aphasia.
Broca's area in the frontal lobe is linked to speech production and connects to Wernicke's area through a fiber bundle.
Today, language is thought to be processed not at a single point but in a broad network linking these areas.
Just behind Heschl's gyrus, a triangular area extends over the upper surface of the temporal lobe.
This area is called the planum temporale, or temporal plane; it contains part of Wernicke's area.
In the 1968 study by Geschwind and Levitsky of one hundred brains, this area was larger on the left in about sixty five percent.
It was larger on the right in about eleven percent of brains, and equal on both sides in about twenty four percent.
This asymmetry was found to fit with language being processed in the left hemisphere in most people.
According to current models, after the auditory cortex speech is processed in two main streams.
The ventral stream runs along the temporal lobe and links the sound heard to words and meaning.
This stream is thought to work in both hemispheres.
The dorsal stream extends to the parietal and frontal lobes and links the sound heard to speech movements.
In most people, this stream is clearly dominant in the left hemisphere.
The two hemispheres process auditory information together, but one leads in some tasks; this is called hemispheric specialisation.
Language is processed mainly in the left hemisphere in about ninety five percent of right-handers and in most left-handers.
The left hemisphere is thought to be more sensitive to fast temporal changes, such as consonant transitions in speech.
The right hemisphere leads for melody, pitch, music, and the stress and emotional tone of speech.
The right hemisphere has also been reported to play an important role in recognizing a familiar person by their voice.
The auditory areas of the two hemispheres are connected by the corpus callosum.
The corpus callosum is the largest fiber bundle in the brain and allows constant exchange of information between the hemispheres.
Fibers carrying auditory information pass mostly through the back part of this structure.
Myelination of the corpus callosum continues through childhood and is not complete until adolescence.
In dichotic listening, different stimuli reach the two ears at once, for example one number in the right ear and another in the left.
In this situation, information from the two ears competes in the auditory pathway.
During this competition the crossed pathways become dominant, while the same-side pathways are largely suppressed.
A word given to the right ear reaches the left hemisphere, dominant for language, directly through the crossed pathway.
A word given to the left ear first reaches the right hemisphere, then crosses the corpus callosum to the left hemisphere.
That is why the right ear usually scores slightly better for verbal stimuli; this is called the right ear advantage.
This finding was first described in 1961 in Doreen Kimura's dichotic listening studies with digits.
The right ear advantage is more marked in young children and decreases as the corpus callosum matures.
In older age, a marked decline in left ear scores can appear again.
Tests can be done in two ways: the person repeats both ears, or attends to one ear first; the two methods give different information.
Free recall gives a natural picture, while directed attention helps reveal the true capacity of the weaker ear.
Dichotic tests are among the core parts of a central auditory processing assessment.
The dichotic digits test gives two numbers to each ear at the same time and can be run quickly.
The Staggered Spondaic Word test, or SSW, presents two-syllable words to the two ears so that they partly overlap.
Competing sentences and dichotic syllable tests also provide similar information at different levels of difficulty.
Because these tests depend on language, versions developed and normed in the language being used are needed.
Because each ear is represented in both hemispheres, a one-sided cortical problem may leave the audiogram normal, as in this example.
In such a case, dichotic test scores often drop in the ear opposite the affected hemisphere.
When the corpus callosum is affected, left ear information cannot reach the language hemisphere, so left ear scores drop markedly.
When the auditory areas of both hemispheres are damaged, the rare condition of cortical deafness can occur.
The auditory cortex is not ready at birth; it needs auditory stimulation to develop.
The first years, when this development is fastest, are called the sensitive period.
Auditory areas that receive no stimulation in this period can partly take on functions of other senses, such as vision.
That is why early identification of hearing loss and early support with hearing aids or a cochlear implant are important.
A tip from the clinic: always look at the pure-tone thresholds before interpreting dichotic test results.
If there is a peripheral threshold difference between the ears, an ear difference on the dichotic test can look like a central finding.
So dichotic tests give the most reliable results in people whose thresholds are normal or symmetric between the ears.
Always compare the results with age-appropriate norms.
The primary auditory cortex lies on Heschl's gyrus on the upper surface of the temporal lobe and carries an orderly frequency map.
Wernicke's area and the planum temporale matter for understanding speech; the planum temporale is larger on the left in most brains.
Language is processed mostly on the left, melody and emotional tone more on the right; the corpus callosum connects the two hemispheres.
Because the crossed pathways dominate in dichotic listening, a right ear advantage appears for verbal stimuli.
Test yourself: in dichotic listening, which ear is usually ahead for verbal stimuli?
The correct answer is A: the right ear.
Right ear information reaches the left hemisphere directly through the crossed pathway; left ear information must cross the corpus callosum.
In the next lesson we will look at the efferent auditory system, which runs down from the brain to the ear, and the olivocochlear bundle.
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Prerequisites

LESSON 45 The central auditory pathway