Know Your Brain: Auditory Cortex
October 25, 2020
Where is the auditory cortex?
The auditory cortex is found in the temporal lobe. Most of it is hidden from view, buried deep within a fissure called the lateral sulcus. Some auditory cortex is visible on the external surface the brain, however, as it extends to a gyrus called the superior temporal gyrus.
The auditory cortex can be subdivided into multiple regions, although there is still some question about the most appropriate way to create those subdivisions in the human brain. There is general agreement, however, that the auditory cortex consists of a primary area—which is often referred to as the core region—as well as multiple non-primary areas.
The primary auditory cortex in humans is hidden within the lateral sulcus on a collection of gyri known as Heschl’s gyri (aka the transverse temporal gyri). The precise location of the primary region in humans is variable, however, as is the arrangement of Heschl’s gyri (some people have one of these gyri, while others have two or three). For example, in some individuals the primary auditory cortex seems to occupy one Heschl’s gyrus, while in others it may extend past that gyrus into a neighboring sulcus (or beyond).
The region adjacent to the core is often referred to as the belt region, and surrounding that is an area often called the parabelt region. These neighboring areas are mostly buried within the lateral sulcus as well, but may extend out to the superior temporal gyrus. The demarcations of the auditory cortex in general, however, are imprecise.
What is the auditory cortex and what does it do?
The auditory cortex plays a critical role in our ability to perceive sound. It is thought to be integral to our perception of the fundamental aspects of an auditory stimulus, like the pitch of the sound. But it is also important in various other aspects of sound processing, like determining where in space a sound originates from as well as identifying what might be producing the sound. The auditory cortex is also thought to be involved in higher-level auditory processing, such as recognizing aspects of sound that are specific to speech. Damage to the auditory cortex can disrupt various facets of auditory perception. For example, damage (e.g., like that caused by a stroke) might cause deficits in the ability to detect changes in pitch, localize sounds in space, or understand speech.
The auditory cortex primarily receives auditory information from a nucleus in the thalamus called the medial geniculate nucleus, which is where all incoming information about hearing is sent before it is processed by the cerebral cortex. Cells in the primary region of the auditory cortex (and in some parts of the non-primary regions as well) are arranged so they form what is known as a tonotopic map. What this means is that different areas of the auditory cortex are involved in processing different sound frequencies. (Frequency, when referring to sound waves, is related to pitch. High frequency sound waves lead to high-pitched sounds.)
The tonotopic arrangement of the primary auditory cortex is similar to what is seen in the cochlea, where sound processing begins. Thus, it can be said that the core of the auditory cortex contains a map of the cochlea, with each point in the cochlea corresponding to a strip of cells in the auditory cortex. This type of arrangement is similar to what is seen in other sensory cortices (e.g., the primary somatosensory cortex contains a map of the sensory receptors of the body).
There are thought to be other patterns of functional organization of the primary auditory cortex as well, although the importance of these patterns in hearing is still poorly understood. For example, there seem to be cells (EI cells) in the auditory cortex that are activated only by stimuli coming from one ear; they are inhibited by stimuli coming from the other ear. Other cells (EE cells) are activated by information coming from both ears.
Sensory processing in the areas surrounding the primary auditory cortex is also not well understood, but it is thought that cells in the belt and parabelt are involved in higher-level processing of complex sounds. For example, research suggests these non-primary areas of the auditory cortex may be important in recognizing species-specific vocalizations, which implies they play a role in processing speech in humans. Studies have also found the non-primary auditory regions to be involved in distinct aspects of speech perception, like detecting the beginning and end of syllables, attending to one voice while ignoring another, and processing sounds in a logical temporal order.
Overall, there is still much to be learned about the fine details of auditory processing in the auditory cortex. It is clear, however, that this part of the brain plays a critical role in creating the rich sensory experience of hearing.
References (in addition to linked text above):
Amunts K, Morosan P, Hilbig H, Zilles K. Auditory System. In: Mai JK and Paxinos G, eds. The Human Nervous System. 3rd ed. New York: Elsevier; 2012.
Moerel M, De Martino F, Formisano E. An anatomical and functional topography of human auditory cortical areas. Front Neurosci. 2014 Jul 29;8:225. doi: 10.3389/fnins.2014.00225.
Pickles JO. Introduction to the Physiology of Hearing. 4th ed. Bingley, UK: Emerald Group Publishing Limited; 2012.
Purves D, Augustine GJ, Fitzpatrick D, Hall WC, Lamantia AS, Mooney RD, Platt ML, White LE, eds. Neuroscience. 6th ed. New York. Sinauer Associates; 2018.