The auditory cortex is divided into three separate parts: the primary, the secondary and the tertiary auditory cortex. The primary auditory cortex, located in between the two others, is mainly composed of two areas named the Brodmann areas 41 and 42. It is located in the temporal lobe, right above the ears. The function of the primary auditory cortex is to process sound. It processes such information as pitch, volume and the location of a sound, and is crucial for understanding language.
The neurons in the primary auditory cortex are organized tonotopically, meaning that the neurons in the auditory cortex react best to specific frequencies. At one end of the auditory cortex, neurons react best to low frequencies, and at the other end, they react to high frequencies. Thus, a lesion to a certain area of the primary auditory cortex is likely to cause a loss of certain frequency perception. The primary auditory area is mainly responsible for processing such simple elements in sounds as pitch. The secondary auditory cortex, on the other hand, processes more complex sound properties such as rhythmic patterns.
Dr. Wernicke, renowned German neurologist and psychiatrist, noticed that a lesion on the left temporal hemisphere caused his patient to lose his ability to understand speech. This area was later named for the doctor, and it was determined that right-handed people typically have Wernicke’s area, or a language understanding area in their left primary auditory cortex. Besides processing speech, this area also processes written language. The same area on the right side is typically involved in understanding tone of voice.
Damage to the Primary Auditory Cortex
If the whole primary auditory area is damaged, a person will not be aware of what he hears. Yet, an ability to react reflexively to sounds remains. There are extensive connections from both ears to both the left and right primary auditory cortexes. That is why a person who has had a lesion on one side of the auditory cortex only, is able to discriminate sound frequencies quite well. However, a dramatic disruption in his abilities to localize sounds can be seen after such an accident. This is because each primary auditory cortex is mainly responsible for localizing sounds on the opposite side.
Primary Auditory Cortex Development
The auditory system develops surprisingly early, as compared with the visual system. Yet, the development of such complex functions as sound localization can take several years, as shown by Drs. L.A. Werner and E.W. Ruber in their book “Developmental Psychoacoustics.” Typically, infants find it harder to discriminate between high frequency sounds than low frequency sounds. However, while thresholds are initially most adult-like at low frequencies, high-frequency sensitivity improves more rapidly.
Other Areas of Sound Processing
Besides the primary auditory cortex, sound processing also takes place in other areas of the cortex, such as the frontal and the parietal lobes. The parietal lobe, for example, plays a role in language acquisition. Moreover, Martha W. Burton, Ph.D., demonstrated in a study published in “Cognitive Science” that the frontal lobe is activated in speech discrimination tasks.
- Ann. N.Y. Acad. Sci.: Neurophysiology and Neuroanatomy of Pitch
- Cerebral Cortex: Principles Governing Auditory Cortex
- Carl Wernicke
- "Developmental Psychoacoustics"; L.A. Werner, E.W. Ruber (Eds); 1992.
- "Cognitive Science": The role of inferior frontal cortex in phonological