Most people do not think much about what happens to sound after it reaches their ears. They assume that the ears are the organ responsible for hearing and that the person will hear perfectly as long as the ears are functioning. The truth is that although the ears receive sounds, the auditory information must be processed in order to make sense of it. This is the function of the auditory cortex.
The auditory cortex functions to analyze and decode auditory information collected by the ears and relayed along the auditory nerves. Wernicke's area is included in the auditory cortex and is responsible for language comprehension. The auditory cortex is found bilaterally in the temporal lobes.
The auditory cortex is a highly complex area with specialized functions. Damage to the auditory cortex produces some interesting results.
Where Is The Auditory Cortex Located?
The auditory cortex occurs in the right and left temporal lobes. The temporal lobes are the second biggest lobes in the brain and are situated on the sides of the brain behind the ears and temples.
There is some discussion as to how the auditory cortex s divided. For this article, we will stay with the simpler model of a primary auditory cortex and the auditory association cortex.
It is important to remember that the brain is divided according to function. This means that if you were to dissect a brain, you would not find clear structural divisions or membranes that separate the areas.
The primary auditory cortex is situated on the surface of the temporal lobe, extending into a deep cleft in the temporal lobe known as the lateral sulcus. Numerous gyri (ridges) in the lateral sulcus form Heschl’s gyri or the transverse temporal gyri. The primary cortex spreads across Heschl’s gyri.
Each person’s primary auditory cortex occurs in a slightly different place. Some people may have only one or two Gyri of Heschl, and some may have three or more. In some individuals, their primary auditory cortex may extend into a neighboring sulcus.
The auditory association cortex is found lateral to the primary auditory cortex. It contains a critical region known as Wernicke's area.
What Is The Function Of The Primary Auditory Cortex?
The primary auditory cortex neurons are tonotopic. This means that the neurons are specialized to respond to different frequency sounds. Neurons that respond best to the lowest frequencies are grouped at one end, and neurons responding to high frequencies at the other end. Sound frequency is sometimes called pitch.
The auditory cortex can be represented by a tonotopic map – a diagram showing where each frequency is received. The cochlea is arranged in a similar fashion. The specific frequency areas of the cochlea and auditory cortex are synchronized. Tonotopic neurons in the auditory cortex allow it to detect changes in pitch.
Another key function in the primary auditory cortex is the ability to localize sound by interpreting auditory and spatial information together. The use of dual sensory input enables us to determine the direction of the sound, its origin, and the distance.
The ability to determine the distance sound has traveled involves complex neurological computations, which include amplitude or the loudness of sounds.
Amplitude, frequency, and localization are considered the basic perception of sound. Complicated sounds such as speech and music are also interpreted in the auditory cortex requiring auditory perception skills. The auditory cortex processes language without any awareness on our part.
Are The Right And Left Auditory Cortex The Same?
Sound received by the right ear is directed primarily to the left temporal lobe, and information from the left ear is relayed primarily to the right temporal lobe.
Dominance refers to which side of your body is dominant and is reflected in whether you are left-handed or right-handed. The auditory cortex responsible for perceiving and comprehending speech is usually found opposite the dominant side of the body. This rule was established by Broca but later amended by Luria.
For example, if the person is right-handed, the language comprehension center will be in the left temporal lobe. This fact holds true for all right-handed people. Luria discovered that 60% of left-handed people also have their language center in the left temporal lobe. Only 40 % of left-handed people have their language center in the right temporal lobe.
The opposite temporal lobe is responsible for recognizing the voices of individuals and identifying non-verbal sounds such as different bird sounds or other noises. This temporal lobe allows you to recognize a bell ringing, an alarm clock, and a dog barking.
The brain shows many instances of crossmodal connectivity. The fusiform gyrus is an area adjacent to the auditory cortex in the temporal lobe. The fusiform gyrus is critical in recognition of faces. The close proximity of the fusiform gyrus and auditory cortex allows us to connect an individual's voice with their face. In addition, the language comprehension area will add information on their language usage patterns which further aids in identifying the person.
What Is The Result Of Damage To the Auditory Cortex?
Trauma to the brain can give some valuable insights into function. Unilateral damage or lesions in the auditory cortex seldom result in hearing impairment.
Bilateral lesions rarely result in total hearing loss. The individual usually retains the ability to hear pure tones. They lose the ability to understand speech, localize sounds, and identify sounds or their association with specific objects. This condition is known as auditory agnosia.
Auditory agnosia can come in various forms.
What Is Auditory Verbal Agnosia?
Auditory verbal agnosia is sometimes called word deafness or Wernicke's aphasia and directly affects receptive language. Wernicke’s area is the region of the auditory cortex responsible for comprehending speech and written language.
Damage to the auditory cortex by disease or trauma can result in Wernicke's area becoming isolated from stimulation relayed through the auditory cortex. The result is that although the individual can hear speech, they cannot understand the language.
Even though receptive language is directly compromised, expressive language is usually indirectly affected. The person can speak fluently, with normal articulation, grammar, and prosody, but the speech makes no sense. It is as if the inability to comprehend words makes them unable to select the correct words in language production.
Neologisms (made-up) words are common in the speech of people with Wernicke's aphasia. Their speech is often described as a word salad as the words appear randomly thrown together. People with this deficit are unaware of their errors.
It is difficult to know how much cognition occurs in people with Wernicke's aphasia as it is impossible to assess due to the severe language barrier. The complex age-old question of whether you can have thought without language arises. To date, there is still no clear answer.
Do people with Wernicke’s aphasia still have internal language that cannot be expressed? Do they experience some form of shut-in syndrome? It is difficult to say what a person with Wernicke's aphasia experiences. Some seem to show no frustration, while others seem to have an emotional response.
What Is Auditory Nonverbal Agnosia?
Auditory nonverbal agnosia is a rare condition in people with bilateral or right hemisphere damage to the auditory cortex. In this form of agnosia, individuals cannot identify environmental sounds and sounds that do not incorporate words.
The person will acknowledge they heard a sound, but they cannot identify what makes the sound. This inability will include sounds such as a person sighing, yawning, or laughing. They cannot identify music, animal sounds, bells, clocks, or running water.
Although the deficit has minimal impact on communication, it can compromise safety. Someone with nonverbal auditory agnosia crossing the road will not identify the approaching truck noise.
We rely every day on our ability to identify environmental sounds such as running water, a kettle boiling, or the sound of something falling. Without these auditory cues to inform us of events that could affect safety, we could easily be injured.
What Is Phonagnosia?
Phonagnosia is the inability to identify people by their voices. It was originally thought phonagnosia originated from lesions in the right auditory cortex. Recent studies indicate that lesions in either auditory cortex could produce this form of agnosia.
What Is Amusia?
Amusia can be congenital or acquired and is a musical disability. It is the inability to process pitch variations, recognize and remember musical tunes and remember and reproduce rhythms. People with this condition are sometimes described as being tone-deaf.
What Is Auditory Perception?
The auditory cortex is responsible for auditory perception, which is the process of analyzing sounds. A wide variety of skills are involved in auditory perception, which have consequences beyond just understanding speech. Difficulties in auditory perception affect reading, writing, spelling, and concentration ability.
It is interesting to note that neonates are not born with auditory perceptual skills. Their ears and auditory cortex are fully developed, but auditory perceptual skills only develop with exposure and experience.
Why Is Auditory Discrimination Important?A crucial skill is auditory figure-ground discrimination. This ability allows the person to identify target auditory signals and isolate them from other sounds such as environmental noise or other people talking.
Individuals with poor auditory figure-ground skills perceive all sounds as of the same value and struggle to pay attention to the target sound. This is commonly seen in some children with Attention Deficit Disorders with or without hyperactivity.
It is also the cause of some learning disorders. Children unable to discriminate a target sound from background noise will have difficulty paying attention in class. They are frequently accused of not listening by both their parents and teachers. They become tired quickly and receive fractured information due to the interference of environmental noise.
Auditory discrimination is the ability to discriminate similar sounding words or sounds. People with this disability confuse words such as thirty or thirteen. The confusion can impact communication significantly and also plays a role in spelling disorders. People speaking a foreign language frequently struggle with word discrimination.
The auditory discrimination of sounds develops according to exposure. Children learning English will learn the phonetic sounds that accompany English. It will be much harder for adults to learn a language that uses alternative sounds.
For example, tonal languages such as Chinese or click language such as Xhosa can be almost impossible for some people to learn. Their auditory cortex has not developed the skills to identify and understand those novel sounds or, in tonal languages, the subtleties of different tones.
Children exposed to different languages will learn the different sounds. Some individuals have a more developed ability to perceive and understand differences in sound. These people are often good at learning new languages and may have advanced musical abilities. Children’s musical abilities can be improved by exposure to music to allow the auditory cortex to develop musical perception.
How Are Auditory Memory And Auditory Sequencing Related?
Auditory memory is the ability to remember auditory information. Sounds are temporal. A sound is presented for a brief period and then is gone. Generally, a visual stimulus can be looked at for longer and studied. Unlike vision, sound comes and goes very rapidly. The auditory memory has a very brief time to encode and store the information.
Auditory memory must be developed. Babies have limited auditory memory and must acquire the skills over time. One memory skill is remembering the auditory information in the correct sequence.
Sequential memory is a critical skill for speech. The child must remember the sounds and syllables in the right order to produce the word. Children often get this wrong when their auditory sequencing memory is still weak. Multisyllabic words are often produced in the wrong order. For example, hospital becomes hopsital.
Auditory memory and auditory sequencing are vital for education, following directions, and following conversations.
What Are Auditory Analysis, Synthesis, And Closure?
Auditory analysis is the ability to break sounds and words down into their component parts. Auditory synthesis is the ability to blend the parts of sounds and words. These two skills are foundational in learning to speak, read and spell.
Auditory closure is a crucial skill for conversation. We do not always hear every sound in speech. Environmental noise interferes with our ability to hear speech, and we may miss words or sounds. The auditory cortex accesses the auditory memory and predicts the missing sounds, allowing us to infer the correct words. Without this skill, many verbal interactions would grind to a halt.
Conclusion
The auditory cortex functions to allow us to perceive and understand the sounds and speech around us. Without it, we would be assaulted by a cacophony of unintelligible, disordered sounds that would only confuse and disorientate us.
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