Your brain is responsible for nearly all functions of your body and for interpreting sensory information from the world around you.
Your brain has many parts but speech is primarily controlled by the largest part of the brain, the cerebrum.
The cerebrum can be divided into two parts, called hemispheres, which are joined by a band of nerve fibers called the corpus callosum.
Your speech is typically governed by the left side of your cerebrum. In about a third of people who are left-handed, however, speech may actually be controlled by the right side.
In recent decades, there has been an explosion of research into language processing in the brain. It’s now generally accepted that the control of speech is part of a complex network in the brain.
The formation of speech requires many different processes, from putting thoughts into words, forming a comprehensible sentence, and then actually making the mouth move to make the correct sounds.
There are several areas of the brain known to play a role in speech:
Each hemisphere of the cerebrum can also be divided into regions called lobes, which include the frontal, parietal, temporal, and occipital lobes.
The lobes located in the front and side of your brain, the frontal lobes and the temporal lobes, are primarily involved in speech formation and understanding.
Broca’s area is located in the front part of the left hemisphere of your brain. It has an important role in turning your ideas and thoughts into actual spoken words. Broca’s area has been found to be
Broca’s area also helps to pass the information to another part of your brain called the motor cortex, which controls the movements of your mouth. It’s named after French doctor, Pierre Paul Broca, who discovered the region of the brain in 1861.
Wernicke’s area is mainly involved in the understanding and processing speech and written language. Wernicke’s area was first discovered by Karl Wernicke in 1876. It’s located in the temporal lobe, just behind your ears. The temporal lobe is also the region where sound is processed.
The arcuate fasciculus is a band of nerves that connects Wernicke’s area and Broca’s area. It helps you form words, speak clearly, and understand concepts in language form.
The cerebellum is located at the back of your brain. The cerebellum is involved in coordinating voluntary muscle movements like opening and closing your mouth, moving your arms and legs, standing upright, and maintaining balance. It also controls language processing.
A review published in the American Journal of Speech-Language Pathology suggests that the cerebellum is actually more important to language processing than previously thought.
To speak clearly, you must move the muscles of your mouth, tongue, and throat. This is where the motor cortex comes into play.
Located in the frontal lobe, the motor cortex takes information from Broca’s area and tells the muscles of your face, mouth, tongue, lips, and throat how to move to form speech.
What happens if one or more of these parts is injured, damaged, or abnormal?
If you have a problem speaking or understanding speech, it’s a condition called aphasia. If you have trouble putting together the correct muscle movements necessary to produce speech, it’s a condition called apraxia.
Both aphasia and apraxia are most often caused by a stroke or trauma to the brain, usually when the left side of the brain is affected. Other less common causes are brain tumors and infections.
Symptoms of aphasia or apraxia depend on where the damage occurs in the brain and the severity of the damage. These symptoms include:
Speaking slowly or slurring words
If Broca’s area is damaged, a person might find it difficult to produce the sounds of speech or may speak very slowly and slur their words. Speech is often limited to short sentences of less than four words. This is called Broca’s aphasia or nonfluent aphasia.
Another cause is if stroke or injury damages the areas of the brain that control movements of the muscles of the mouth or tongue.
Speaking in long and nonsensical sentences
Damage to Wernicke’s area may cause someone to make up nonsense words or speak in long sentences that have no meaning. The person also may not realize that others can’t understand them. This is called Wernicke’s aphasia or fluent aphasia.
Inability to repeat words you just heard
If the arcuate fascilicus, the bundle of nerves that connects Broca’s area and Wernicke’s area, is damaged, a person may not be able to repeat language previously heard. This is called conduction aphasia.
General inability to speak and understand language
Widespread damage to the brain’s language centers can result in global aphasia. People with global aphasia will have an extremely hard time expressing and understanding language.
People with neurodegenerative diseases, such as Alzheimer’s disease, often experience loss of speech slowly over time. This is called primary progressive aphasia (PPA).
PPA is not Alzheimer’s disease but can be a symptom of Alzheimer’s disease. PPA can also be an isolated disorder without the other symptoms of Alzheimer’s disease. Some people with PPA have normal memories and can continue leisure activities and sometimes even work.
Unlike aphasia that results from stroke or brain trauma, PPA results from slow deterioration of one or more areas of the brain used in speech and language.
Speech relies on the activation of multiple areas of the brain working together cooperatively.
Broca’s area and Wernicke’s area are considered the major components of the brain involved in speech, but other parts of the brain also play an important role in coordinating the muscles of the mouth to create spoken words. For most people, speech-related brain activity happens on the left side of the brain.
Damage or injury to any of these parts can lead to speech problems known as aphasia or apraxia. Speech-language therapy is often helpful for people with these conditions. Although restoring full speech abilities after brain damage isn’t always possible, improvements can be made.