The human brain consists of two halves, the right and left hemispheres (Kennison, 2013). Also known as the logical brain, the left hemisphere regulates language and analysis. On the other hand, the right hemisphere, otherwise known as the creative brain, regulates imagination and daydreaming (Kennison, 2013).
Researchers in the early nineteenth century discovered that an injury on the left hemisphere caused the loss of language and speech abilities (Kennison, 2013). Conversely, injuries on the right hemisphere did not cause the loss of language and speech abilities. Further studies have supported the hypothesis that language and speech abilities are located in the left hemisphere. The majority of these studies have shown that in many people, close to 97 percent, these abilities are situated in the left hemisphere (Kennison, 2013). However, the scenario is a bit different in left handed people. Nineteen percent of this group have these abilities located in the right hemisphere (Kennison, 2013). In other left-handed individuals, about 68 percent, language and speech abilities are situated in both left and right hemispheres (Kennison, 2013).
Speech and language regions of the brain
The angular Gyrus
This region takes care of the sensory, auditory (hearing) and visual aspects associated with language and speech (Skipper, Goldin-Meadow, Nusbaum & Small 2007). This region helps people to associate a perceived word with different ideas, sensations and images. The auditory and visual complexes fall here.
The auditory complex
It is located in the cerebral cortex, and it processes auditory processes. It is also part of the sensory system for hearing. This region is responsible for basic and advanced hearing functions (Skipper et al., 2007).
The visual cortex
This part of the brain is situated in the cerebral cortex, and it processes visual information (Skipper et al., 2007).
It is situated in the frontal lobe (in the left hemisphere), and it is responsible for the production of speech. This region controls basic and complex aspects of grammar (Skipper et al., 2007). This region was discovered by French researcher and neurologist Paul Broca. He dealt with patients who had severe language difficulties. In his study, Broca discovered that one of his patients could understand the speech of others (Skipper et al., 2007). However, that patent could only utter one word, tan. Following the death of that patient, Broca conducted further investigations and he realized that the patient’s frontal lobe had been serious damaged. This research confirmed his earlier hypothesis that this region controls speech production (Skipper et al., 2007).
Malfunctions in this area cause a disorder called expressive or Broca’s aphasia (Skipper et al., 2007). Patients with this disorder have little difficulties in understanding the speech of others, but they are unable to speak. Their speech is not only slow and laborious, but also has slurred word sequences (Skipper et al., 2007).
This area is situated in the cerebral cortex of the brain, and it regulates the written and the language aspects (Skipper et al., 2007). This area was discovered by a German neurologist, Carl Wernicke. Carl conducted a study on a patient who could speak quite well but had difficulties in understanding the speech of others. After the death of that patient, Carl conducted an autopsy and he realized that the area at the tip of the temporal lobe and above the auditory cortex had been severely damaged. This area was later named after him. Carl later confirmed his earlier hypothesis that this area is involved in speech comprehension (Skipper et al., 2007). Malfunctions in this area cause receptive or Wernicke’s aphasia. When patients with this disorder are asked a question, they can respond in a sentence that can be either grammatical or not. Howver, the words spoken have no or little meaning to the question asked (Skipper et al., 2007).
The figure below illustrates these regions.
Figure 1: Brocas, Wernicke’s and angular gyrus areas (Corballis, 2003).
Brocas and Wernicke’s areas are not situated in the same lobe, but they are situated close to each other. In addition, they are connected to each other through a tract of nerves, arcuate fascilicus. The figure below illustrates further.
Figure 2: arcuate fascilicus (Corballis, 2003).
Malfunctions in these nerves could cause conduction aphasia (Corballis, 2003). This disorder is less complex than expressive and receptive aphasias. Patients with conduction aphasia can produce coherent speech although with difficulties. They can equally understand speech. However, they lack the capacity to repeat the words or sentences that they hear (Corballis, 2003).
It is essential to note that reading and writing are essential aspects of grammar. The brain region that controls these aspects is still poorly understood (Corballis, 2003). Malfunctions in the angular gyrus region could cause other less popular language and speech disorders. The inability to read known as alexa, difficulties with reading known as dyslexia and the inability to write known as agraphia are the disorders associated with malfunctions in the angular gyrus region. The hypothesis that malfunctions in this cause the aforementioned disorders has been proven by PET scans. PET scans from people with alexia, agraphia and dyslexia show a less active angular gyrus than that of people with active reading and writing.
Corballis, M.C. (April 2003). From mouth to hand: gesture, speech, and the evolution of right- handedness. Behav Brain Sci., 26 (2): 199–208.
Kennison, S. (2013). Introduction to language development. Los Angeles: Sage.
Skipper, I., Goldin-Meadow, S., Nusbaum, C., and Small, L. (2007). Speech-associated gestures, Broca's area, and the human mirror system. Brain and Language, 101 (3), 260–277.