Lateralization of brain function
The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. The median longitudinal fissure separates the human brain into two distinct cerebral hemispheres connected by the corpus callosum. Both hemispheres exhibit brain asymmetries in both structure and neuronal network composition associated with specialized function.
Lateralization of brain structures has been studied using both healthy and split-brain patients. However, there are numerous counterexamples to each generalization and each human's brain develops differently, leading to unique lateralization in individuals. This is different from specialization, as lateralization refers only to the function of one structure divided between two hemispheres. Specialization is much easier to observe as a trend, since it has a stronger anthropological history.
The best example of an established lateralization is that of Broca's and Wernicke's areas, where both are often found exclusively on the left hemisphere in the vast majority of people. Function lateralization, such as semantics, intonation, accentuation, and prosody, has since been called into question and largely been found to have a neuronal basis in both hemispheres. Another example is that each hemisphere in the brain tends to represent one side of the body. In the cerebellum, this is the ipsilateral side, but in the forebrain this is predominantly the contralateral side.
Lateralized functions
Language and speech
functions are lateralized to the left hemisphere in 96% of right-handers and 60% of left-handers.Meaning of words, called lexicon, is processed bilaterally which has been tested through the word superiority effect. This finding is consistent with the distributed memory and knowledge systems required for lexical entries; however, each hemisphere's lexicon is considered unique since it may be organized and accessed differently. For example, the right hemisphere lacks letter recognition, and cannot judge lexical relationships such as superordinate words or antonyms.
The permitted organization of words, called grammar, is lateralized in only one hemisphere, typically the left one. These functions include "understanding verbs, pluralizations, the possessive, and active-passive differences" and understanding changes in meaning due to word order. However, the right hemisphere is able to judge when a sentence is grammatically correct, which may indicate that patterns of speech are learned by rote rather than applied through understanding rules.
Speech production and language comprehension are specialized in Broca's and Wernicke's areas respectively, which are located in the left hemisphere for 96% of right-handers and 70% of left-handers. However, there are some cases in which speech is produced in both hemispheres in split-brain patients, also lateralization can shift due to plasticity over time. The emotional content of language, called emotional prosody, is right-lateralized.
In writing, studies attempting to isolate the linguistic component of written language in terms of brain lateralization could not provide enough evidence of a difference in the relative activation of the brain hemispheres between left-handed and right-handed adults.
Sensory processing
Sensory processing for the left and right sides of the body is often lateralized to the contralateral hemisphere due to nerve fiber decussation.Because of the functional division of the left and right sides of the body, the processing of information in the sensory cortices is essentially identical. That is, the processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally. Numerical estimation, comparison and online calculation depend on bilateral parietal regions while exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing.
Vision
In vision, retinal ganglion cells undergo partial decussation at the optic chiasm, where axons from the nasal retinas cross to the opposite hemisphere, while axons from the temporal retinas remain on the ipsilateral side. As a result, visual input from the left visual hemifields are processed by the right hemisphere's visual cortex, while input from the right visual hemifields are processed by the left hemisphere's visual cortex.Hearing
In hearing, spiral ganglion neurons in the vestibulocochlear nerve project to the ipsilateral cochlear nuclei in the medulla. However, second-order axons from the ventral cochlear nucleus branch to both the ipsilateral and contralateral superior olivary complexes. Consequently, hearing is strongly lateralized only at the ipsilateral cochlear nuclei, while further processing in the inferior colliculi, the medial geniculate nucleus of the thalamus, and the auditory cortex occurs bilaterally with a slight contralateral dominance. This lateralization explains why damage to one cochlear nucleus causes deafness in the ipsilateral ear, whereas damage above the cochlear nucleus typically results in only slight hearing loss.When tasked to repeat words in a dichotic listening task, individuals tend to say words played in their right ear, a phenomenon called right-ear advantage. Since hearing is slightly contralateral dominant, this effect is consistent with the left hemisphere lateralization of language. When tasked to recall melodies in a dichotic listening task, people instead tend to have a left-ear advantage.
Touch
In the somatosensory system, sensations of touch, vibration, pressure, pain, and temperature are primarily processed in the contralateral somatosensory cortex of the brain. Mechanoreceptors responsible for touch and vibration transmit signals through the dorsal column-medial lemniscal pathway, where they decussate at the dorsal column nuclei in the medulla before ascending. Touch from the face and top of the head follows the trigeminal touch pathway, where second-order neurons decussate at the trigeminal nucleus.Pain and temperature signals from nociceptors travel a different pathway, the spinothalamic pathway, where second-order neurons decussate earlier in the spinal cord. For pain and temperature in the face and top of the head, second-order neurons decussate at the spinal trigeminal nucleus of the brainstem. The earlier decussation of pain signals compared to touch explains Brown-Séquard syndrome, a condition in which damage to one half of the spinal cord leads to ipsilateral insensitivity to touch but contralateral insensitivity to pain and temperature.
Motor system
Voluntary movement is lateralized to the contralateral motor cortex, so the right hemisphere controls the left side of the body, while the left hemisphere controls the right side.In the two lateral pathways, the corticospinal tract is responsible for control of distal muscles and begins at the contralateral motor cortex or contralateral somatosensory areas, and decussates between the medulla and spinal cord. The rubrospinal tract responsible for distal muscle and posture begins at the contralateral red nucleus and quickly decussates in the pons.
In the four ventromedial pathways, the vestibulospinal tract responsible for head balance begins at the ipsilateral vestibular nucleus of the medulla and splits into a bilateral and ipsilateral path. The bilateral path controls neck and back muscles for head balance, while the ipsilateral path maintains upright posture of the legs. The tectospinal tract responsible for orienting the head toward sensory stimuli begins at the contralateral superior colliculus and quickly decussates at the red nucleus. The reticulospinal tracts responsible for controlling muscles against gravity begin at the ipsilateral reticular formation and do not decussate.
Value systems
Rather than just being a series of places where different brain modules occur, there are running similarities in the kind of function seen in each side, for instance how right-side impairment of drawing ability making patients draw the parts of the subject matter with wholly incoherent relationships, or where the kind of left-side damage seen in language impairment not damaging the patient's ability to catch the significance of intonation in speech. This has led British psychiatrist Iain McGilchrist to view the two hemispheres as having different value systems, where the left hemisphere tends to reduce complex matters such as ethics to rules and measures, and the right hemisphere is disposed to the holistic and metaphorical.Clinical significance
Depression is linked with a hyperactive right hemisphere, with evidence of selective involvement in "processing negative emotions, pessimistic thoughts and unconstructive thinking styles", as well as vigilance, arousal and self-reflection, and a relatively hypoactive left hemisphere, "specifically involved in processing pleasurable experiences" and "relatively more involved in decision-making processes". Additionally, "left hemisphere lesions result in an omissive response bias or error pattern whereas right hemisphere lesions result in a commissive response bias or error pattern." The delusional misidentification syndromes, reduplicative paramnesia and Capgras delusion are also often the result of right hemisphere lesions.Hemisphere damage
Damage to either the right or left hemisphere, and its resulting deficits provide insight into the function of the damaged area. There is truth to the idea that some brain functions reside more on one side of the brain than the other. We know this in part from what is lost when a stroke affects a particular part of the brain. Left hemisphere damage has many effects on language production and perception. Damage or lesions to the right hemisphere can result in a lack of emotional prosody or intonation when speaking. The left hemisphere is often involved with dealing of detail-oriented perception while the right hemisphere deals mostly with wholeness or an overall concept of things.Right hemisphere damage also has grave effects on understanding discourse. People with damage to the right hemisphere have a reduced ability to generate inferences, comprehend and produce main concepts, and a reduced ability to manage alternative meanings. Furthermore, people with right hemisphere damage often exhibit discourse that is abrupt and perfunctory or verbose and excessive. They can also have pragmatic deficits in situations of turn taking, topic maintenance and shared knowledge.. Although both sides of the hemisphere has different responsibilities and tasks, they both complete each other and create a bigger picture.
Lateral brain damage can also affect visual perceptual spatial resolution. People with left hemisphere damage may have impaired perception of high resolution, or detailed, aspects of an image. People with right hemisphere damage may have impaired perception of low resolution, or big picture, aspects of an image.