Chapter 4: Hemispheric Specialization Flashcards
Amobarbital
Book definition: “A barbiturate used to produce rapid and brief anesthesia. (p. 125)”
Amobarbital was used by Japanese Canadian Juhn A. Wada in the late 1950’s to confirm the dominant role of the left hemisphere in language.
In the Wada test, a patient is given an injection of amobarbital into the carotid artery, producing a rapid and brief anesthesia of the ipsilateral (same side) hemisphere. The patient is then tested in a series of tasks related to language and memory.
When the Wada test is applied to the left hemisphere, the patient’s ability to speak or comprehend speech is disrupted for several minutes, indicating the left hemisphere’s vital role in language.
Anterior commissure
Book definition: “The nerve bundle connecting the left and right cerebral hemispheres and that is located anterior to the corpus callosum. (p. 128)”
The anterior commissure is a bundle of nerve fibers (white matter) about a tenth the size of the corpus callosum and is found inferior to (below) the anterior (frontal) part of the corpus callosum, also known as the genu.
Together with the smaller posterior commissure, it links the two cerebral hemispheres, connecting the amygdalae and temporal lobes.
The anterior commissure plays a key role in pain sensation and contributes to memory, emotion, speech, and hearing. It also contains decussation fibers from the olfactory tract, involving it in olfaction (the sense of smell).
Cerebral specialization
Book definition: “The adaptation of the activity in a particular brain region to subserve a given cognitive function or behavior. (p. 132)”
Corpus callosum
Book definition: “A fiber system composed of axons that connect the cortex of the two cerebral hemispheres. (p. 128)”
The corpus callosum is a wide band comprised of 250-300 million axonal fibers that cross from one side of the brain to the other, facilitating interhemispheric communication.
It is located beneath the cortex, running along the longitudinal fissure (i.e. the “crack” between the two hemispheres) and is divided into three parts:
- The genu, located anteriorly (frontally). Involved with higher order transfer of semantic information.
- The body, making up the larger middle portion. Partial sectioning of the corpus callosum has shown that tactile information (touch) is transferred by fibers in the posterior region of the body.
- The splenium, located posteriorly (caudally). Involved with transferring visual information between the occipital lobes. When this section is spared in a callosotomy, pattern, color, and linguistic information can still travel between hemispheres.
The size of the crossing fibers increase from the front (about 0.4 micrometers) to the back (about 5 micrometers).
Surgically severing the corpus callosum is called corpus callosotomy and results in so-called split-brain patients. While these patients report normal cognition and consciousness, studies show that the two hemispheres appear to have distinct personalities and abilities, such as only the left hemisphere being able to verbally respond to stimuli.
Other studies have found examples of the two hemispheres using external cues such as body movement or speech to subconsciously communicate: e.g., when presenting the non-speaking hemisphere with one of two colors and asking the speaking hemisphere to identify it, the speaking hemisphere would make a blind guess and start saying a color name. If the answer that the non-speaking hemisphere heard did not match the presented color, it would interrupt with a slight shaking of the head, after which the speaking hemisphere would correct its mistake.
Dichotic listening task
Book definition: “An auditory task in which two competing messages are presented simultaneously, one to each ear, while the subject tries to report only one or both messages. The ipsilateral projections from each ear are presumably suppressed when a message comes over the contralateral pathway from the other ear. (p. 151)”
Studies have found a tendency for participants in dichotic listening tasks to produce more words presented to the right ear – an effect dubbed the right-ear advantage. This seems to supports the idea that the left hemisphere is dominant for language.
Functional asymmetries
Book definition: “Differences in the functions that each hemisphere subserves. (p. 126)”
In language-associated regions, the left hemisphere has greater high-order branching of dendrites than that of the homologs of the right hemisphere, which in turn has more low-order branching of dendrites. This might hint at an explanation to the left hemisphere’s leading role in higher cognitive, analytical function.
Handedness
Book definition: “The tendency to perform the majority of one’s manual actions with either the right or left hand. (p. 156)”
Heterotopic areas
Book definition: “Noncorresponding areas of the brain. Usually such areas are referred to as heterotopic because of their connections with one another. For instance, a connection between M1 on the left side and V2 on the right side would be a connection joining heterotopic areas of the brain. Compare homotopic areas. (p. 128)”
Hierarchical structure
Book definition: “A configuration that may be described at multiple levels, from global features to local features; the finer components are embedded within the higher level components. (p. 144)”
An example of a hierarchical structure is a house. At the highest level, it can be viewed as a house; on a lower level, it could be viewed as a series of components, i.e. doors, windows, walls, roof and so on; on an even lower level, a wall could be viewed as a collection of bricks, or a window as a collection of glass panes and wooden panels.
Homotopic areas
Book definition: “Areas in corresponding locations in the two hemispheres. A connection between M1 on the right side and M1 on the left side would be joining homotopic areas. Compare heterotopic areas. (p. 126)”
Interpreter
Book definition: “A left-brain system that seeks explanations for internal and external events in order to produce appropriate response behaviors. (p. 146)”
In tests where a split-brain patient (i.e. a patient with a surgically severed corpus callosum) is given a task that only the right cerebral hemisphere is aware of, the patient may act on the task and have their left hemisphere try to rationalize it afterwards.
As an example, when giving a patient the command to stand up, he would stand up. When the researcher asked the patient why they stood up, they confabulated the story “Oh, I felt like getting a Coke” to explain their response.
The important thing to note here, is that these patients are not lying – their own brain is simply making up a story to try to explain their behavior.
This effect is not limited to split-brain patients. Your own brain can, in essence, constantly make up reasonable narratives to explain your own behavior – narratives that you fully believe are the cause of your actions and not vice versa.
…Think about that for a second.
Module
Book definition: “A specialized processing unit of the nervous system. Modules are hypothesized to perform specific computations; for example, some theorists believe there are dedicated modules for speech perception, distinct from those used for auditory perception. (p. 156)”
Planum temporale
Book definition: “The surface area of the temporal lobe that includes Wernicke’s area. The planum temporale has long been believed to be larger in the left hemisphere because of the lateralization of language function, although this theory is currently controversial. (p. 126)”
While some studies show that people on average have a larger left planum temporale than right, symmetry in their sizes has also been linked to dyslexia. In addition, an even greater size of the left planum temporale was found in musicians with perfect pitch and control compared with musicians without those abilities.
Posterior commissure
Book definition: “Located above the cerebral aqueduct at the junction of the third ventricle, this carries interhemispheric fibers that contribute to the papillary light reflex. (p. 128)”
Splenium
Book definition: “The posterior area of the corpus callosum that interconnect the occipital lobe. (p. 128)”
The splenium is located posteriorly (caudally) on the corpus callosum and is involved with transferring visual information between the occipital lobes.
When this section is spared in a callosotomy, pattern, color, and linguistic information can still travel between hemispheres.
Sylvian (or lateral) fissure
Book definition: “Also lateral fissure. A large fissure (sulcus) on the lateral surface of the cerebral cortex first described by the anatomist Franciscus Sylvius. The Sylvian fissure separates the frontal cortex from the temporal lobe below. (p. 125)”
Wada test
Book definition: “A clinical procedure in which a barbiturate is injected to temporarily disrupt function in one of the cerebral hemispheres. This procedure, used to identify the source of epileptic seizures, provided important initial insights into hemispheric specialization. (p. 125)”
Pioneered by Japanese Canadian Juhn A. Wada, the Wada test was used in the late 1950’s to confirm the dominant role of the left hemisphere in language.
In the test, a patient is given an injection of the drug amobarbital into the carotid artery, producing a rapid and brief anesthesia of the ipsilateral (same side) hemisphere. The patient is then tested in a series of tasks related to language and memory.
In most cases, when the Wada test is applied to the left hemisphere, the patient’s ability to speak or comprehend speech is disrupted for several minutes, indicating the left hemisphere’s vital role in language.
