Hempispheric Specialisation Flashcards
Describe the anatomy of the brain hemispheres
A longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The two sides resemble each other and each hemisphere’s structure is generally mirrored by the other side. Yet, despite the strong anatomical similarities, the functions of each cortical hemisphere are managed differently.
Describe anatomical asymmetries of the brain
MACROSCOPIC AYSMMETRIES
- SIZE/VOLUME: The right protrudes in the front and the left protrudes in the back (difference in sizes): implications of this difference remains uncertain, as the mass of the two hemispheres still remain the same.
- - However, the right has a higher volume in the frontal region whilst the left is larger posteriorly in the occipital region which can nudge the right hemisphere off centre and bends the longitudinal fissure - PLANUM TEMPORALE: Asymmetry in the superior aspect of the temporal lobe known as the planum temporale (involved with understanding language).
This area was significantly larger on the left side in about two-thirds of human subjects studied postmortem. (GESCHWIND, 1968). - HESCHL GRYI: There are two Heschl’s gyri on the right side and only one on the left which complements the asymmetry of the planum temporale
MICROSCOPIC AYSMMETRIES
- There are differences in the cortical microcircuitry between the two hemispheres in both the anterior and posterior language associated cortex
- - the left hemisphere has greater higher order dendritic branching
- - In the Wernickes area of left there are aysemmteries of the cortical columns - Examination of brains at autopsy has revealed some consistent differences, in the number and size of certain neurons between the two hemispheres. A region of the temporal lobe that is part of the auditory association cortex, which is involved in higher-level processing of auditory information and especially speech sounds, is larger on the left side of the brain. And an area lying mainly on the angular gyrus between the temporal and parietal lobes was also found to be larger on the left side. Lesions to this area have been associated with problems in naming objects and in word-finding tasks
Evaluate the anatomical asymmetries of the brain
+ There is high practical application in understanding these differences in anatomy of the two hemispheres because it helps to develop the understanding of whether differences in neural circuits or anatomical differences underlie functional asymmetries in tasks such as language. The asymmetries involving the planum temporal and Heschl’s gyri might provide an anatomical basis for the asymmetry in language production: Suggests that these areas evolved to subserve the production of language.
- However, the fact that a detectable planum asymmetry is present in only 67% of human brains, whereas the preeminence of language in the left hemisphere is evident in 97% of the population, argues that this association should be regarded with caution. In fact, recent studies using noninvasive imaging have indicated that there is less variability in the planum temporale than originally described.
What is the basic hemispheric specialization theory
The brain is divided into two separate hemispheres – right and left. The hemispheres cannot communicate except through a structure called the corpus callosum. When stimuli are presented to the right visual field, because of the way the neural pathways from eye to brain are configured, this right visual field is sent directly to the left hemisphere and only the left hemisphere. The reverse is true for the left visual field.
SPERRY+ORNSTEIN: Hemispheric specialization theory says that certain tasks can be processed only by the right hemisphere or only by the left hemisphere (but not by both hemispheres). If the information that arrives in a hemisphere can be processed by that hemisphere, then the response time will be shorter and the processing will be more accurate than if the information cannot be processed by that hemisphere and must be sent, via the corpus callosum, to the other hemisphere.
What are the major functional differences between hemispheres
LEFT:
- Language processing + speech
- Problem solving + logical reasoning
RIGHT:
- Visuospatial processing such as tasks of drawing cubes
- Facial processing + recognising faces
- Music analysis
- Emotional processing
What are the three main approaches to conducting research on hemispheric specialisation
- Studying people with brain damage
- Studying people after a split brain operation
- Studying people with intact brains (Wada + Dichotic)
What is the corpus callosum
The brain is divided into the right and left hemisphere, and the two halves are connected by the corpus callosum. This bundle of nerve tissue contains over 200 million axons (nerve fibers that carry electrical impulses from neurons’ cell bodies) by rough estimate. This neural tissue facilitates communication between the two sides of the brain.
The corpus callosum is the largest collection of white matter within the brain, and it has a high myelin content. Myelin is a fatty, protective coating around nerves that facilitates quicker transmission of information. White matter should not be confused with gray matter. The brain uses gray matter for computation, thinking, memory storage, and more. White matter, like the corpus callosum, allows different parts of the brain to communicate with each other.
Describe split-brain research
When the corpus callosum connecting the two hemispheres of the brain is severed to some degree. The surgical operation to produce this condition results from transection of the corpus callosum, and is usually a last resort to treat refractory epilepsy.
After the right and left brain are separated, each hemisphere will have its own separate perception, concepts, and impulses to act.
Evaluation of the split-brain method
Studying split-brain patients to understand hemispheric lateralisation comes with its advantages and disadvantages.
- There remains an inconclusiveness in research as the patients were not neurologically normal before their surgeries; most were epileptics or experienced seizures which may have caused neurologic damage. Furthermore, the transcortical connections may not have been completely sectioned and some fibres may still remain intact. Caution must be taken that accurate documentation of a callosal section is crucial for learning about the organisation of the cerebral commissure.
Therefore, even if these patients showcase abnormal or specific behaviours after the operation, it is uncertain as to whether a causal relationship can be determined, reducing its usefulness. - Ethical concerns must be considered as such studies may not be acceptable now. People who undergo this surgery then are subjected to years of testing which can have an impact on their psychological well0being – Extremely invasive procedure: Today hemisphere interaction can be studied using devices that measure the electric or magnetic fields surrounding the skull.
+ Nevertheless, split-brain patients remain an invaluable resource. Imaging tools can confirm, for example, that the left hemisphere is more active than the right when processing language. But this is dramatically embodied in a split-brain patient, who may not be able to read aloud a word such as ‘pan’ when it’s presented to the right hemisphere, but can point to the appropriate drawing. “That gives you a sense of the right hemisphere’s ability to read, even if it can’t access the motor system to produce speech,” Ivry says. “Imaging is very good for telling you where something happens,” he adds, “whereas patient work can tell you how something happens.”
What are unilateral cortical lesions
People with lesions to only one side of the brain: experiments using these individuals compare patients with right hemisphere lesions to patients with left hemisphere lesions
What is the wada test
In the Wada test each hemisphere is put to sleep at different times by injecting anesthetic into a major artery that leads only to that hemisphere. While one hemisphere is anesthetized, the other remains awake and conscious. Once half the brain is anesthetized, doctors can interview the other half (if the patient is capable of talking) or give instructions to perform simple tasks. This helps to determine what skills or abilities might be confined to one hemisphere.
Evaluation of using unilateral cortical lesion patients to study lateralisation
+ There is no need to lateralise the stimuli to one side or the other as laterality effects are assumed to arise because of the unilateral lesions. If lesions to the the left hemisphere result in an effect then it can be attributed to the hem’s specialisation in the process.
- Lesions rarely are localized and can affect large areas of the brain. Processing in the brain is generally not localized and requires many areas of the brain to process. Furthermore, lesioning may interfere with pathways that span the lesion site. Thus, lesions are not always a good way to determine what specific brain areas do. Therefore, a degree of skepticism should be kept in mind when viewing lateralization data from lesion studies. (KOLB+WISHAW, 1990).
+ It has been critiqued that functions can not be attributed to certain areas of the brain simply as a result of lesions. However, this has been overcome using a method called double association which can demonstrate lateralisation of a function. This is an inferential technique that demonstrates that lesions in one hemisphere can produce functional deficits such as to language as they do not follow after lesions to the other hemisphere. Thus the functions of the two hemispheres can be said to be dissociated and it can be concluded that there is lateralisation.
Evaluation of studying normal patients for lateralisation
- (KOLB+WISHAW, 1990) Experiments demonstrating hemispheric asymmetry show that in normal people, anatomical asymmetry is relative rather than absolute. The data from studies such as dichotic listening should not be interpreted as showing evidence for hemispheric abilities as there is still a connection through the corpus callusom - a more likely attribution is that routes into one hemisphere is inefficient and info is lost.
Evaluation of brain lateralisation concepts
- DRENTH 2003 There are significant negative implications of this research in the real world as many individuals misapply the concept. Broad generalizations are often made in “pop” psychology about one side or the other having characteristic labels, such as “logical” for the left side or “creative” for the right. These labels are not supported by studies on lateralization, as lateralization does not add specialized usage from either hemisphere. This research has been exaggerated and mistakenly used in psychological interventions such as EMDR and neurolinguistic programming, brain training equipment, or management training
- Caution must be taken when generalising findings regarding lateralisation of certain functions; laterality is relative and not absolute as both hemispheres play a role in every behaviour. So, although functions such as language are more dominantly produced in the left hemisphere, the right still has some language capabilities.
- Furthermore, there is often contradictions that occur within lateralisation research which demonstrates the inconclusiveness of findings presently and that more work needs to be done.
For example, in terms of emotional processing it is suggested to be right hemisphere dominant BUT In contrast, the valence hypothesis claims that the right hemisphere is specialized for processing negative emotions whereas the left hemisphere is specialized for positive emotions (Ross, 1984). Yet contrary to both lateralization hypotheses, results from other studies have failed to find hemispheric specialization for emotional processing (Braun et al., 2005; Tamietto et al., 2007).
One possible explanation for the mixed findings in the literature is that the lateralization of emotion has been studied primarily at a broad level. for example, comparing positive and negative emotion. A more narrowed focus on the functional domain of an emotion might lead to more nuanced patterns of how emotion is processed in the brain.
Describe the lateralisation of language
Language is primarily localized in the left hemisphere: processed in areas of the temporal lobe, most commonly on the left side of the head.
Neuroscientists generally agree that around the lateral sulcus in the left hemisphere of the brain, there is a neural loop involved both in understanding and producing spoken language. At the front end or beginning of this loop lies Broca’s area, which is usually associated with the production of language, or language outputs. At the other end, or specifically in the superior posterior temporal lobe, lies Wernicke’s area, which is associated with the processing of words that we hear being spoken, or language inputs. Broca’s area and Wernicke’s area are connected by a large bundle of nerve fibres called the arcuate fasciculus.
SPLIT BRAIN PATIENTS
1. GAZZINGA+SPERRY studied the contributions of each hemisphere to various cognitive and perceptual processes. Main finding was that the right hemisphere was capable of rudimentary language processing, but often has no lexical or grammatical abilities.
Split-brain patient had words and images presented on either side of screen and the visual stimuli would go to either the right or left visual field, and thus the left or right brain. Observed that if a patient was presented with an image to his left visual field (right brain), he would report not seeing anything. If he was able to feel around for certain objects, he could accurately pick out the correct object, despite not having the ability to verbalize what he saw.
– Confirmed that the left brain is localized for language while the right brain does not have this capability, and when the corpus callosum is cut and the two hemispheres cannot communicate for the speech to be produced.
NORMAL BRAINED PATIENTS
- KIMURA 1973: Studied auditory perception using dichotic listening tasks that isolates input to one hem. Stimuli is presented monaurally where two competing messages are presented simeltanesouly not to each ear and the patient has to report both messages. It is found that ppts consistently produce words that had been presented to the right ear suggesting that the left hem is dominant for language.
- - Also showed that when musical notes were presented to the two ears through earphones, it was more easily analysed when presented to left ear suggesting lateralisation to the right side of the brain. - Evidenced by the Wada test where the patient is tested on language tests before an elective surgery after being given an injection that produces anaesthesia on one side of the brain.
- - However, BARRETT 1999 reported that there is a double dissociation between language and emotional prosody whereby the right hempishere is specialised for comprehending emotional expressions of speech; evidence by brain lesion patients where damage to left showed difficulty comprehending words but showed little deficit in interpreting meaning of emotion
