Chapter 13 - Cognitive Neuroscience Flashcards
(32 cards)
LATERALIZATION
The right and left hemispheres differ in function as well - the localization of a function in one hemisphere or the other is known as LATERALIZATION.
Generally speaking:
- the LEFT HEMISPHERE is specialised in language and logical processing.
- the RIGHT HEMISPHERE is specialised in prosody, musical abilities, spatial abilities, facial recognition, intuition and artistic skills.
Evidence for brain lateralization comes from:
1) SPLIT BRAIN OPERATIONS;
2) WADA TESTS;
3) DICHOTIC LISTENING TASKS.
Today, lateralisation can be observed using fMRI and DTI.
SPLIT BRAIN OPERATIONS
In the SPLIT BRAIN OPERATION - used to reduce the severity of seizures - pathways connecting the right and left cerebral hemispheres are severed. Usually only the CORPUS CALLOSUM is removed, but sometimes also the MASSA INTERMEDIA, ANTERIOR COMMISSURE and HIPPOCAMPAL COMMISSURE are severed.
SPLIT BRAIN TASKS
In a classic split brain task, a patient is asked to fixate their gaze on a dot in the centre of their visual field. Then, a word is briefly displayed on a screen, with the first half of the word to the left of the dot and the second half to the right.
When the participant is asked which word has been presented, she responds with the right half of the word - this is the half processed by the left hemisphere, which in most people processes language.
If the participant is asked to point with her left hand to the word she saw, she will point to the left half of the world - this is the half processed by the right hemisphere, which controls movement of the left hand.
Similarly, if a stimulus is displayed only on the left half of the screen, the participant claims not to know what the stimulus presented was, but still maintain the ability to draw it with their left hand.
LATERALISATION of MOTOR and VISUAL SYSTEMS
Both the motor and the visual system show lateralisation:
1) in the VISUAL SYSTEM, input captured by NASAL HEMIRETINA goes to the opposite hemisphere, whereas input captured by the TEMPORAL HEMIRETINA stays in the same hemisphere. In other words, information from the right half of the visual field is transmitted to the left visual cortex, and vice versa.
2) in the MOTOR SYSTEM, movements of each half of the body are controlled by the opposite hemisphere.
WADA TESTS
WADA TESTS were typically performed before surgery in order to determine which hemisphere is involved in a specific task. They consists of injection of a sedative in one hemisphere and subsequent observation of cognitive changes.
DICHOTIC LISTENING TASKS
DICHOTIC LISTENING TASKS feature different sounds presented simultaneously to the left and right ears. If two different stimuli are presented simultaneously to both ears and participants are asked to report what they have heard, they mainly report info from the RIGHT EAR.
This phenomenon can be explained by the LATERALISATION of language processing; WERNICKE’S AREA, which is specialised in SPEECH COMPREHENSION, is found in the left temporal lobe. Although information from one ear is processed by both cerebral hemispheres, it is processed more rapidly by the CONTRALATERAL AUDITORY CORTEX.
This means that input from the left ear is first processed by the right auditory cortex and then sent to the left temporal lobe, whereas input from the right ear is directly sent to the left hemisphere - this difference in “distance travelled” favours input from the right ear, which reaches wernicke’s area in a shorter amount of time.
HEMISPHERECTOMY
HEMISPHERECTOMY - a surgical procedure in which the cortex from one entire cerebral hemisphere is completely removed - provides insight on the power of NEURAL PLASTICITY during the developmental phase. Treated children - usually suffering from severe seizure attacks in only one hemisphere - do not show cognitive deficits, but only minor motor impairments.
the EVOLUTION of LATERALIZATION
LATERALIZATION might have evolved in both vertebrates and invertebrates because it provides advantages:
- individual benefits include ENHANCED SKILL PERFORMANCE, FASTER REACTION TIME
- social benefits include COORDINATION: population lateralization is more common among social species, where it may coordinate responses to a predator.
the DEVELOPMENT of LATERALIZATION
Two hypotheses of LATERALIZATION DEVELOPMENT have been advanced:
1) the first hypothesis suggested that PRENATAL ANDROGENS play key roles in the lateralization of language and visuospatial skills and that high levels of androgens promote left-handedness.
2) the second hypothesis maintains that structural and functional lateralization originates in different GENES involved in the development of the right and the left hemisphere - genes involved in the development of axons and dendrites are expressed differently in the right and left hemispheres between 3 and 5 months of life.
Not only exposure to androgens and genes seem involved in this development, but also exposure to LIGHT affects lateralisation - chicks incubated in dark environments show poor lateralisation compared to chick incubated in the presence of light.
the LATERALIZATION of EMOTION and MUSICAL ABILITIES
EMOTION and MUSICAL ABILITIES show lateralisation.
1) The RIGHT HEMISPHERE plays a larger role in expression of EMOTION and the perception of emotion in others’ faces.
On the other hand,PROSODY, or the ability to produce and detect emotional tone in language, appears to involve.
2) Different aspects of MUSIC appear to produce more activity in one hemisphere or the other.
In DICHOTIC LISTENING TASKS with musical stimuli, NON-MUSICIANS typically remember more notes from LEFT ear (input processed by the right hemisphere), whereas MUSICIANS typically remember more notes from RIGHT ear (input processed by the left hemisphere). This could be explained by the fact that musical training leads musicians to process music with similar brain areas involved in verbal language. Furthermore, musicians show neural STRUCTURAL DIFFERENCES: the PLANUM TEMPORALE - parto of the auditory cortex - is usually larger in the left hemisphere than in the right. However, the difference between the left and right planum temporale is about twice as large in musicians with perfect pitch as in non-musicians.
IDIOTS SAVANTS
IDIOTS SAVANTS are individuals with mild mental retardation who show exceptional skills and talents in specific tasks, usually in the arts. Some research maintains that these skills could be due to abnormalities in the left hemisphere, which, in typical individuals, overshadows such artistic abilities of the right hemisphere.
HANDNESS, LANGUAGE and LATERALIZATION
One of the strongest correlations regarding lateralization is the association between handedness and the localization of language - of the 90 percent who are right-handed, 96 percent localize language to the left hemisphere. Among the 10 percent who are left-handed, only 70 percent localize language to the left hemisphere, with the remaining 30 percent localising language in the right or both hemispheres.
LEFT-HANDNESS is an advantage in forward facing sports, such as fencing, tennis, and baseball, but not in non-interactive sports. This is because left-handedness provides an advantage in physical confrontation, since the majority of the population is right-handed - in support of this theory, it has been found that left-handedness becomes more frequent in societies engaged in sustained wars.
LANGUAGE
LANGUAGE is defined as a system for communicating thoughts and feelings using arbitrary signals, such as sounds, gestures, or written signals. Several criteria have been identified to discern language from simple communication:
1) SEMANTICITY, or the use of symbols to refer to objects and actions;
2) ARBITRARINESS, or the absence of a connection between a symbol and what it refers to;
3) SPONTANEOUS USAGE;
4) DUALITY, or use of different sounds or different orders of sounds;
5) DISPLACEMENT, or the ability to communicate about objects and events that are distant in time and place;
6) STRUCTURE-DEPENDENCE, or the use of grammar;
7) CREATIVITY, or the ability to create novel utterances;
8) TURN-TAKING, the existence of social communicative rules.
The ORIGINS of LANGUAGE
CLICK LANGUAGES might represent the earliest form of human language - studies of genetic relatedness among African groups using click languages show that the origin of click languages occurred long before human beings settled down to begin agriculture.
Several lines of reasoning point to a BIOLOGICAL ORIGIN or human language:
- no human culture on earth exists without language;
- no specific instruction is needed to learn spoken language;
- language learning does not always correlate with intelligence - there might be an independent “language module” in the brain.
One GENE associated with speech and language disorders is the forkhead box P2 gene (FOXP2), located on chromosome 7 - mutations of this gene lead to severe difficulties in speech production. This gene could be responsible for transferring the control of vocalization from the subcortical level - as it is for primates - to the cortical level in hominis.
COMMUNICATION in nonhuman ANIMALS
Many animals communicate with one another - communication, however, is not the same as language. If we are to find an animal precursor to human language capability, the most logical place to start is with our nearest relatives, the great APES. The most important question researchers asked themselves was whether the reason why animals do not produce language was a CENTRAL ISSUE - a lack of cognitive skills - or a PERIPHERAL ISSUE - a lack of anatomical structures to support extensive vocalisation.
Researchers have attempted to teach human-like languages to apes with ambiguous results: while some chimpanzees were able to learn and recognize a few words, the results were scarce and the learning process very laborious. Attempts to teach chimps SIGN LANGUAGE have been somewhat successful.
1) GUA - a baby CHIMPANZEE - did not show significant results in learning human speech;
2) WASHOE - a CHIMPANZEE - successfully learned 132 signs of ASL;
3) KANZI - a BONOBO CHIMPANZEE - learned to associate geometric symbols with words;
4) PANBANISHA - a BONOBO CHIMPANZEE - learned to use a keyboard to produce grammatical sentences.
MULTILINGUALISM
MULTILINGUALISM refers to proficiency in more than one language.
Research into the effects of brain damage in multilingual patients provides fascinating insights into the way the brain manages multiple languages and suggests that multiple languages use some of the same areas of the brain but that the degree of overlap is not 100 percent.
Although different languages share the same brain areas, we are able to keep them separate - according to the “LANGUAGE SWITCH” hypothesis, switching languages involves an executive function of the DORSOLATERAL PREFRONTAL CORTEX - thus, it would be more costly to switch language than not.
Gray-matter density in the LEFT INFERIOR PARIETAL REGION - involved in attentional processes related to language - is (1) positively correlated with proficiency in a second language and (2) negatively correlated to the age of acquisition of such language.
SIGN LANGUAGE
SIGN LANGUAGE is a language not of sounds but of sight and movement.
In spite of the spatial nature of sign language - for which it would be better processed by the right hemisphere - the LEFT HEMISPHERE is the likely place for sign language to be processed. In fMRI studies, the same areas of the brain are activated during language tasks regardless of whether the person uses spoken English or sign language.
COMMUNICATION DISORDERS
COMMUNICATION DISORDERS include:
1) APHASIAS;
2) disorders of READING and WRITING;
3) STUTTERING.
APHASIAS
APHASIAS are neurological disorders that result in total or partial loss of the ability to either produce or comprehend spoken language. There are FIVE types of aphasia: 1) BROCA's APHASIA; 2) WERNICKE's APHASIA; 3) CONDUCTION APHASIA; 4) GLOBAL APHASIA; 5) TRANSCORTICAL APHASIAS.
BROCA’S APHASIA
PAUL BROCA made some of the earliest observations of the localization of language in the brain. He studied the case of a patient known as TAN who showed severe impairment in SPEECH PRODUCTION, but retained the ability to understand most of what was said to him. Broca found significant damage to the patient’s left inferior frontal region, now referred to as Broca’s area.
BROCA’S APHASIA - or production aphasia - is the result of damage to BROCA’S AREA (brodmann 44) in the FRONTAL LOBE.
Main symptoms of the disorder are:
- difficulty in producing speech, which is very slow and has a telegraphic quality;
- difficulty in writing;
- ANOMIA, or difficulty in retrieving the words for the ideas one wishes to express;
- comprehension is slightly affected as well.
Broca’s patients seem to be able to fluently sing song they know very well - this, in addition to the fact that their writing shows the same limits of their speech, provides evidence that Broca’s aphasia is not only a motor impairment.
WERNICKE’S APHASIA
WERNICKE’S APHASIA affects an area of the brain known as WERNICKE’S AREA (brodmann 22) , located on the superior surface of the temporal lobe, adjacent to the primary auditory cortex. Main symptoms of the disorder are:
- difficulty in speech comprehension, usually for both written and spoken language;
- fluent but virtually meaningless speech production;
- difficulty in repeating sentences;
- no apparent distress and unawareness of the disorder.
CONDUCTION APHASIA
CONDUCTION APHASIA results from damage to the ARCUATE FASCICULUS, a band of fibers connecting Broca’s and Wernicke’s areas. The main symptom of this disorder is a major impairment in the REPETITION of words, reflecting the damage in the connection between the two areas - or the connection between comprehension and production.
These patients are less impaired in language function than patients with either Wernicke’s or Broca’s aphasia - speech remains fluent, and comprehension is fairly good.
GLOBAL APHASIA
GLOBAL APHASIA combines all of the deficits of Broca’s, Wernicke’s, and conduction aphasia. It is the result of substantial cortex damage involving at least BROCA’s AREA, WERNICKE’s AREA and the ARCUATE FASCICULUS. Most cases of global aphasia are caused by damage to the MIDDLE CEREBRAL ARTERY, which serves the language centers of the left hemisphere.
Individuals with global aphasia lose all language functions - abilities to speak, comprehend, read and write.
TRANSCORTICAL APHASIAS
TRANSCORTICAL APHASIAS is caused by damage to connections between the major language centers, which results in isolation of these areas from other parts of the brain. There are two type of transcortical aphasia:
1) TRANSCORTICAL MOTOR APHASIA results from damage of the DORSOLATERAL PREFRONTAL CORTEX or of the supplementary motor area (adjacent to the primary motor area). These areas are involved in the executive control and initiation of speech.
Patients show symptoms similar to those of Broca’s aphasia (production aphasia), yet they are can accurately repeat even complex sentences.
2) TRANSCORTICAL SENSORY APHASIA results from damage to areas at the intersection of the temporal, occipital and parietal lobes. These areas connect the language centers to brain areas responsible for word meaning.
Patients show symptoms similar to those of Wernicke’s aphasia (comprehension aphasia), yet they are can accurately repeat even complex sentences.
