Nature and nurture of language development Flashcards
What is the difference between language development and language ability?
language development - the process of acquiring language
language ability- can be assessed with specific tests
Language ability:
- when does it appear?
- across?
- without?
- what ability?
- Appears in the first three years of life
- Across different languages and cultures- it’s universal
- Without need of explicit instructions
- Innate ability- genetic factors
Language development:
What are early language skills important for?
- children’s readiness for school
- qualifications gained at school
- earnings, literacy skills and mental health in adulthood
- for health and wellbeing
Language ability and disability:
- what is the most widely used test?
- what does it show?
- what is the distribution of scores?
- high vs low performers?
The most widely used test by all speech pathologists is the Clinical Evaluation of Language Fundamentals (CELF): child’s understanding and use of language (receptive and expressive language skills) and compares this to their peers of the same age (percentile rank).
Shows high variation in the population
Distribution of the scores will follow the normal distribution- majority of children will be in the middle of the distribution.
High performers: Some children will perform much better than their peers at the same age.
Low performers: much lower level of development in comparison to peers of the same age.
Nature vs nurture
Nature = genetic factors
Nurture = environmental factors
What is the evidence for the role of genetic factors?
- Twin studies
- KE family with members with language disability
- Comparative genomic studies
- Genetic association studies
Twin design: MZ and DZ twins
Monozygotic (MZ): share ~100% of genes
Single egg fertilised by single sperm then splits into two
Dizygotic (DZ): share ~50% of genes
Separate eggs fertilised by separate sperm
How can you compare resemblance (similarity) of MZ and DZ twins on specific traits (eg. language ability)?
By looking at correlations
Definitions of heritability, common environment and unique environment
A = heritability = genetic influence = the proportion of variance in a population attributable to genetic differences between people
C = common environment = shared environmental influence = the degree to which siblings in a family are similar to one another, once their genetic similarity has been accounted for
E = unique environment = non-shared environmental influence = the degree to which siblings in a family are different from one another, once their genetic differences have been accounted for
How do you calculate heritability?
2(rMZ - rDZ)
(We expect the similarity of MZ twins to be higher-
twice as high)
Twin studies results
These bars show the % of individual diffs that can be explained by genetic factors
- height and weight- around 80% therefore genetic factors substantially and significantly contribute to the development of height and weight
- math, language - the contribution of genetic factors is low in comparison to height and weight. eg. language- 40% explained by genetic so 60% by environmental
- none of these traits show 100% heritability- this shows all these traits are also influenced by environmental factors
Twin studies of language ability
- what is language ability?
- age 2
- age 4
- what was obtained at age 16
- what can we conclude about language abiloty
Language ability is a heritable trait
- At age 2 years, heritability for vocabulary = 26%, and for grammar = 52% (in boys) & 43% (for girls)
- At age 4, the heritability estimates were even higher (45%-68%) and further increased by age 6 (44%-92%)
- Convergent evidence for moderate-to-high heritability was obtained at age 16 in a different sample (TEDS)
- Language ability is not 100% heritable; environmental influences also play an important role
Contribution of genetic factors and heritability
- the contribution of genetic factors will change across development
- heritability increases as children develop
- co-heritability is significant, it may change across the lifespan
Twin studies of language ability
Limitations
(a) Large sample sizes are required- we need one of MZ and one of DZ twins
(b) Population-based samples are preferred, to avoid overrepresentation of MZ pairs
(c) It can be difficult to identify twins
when measuring specific traits language effect for example is harder to measure?
(d) Language phenotypes are often limited and lacking in metrics interpretable for comparisons to singleton age peers
(e) Possible twinning effect (twins are more likely to have delayed speech and language skills than singletons)
Behavioural phenotype and neural phenotype
Behavioural phenotype: for every test of speech and language that was administered, the group of affected family members was, on average, significantly impaired relative to the unaffected group.
Neural phenotype: Bilateral voxel-based morphometry (VBM) analyses showing some of the regions in which affected KE members have significantly reduces grey matter
KE family & Developmental verbal dyspraxia
What is verbal dyspraxia?
Verbal dyspraxia is a condition where children have difficulty in making and co-ordinating the precise movements needed to produce clear speech with their mouths; and without any signs of damage to nerves or muscles. Verbal dyspraxia is also sometimes called Childhood Apraxia of Speech.
KE family & Developmental verbal dyspraxia
What did fMRI scans of the KE family show?
Underactivation of broca’s area while speaking, but also deficits in the basal ganglia and its control of the vocal tract.
KE family & Developmental verbal dyspraxia:
What did the family tree and structural MRI show?
family tree diagram- each level represents generations. They showed theres developmental global destructionUnaffected - green, affected- blue. SO half of this family have shown this sever impairment.
structural MRI showing many members of this family had significantly reduced grey matter. There was also some under activity of the brocas area. neural phenotype- suggesting particular brain area was impaired in these family members.
What gene is responsible for the speech and language disorder in the KE family?
FOXP2 gene
What does a single mutation in the KE family lead to?
A single mutation in the KE family leads to a loss of function of one copy of the FOXP2 gene, and that the one copy that remains is insufficient for normal brain development (haploinsufficiency), leading to the speech and language disorder
Mutation in KE family
Monogenetic trait = caused by a mutation in a single gene
In this family, the mutation is single for a single gene, resulting in the language impairment
In the majority of cases, the language ability in the population is not caused by the mutation in this gene
What did a comparison of the FOXP2 gene of humans, other primates and other placental mammals show?
That FOXP2 is among the most highly conserved 5% of proteins, indicating that it has a fundamental role in mammals (Enard et al., 2002)
Evolution of the FOXP2 gene
Different human populations show essentially no variations in amino acid sequence, indicating that the present FOXP2 sequence is fixed in modern humans. Although a number of nucleotide changes have accumulated in FOXP2 since the human and mouse lineages diverged, around 70 million years ago, only three amino acids have changed in the FOXP2 sequence.
Strikingly, two of these three changes (threonine to asparagine at position 303 and asparagine to serine at position 325) are present uniquely in humans, but not in chimpanzees, gorillas or orangutang. Hence, these amino acid substitutions arose and became fixed in the FOXP2 sequence since the human lineage diverged from the chimpanzee lineage, only 4 to 6 million years ago. This rate of amino acid change is significantly greater than that expected by chance, given this period of evolutionary time.
Moreover, the two amino acid changes in the FOXP2 sequence satisfy all the criteria for a relatively recent selective ‘sweep’, in which this putatively advantageous genotype spread rapidly in all human populations. Indeed, it has been estimated that the spread was completed within the past 100,000–200,000 years, close to the time that anatomically modern humans appeared.
Gene-culture co-evolution of FOXP2 gene and communication
Genetic association studies
Usually this design is 2 groups:
- cases- shows for example a specific trait
- we look at the genetic variance in the group of cases. We expect to see that in the group of the cases, the genetic variance that puts the children of the highest of developing the language disability will be preset with the higher frequency than in the group of the children without language disability.
- so we compare the frequency of specific genetic variance in these 2 groups
- if we find evidence that the variance frequency is higher in cases than controls, we do the chi squared test
Understanding language development
There will be multiple genetic factors and multiple environmental factors
This trait is called multifactorial trait
Genetic association studies of language
- what chromosome was identified?
- what is a monogenetic and polygenetic trait
Chromosome 7- one of the genes that was identified in the genetic association studies
Others have also shown to be responsible for the language development
monogenic trait- single gene causes specific trait
polygenic trait- multiple genes lead to the development of the trait (which is in this case)
