Biological explanations of autism

Genetics and neurological correlates

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Basic overview

There are TWO biological explanations of autism: GENETICS and NEUROLOGICAL CORRELATES

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Genetics- basic overview

We inherit our genetic material from our parents - 23 chromosomes from our mother, and 23 chromosomes from our father. Research has shown if we are related to someone autistic, there is a higher chance of members of the family developing the disorder. Autism is more common in boys than girls.

Concordance studies (the extent to which a pair of twins share similar traits) are often used to determine the extent to which a condition is inherited.

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Genetics- Siblings

Folstein and Piven (1991) reported a concordance rate for autism at 2-3% for siblings, which is much higher than the general population. However, the strongest evidence for genetic factors comes from twin studies, some of which indicate the concordance rate for monozygotic (Mz/ identical.) twins as high as 96% and a concordance rate for dizygotic (dz/non identical) similar to that of ordinary siblings.

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Genetics - Folstein and Rutter 1977

AIM- a twin study to investigate the genetic cause of autism

METHOD- co-ordnance rates for autism were investigated in 21 pairs of twin boys, 11 MZ twins and 10 DZ twins

RESULTS- concordance rates for MZ twins was 4 out of the 11 pairs, and with the DZ twins it was 0 out of the 11 twins. In the non concordant MZ group, many brothers did have a milder form of autism and their symptoms became more apparent with age. Concordance using a wider criteria for MZ twins was 90%.

CONC- this study provides some evidence for a genetic cause of autism.

EVALUATION- the milder symptoms experienced in the non concordant MZ group suggest autism might be best explained with respect to be a broader phenotype, which is consistent with the idea of an autistic spectrum.

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Genetics - evaluation

o Twin study evidence suggests a strong genetic basis for autism

o Twins share the same environment and therefore any similarities may be due to environmental factors

o The suggest

Ion that autism has a genetic cause is consistsent with the findings that show a strong link between autism and other genetic disorders - eg, it's been found 10% of children with autism have a chromosome abnormality (fragile x syndrome) Bee 1989.

o autism has also been associated with other genetic disorders, such as Tourette's. Comings and Comings (1991) noted sufferers of Tourette's and autism show many similar symptoms, such as ritualised behaviours and stereotypical movements.

o Chromosomal analysis studies are as yet inconclusive.

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Neurological correlates - basic overview

This theory proposes that autistic individuals have one or more abnormalities in the brain. The area of the brain damaged, or with structural abnormalities in children diagnosed with autism appears to correlate with those areas responsible for the development of normal communication, social functioning and play.

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Neurological correlates- post mortem studies

This is an invasive method after the individuals death. Such studies show abnormalities in the frontal lobes (planning and control) the limbic system (emotional regulation) brain stem and cerebellum (moot co-ordination.) from these studies no single abnormality has been found and it is uncertain which are specific to autism. Also,

o most post mortems are performened on adult brains, so it's hard to tell cause and effect

o abnormalities in the brain may be a result of life injuries, like knocks on the head, and not necesarily autism.

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Neurologial correlates - PET scan

(positron emission tomography)

Slightly radioactive glucose is injected into a patients arm. Once in the blood stream, it gets carried to the brain and sensors in the PET scan can detect the radioactivity as it moves through the brain. PET can provide image of blood flow which gives reliable measure of brain activity - rested or stimulated. It also gives a very sensitive analysis of the brain.

ZIBOVICUS ET AL (2000) used PET scan in his study. He found 75% of the autistic children examined had functional abnormality in the temporal cortex but, a mentally impaired comparison group showed no abnormalities.

As PET scans require blood samples, and radiation, SPECT scans are preferred for children.

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Neurological correlates - SPECT scan

(single photon emission computed tomography)

Practically the same a PET scans but use less radiation. ONISHI et al (2000) used SPECT and identified different regions of the autistic brain. A positive correlation was observed between:

o Impairments in communication, social interaction and blood flow in the frontal cortex

o Obsessive desire for sameness and blood flow in the right hippocampus and the amygdale (parts of the limbic system)

The usually high blood flow in these parts was interpreted as being a cause of the qualitative impairments in social interaction seen in children.

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Neurological correlates - MRI scan

(magnetic resonance imaging)

MRI is the most used scan for children as they have no radiation effect - magnetic fields and radio waves produce high quality 3D images of the brain.

MRIs have found abnormalities in the cerebellum - the area thought to be involved in the control of attention, and this abnormality could explain why autistic people struggle to focus.

COURSCHEME ET AL (1994) looked at some autistic patients and found a significant reduction in vermal lobules size in the cerebellum of autistic people, with or without mental retardation.

Further studies have supported the findings that the cerebellum and brain stem are significantly smaller in autistic individuals.

PIVEN ET AL (1995) measured brain volume using MRI. autistic people had a notably larger total brain volume (when compared to a control group.) As a result, that has been replicated a number of times since and there is now converging evidence that autistic individuals have brain enlargement.

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Neurological correlates - evaluation

o The advance in technology allows researchers to fully investigate the brain and it's structures in more detail, and findings give more info on autism and it's biological causes and have provided evidence for differences between autistic people and 'normal' people.

o The different techniques have their own merits but MRI is the most advanced and allows more sensitive observations.

o Scans are hard do and the participant has to stay still for a long time, meaning that most studies use small samples so it's inappropriate to generalise results to all cases of autism.

o Advances in technology have allowed us to identify regions of the brain which may be responsible for autistic symptoms eg, social impairment, but none of the studies account for the full range of symptoms.

o Methological factors can confound the comparison of studies as factors such as age, gender and IQ differ greatly between autistic samples and controls both within and between studies

o Most evidence is correlational and we cant assume cause and effect from correlations.

o Biological differencs may be due to a disorder rather than a cause

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