Over the past years, individuals have been using the terms "autism" and "autism spectrum disorder" like they mean the same thing. Autism is a spectrum disorder. This means that some people who have autism are only mildly affected. These people may go to regular schools, go to work, and have partners and families. Some people are affected worse than others. These people may be able to take care of most of their own needs at home like dressing and getting food, but not be able to have a regular job or travel alone. A person who has severe autism may need to be cared for all their life. Few people with autism are extraordinarily gifted or talented. These people are said to have savant syndrome. They are often very good at just one thing in particular, like mathematics, playing the piano or remembering football scores.
The overall category of the Autism Spectrum Disorder is called Pervasive Developmental Disorder. The name describes medical conditions with developmental problems in a number of fields. In most cases, when referring to the Autism Spectrum Disorder, we are referring to the most common disorders:
The different categories, the so-called “International Classification of Diseases” (ICD), are defined by the World Health Organization. As shown in the table, the diseases vary in several points. These are the age of onset, social interaction, communication and behaviour. An individual belongs to a certain category, depending on the areas of impairment. All three may seem similar in symptoms, yet are not the same. Still, people within one group show similar symptoms. The groups can be used to search for an underlying cause of the disease and to provide a better treatment.
Scientists do not know exactly what causes autism. There may be many different causes for the different types of Autism Spectrum Disorder. Scientists do know about some things that make a person more likely to have an Autism Spectrum Disorder.
The autism spectrum disorders are highly heritable disorders. This means that it is very likely (over 90%) that the child of an autistic person will also be on the autism spectrum. The reason for this are certain genes. These genes are linked to autism and are passed on from a parent to the child. Already Leo Kanner and Hans Asperger noticed that fathers of autistic children often preferred being on their own over being with people. Therefore, Kanner and Asperger suspected a genetic cause. Studies with families with a child on the autism spectrum were done to investigate this theory. They found that sometimes the children’s parents (fathers as well as mothers) also show social difficulties, shyness and problems with understanding contextual information when talking to someone. Some studies have found this in only 10 out of 100 families, some in as much as 45 out of every 100 families.
Other than the studies done on children who had autistic parents, recently a couple of studies were done on children who either had a sibling or a twin showing autistic signs. Similar to the outcomes in the autistic parents studies, most children having autistic siblings/ twins were found out to be showing autistic signs. According to a recent study, around fifty percent of infants with autistic siblings showed some kind of atypical behaviour, around one forth of them showed delays and deficits in achieving infancy milestones and around 17% of all were later diagnosed with ASD (Autism Spectrum Disorder).
Moreover, twin studies showed that it is much more likely that identical twins are both on the autism spectrum than that non-identical twins are both on the autism spectrum. Identical twins have nearly the same DNA. Therefore, it is highly probable that they are either both autistic or both not autistic. Non-identical twins share only half of their genes which makes it less likely that they both have the disorder. Scientists have shown that the recurrence risk (the likelihood that a child is on the autism spectrum if it has a sister or a brother that is autistic as well) is 20 to 80 times higher than the probability for having the disorder in the average population.
However, it is not a single gene that increases the risk for autism spectrum disorders. Several genes have been linked to the disorder. All of the identified genes can in sum only explain one or two out of every ten autism cases. None of the genes that have been found to play a role in autism can explain more than one out of every 100 cases.
The genetic influences include:
The affected genes influence:
Researchers have not found a single neuronal correlate for autism spectrum disorders, i.e. no abnormal functioning of the brain is the only cause of autism. Nevertheless, scientists found some differences between the brains of autistic people and the typical human brain. These differences lie in the anatomy of the brain, the activity in certain brain regions as well as in the connections between brain regions. It is a general tendency that autistic people use the same brain regions as other people but show different activity. In some areas the activity is less, in others it is stronger. However, this may only be true for male autists. Throughout the last years, studies have shown that brains of autistic women might be like normal men's brains.
Some researchers grouped autistic people's difficulties into three main areas in order to investigate their neuronal basis.
Researchers say that building a Theory of mind is one of the main problems for autistic people. This makes it difficult for them to interact with others. Problems in social interaction are probably worsened by low preference for social stimuli, such as faces. This tendency has been found in studies tracking the person’s eye movements and in EEG studies. EEG studies showed lower than normal activity in the fusiform face area (FFA), a brain region that is usually active when seeing faces. However, the results of other studies could not show this.
Also, scientists measured brain activity of people on the autism spectrum with fMRI while they were thinking about their own emotions. They found activity in the same brain regions as in non-autistic people. The difference was that some regions which are important for thinking about oneself and emotions (especially the anterior insula) were less active in autistic people. Other brain regions were much more active. These findings may explain the difficulties people with autism experience when trying to understand complex facial expressions and emotions (such as shame, jealousy).
People on the autism spectrum show a lower ability for executive function. Executive function means physical, emotional and cognitive self-control. This includes planning actions, focusing, shifting attention and flexibility of behaviour and thinking. Autistic people's ability can improve but it will remain below non-autistic people's ability in executive function.
Problems with executive function are probably caused by large networks in the brain. Grey matter and white matter irregularities have a negative influence on the way different brain regions work together (functional integration). A smaller corpus callosum leads to abnormalities in working memory (an active process of keeping a memory until it is needed) and planning actions.