Can autism be explained by sensory limitations and an inability to make associative links?
Biological research suggests that autism is two separate conditions and that there are two sets of genes responsible (Plomin, 2003). Twin studies have shown that there is an inherited factor (Folstein and Rutter, 1977 and Plomin, 2003), and this may involve a predisposition to a way of thinking and a process of natural selection which favours the extreme male brain (Simon Baron-Cohen, 1999). Some people naturally inherit a multi-tasking and intuitive brain, and others inherit a more logical brain with greater ability to focus. Within this continuum there are extremes which still lie within the normal range.
There are also different reasons why autism may develop along a particular path. The inherited predisposition may sometimes indicate a faulty gene or genes, where a transmitting chemical does not come into play when the brain is developing. Factors affecting the development of autism can include a difficult birth, causing lack of oxygen, and/or a virus contracted either at a stage when the brain and nervous system are developing, or after development, causing regression. There may also be a physical reason why the brain cannot process multiple stimuli, for example as a result of a known condition, such as Prader Willi syndrome, Rett syndrome, neurofibromatosis or tuberous sclerosis.
There is confusion, because autistic spectrum disorder (ASD) is both a continuum and a spectrum disorder. To understand what the common thread is and explain the differences, it is necessary to look at sensory perception and cognition. I would hypothesise that neurological damage may cause the transmission pathways to narrow and to affect perception, by only allowing the processing of a limited number of stimuli. This can affect any of the senses and might happen for a number of reasons and to varying degrees.
Baron-Cohen and Frith (1987) considered that perception was most productively viewed as a cognitive process with lower and higher levels of processing. They came up with two main methods of viewing the perceptual process: “bottom up”, where information perceived is registered at the extremities or sensory organs and taken up to the brain, and “top down”, where the information is sent from the top in the brain and travels down the nervous hierarchy. The information carried along these channels is open to distortion, with the visual and auditory senses most susceptible, although all of the senses could be distorted in people with autism (Grandin, 1987 and Gillingham, 1991).
I would suggest that it is most useful to look at the “bottom up” process of sensory perception. If the path for sensory perception is limited, causing it to be narrow, this could result in either over or under stimulation of the senses. The stimulus either forces through a limited narrow path, making it too strong and painful, or it is blocked and not registered at all. Both of these outcomes would result in limited links being made.
The distortion caused by strong and weak perception (lack of modulation) leads to partial reception and cognition. Strong over stimulation of the senses is experienced as over forceful and leads to narrow, limited, intense or focused perception and cognition. Under stimulation leads to limited correlation of stimuli and partial or no perception for cognition. In the first case, over reaction to stimulation occurs, while the later causes under stimulation. Either way, a lack of sensory co-ordination is the result.
The processing of sensations in minute detail, without linking them to the bigger picture, can occur because sensations are so intensely focused and because some stimuli are perceived weakly or not at all. Furthermore the central nervous system and sensory systems can be over or under stimulated for the olfactory, visual, tactile, vestibular, proprioceptive or gustatory processes.
In Central Coherence Theory (1989), Frith hypothesises that the inability to generalise is the result of not pulling information together in spite of perceived similarities, due to a weakness in the drive for central coherence. I would suggest, though, that it is not due to a weakness in central coherence, but due to a limited perception of input. Input perceived is blocked, to a greater or lesser extent, across all or any of the senses. Similarities and differences are therefore not perceived across whole scenarios but are only perceived, contrasted and compared amongst information received in fragmented parts or details. This leads to an inability to process complex impressions. The autistic cognitive dysfunction results from a sensory dysfunction of how much data can be taken in, discriminated and then related into a meaningful pattern. Associations and links are made between data which are easily related along single or limited tracks of thinking. How modified the links have to be before they are associated would explain different levels of flexibility in thinking.
Hirschfield et al (2007) found, in their study of children with ASD, that, while the understanding of others in terms of their mental states was impaired, understanding others in terms of their group membership was not. For example, children with ASD showed good knowledge in terms of gender and racial stereotypes. This could be because categorising people according to what they look like takes into account data which is easier to process, without having to be flexible and draw several comparisons together. It requires a smaller number of easily observable similarities and differences which can be pictured and remembered as a single or discreet visual memory. It does not require the correlation of impressions of people, and it is the ability to form impressions which I believe is impaired or restricted at all levels of autism.
Although, traditionally, research has been mostly with children with ASD onset under three years of age, there is no reason why onset cannot occur at any age, if triggered by neural damage. The age of onset and the stage of development reached are likely to affect outcome and behaviour, and the perceptual limitations and deficits of associative links will consequently present in different ways. Autism is a development disorder, a continuum and a spectrum disorder. The degree of flexibility in the ability to make cognitive links, and how similar data has to be before it is linked, will affect the number of links made. The common thread of autism is the limitation in the number of cognitive links. The differences in autism lie in degree, sensory-cognitive area, type of sensory sensitivity, cause and age of onset.
Further information and references
Sara Rackow worked as a teacher and advisory teacher in the field of autism for 30 years and has written widely on the subject. She currently works as an autism consultant.
Baron-Cohen,S and Frith, U (1987): Perception in Autistic Children. Handbook of Autism and Pervasive Development Disorders.
Baron-Cohen, S (1999): The Extreme Male Brain Theory of Autism.
Folstein, S and Rutter, M (1977): Infantile Autism. A genetic study of 21 twin pairs. Journal of Child Psychology and Psychiatry.
Frith, U (1989): Autism: Explaining the Enigma.
Gillingham (1991) op cit Lowden, G: Some Thoughts on the Nature of Perception in Autism. Communication 25 (3) December 1991.
Grandin, T (1987): An Autistic Person’s View of Holding Therapy. Communication 23 (3).
Hirschfield, L, White, S, Bartmess, E, Frith, U (2007): Can Autistic Children Predict Behaviour by Social Stereotypes. Current Biology, 17 (12).
Plomin, R, (2003): Institute of Psychiatry op cit Autism is Two Separate Illnesses. The Observer 16 March 2003.
Article first published in SEN Magazine issue 44: January/February 2010.