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Discuss the control of prehension from the perspective of developmental disorder

Prehension describes the act of reaching out to grasp an object. In particular, theories of prehension ask why it is that humans reach out and grasp in a specific way - given the infinite number of ways they could potentially perform this operation. Firstly, basic theories of prehension are reviewed and then some of the research from developmental disorders is discussed.

Prehension has been broken into two components: transport, the movement of the hand to a new location, and grasp, the adjustment of the gap between thumb and forefingers in preparation to grip the object (Mon-Williams & Tresilian, 2001). Researchers have come up with a variety of theories to explain how the two separate parts of this operation are co-ordinated so precisely. Mon-Williams & Tresilian (2001) themselves put forward a proportional rule between the two factors that relates them mathematically, and relatively simply. They argue that this mathematical relationship fits the established data well.

Further research has looked more closely at how some aspects of the object affect prehension. Mon-Williams & Murray (2000) looked at the affect of visual size cues. Their findings confirmed previous research that visual cues are important in the production of an efficient precision grip. However, when examining the size-weight illusion, they found that motor programming is not to blame but instead might be a cognitive artefact. The size-weight illusion is that the smaller of two objects of equal weight is judged the heaviest after lifting. Similarly Flanagan & Beltzner (2000) found that a mismatch between sensory and actual feedback was excluded as an explanation for the size-weight illusion. Instead they suggest that it results from perceptual and cognitive factors.

An important line of research in prehension has examined people with developmental disorders. Mari, Castiello, Marks, Marraffa & Prior (2003) examine prehension in children with an autistic spectrum disorder. Mari et al. (2003) report evidence from Leary & Hill (1996) that places movement disorders at the centre of autism. Important for this study was the earlier finding that the closer an object is, the more likely it is that the hand will be opened wider at an earlier stage. Also, the smaller an object is, the slower the hand moves towards it so as to be able to adopt the appropriate precision grip.

Mari et al. (2003) compared the reach to grasp movements of autistic children with those of normal age-matched controls. The researchers hypothesised that, because of reported problems autistic children have with goal planning, they would not demonstrate some of the aforementioned size and scaling adjustments. Mari et al. (2003) found that there was little difference between the control group and the autistic children in reactions to the distance and scale of the object. This research confirmed many of the previous findings in this area. There was, in fact, a difference between the two groups but this was ascribed to the lower IQ found in the autistic children - this manifested itself in a generalised slowness.

Research has also looked at prehension in adults with Down syndrome. Mon-Williams, Bell, Coppard, Jobling, Tresilian & Carson (2001) compared ten adults with Down syndrome (DS) with two other groups, one comprising 12 adult controls and one comprising 10 children matched in intellectual ability to the DS adults. This study looked, in particular, at the ability of those with DS to use information given to them before reaching-to-grasp. Participants were exposed to three different conditions. One in which complete information about the position of perspex blocks was available, one in which only partial information was imparted and one in which none was imparted. This study found that the adults with DS were able to use the cuing information effectively in the unambiguous condition but were unable to do so when given only partial information. Meanwhile the controls and children were able to utilise the partial information effectively. Mon-Williams et al. (2001) consider some explanations for their results. They posit that DS people are unwilling to use a high-risk strategy when the information is ambiguous, despite the fact that they will use it when the advanced information is unambiguous. This is explained by the idea that DS people simply find it too complex to make adjustments to their prehension while they are in motion.

Cole, Abbs & Turner (1988) examined how DS people adapt their grip to the properties of the object they are trying to life. They found that DS people generally gripped objects much more tightly and failed to adapt their grip to the thing they were holding normally. Eliasson, Gordon & Forssberg (1995) examined the same skill of gripping objects, but in children with cerebral palsy. When compared with controls, those children with cerebral palsy could in fact adapt their grip successfully to what they were holding. It did, however, require repeated attempts and predictable circumstances before they could learn to do so effectively.

In conclusion, the study of prehension has shown that reaching and grasping is made up of two separate components that are combined: reaching and grasping. These findings have been examined in people with Down syndrome, those with an autistic spectrum disorder and those with cerebral palsy. In most cases these developmental disorders seem to cause subtle deficits in the prehension ability - in some cases perhaps the result of limited cognitive capacity. Some of the research suggests - especially that on autistic spectrum disorders - that prehension deficits could be used in early diagnosis.


References

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