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Visual evoked potentials study

What are Visual evoked potentials?

Visual evoked potentials, V.E.Ps for short, measure the response of the brain to visual stimulation. Visual information flows from the eyes along the nerve pathways to the brain. These nerve pathways are connected to the brain at the back of the head.

By placing sensors on the scalp on this part of the head it is possible to record this flow of visual information to the brain. Black and white stripes are used, that flicker at a known rate and we record the strength of the corresponding activity in the brain at that rate. The more visible a pattern is, the stronger the signal we record from the brain. This relationship is used to work out which patterns are visible to the child, as each individual is shown a succession of patterns on a computer monitor. A pattern with large high contrast stripes elicits a strong signal to the brain, whereas a pattern with very fine (or faint) stripes will give a weaker signal.

What was the purpose of the study?

Many things can hinder the flow of information from the eye to the brain; therefore the purpose of this study was to discover whether anything is disrupting this flow of visual information in children with Down's syndrome. When tested with traditional tests for visual ability, children with Down's syndrome often appear to be less sensitive to fine detail (reduced visual acuity and reduced contrast sensitivity).

It has been suggested that people with learning disabilities have difficulty understanding the demands of traditional vision assessment, and that this could influence their results. Therefore, the V.E.P technique is potentially a more objective way of testing a child's detail vision, as it doesn't require any cognitive response from the child beyond that of looking at a target on a screen. However, this study incorporates both methods of vision testing, in order to obtain a full measurement of each child's vision. This is the first study to date that has measured vision in children with Down's syndrome using the V.E.P technique.

What did we find out?

The results obtained are in agreement with findings from previous studies that have used conventional tests. Children with Down's syndrome have reduced detail vision (visual acuity), and reduced contrast sensitivity when compared to children without Down's syndrome.

The difference between the two groups of children is less pronounced during infancy, this implying that the development of vision during the first few months of life is as expected. However, as children with Down's syndrome develop, their fine detail vision and contrast discrimination doesn't reach the same level as their peers. This could be due to a number of contributing factors, for example uncorrected long or short-sight, reduced accommodation (poor focusing), and conditions like strabismus / squint and nystagmus / wobbly eyes, that are far more common in people with Down's syndrome than in the average population. However, children with Down's syndrome who have no known eye problems also tend to have reduced vision. This is most likely due to differences in the visual pathways beyond the eyes.

It is important to note that the correction of any visual problem (where possible) will improve detail vision. It is therefore important that people with Down's syndrome have regular eye checks from infancy, and that people are made aware of possible visual restrictions in order to optimise visual input, be it in an educational, social or home environment.

This study was conducted by Ffion John and Nathan Bromham, in collaboration with Rowan Candy from Indiana University School of Optometry. It was funded by NERC and Mencap.