Vision for orientation and mobility

Vision for orientation and mobility needs to be assessed both inside day care centres and schools, and outside. The child's functioning is observed at different luminance levels both in places that the child knows well (in order to record how well (s)he can use visual memory for orientation) - and in places where (s)he has not been (in order to determine the capability in recognising common structures and to draw conclusions from that information). The measurements of basic visual functions are useful though, e.g. if contrast sensitivity is found to be low, it means that spaces with faint shadows at the corners of rooms or open spaces, are not seen as being square, as we see them, but round, because the low contrast shadows are not seen. It is much more difficult to have a visual "grasp" of a round space than of a square space, so this feature of identifying the spatial structures should be assessed carefully.

Changes in colour vision may make some "obvious" landmarks less obvious. This is not always thought of even in information for the normally sighted. For example, the signs guiding to exits are commonly painted with bright yellow on dark blue. In the case of electric power failure - the most likely situation in an emergency - the signs will not be seen in twilight, because these two colour surfaces are equiluminant (=of equal luminance) and thus indistinguishable from each other. In each case of colour deficiency it is crucial to remember which colours the child confuses, otherwise we may think that the child's general ability to observe is inadequate.

Mobility instructors usually have developed lists of local orientation marks and details which are difficult to notice. By evaluating how far away the child can recognise these landmarks and how well (s)he deduces his/her position, we learn about vision for O&M. The difficult-to-notice details are usually low contrast details, like curbs and stairs. They can be hanging objects that may be missed because of visual field defects.

We do not yet measure one very important visual function, motion perception in our clinical examination, but this ability should be remembered, especially in diffuse retinal lesions and in vision impairment due to brain damage. Inadequate motion perception makes the assessment of the distances of moving objects inexact and may lead to collisions and problems with reaching for and grasping moving objects. You may make important observations during measurement of grating acuity (see Instructions LEA GRATINGS) and when testing with Hiding Heidi.

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