2.7.4. Williams Syndrome and other genetic developmental disorders As well as developmental disorders acquired through brain injury, there has been extensive research interest in visual problems associated genetically based neurodevelopmental disorders, both because of the clinical and educational issues these children face, and because of the insights they might yield on the genetic basis of visual brain development. A particular focus has been on Williams Syndrome (WS), a rare disorder arising from a deletion of around 30 genes on one arm of chromosome 7 (reviewed by Atkinson and Braddick (2011)). WS is characterised by a very uneven cognitive profile, with relatively good language abilities, good face recognition, and object recognition in line with their mental age (Landau, Hoffman, & Kurz, 2006) but very poor performance on visuospatial and visuomotor skills such as drawing and block construction (e.g. Bellugi, Lichtenberger, Mills, Galaburda, & Korenberg, 1999). In a large scale study of 73 young children with WS (Atkinson et al., 2001), a high incidence of binocular disorders, reduced acuity and refractive errors (usually hyperopia rather than myopia) was found in around 50% of the group. Marked deficits were also found on many subtests of the ABCDEFV, with particularly poor performance on the block construction copying task. However, the severity of spatial deficits was not well correlated with sensory visual deficits such as strabismus, suggesting no direct causal link between sensory visual loss and problems of spatial cognition. The discrepancy between face and object recognition abilities and spatial cognition, supported by findings on global motion vs. global static form perception and on the contrast between good orientation matching vs. poor visuomotor alignment to the orientation of a postbox slot, led to the hypothesis of ‘dorsal stream vulnerability’, that the dorsal stream was a focus of this disorder (Atkinson et al., 1997). Many later findings are consistent with this idea of a dorsal stream loss, examples are a massive delay on all visuo-motor planning tasks (e.g. Atkinson et al., 2001 and Atkinson et al., 2003), problems of using visual judgments of step height in stair descent to scale leg and foot movements (Cowie, Braddick, & Atkinson, 2010), poor spatial location memory for hidden objects when allocentric frames of reference (relative to the external environment) need to be used (Nardini, Atkinson, Braddick, & Burgess, 2008), better face processing than processing the location of faces (Paul, Stiles, Passarotti, Bavar, & Bellugi, 2002) and neuroimaging evidence (Eckert et al., 2005 and Meyer-Lindenberg et al., 2004). However, although some of these tests have isolated dorsal from ventral processing, many of the spatial defcits in WS may depend heavily not on dorsal stream functioning alone but on integration of ventral stream information with dorsal and it may well be this integration which is the main block for these children learning to overcome their difficulties.
Posted on: Tue, 24 Sep 2013 17:48:32 +0000
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