Orientation discrimination in WS 4
stream deficit is the only neural basis for cognitive and performance anomalies in
WS.” (p. 1921). It therefore appears that there may be a number of factors that
contribute to the profile of visuo-spatial abilities in WS.
These two competing hypotheses can be investigated by examining the factors
that constrain performance on the Block Design task of the Wechsler scales (e.g.,
Wechsler, 1974, 1981). Individuals with WS typically perform less well on this task
than on other visuo-spatial tasks such as the Picture Completion and Picture
Arrangement subtests of the Wechsler Intelligence Scale for Children (WISC;
Wechsler, 1974) and the Children’s Embedded Figures test (CEFT; Witkin, Oltman,
Raskin, & Karp, 1971) (e.g., Dall'oglio & Milani; Farran et al. 2001; Udwin & Yule,
1991), despite the fact that these other tasks themselves already show a substantial
impairment relative to verbal ability (e.g., Arnold, Yule & Martin, 1985; Howlin,
Davies & Udwin, 1998). In the Block Design task, the participant must put a number
of blocks together so that the upper faces resemble a model image. This involves two
major stages; individuals must first perceptually segment the model image into
component parts and second, integrate these to resemble the model image (using both
visuo-motor and perceptual processes) (see Kohs, 1923). These two stages both
require processing at local and global levels, hence why, through manipulation of the
local processing requirements, the Block Design task is known to index extent of local
processing bias (e.g., Shah & Frith, 1983). Both stages may well also involve mental
imagery - a property of the dorsal visual stream (Alivisatos & Petrides, 1997; Larsen,
Bundesen, Kyllingsbaek, Paulson, & Law, 2000).
Preliminary support for the suggestion that a local processing bias in WS
might lead to impaired Block Design performance in WS comes from work by
Bellugi and colleagues (e.g., Bellugi et al., 1988a; Bellugi, Sabo, & Vaid, 1988b;