Orientation discrimination in WS 3
Exploring Block Construction and Mental Imagery: Evidence of atypical orientation
discrimination in Williams syndrome
Introduction
Williams syndrome (WS) is a rare genetic disorder occurring in approximately 1 in
25,000 births (Morris & Mervis, 1999), that is characterised by a particular cognitive
phenotype in which visuo-spatial abilities are impaired in comparison to relatively
superior verbal abilities (e.g. Mervis, Morris, Bertrand, & Robinson. 1999). Two
dominant hypotheses have been put forward to explain the deviance in visuo-spatial
cognition. The first, the local processing bias hypothesis (e.g., Bellugi, Sabo & Vaid,
1988), asserts that individuals with WS prefer to attend to the parts or details of an
image, rather than the image as a whole. In fact, individuals with WS do show a local
bias in drawing and construction tasks where they tend to sacrifice global accuracy for
local detail, but not on perceptual tasks, where the balance of local and global
processing appears to be typical in WS (Farran & Jarrold, 2003; Farran, Jarrold &
Gathercole, 2001, 2003; Pani, Mervis, & Robinson, 1999).
The second hypothesis relates to differences in the function of the ventral and
dorsal visual streams in WS, which are thought to be responsible for perception and
action respectively (Milner & Goodale, 1995). Atkinson et al. (1997) demonstrated a
relative impairment in WS performance on a „dorsal stream’ post box task, in which
the individual posts a card into a slot, compared to a „ventral stream’ version of the
task, where the participant must indicate the orientation of the posting slot. As a
consequence they suggested that individuals with WS have a deficit in dorsal stream
processing, which might explain their difficulties in drawing and construction,
although they also noted that “.. .their pattern of deficit, make it unlikely that a dorsal