Orientation discrimination in WS 24
Atkinson et al. (1997) involve processing orientation. We suggest that it is the
orientation processing properties of these tasks that lead to impaired performance,
rather than a general deficit in dorsal stream processing.
Our conclusion that orientation discrimination is impaired in WS appears to
contradict the findings of Stiers et al (2000) who found that the ability to identify a
target line amongst distracters may be no poorer than overall non-verbal level of
ability in WS. However, this difference in results could reflect the choice of targets
and distracters. In the Squares tasks, orientations are perpendicular to one another, or
are the same. This may lead to greater demands being placed on the orientation
discrimination system than in a task involving comparison of line orientations.
In addition to the implications that the present results have for understanding
WS, this study also adds to the literature regarding the fractionation of mental
imagery into separate subsystems. In particular, the data question whether all mental
image transformations are served by one subsystem (cf. Kosslyn & Koenig, 1992), or
at least question whether other factors (such as orientation discrimination) contribute
more to mental rotation than size transformation.
Individuals with WS show an overall impairment in visuo-spatial cognition.
Even on the tasks mentioned here where performance is at the level of controls, the
control groups are many chronological years younger than the WS group. However,
this deficit in visuo-spatial abilities appears to comprise a mixture of simple delay on
some tasks, but also of a pattern of deviant performance on others. Indeed, the
present study has revealed two important findings. First, individuals with WS do not
have typical perceptual abilities - they show deviant orientation processing abilities. It
appears then, that a local bias in construction is not the only contributor to poor visuo-
constructive abilities in WS. Second, dorsal stream functioning in individuals with