While most diagnostic criteria of Pervasive Developmental Disorder (PDD) are characterised by impairments, mostly with respect to social and communicational abilities, subjects with PDD excel on certain visuo-spatial tasks that supposedly reflect superior processing of fine detail (see Dakin & Frith,
2005, for a review). An especially robust finding seems to be the behavior of subjects with PDD on visual search tasks (Plaisted, O’Riordan, & Baron-Cohen,
1998; Plaisted, Saksida, Alcantara, & Weisblatt,
2003; O’Riordan, Plaisted, Driver, & Baron-Cohen,
2001). In such tasks, subjects are required to detect a target in a display containing a variable number of distracters. If the task difficulty is increased by adding distracters, usually RTs increase. In studies on PDD, two search task versions that differed in difficulty have been used. In both tasks children with PDD showed shorter RTs, as compared with normally developing controls (Plaisted et al.,
1998; Plaisted et al.,
2003; O’Riordan et al.,
2001). The reason for this superior performance is not clear, but has been related to enhanced ability to discriminate between stimulus elements in subjects with PDD (O’Riordan & Plaisted,
2001), an explanation that has also been proposed for the superior performance of subjects with autism on other visuo-spatial tasks (Plaisted et al.,
2003, see also Bertone, Mottron, Jelenic, & Faubert, 2001).
So far, however, there have been no studies that have validated the claim for enhanced stimulus discrimination as an explanation for superior performance in search tasks in PDD more directly. It is well-known that stimulus discriminability affects eye movement parameters, especially number of fixations and fixation duration (Hooge & Erkelens,
1999). Therefore, studying eye movements during search tasks in subjects with PDD is a first step to gain more insight into the neurocognitive mechanisms of atypical visuo-spatial processing in this group. If increased stimulus discriminability is indeed the underlying factor for superior performance of subjects with PDD, it is expected that they need fewer and/or shorter fixations to identify the target. Also, eye movement parameters can be used to test alternative hypotheses for shorter reaction times in search tasks in PDD. More specifically, it has been argued that (healthy) subjects in a visual search task have a tendency to move their eyes even though in some situations it would be a better strategy not to do so, since longer fixations allow better extraction of (peripheral) information (Hooge & Erkelens,
1999; see also Rayner,
1998). It is well possible that individuals with PDD use a different search strategy in which they show longer, but less, fixations than controls.
The first aim of the present study was to use eye movement parameters to test the two hypotheses described above for superior visual search in subjects with PDD. A second aim was to replicate the findings of Plaisted et al. (
1998) and O’Riordan and Plaisted (
2001) in high functioning adults with PDD. For these reasons we used the same search tasks as described in O’Riordan et al. (
2001, second experiment) in a group of high functioning adults with PDD, matched on gender, IQ, and age to a control group.