Timed Perceptual Tasks: Better Index of Processing Speed in Autistic Children?

Background: WISC-IV profile in autism is characterized by trough and peaks of abilities. One trough that has been reported is on the subtests of the Processing Speed Index (PSI) (Oliveras-Rentas et al., 2012). Some suggested that this trough might be explained by the timing constraint of these subtests (McGonigle-Chalmers & McSweeney, 2013). However, many perceptual tasks on which autistics excel are timed, such as Children Embedded Figure tests (CEFT) (Jolliffe et al., 1997) or Visual search (O'Riordan, 2001).

Objectives: To document the association between timed perceptual tasks and processing speed in children on the autism spectrum and typically developing children (TD).

Methods: Fifty-three children aged between 6 and 12 years participated in the study. These comprised 26 autistic children (20M and 6 F) and 27 TD children (19M and 8F), closely matched on chronological age (p= .88). All children were recruited in regular classes and specialized classes for children on the autism spectrum from a public elementary school in Montreal (Canada). Diagnosis of autism spectrum disorder was given by multidisciplinary teams using ADOS-G and/or ADI-R in most cases. All children were assessed using Wechsler Intelligence Scales for children (WISC-IV), CEFT and a visual search task. The three tests were administered in a small isolated room at the school, on separate occasions and in a counterbalanced order. 

Results: WISC-IV full scale IQ was significantly lower in the autistic group (p< .001). As found in previous studies, autistic children (M= 77.5, SD=15.5) performed lower than TD children (M= 94.7, SD=12.0) on the PSI of the WISC-IV (p< .001) and on the two subtests comprised in the PSI (coding: p< .001, symbol search: p< .01). For the perceptual tasks, no difference was found between groups on either visual search time (p=.15) or CEFT score (p=.12), while autistic children were significantly faster than TD children on CEFT searching time (calculated for successful trials) (p< .01). The results on these three perceptual indexes were significantly correlated to PSI in the TD group (r from .41 to .64, all p< .05), but not in the autistic group (all p> .07).  

Conclusions: Autistic children performed faster than TD children on some perceptual tasks, despite having lower “processing speed” as measured by the Wechsler Scales. Contrary to TD children, their performance in the perceptual tasks was not correlated to their PSI score, suggesting that speed of processing perceptual information would not be responsible for their poor scores on the PSI index. Some motor problems or visuomotor coordination problems are more likely to contribute to poor measured “processing speed” in autistic children. Our data therefore corroborate other findings that one or more NON-processing speed-related factors contribute to lower measured intelligence in autistic children (Wallace, Anderson & Happé, 2009).