Inspection time and specific learning differences

Inspection time (Vickers, Nettelbeck & Willson, 1972), widely believed to evaluate processing speed (e.g., Anderson, 2008), is the time needed to discriminate between mirrored backward masked stimuli. Most children with developmental dyslexia have typical inspection times (e.g., McLean, Stuart, Coltheart & Castles, 2011), whereas in children with developmental coordination disorder (DCD) inspection times are atypically long (Piek, Dyck, Francis & Conwell, 2007). To date, there have been no comparative studies of inspection time or the shared variance between inspection time and other chronometric, psychometric or attainment measures in adults with these specific learning differences (SpLDs). In the current investigation, the first aim was to compare inspection times in groups of adults: with typical development, dyslexia, DCD and co-occurring dyslexia/DCD. Participants (N = 141) completed inspection time tasks with standard, pi-figure stimuli and un-speeded versus speeded responses; and non-standard stimuli of left/right versus up/down discrimination. The second aim was to illuminate the nature of inspection time by exploring its relationships with alternative measures of information processing speed, namely, symbol-digit coding and decision time, as well as IQ, working memory, visual discomfort, and literacy attainment. Results showed that standard inspection time was significantly longer than typical only in the group with dyslexia/DCD with this group difference remaining reliable after controlling for IQ, working memory and visual discomfort. In contrast, all the SpLD groups were slower than typical on inspection time from the non-standard stimuli, the symbol-digit coding test of processing speed and choice decision time. Whether the responses were speeded or un-speeded, or whether non-standard stimuli required and left-right or up-down discrimination, did not affect inspection time in any group but there was more variability between tests in the SpLD groups. All measures of processing speed shared significant variance with visual discomfort and, after controlling for group, there remained significant shared variance between standard and non-standard inspection time and visuospatial working memory. Conclusions were that (1) among adults with dyslexia, DCD or both, slow standard inspection time is characteristic only of co-occurring dyslexia/DCD, not dyslexia or DCD alone, and (2) in diagnostic assessment, standard inspection time may be a more specific measure of processing speed than either symbol-digit coding or decision time.