Seeing problems that may not exist: A reply to West et al.'s (2018) questioning of the procedural deficit hypothesis

In a recent paper (“The procedural learning deficit hypothesis of language learning disorders: We see some problems”, Developmental Science, 2018), West, Vadillo, Shanks, and Hulme (2018) aimed to test the procedural deficit hypothesis (PDH) of specific language impairment (SLI) and dyslexia. This hypothesis proposes that abnormalities of brain structures underlying procedural memory can largely explain SLI, and perhaps developmental dyslexia (Ullman, 2004; Ullman & Pierpont, 2005; see also Nicolson & Fawcett, 2007, 2011). West et al. examined aspects of declarative and procedural memory in a representative sample of 7‐ to 8‐year‐old children, in relation to language, literacy, and mathematical abilities. They emphasized two findings. First, their declarative but not procedural learning measures correlated with language, reading, and math scores. Second, their procedural learning measures demonstrated relatively low reliability. West et al. concluded that their results “seriously question the suggestion that the construct of a ‘procedural learning system’ can be reliably measured and cast strong doubts on claims from earlier studies that deficits in such a system are related to language learning difficulties” (pg.10). Their study raises important questions about the nature of procedural learning, how best to measure it, and whether it is related to typical and atypical language and cognition. However, we suggest that there are a number of weaknesses with the study that invalidate their argument.