Neuropsychologia 45 (2007) 1921–1930 Prisms throw light on developmental disorders Rebecca L. Brookes ∗ , Roderick I. Nicolson, Angela J. Fawcett Department of Psychology, University of Sheffield, Western Bank, Sheffield S10 2TP, United Kingdom Received 15 June 2006; received in revised form 22 November 2006; accepted 25 November 2006 Available online 30 January 2007 Abstract Prism adaptation, in which the participant adapts to prismatic glasses that deflect vision laterally, is a specific test of cerebellar function. Fourteen dyslexic children (mean age 13.5 years); 14 children with developmental coordination disorder (DCD): 6 of whom had comorbid dyslexia; and 12 control children matched for age and IQ underwent prism adaptation (assessed by clay throwing accuracy to a 16.7 ◦ visual displacement). All 8 DCD children, 5 of the 6 children with comorbid DCD and dyslexia and 10 of the 14 dyslexic children showed an impaired rate of adaptation, thereby providing strong evidence of impaired cerebellar function in DCD and developmental dyslexia. Taken together with other emerging evidence of overlap between developmental disorders, these findings highlight the importance of complementing research on the individual disorders with research on the commonalities between the disorders. © 2006 Elsevier Ltd. All rights reserved. Keywords: Dyslexia; Cerebellum; DCD; Learning; Adaptation; Comorbidity 1. Introduction Thirty years ago, the developmental disorders were char- acterised in terms of minimal brain dysfunction (Clements & Peters, 1962; Wender, 1978) or ‘soft neurological signs’ (Touwen & Sporrel, 1979). Subsequent research has focused primarily on analyses of the individual disorders, of which the most prevalent are now termed attention deficit and hyperactivity disorder (ADHD), dyslexia, developmental coordination disor- der (DCD), specific language impairment (SLI), and autism. There remain significant difficulties in diagnosing these devel- opmental disabilities with the DSM-IV (American Psychiatric Association, 1994) and the ICD-10 (World Health Organization, 1992) classifying the disorders primarily in terms of behaviour rather than underlying aetiology. It is well known that many symptoms seem to co-occur. Single-disorder theorists have attempted to find specific and pos- itive criteria for diagnosis of a given disorder, but in general (with the possible exception of autism) such endeavours have been unsuccessful. A valuable example is the ‘phonological- core, variable difference’ model of dyslexia (Stanovich, 1988), which held that dyslexic children have a specific problem in ∗ Corresponding author. Tel.: +44 114 2226545. E-mail address: r.l.brookes@shef.ac.uk (R.L. Brookes). phonological skills, and though they might have additional problems in other domains (such as motor skill) these were not ‘core’ problems. Subsequent research has revealed that almost all children with dyslexia do indeed have phonological difficulties (Vellutino, Fletcher, Snowling, & Scanlon, 2004). Unfortunately, in terms of this theoretical perspective, many children without dyslexia also have phonological difficulties (Morris et al., 1998; Stanovich, 1993; Stuebing et al., 2002). Furthermore, a high percentage of dyslexic children (at least 50 percent) show motor problems early on (Fawcett & Nicolson, 1995; Ramus, Pidgeon, & Frith, 2003a; Wolff, Michel, Ovrut, & Drake, 1990). The above analyses reflect the high ‘comorbidity’ (overlap) between dyslexia and other developmental disorders (Fletcher, Shaywitz, & Shaywitz, 1999; Gilger & Kaplan, 2001; Jongmans, Smits-Engelsman, & Schoemaker, 2003). Comorbidity problems appear to be particularly difficult in the case of DCD, which has been shown to be linked to ADHD, SLI, and dyslexia (Cantell, Smyth, & Ahonen, 2003; Dewey, Kaplan, Crawford, & Wilson, 2002; Henderson & Henderson, 2002; Kaplan, Dewey, Crawford, & Wilson, 2001; Kaplan, Wilson, Dewey, & Crawford, 1998; Macnab, Miller, & Polatajko, 2001; Visser, 2003). Gillberg (2003) refers to one grouping as deficits in attention, motor control and percep- tion (DAMP). Hadders-Algra (2002) proposes the terminology MND (minor neurological dysfunction). Gilger and Kaplan (2001) propose the term atypical brain development (ABD). 0028-3932/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2006.11.019