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Article Excerpt
The present research seeks to build upon a body of research that has attempted to underpin developmental attention difficulties by identifying cognitive processing deficits that reliably differentiate children with attention problems from children who do not have attention problems (e.g. Castellanos & Tannock, 2002). Recent evidence suggests that children with attention problems experience significant difficulties with tasks requiring central attention resources (Adams & Snowling, 2001; Barkley, 1997; Nigg, Hinshaw, Carte, & Treuting, 1998; Pennington, Groisser, & Welch, 1993; Roodenrys, Koloski, & Grainger, 2001; Sonuga-Barke, Taylor, & Hepinstall, 1992). Children with these problems have also been argued to have difficulty in inhibiting pre-potent (but currently inappropriate) responses and in planning, sequencing and executing complex tasks compared with typically functioning peers (Wilding, Cornish, & Munir, 2002; Wilding, 2003). Castellanos and Tannock argue that these response inhibition difficulties can be characterized as the behavioural representation of an underlying neuropsychological deficit that is specific to attention deficit hyperactivity disorder (ADHD).
It has also been argued that children with attention difficulties experience problems performing two tasks simultaneously, even if those tasks utilize distinct sense modalities. Theoretical models of attention based upon single capacity models (e.g. Broadbent, 1958; Posner & Rafal, 1987), which assume that dual tasks call upon general attention/executive resources, as well as multiple-resources theories (e.g. Baddeley, Della Sala, Gray, Papagno, & Spinier, 1997; Baddeley & Hitch, 1994; Della Sala & Logie, 2001; Neisser, 1967), which assume that resources are relatively specific to some cognitive/perceptual processes, both predict dual-task deficits in children with attention difficulties. While not all research studies have reported clear evidence of dual-task deficits in children with ADHD diagnoses, at least some empirical evidence supports the existence of deficit in this domain (e.g. Adams & Snowling, 2001; Dige & Wik, 2005; Karatekin, 2004; Li, Lin, Chang, & Hung, 2004).
Baddeley and Hitch's well-known working memory (WM) architecture has also been used more directly as a potentially useful framework for discussing individual differences in attention, including developmental disorders of attention. In most models of WM, the central executive is generally viewed as being akin to a central supervisory attention processor that operates to schedule competing action plans (e.g. Baddeley, 2004). A good deal of experimental work on normal adults supports the plausibility of this general assertion (e.g. Conway & Engle, 1994; Cowan, 1988, 1997; Kane & Engle, 2000). Evidence from typical adult functioning also suggests that the central executive system is related to the control of attention through habituation or inhibition processes (e.g. Conway & Engle, 1994; Cowan, 1988; Kane & Engel, 2000). Conway and Engle, for example, report that WM differences were only evident in probed recall tasks that lead to interference or response competition. Conway and Engle also note that the capacity to inhibit which serves to limit entry to WM is likely to be relevant to developmental WM and attention problems. Consistent with this general view, problems on cognitive tasks designed to assess Central executive functioning has also been reported among children with clinical attention deficits (e.g. Barnett et al., 2001; Kempton et al., 1999; Nigg, Blaskey, Huang-Pollock, & Rappley, 2002; Schachar, 1991). Some of these studies have also implicated visuospatial WM deficits in ADHD.
Recently, this developmental pattern has been somewhat complicated by evidence suggesting that all three of the cognitive deficits described above: namely, dual-task processing, response inhibition and WM deficits, may all be related to reading disability (RD) rather than, or as well as, attention problems. Purvis and Tannock (2000) contrasted children experiencing either ADHD or RD alone, or RD and ADHD with typical controls. They found that both ADHD + RD and RD samples of children showed deficits in speeded processing tasks but not on a continuous performance task used to measure response inhibition. Children with RD did show poorer performance than non-RD children on this measure. This pattern led Purvis and Tannock to conclude that inhibition tasks may not be unique cognitive markers for ADHD (see also Adams & Snowling, 2001; Dige & Wik, 2005; Karatekin, 2004; Li et al., 2004). Purvis and Tannock suggest that children with ADHD may have a more pervasive inhibitory deficit shown in behavioural impulsivity, whereas RD children may experience inhibition problems more specifically with rapidly presented material.
Some reading researchers have suggested that dual-task performance might also distinguish good and poor readers (e.g. Nicolson & Fawcett, 1990, 1995, 2000; Nicolson, Fawcett, & Dean, 2001; Yap & Van Der Leij, 1994). In these experimental studies, developmental dyslexics were asked to carry out two tasks such as balancing on a beam and counting backwards, or playing a computer game while also concurrently responding to on-line signals. Children completed each of these tasks first separately, and then simultaneously. Dual-task performance deficits have been reported on a range of well-learned tasks for developmental dyslexics compared with regular reading controls. As the tasks used in these studies do not require use of reading, spelling or phonological skills, this pattern is therefore consistent with a general dual-task deficit in developmental dyslexia. This work has itself been criticized on methodological grounds (Savage, 2004). Failures to replicate this dual-task deficit in children with dyslexia have also been reported (e.g. Wimmer, Mayringer, & Lander, 1998; Wimmer, Mayringer, & Raberger, 1999).
The specificity of associations between developmental attention difficulties and WM component systems such as the central executive and visuospatial processing has also been questioned. Swanson (1993), for example, suggests that problems at all three postulated levels of WM architecture characterize children with a wide range of developmental problems. Others have reported that there are no clear associations between attention problems and visuospatial processing (Kerns, McInerny, & Wilde, 2001; Sonuga-Barke, Dalen, Daley, & Remington, 2002). Similarly, the reliability of deficits in central executive processing in attention disorders has also been questioned by other research. Roodenrys et al. (2001) contrasted 16 children with ADHD plus RD and 16 children with pure RD and non-disabled controls. Both the ADHD/RD and the RD groups showed phonological loop deficits compared with controls. Deficits were also evident for both groups on tasks tapping central executive function. August and Garfinkel, (1990) and Halperin, Gittelman, Klein, and Rudel (1984) have failed to find differences between ADHD and co-occurring ADHD plus poor reader groups sampled from clinics on attention measures. Finally, Pennington et al. (1993) contrasted four groups of children drawn from schools, parent networks and clinics. Children with pure ADHD (N = 16) and reading problems (N = 15; hereafter called RD) were contrasted with a group with both ADHD and RD (N = 16) and controls (N = 23). The pure RD and ADHD + RD groups both showed phonological processing deficits. Pennington et al. also report that the co-morbid ADHD and RD group did not show executive function deficits compared with the pure ADHD group who did show deficits. Pennington et al. argue on the basis of this associational data that this pattern suggests that a primary reading problem might frequently lead to a secondary attention difficulty that reflects reading problems in conjunction with environmental risk, rather than a ccentral executive deficit. The broader environmental risk factors identified by Pennington et al. include lower maternal education, mother-only households and greater family psychiatric difficulty. Pennington et al. term this general notion the phenocopy hypothesis.
In summary then, existing studies provide at best mixed support for clear cognitive profiles of dual-task speeded response inhibition and WM deficits underpinning developmental attention problems. Furthermore, there is some evidence that a number of the purported deficits in developmental attention disorders may be most closely related to cognitive processes in reading acquisition (e.g. Nicolson & Fawcett, 1990, 2000; Pennington et al., 1993; Purvis & Tannock, 2000).
Caution is currently also necessary in interpreting all deficit patterns because of a number of important methodological limitations in many of the existing studies described above. Firstly, existing studies have often involved comparisons of relatively small cells of between 10 and 25 ADHD and poor-reading children. Arguably, more reliable patterns among WM and other theoretically important attention processing measures are likely to emerge only from somewhat larger studies.
Secondly, many studies have also been based exclusively on children with multiple clinical problems that are more likely to come to the attention of services. Given genetic evidence that, for example, the behavioural traits associated with ADHD may be distributed in a continuum across the population (Gillis, Gilger, Pennington, & DeFries 1992; Levy, Hay, McStephen, Wood, & Waldman 1997), the literature may benefit from the use of use of complimentary non-clinical populations as clinical populations may bring with them issues such as referral bias (Adams & Snowling, 2001; Pennington et al., 1993).
Thirdly, the very mixed patterns of association reported between variation in reading and attention on the one hand and cognitive processes on the other hand may well reflect problems with specific statistical and methodological models used. One reason for the mixed patterns of effects reported to date may be that multiple statistical contrasts are undertaken on related cognitive measures. As a result, findings may be unreliable. A further methodological issue in this literature is that cognitive constructs are operationalized in a range of ways across studies, potentially adding further to measurement error and unreliability. A superior statistical approach may be to explore a more limited number of latent variables derived from preliminary...
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