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Aim Developmental coordination disorder (DCD) is a significant disorder of childhood, characterized by core difficulties in learning fine and/or gross motor skills, and the attendant psychosocial problems. The aim of the meta-analysis presented here (the first on DCD since 1998) was to summarize trends in the literature over the past 14 years and to identify and describe the main motor control and cognitive deficits that best discriminate children with DCD from those without.
Method A systematic review of the literature published between January 1997 and August 2011 was conducted. All available journal papers reporting a comparison between a group of children with DCD and a group of typically developing children on behavioural measures were included.
Results One hundred and twenty-nine studies yielded 1785 effect sizes based on a total of 2797 children with DCD and 3407 typically developing children. Across all outcome measures, a moderate to large effect size was found, suggesting a generalized performance deficit in children with DCD. The pattern of deficits suggested several areas of pronounced difficulty, including internal (forward) modelling, rhythmic coordination, executive function, gait and postural control, catching and interceptive action, and aspects of sensoriperceptual function.
Interpretation The results suggest that the predictive control of action may be a fundamental disruption in DCD, along with the ability to develop stable coordination patterns. Implications for theory development and intervention are discussed.
Motor clumsiness (or developmental coordination disorder [DCD]) affects between 5% and 10% of all children1 (all references published online in Supporting Information). Efforts to understand the developmental precursors of DCD are important to avoid continued disruptions to skills development, secondary impacts on self-esteem and participation, and associated issues such as obesity, poor physical fitness, and social isolation.2,3 As a result, DCD has received considerable attention from researchers across disciplines including kinesiology, occupational therapy, paediatrics, physiotherapy, and psychology.4,5 Understanding the underlying mechanisms of DCD has been an important focus in an effort to better model atypical development generally, to optimize therapy, and to avoid some of the negative consequences of the disorder. Unfortunately, despite continued efforts to understand DCD, the aetiology of the disorder remains unclear.
The last comprehensive review of the literature on deficits underlying DCD was published in 1998.6 Since then, there has been a substantial growth in research designed to understand the disorder.7 Furthermore, approaches to the study of DCD have evolved rapidly since the mid-1990s. We have seen a sharp growth in research conducted from a more cognitive neuroscientific perspective, as well as in dynamic systems and hybrid approaches. This has broadened the domains of research and heralded something of a revolution in the choice of research paradigm, which is worth tracing.
The cognitivist approach, which dominated early research, seeks to understand behaviour by defining the set of internal cognitive processes that support it. The traditional metaphor used to represent the mind is the computer, generating a set of rule-governed computations. From a cognitivist perspective, simple chronometric and neuropsychological measures were dominant in DCD research up to the mid-1990s,6 and maintain some presence in the literature. Notable among these studies are those assessing aspects of executive function, drawing on the models of Baddeley8,9 and Shallice,10–12 for example. Working memory has been explored in a series of studies by Alloway et al.13–15 and by other groups,16–19 with the weight of data said to implicate visuospatial deficits. Problems in response inhibition have also been detected,20–23 as well as a more generalized impairment across set-shifting, working memory, and inhibition.16 At the level of executive attention (or supervisory control), deficits have been reported on dual-task performance,24 set-shifting,25 cognitive abilities such as planning and flexibility,26,27 and meta-cognitive awareness.28,29
Poor cross-modal integration has been implicated in a number of studies since 1996, although the exact pattern of impairment in DCD across conditions is variable.30–36 Others have reported problems in (intramodal) proprioceptive matching.32,37,38 Unfortunately, the strength of these effects has not been calculated or compared across different task conditions and processing domains.
Other aspects of sensoriperceptual dysfunction in DCD have also been reported on a consistent basis in the literature. These areas include basic visuosensory processing,39 visuospatial processing,16–19,31,40–45 tactile perception,31,46 kinaesthetic perception,32,34,47 and basic processing speed.48,49 Finally, at the level of procedural learning (a form of implicit memory), results have been mixed, with some reporting intact learning50 and others not.51
The integration of brain and behaviour under a single conceptual scheme is encapsulated by cognitive neuroscience. This approach has been revolutionary in the study of typical and atypical development, mapping the neural networks that support human cognition and action. Some of the earliest studies of DCD to adopt a cognitive neuroscience perspective appeared around the mid-1990s.52,53 The concept of predictive motor control (also known as internal forward modelling) has been particularly influential under this approach,54–59 and has been explored either directly or indirectly by several groups. Wilson et al. have used converging paradigms including motor imagery,60–66 double-step saccade task,67 and step perturbation reaching.68,69 Importantly, other groups have also used sequential reaching tasks, but with mixed results.70,71 Other methods used to explore internal modelling in DCD, and predictive control more specifically, include anticipatory postural adjustments during bimanual lifting,72 perceptual–motor adaptation,73–78 coupling of grip and load force during manual lifting,46,79 and smooth pursuit (eye) tracking.80 Deficits in covert orienting of visuospatial attention have also been taken to indicate problems of movement preparation and/or prediction,52,81 while others infer problems of inhibition.82–85 Note that force control has also been investigated more generically using isokinetic tasks86,87 and measures of peak output.88
Different aspects of computational motor control (i.e. attention, feedback, and feedforward processes) have been imbued with the language of neuroscience under the cognitive neuroscience approach. Mechanisms of control have been inferred from studies of overt orienting of attention during reaching,89,90 hand–eye coordination tasks,90 and manual steering/tracking.91–93 Feedback and feedforward processes have been implicated, but the magnitude of effects has never been compared across studies.
We have seen significant growth in studies of DCD from a dynamic systems perspective over the past 15 years. This approach has its roots in biological systems theory and ecological psychology. The main working assumption here is the interaction of multiple task, individual, and environmental constraints in the organization of movement.94 The timing and coordination of movement are viewed as emergent properties of the (individual) physical system in its interaction with the immediate environment,95 and not as centrally stored and generated outcomes. Researchers have endeavoured to describe the nature of rhythmic coordination and timing in DCD. This work includes the following: self-paced rhythmic coordination of unimanual and bimanual actions;96–98 rhythmic perceptual–motor coordination,a namely visuomotor96,98 and auditory–motor synchronization;99 stability of coupling in response to perturbation;100 and interlimb coordination between arms and legs;101,102 see also the work of Volman et al.103 Taken together, aspects of dynamic pattern stability and interlimb coupling are shown to be deficient in children with DCD. However, the magnitude of deficits across different task constraints has not been evaluated. Note also that dynamic planning (as distinct from motor execution) has been examined from a dynamical perspective and implicated in DCD.104
Hybrid approaches to DCD are those that blend conceptual schemes and paradigms from cognitivist, cognitive neuroscience, and dynamic perspectives.87,105 In other words, the conceptual framework is not pure but rather is integrative, defined by current trends in thinking across a variety of literature areas. More than ever, researchers are willing to integrate ideas from cognitive, learning, developmental, and neuroscience theories, as well as consider how the dynamics of movement might be instantiated neurally in the system.73,106 Hybrid approaches to DCD have covered different forms of manual control (reaching and catching, in particular), gait and posture, timing, and aspects of praxis (including imitation). Work on manual control includes kinematic studies of target-directed reaching under different task constraints,107–110 reach–grasp dynamics,109,111 the sensory control of manual pointing,112 head–torso–hand coordination during reaching,105 aiming using a stylus or pen,113 manual interception of simple visual stimuli,114 and graphomotor control.115–118 Prospective judgements of reaching/grasping have also been examined.34,119,120
The kinematics and dynamics of catching under different task constraints has also been an area of intense interest, mainly by Astill et al.,121–124 Deconinck et al.,125 and others.126–128 Problems of interlimb coupling have been reported, along with the temporal control of grasp or interception.
The spatiotemporal control of gait has been investigated under conditions of normal walking129–131 and under restricted vision;132 it has been suggested that individuals with DCD place greater reliance on visual control, but findings appear to be equivocal. Similarly, control of static posture has been investigated under normal conditions133 and under altered constraints and perturbation, that is, visual,133–138 physical,136,139,140 and cognitive.44,141,142 In general, postural deficits are suggested to be greater with added external constraints. Similar arguments have been made for dynamic postural control while reaching, leaning,143 or lifting.144
Motor timing issues have been reported in DCD using a blend of traditional and neuroscience methods: finger tapping paradigms,145 rhythmic finger synchronization,146 and rhythmic arm movements.147 Increasingly, timing is modelled as a distributed system of neuromuscular control, one involving primary sensorimotor cortex, posterior superior temporal gyrus, cerebellum, and supplementary motor area.148
Praxis deficits to imitation and verbal command have also been reported,149–152 suggesting disruption to motor planning. Praxis to imitation, in particular, has been linked to left parietal dysfunction. And, finally, associated movements (or motor overflow) suggest poor synergies in movement production, even for relatively well-rehearsed skills such as running;153,154 their neurological status is unclear.
The full complement of performance categories under each approach is listed in Table SI (supporting information published online), together with common outcome measures. We see, in part, the evolution in approaches to DCD, as well as the sheer range and complexity of paradigms and their associated metrics. Clearly, there is a need to synthesize this vast volume of data, drawing out the common threads in findings that will help push the field forward.
Also motivating this review was an initiative from representatives of the Association of the Scientific Medical Societies in Germany and the European Academy of Childhood Disability to develop Clinical Practice Guidelines for DCD in German-speaking countries.155 Because motor clumsiness in children is defined somewhat differently across countries, it was necessary to initiate an international consensus to confirm and/or modify a previous international consensus (London Consensus, Leeds Consensus). Chaired by Professor Rainer Blank, a panel of international researchers was established to look at three key aspects of DCD: aetiology, diagnosis, and treatment. A core aspect of examining aetiology concerned a systematic review of findings of impaired functions or underlying mechanisms.
A meta-analytical approach was used because, by combining estimates from multiple studies, meta-analysis increases the total number of primary units for analysis (i.e. participants), reduces the sampling error of an association,156 and permits evaluation of variables that moderate group differences, such as age, sex ratio, and severity of the primary condition.157 It is one of the primary means of data synthesis, and a means of building knowledge in a given domain.
The specific aims of this meta-analysis were (1) to present a quantitative review of the DCD performance literature since January 1997; (2) to identify the deficits that best discriminate between children with and without DCD; (3) to identify patterns in the constellation of deficits that suggest causal mechanisms or that fit particular accounts of the disorder; (4) to determine the moderating effect, if any, of sex and age; and (5) to identify areas where more systematic research is needed to build knowledge and inform our approaches to treatment.158
- Top of page
- What this paper adds
- Supporting Information
The meta-analysis presented here explored the pattern of performance deficit in DCD across a range of tasks measuring motor control, learning, and cognition. There was a generalized level of impairment across tasks of large magnitude (dw=0.97). This raises the possibility that the reduced ability to learn motor skills is associated with a baseline level of neuromaturational delay or dysfunction, affecting cortical networks associated with the control of action. However, there were several clusters of even more pronounced deficit in DCD: internal modelling of action (i.e. predictive control), rhythmic coordination and timing, executive function, dynamic control of posture and gait, and interceptive action (catching and manual interception). While seemingly diverse, the pattern of deficits in DCD supports several converging lines of argument about underlying mechanisms. The first argument concerns the (domain-general) concept of predictive control in its broadest sense,e both at the level of real-time control and with respect to learning and utilizing internal models for action (or predictive mapping, more specifically). The difficulties we see with motor prediction under tight temporal constraints are manifest across target-directed reaching, the coupling of grip and load force, anticipatory postural control, and interceptive actions. Problems developing new internal models for action over repeated trials were also evident during perceptual–motor adaptation. And, there is a strong suggestion that difficulties in rhythmic perceptual–motor coupling may also reflect a more fundamental issue in the development of internal maps for action. This argument is notable because it represents a prototypically hybrid view on motor control: the paradigm on which this type of work is based has its roots in the dynamical systems approach, but the interpretation draws on neurocomputational theory. The notion of increased noise within parietocerebellar networks is one possible explanation for deficits in predictive control.
The second converging argument is that a more basic issue in rhythmic coordination and timing is seen consistently, to a point where it is almost synonymous with DCD. It is perhaps then not merely a coincidence that the term ‘coordination’ is part of the diagnostic label according to DSM. At face value, the term suggests that the movement of these children lacks fluency and efficiency: the parts of an action are not put together in the right way, but rather require much effort, even for the simple skills that most children take for granted. This is apparent both to the trained eye of the clinician and to most parents and teachers. At the level of motor control, we dissect the notion of ‘coordination’ further, and see pronounced difficulties with intra- and interlimb coupling and stability. From a dynamic perspective, the control issue may reflect reduced stability of the coupling to an attractor state (Clark J, Whitall J, personal communication, 2011). However, Clark and Whitall suggest that this interpretation does little to clarify the underlying mechanism. In computational terms, children with DCD appear to have a reduced ability to form internal models for action and to use these stored estimates in a predictive manner in order to synchronize to an external entraining signal. Indeed, children with DCD tend to ‘live on feedback’ (Clark J, personal communication, 2011). Dysfunction at a corticocerebellar level is likely to explain this fundamental deficit in timing. Indeed, cerebellar dysfunction sees a constellation of deficits in timing, predictive control, fine motor coordination, and basic cognitive functions.182,194 This fits the profile of deficits that were highlighted in this meta-analysis and suggest, also, important rate-limiting factors in mainstream motor development.195
The third (domain-general) area of deficit was in executive function. Deficits here were evident in most aspects of executive control including working memory, inhibition, and executive attention. Metacognitive aspects of action planning were also affected, suggesting a generalized level of impairment in this area. That the degree of dysfunction parallels or even exceeds that which is seen in attention-deficit–hyperactivity disorder is striking and demands future investigation.
Although it is beyond the scope of this paper to comment on intervention, it is fair to say that effective remediation of DCD should be critically tied to current theory.94 What this review suggests is perhaps two main avenues for intervention research. The first concerns ways of improving predictive control. Simple forms of augmented feedback and motor imagery training may provide starting points for training internal models for action and associated body schema.183 The second avenue for intervention work concerns rhythmic coordination and timing within and between limbs, targeting cerebellar function more specifically. When concurrent augmented feedback is provided in synchrony with voluntary, rhythmic movements, the stability of coordination is often enhanced.106 This modality requires investigation in DCD.