Abnormalities of motor and praxis imagery in children with DCD

In an earlier study using the visually guided pointing task (VGPT) the auth ors showed that the timing of imagined movement sequences in children with developmental coordination disorder (DCD) does not conform to the conventio nal speed-for-accuracy trade-off (or Fitts' law [P.M. Fitts, Journal of Exp erimental Psychology 47 (1954) 381-391]) that occurs when the distance and accuracy requirements of movements are varied [P. Maruff, P.H. Wilson, M. T rebilcock, J. Currie, Neuropsychologia 37 (1999b) 1317-1324]. The present s tudy sought to replicate this earlier finding and to examine (using a weigh t manipulation) whether this deficit was also attributable to inaccurate pr ogramming of relative force. The chronometry of real and imagined movements was investigated in a group of 20 children with DCD aged between S and 12 years and a group of controls matched on age and verbal IQ (VIQ). Movement duration was tested for real and imagined movements using the preferred han d, with the VGPT performed under two load conditions: with and without the addition of a weight attached to a pen. Group means of each subjects' mean movement duration were calculated and plotted against target width for each of the four conditions [Movement type (2) x Load (2)] and a logarithmic cu rve was fitted to the data points. In the control group, the speed-for-accu racy trade-of for both real and imagined performance conformed to Fitts' la w under each load condition. In the DCD group only real movements conformed to Fitts' law. Moreover, the effect of load differed between groups - for real movements, movement duration did not differ between load and no-load c onditions for either group. while for imagined movements, movement duration increased under the load condition for the control group only. These resul ts replicate and extend the results of our earlier study. This pattern of p erformance suggests that children with DCD have an impairment in the abilit y to generate internal representations of volitional movements which may re flect an impaired ability to process efference copy signals. The ability to programme both relative force and timing appears to underly this difficult y. Results have implications for the use of (guided) motor imagery training in order to facilitate the development of motor skill in children with DCD . (C) 2001 Elsevier Science B.V. All rights reserved.