The planning and control of action in normal infants and children with Williams syndrome
The development of action control was studied in normal infants between 5-15 months and in children with Williams syndrome, a group in whom it is thought that the visual processing stream controlling action (the dorsal stream) may be specifically impaired. In the first set of experiments, looking and reaching measures were taken as infants were resented with pairs of objects of different sizes. An increase in frequency of reaches to raspable objects and in the time spent inspecting objects before reaching was observed after 8 1/2 months, reflecting a developing ability to use visual information to predict the 'graspability' of objects. There were concurrent changes in the kinematics of the reach; infants at this age exhibited slower reaches, with an extended 'homing-in' phase. referential looking measures showed that as infants get older, visual orienting to objects in personal space becomes less dependent on the visual salience of the object. Lastly, there was evidence that reaches tended to follow the direction of the first look in any given trial. Flexibility in the choice of a target for reaching may therefore depend upon the development of an ability to selectively inhibit neural circuits controlling looking and reaching. The second set of experiments investigated how far the visuo-motor difficulties experienced by children with Williams syndrome are due to atypical dorsal stream development, or in the ability to plan ahead in simple actions. The ability to adapt grip size to object size (a 'dorsal stream') function was compared with the ability to make a perceptual size judgment (a 'ventral stream' function). Deficits were observed in both tasks, with no relative deficit in dorsal stream, compared to ventral stream function. Tests of motor planning indicated that children with WS are less likely than controls to select an initial grip that ensures a comfortable end-state posture of the arm. However, this ability seems to be unevenly distributed in children with WS. Further work will need to be done to identify whether these differences between individuals are related to different compensatory strategies or are correlated with the extent of the genetic deletion in WS.