Evidence for deficient motor planning in ADHD

Abstract

We compare motor planning mechanisms of ADHD and control subjects based on their effect on later observed kinematic characteristics. We monitor hand movement following planning conditions that differ in preparation time, and evaluate the differences across conditions and participants with/without ADHD. Our findings show that when there is sufficient planning time, people without ADHD seem to have a motor plan ready, and immediately initiate a planned movement after a 'GO' cue, with a bell shaped velocity profile. When planning time is not sufficient, they start the movement in a delayed time, possibly indicating that they needed to complete a movement plan. However, people with ADHD, did not start movement immediately after the cue, even when provided with a long preparation time, possibly indicating that even for this planning interval they did not have a motion plan ready. The movement was not only delayed, its velocity profile was not bell shaped and had several peaks. We further found differences between control and ADHD participants in the velocity profile, variability and jitter of movements. Our results suggest that ADHD motion characteristics, are associated with an immature motor plan. Based on the results we propose a paradigm to evaluate deficiencies in motor planning.

Figure 1

Experimental Design. A player performing the experiment. Participants initiate obstacle-avoidance (obstacle are seen as blue dots) movements leading the stylus from the starting point toward the target (in red). They control the movement, in the virtual environment, by manipulating the pen-like stylus of the Phantom robotic arm.

Figure 2

Experimental timing illustration. Subjects were trained to leave the starting point at the last in a sequence of four beeps. Beeps were separated by 700 ms intervals. Targets were presented at one of two time points: 25 ms or 350 ms prior to the final auditory ‘GO’ signal.

Figure 3

Velocity profile. ADHD vs Control. Left: Velocity profile for ADHD subject s.g. Blue: long planning condition. Red: short planning condition. Right: Velocity profile for control subject n.s Blue: long planning condition. Red: short planning condition.

Figure 4

Start time. Start time of movement for different conditions for control and ADHD subjects. Control subjects started movement shortly after the cue in the long planning condition. This did not occur for ADHD subjects. Each line connects the values of start time for long and short durations for each subject.

Figure 5

Variability of start time. Variability of start time within subject trials. Intrasubject variability was reduced in the long planning condition. This did not occur for ADHD subjects, where intrasubject variability was generally higher. Each line connects the values of variability in start time for long and short durations for each subject.

Figure 6

Timing Diagram. Timing from appearance of target to start of the participant’s hand motion in the short and long conditions. Blue: long planning condition. Red: short planning condition. Planning time is available before the 4th beep. Further time is obtained after the beep and before actual movement.