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Evidence for abnormal motor planning in ADHD

Anat Dahan1* and Miriam Reiner1
  • 1 Technion institute of Science, Israel

It has been shown that individuals who are diagnosed with ADHD have atypical motor development and behavior. They have been shown to have slower performance on timed motor tasks (Piek et all 1999), greater variability in speed (Castellanos & Tanock 2002; Gilden 2007), poor locomotor skills (e.g. running, hopping, leaping, sliding) and control of objects (kicking, catching, throwing) (Harvey et al., 2007) and poor fine motor ability (Pitcher Piek & Hay 2003). Although a variety of clinical tests have shown that a motor dysfunction is present in ADHD, they do not indicate which specific processes are involved, as motor planning and/or motor adjustment (sergeant et al, 2006). Few studies have looked at the aspect of preparation for movement. Wood et al (1999) contrasted task performance with brief (100-150ms) versus a longer (350-800ms) interval between warning cue and target. They found that the longer interval increased differences between ADHD and control groups with respect to susceptibility to invalid warning cues. This suggests that individuals with ADHD do not benefit to the same extent from, or were adversely affected by, increased time to prepare a response. Deficient response preparation in ADHD was reported by Klimkeit et al (2005), who showed that children with ADHD were slower than typical controls with no ADHD symptoms, in reacting to a response cue by releasing a button, but did not differ in the time taken up by the actual execution of motor responses. They suggested that ADHD is characterized by slow motor preparation but not by actual slow motor execution. Eliason et al investigated the ability of children with ADHD to program and execute goal directed movements. Start and End positions were always visible while the curser was either visible (visual feedback) or hidden (without visual feedback). They found that movement control was impaired for children with ADHD, especially during the without-feedback condition, indicating poorer motor programming in ADHD. Dahan Rydar and Reiner (2016), suggested to look at the aspect of motor deficiencies in the prism of four partly overlapping components: movement planning, execution, attention to task and motion monitoring. Hence we seek for a methodology that would allow measurements of the entire movement process, from attending a target, planning and then execution of the movement. Procedure In the present study we used a paradigm introduced by Gehz (1997) to separate between two motor planning intervals. Subjects had to perform a reaching obstacle avoidance movement towards one of four possible targets after randomly interleaved short and long planning conditions. Movements were performed and recorded in an ecologically valid environment that allows natural and free hand movements, using a 3D virtual environment, and a haptic Phantom robotic arm with haptic feedback and possibility of force and high resolution hand position measurements (desktop 6DOF Phantom by Sensegraphics). Fig 1: Subjects hold a robotic arm, that allows natural movement. Top right: Image of the task, stylus has to be led to red target while avoiding blue obstacles. Experiment was conducted on 12 normal subjects, aged 27-50, (5 female, 7 male), and 13 ADHD subjects, ages 22-38, (5 female, 8 male). Results We compared motor execution for Normal subjects under two conditions: short and long planning intervals. Several aspects of execution were compared to see the effect of the planning interval on the consequent movement. One of the parameters that stood out was the time of reaching a larger that zero velocity threshold, i.e. time of actual start of movement. It seems that the movement does not begin until the brain has some form a movement plan ready. This gap, in beginning of movement timing and in duration of movement, results in a gap of total execution time. Ratio between long and short planning condition was : TTshort/TTlong = 1.41. Fig 2: Velocity profile for normal subject N.S. Movement after long planning interval in blue Movement after short planning interval in red. The movements after the shorter planning interval are clearly delayed. The velocity pattern in ADHD subjects reveals interesting differences in many aspects of the velocity profile and of the smoothness and jitter of the movement. There is a significantly smaller difference in the movement start time between the two planning conditions. Moreover the start time in both conditions is delayed relative to the Normal long –planning condition. Fig 3: Velocity profile for ADHD subject S.G. Blue: long planning condition. Red: short planning condition. Summary There are a few different processes that seem to be impaired in subjects with ADHD. In normal subjects there are clear differences in kinematics and response time between the short planning and long planning condition. It seems that in the long planning condition, subjects already have a "movement plan" ready for execution, while in the short planning condition the movement plan is not yet mature. However in ADHD subjects a smaller difference between these conditions can be found and movement starts in a delayed manner in both cases, possibly indicating that no "movement plan" was ready upon the fourth beep in both conditions. This can be a result of one of the following: The target is not attended when it appears, or movement planning is flawed in a manner that there is no gain in additional planning time. These are not necessarily separate explanations but rather may be strongly connected. The results of the experiment indicate that the difference in execution originate in more than a single cause such as lack of attention to the task. If the target was merely not attended in a timely manner, we could expect the consecutive velocity profile of ADHD subjects to resemble that of the short planning condition of normal subjects. This however is not the case. The resulting velocity profile is not as bell shaped and has more jitter, indicating online corrections . It is plausible that the velocity profile we see indicates a flaw in movement planning and online movement control.

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References


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Keywords: ADHD, Motor planning, kinematics, motor execution, Velocity pattern

Conference: SAN2016 Meeting, Corfu, Greece, 6 Oct - 9 Oct, 2016.

Presentation Type: Oral Presentation in SAN 2016 Conference

Topic: Oral Presentations

Citation: Dahan A and Reiner M (2016). Evidence for abnormal motor planning in ADHD. Conference Abstract: SAN2016 Meeting. doi: 10.3389/conf.fnhum.2016.220.00039

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Received: 01 Aug 2016; Published Online: 01 Aug 2016.

* Correspondence: Mrs. Anat Dahan, Technion institute of Science, Haifa, Israel, anatdhn@gmail.com