The Impact of Movement Complexity on Movement Planning and Execution in Individuals with an Autism Spectrum Disorder

Background:  

Basic fine motor skills provide a foundation for developing more complex communication skills and performing activities of daily living. Difficulties with some aspects of reaching movements are now reported consistently in the literature; however the details are not always consistent (Fournier et al., 2010).  Reaction time (RT) results, a measure of movement planning, consistently indicate that individuals with ASD spend more time preparing to move (Glazebrook et al., 2006; Rinehart et al., 2006). That said, whether people with ASD also spend longer executing their movements, and if these movements are performed as consistently and accurately as age-matched peers, is unclear (Glazebrook et al., 2006, 2009; Rinehart et al., 2006; Papadopoulos et al., 2010). On closer examination of the tasks used for the above work we predicted that the inconsistent results for movement execution may be due to differences in movement complexity.

Objectives:  

The present study sought to determine: 1) If differences in movement complexity account for the inconsistent reports of movement time and variability. 2) If longer RTs were associated with the cognitive aspects of movement planning, or the motor preparation needed to initiate a voluntary movement.

Methods:  

We manipulated movement complexity by comparing sliding (1D and 2D) and aiming (3D) movements. We recruited 11 young adults with ASD and 13 typically developing (TD) age-matched peers to perform three types of reaching movements to targets in the sagittal plane: 1) sliding along a track on a piece of Plexiglas (1D, constrained); 2) sliding along a piece of Plexiglas (2D, constrained); and 3) aiming (3D, unconstrained). Movements were recorded using a 3D motion-analysis system (Optotrak 3D Investigator, NDI) and muscle activity was recorded using surface electromyography (CED 1902 dual system amplifier). RT was parsed into premotor and motor time to assess the relative contributions of cognitive and motor processes respectively. Movement time and the variability of the movement path were derived from the 3D motion analysis data. All dependent variables were submitted to a 2 Group (ASD, TD) by 3 Movement Type (1D, 2D, 3D) mixed analysis of variance.

Results:  

No group differences for RT was found (Fs<0.6). A significant Group by Movement Type interaction for MT was consistent with predictions that there were no differences between groups for sliding movements, but the ASD group took longer to execute unconstrained movements. The TD group also exhibited significantly shorter premotor RTs for less constrained movements, whereas premotor RT did not change as a function of movement type for the ASD group. Finally, the ASD group’s movements were more variable throughout the entire movement.

Conclusions:  

We propose the lack of group difference for RT occurred because the task allowed participants to plan ahead since movement direction was consistent. Based on the group interaction for premotor RT, MT, and other kinematic variables, we suggest that unconstrained reaching movements are more difficult for people with ASD. The present findings will be discussed in the context of brain connectivity models and related preferences for sensorimotor integration.