I Know It's Heavy but I Can't Make Anything of It before I Feel It

Background:  A trustworthy representation of oneself and the world around us necessarily relies on a functional sensorimotor system. ASD children who suffer from a wide range of symptoms, from social communication impairments to restricted, repetitive patterns of behavior (DSM V), often exhibit an atypical sensorimotor development.  

Objectives: To determine the nature and the specificity of motor deficits in ASD, we compared both feedforward and feedback mode of controls in healthy children and ASD children. We investigated the motor development of healthy children aged from 5 to 10 years-old and ASD children aged 9-10 years-old.

Methods:  Children were required to reach and grasp an object in order to displace it to a lateral location. The objects, two visually identical opaque bottles, could be heavy or much lighter. We manipulated the participant's previous knowledge of the object weight: when known, participant might anticipate the consequences of the weight when reaching for the object, prior to contact with it, thus allowing for feedforward control. Conversely, when unknown prior to contact, participants had to adapt to the object weight after contact (feedback control), in the displacing phase of the movement. Movement kinematic was recorded with a high-resolution optoelectronic motion tracking system.  

Results:  Neurotypical children were able to use weight information beforehand as early as age 5: they showed peaks of longer latencies and smaller amplitudes for heavy objects during the reaching phase. Children from 7 to 10 years-old also used weight information beforehand, but exhibited an opposite and more efficient pattern to overcome its effects (shorter latencies and higher peaks for heavy objects). When unknown, the heavy object impacted the displacing phase in the three groups alike: movements were slowed down, displaying later and smaller peaks in the heavy object condition. Preliminary results obtained on 10-year-old children with ASD showed that they were unable to benefit from weight knowledge before they came into contact with the object. Surprisingly though, previous knowledge of object weight was used in the displacing phase enabling the on-line control. In the unknown weight condition in which ASD children had to rely on their on-line control, their performance was comparable to that of healthy children. 

Conclusions:  Our results show that healthy children used a feedforward mode of control since the age of 5, yet this control becomes efficient and adequate only at the age of 7. ASD children were unable to modulate their movements as a function of the upcoming object weight, showing thus an impaired feedforward control. Interestingly, the displacing phase of the movement after somatosensory feedback differed between known and unknown weight conditions, thus testifying that ASD children were nevertheless capable to take weight information into account. These findings shed a new light on the motor control of ASD children by revealing a preserved on-line control and a deviant feedforward mode of control. This incapacity of anticipating their own actions could participate to their trouble in understanding others' actions and behaviors, and therefore could play a role in their lack of communication skills.