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Introduction: Patients, who suffer from autism-spectrum-disorder, often show motor deficits of different extent. It is unclear, which pathophysiology or which brain systems are responsible for these movement abnormalities. Post-mortem studies, structural and functional as well as behavioural studies about autism present evidence that the cerebellum of these patients shows alterations, which might contribute to the described motor dysfunctions. Predictive movement control is supposedly based on so-called internal models, which can predict the dynamic effects of one’s own movements as well as object dynamics and the resulting sensory feedback information, and integrate them into motor commands. There is evidence that regions of the cerebellum represent the anatomical correlate of such internal models. In this study, we examined motor functions of Asperger patients for predictive control mechanisms and movement coordination in a series of experiments.
Methods: Motor performance of 11 patients with Asperger-Syndrome was compared to 11 healthy control subjects. The groups were age, sex and IQ matched, all subjects were right-handed. In Experiment 1 the placement task of the Purdue pegboard was applied. Small metal pegs had to be placed into an array of vertical holes as quickly as possible. Thus, we were able to examine fine motor skills and hand-eye-coordination, and at the same time, rule out major motor deficits. In Experiment 2 we examined predictive motor control on the basis of load and grip force coupling while lifting an object. The subjects had to grasp and lift an object, which incorporated a grip force and acceleration sensor. In another test on predictive motor control, subjects dropped a weight with one hand into a receptacle, which was attached to the test object, held by the contralateral hand. This complied with a condition testing predictive increase of grip force. In Experiment 3 subjects reached and grasped a cube with their index finger and thumb. A reach-to-grasp movement consists of two components: the hand transport and the grip formation. We calculated several parameters that represent the temporospatial coordination of the two movement components. For the three-dimensional recording of movements during the reach-to-grasp experiment, an ultrasonic motion measurement system was used.
Results: Patients showed a significantly lower performance in the Purdue pegboard test than healthy controls. The results were most prominent with the left hand. However, the results did not reflect a functional disorder of the motor system. In the lifting tasks patients showed significantly worse temporal coordination of increasing grip and load force. In the experiment on self-induced weight perturbation both groups generated a predictive increase of grip force. However, patients exhibit a significantly larger initial grip force and a significantly lower absolute increase of grip force prior to weight impact. In the reach-to-grasp tasks patients produced a significantly straighter and lower approach towards the object than healthy controls. Furthermore, patients exhibit a significantly smaller grasp aperture and grasp aperture velocity. The hand transport velocity in the patient group is on average considerably slower than in the control group. However, the temporal coupling between hand transport and grip aperture hardly differed between groups.
Discussion: In our experiments we revealed several significant differences in motor performance and motor behaviour between Asperger patients and healthy controls. This suggests a disturbed predictive movement control and cerebellar disorder in Asperger autism. In particular, we showed, that patients exhibit an altered coupling between grip and load force while lifting an object. Furthermore, the lower and straighter hand trajectories in the patient group in the reach-to-grasp task could result from disturbed temporospatial coordination of proximal and distal muscle groups of the arm and may reflect altered cerebellar processing of forward models. Reach-to-grasp movements were executed slower by patients over all conditions, a finding that correlated with the patients’ performance in the Purdue Pegboard test. We assume that this slowness is compensatory to adjust to an altered processing of internal models and their integration into predictive movement control.