Kinematic movement strategies in primary school children with 22q11.2 Deletion Syndrome compared to age- and IQ-matched controls during visuo-manual tracking

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Abstract

The present study focused on the mechanism subserving the production of kinematic patterns in 21 children with 22q11.2DS (mean age = 9.6 ± 1.9; mean FSIQ = 73.05 ± 10.2) and 21 age- and IQ-matched control children (mean age = 9.6 ± 1.9; mean FSIQ = 73.38 ± 12.0) when performing a visuo-manual tracking task in which they had to track a cursor rhythmically between 2 target zones. Children with 22q11.2DS moved faster (overall) and reached their maximum velocity sooner when compared to controls. However, the number of corrective submovements to attain the target did not differ. Children with 22q11.2DS seem to adopt a young ballistic movement strategy, with a fast ballistic first movement phase, followed by a second movement phase with very little online corrections to attain the target. Children with 22q11.2DS are not able to process the incoming feedback during the second movement phase to maximize the accuracy of the ongoing movement and use this phase to prepare the following. The fact that the parietal cortex and cerebellum are involved in action prediction and internal representation and are implicated in children with 22q11.2DS provides a possible neurological basis for their problems with prospective control and tracking behavior.

Introduction

Although motor difficulties have been described in children with 22q11.2 Deletion Syndrome (22q11.2DS) from the early publications onwards (Gerdes et al., 1999, Oskarsdottir et al., 2005, Shprintzen et al., 1978, Sobin et al., 2006, Van Aken et al., submitted for publicationa, Van Aken et al., 2007) we are still in need of more detailed experimental studies scrutinizing why these children show the specific motor profile with balance, bimanual coordination and visuo-spatial processing skills mostly affected. However until now studies trying to tap into the possible underlying motor processes causing motor deficiency in 22q11.2DS are lacking.

Possibly, the motor problems in 22q11.2DS are related to an increased vulnerability of neural networks that are responsible for integrating sensory-motor information. Therefore, several hypotheses that might explain the motor difficulties, observed in 22q11.2DS, have to be investigated: general slowness, limited capacity, motor control mode and cerebellar deficit hypotheses. With respect to the first (general slowness hypothesis), children with 22q11.2DS are slower overall in performing cognitive and motor tasks when compared to normal developing children (Gerdes et al., 1999, Swillen et al., 1997, Van Aken et al., 2007). Since children with 22q11.2DS show a delayed development with an average FSIQ of 70 (Swillen et al., 1997), Van Aken et al. investigated whether children with 22q11.2DS still showed additional motor difficulties when compared to age- and IQ-matched controls. It was found that the 22q11.2DS group performed worse than the age- and IQ-matched controls on bimanual coordination, static balance and visuo-spatial processing tasks when tested with standardized motor tests (Van Aken et al., submitted for publicationa, Van Aken et al., 2007). Subsequently, an experimental study was set up to try to tap into the underlying processes that may cause these motor deficiencies in 22q11.2DS which was a completely unexplored area of study within the syndrome. In this study the general slowness hypothesis was investigated by comparing the performance of a 22q11.2DS group and an age- and IQ-matched control group using a simple motor response task and a visually guided movement timing task. It was interesting to find that the children with 22q11.2DS achieved similar scores than the age- and IQ-matched controls, meaning that response timing, movement timing and visual processing are related to IQ measures and cannot account for the additional motor problems (bimanual coordination, balance, visuo-spatial integration) found in 22q11.2DS (Van Aken, Caeyenberghs, Smits-Engelsman, & Swillen, 2009). Thereupon we hypothesized that the information-processing capacity in children with 22q11.2DS might be limited (limited capacity hypothesis), meaning that with fewer resources for parallel processing, movement planning may be particularly constrained under high levels of cognitive load. The process of attentional selection implies that the brain has limited capacity to process overlapping sensory, perceptual, cognitive and motor information. In 22q11.2DS attention deficits have been known to be highly prevalent (Gothelf, Schaer, & Eliez, 2008). However, Lavie and Tsal (1994) reported that most attentional deficits are thought to reflect the use of inappropriate strategies for allocating resources rather than a lack of processing resources per se. Since, the limited capacity hypothesis has not been investigated systematically in 22q11.2DS, future research addressing this issue is necessary. Another hypothesis that might explain the motor difficulties in 22q11.2DS is the motor control mode hypothesis. Van Aken et al. investigated the prospective control abilities in children with 22q11.2DS and aimed to find out whether children with a 22q11.2DS are able to build up an internal representation of a required movement using a visuo-manual tracking task. Children with 22q11.2DS showed an impaired visuo-manual tracking performance (higher time and distance errors) when compared to the age- and IQ-matched controls suggesting that the 22q11.2DS group experienced an additional (syndrome specific) processing deficit that can not be attributed to their lower intellectual abilities. In addition it was found that both groups (22q11.2DS and controls) had difficulties anticipating the movement of the target and thus are assumed to rely more on FB instead of on an internal representation of the movement (prospective control deficit) (Van Aken et al., 2010). Finally, the cerebellar deficit hypothesis is put forward (Nicolson, Fawcett, & Dean, 2001) that states that a deficit in cerebellar performance might provide a possible explanation for the motor and neuromotor problems children with 22q11.2DS experience.

The aim of the present study is to further investigate the prospective control abilities in 22q11.2DS and to find out whether children with 22q11.2DS exhibited different kinematic patterns than IQ-matched controls (motor control hypothesis) when performing a time-controlled rhythmic movement. For this purpose the same visuo-manual tracking task as in the previous study was used (Van Aken et al., 2010). Participants had to move a cursor rhythmically between 2 target zones at the pace of a metronome (1 movement per second). In addition to the audible signal, visual online feedback was provided on a monitor screen. Aiming movements are composed of an initial ballistic phase followed by a second phase of normally small corrective movements near the target. The execution of such an aiming movement is thought to mainly depend on motor pre programming in the first phase and feedback in the second one (Hay, 1978). Crossman and Goodeve (1983) suggested a linear relation between the duration of a movement and the number of corrections necessary for the execution of the movement. The faster the primary component is executed the larger the endpoint variability and hence the greater need of further submovements to attain the target. To investigate this kinematic pattern movement time, time to maximum velocity, mean velocity and maximum velocity were calculated. Moreover, the smoothness/fluency of tracking was investigated in two ways: first, the number of zero crossings was counted. A zero crossing is marked when there is a significant deceleration in the acceleration phase (acceleration becomes negative) or when there is a significant acceleration in the deceleration phase (acceleration becomes positive) of the movement. This number reflected the degree to which a child uses a sequential pursuit strategy (Miall et al., 1986, Mounoud et al., 1985), meaning that after a movement has been made to catch up a target, a stop is made to determine on the basis of FB what the next movement will be and thus short parts of the trajectory are planned and executed successively. Secondly, the ratio of peak-over-mean-velocity (Flash & Hogan, 1985) was calculated to quantify the extent to which movements are produced in a more open-or closed-loop mode. Cyclic movements are performed more ballistically while discrete movements are more visually guided, utilizing a predominantly closed-loop strategy (Smits-Engelsman, Duysens, & Van Galen, 2001). Discrete aiming movements will allow corrective movements at the end of the trajectory, whereas such corrections are absent in fast oscillatory movements (Crossman & Goodeve, 1983). Van Aken et al. (2010) found that the 22q11.2DS group heavily relied on FB during movement execution. Therefore, it was hypothesized that the 22q11.2DS group would have more difficulties switching to a feedforward or open-loop strategy resulting in a fast initial movement phase and more corrective submovements in the second phase of the movement. Consequently, it is assumed that the tracking task will be performed less smoothly/fluently by the 22q11.2DS group, resulting in higher values of the ratio peak-over-mean-velocity (approximating 2.0) (Flash & Hogan, 1985).

Section snippets

Participants

Forty-two participants participated in this study, including 21 children with a 22q11.2DS (mean age = 9.6 ± 1.9; mean FSIQ = 73.05 ± 10.2) and 21 control children (mean age = 9.6 ± 1.9; mean FSIQ = 73.38 ± 12.0). The children with a 22q11.2DS were recruited from the multidisciplinary 22q11.2DS-clinic at the Center for Human Genetics (University Hospital Gasthuisberg) in Leuven. Parents and children visit the clinic on a regular basis and are followed up by a multidisciplinary team. Detailed medical data are

Demographics

The children of the 22q11.2DS group were individually matched to 1 control child based on age and FSIQ. No significant age (DF = 1; F = 0.05; p = 0.82) or FSIQ (DF = 1; F = 0.27; p = 0.61) differences between the children with a 22q11.2DS and the control children were found.

Velocity measurements

A significant group effect for total mean velocity was found. The children of the 22q11.2DS moved at higher velocities (on average) than the controls to execute the visuo-manual tracking task (DF = 1; F = 3.91; p = 0.05). No significant

Discussion

Since little is known about the underlying processes causing motor deficiency in 22q11.2DS, the present study focused on the mechanism subserving the production of kinematic patterns in children with 22q11.2DS and age- and IQ-matched controls. Do children with a 22q11.1DS exhibit a different kinematic pattern than age- and IQ-matched controls when performing a visuo-manual tracking task? Children with 22q11.2DS moved faster (overall) and reached their maximal velocity sooner compared to the

Conclusion

Children with 22q11.2DS seem to adopt a young ballistic movement strategy when performing a series of rhythmic movements with a fast ballistic first movement phase, followed by a second movement phase with very little online corrections to attain the target. Children with 22q11.2DS are not able to process the incoming feedback during the second movement phase to maximize the accuracy of the ongoing movement and use this phase to prepare the following. The fact that the parietal cortex and

Role of the funding source

Support for this study was provided through a grant from the Steunfonds Marguerite–Marie Delacroix (Belgium). This funding source had no involvement in study design, collection, analysis and interpretation of the data and in writing and submitting the paper.

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