DISCUSSION
The current study used a series of object-retrieval tasks of increasing complexity and varying context to compare the performance of young children with DS with mental age-matched TD children. Overall, our results indicated that children with DS were more challenged by the tasks compared to the mental age-matched TD group. However, the results were highly task-specific highlighting a strong interaction with the level of task and contextual complexity. To elaborate, for the easier task 1 involving retrieving a toy from a transparent box, children with DS performed similarly to the TD group and did not differ in terms of overall success and hand use strategies. However, with increasing task and contextual complexity, children with DS demonstrated specific difficulties with overall success and performance during the task 2 and 3. For task 2, the DS group had less success, required greater adult assistance, and longer time to plan the actions compared to the TD group. For task 3, the DS group showed less use of asymmetric bimanual strategies and required greater tester assistance compared to the TD group. Lastly, the DS group took longer to execute the tasks compared to the TD group. Interestingly, both groups had similar success rate for task 3 which could be attributed to the floor effects of a complex object retrieval task involving both task and context complexity.
The motor impairments of children with DS and their impact on object retrieval was apparent during task 3. Specifically, children with DS showed limited use of asymmetric bimanual strategies during task 3 compared to the TD group, and no differences for the symmetric bimanual or unilateral hand use. Bimanual coordination is required for variety of daily tasks, and evidence from the typical population suggests later development of asymmetric compared to symmetric bimanual coordination (Brakke et al., 2007), and its dependence on both the individual and environmental factors (Kirschner & Tomasello, 2009; Pellegrini et al., 2004). Additionally, bimanual manipulation is an important achievement for refining consistent hand preference in children (Nelson, 2022). Children with DS have poor bilateral coordination compared to neurotypical children as assessed using standardized motor tests and motor tasks such as walking (Vali Noghondar et al., 2022; Nocera et al., 2021; Jobling, 1998). However, there is limited information on the development and origins of bimanual coordination in infants and young children with Down Syndrome. More importantly, principles guiding bimanual training has only been studied in adults with DS, such as bimanual coordination improves with visual information, stable handedness, and repeated practice in adults with DS (Ringenbach et al., 2012; Mulvey et al., 2011; Latash & Patterson, 2002). Future studies should examine the development of bimanual coordination in infants with DS along with the impact of individual (e.g., severity of motor, attentional deficits) and environmental factors (e.g., task difficulty, visual or auditory stimulus) on performance of infants.
Cognitive impairments, or more specifically differences in executive skills could also have impacted the overall success rate and task performance of children with DS. Global differences in executive skills or its subconstructs such as working memory, cognitive flexibility, and inhibitory control are commonly reported in children with DS (Tungate & Conners, 2021; Daunhauer et al., 2014). Similarly, for the study tasks, children were required to explore multiple strategies, inhibit certain predominant strategies (such as directly trying to grasp the toy without opening the lid), and choose the best strategy (such as asymmetric hand use involving stabilization with one hand and moving with the other hand). Clearly, children with DS were slower in planning, choosing, and executing the strategies compared to TD children, but these challenges became apparent only when the task was complex and/or had additional contextual complexity of reduced transparency level was added to the tasks.
The current study also indicated that children with DS required more time to plan and execute their actions compared to TD children. Slower movement initiation and execution have been commonly reported in individuals with DS (Charlton et al., 1996; de Campos et al., 2010; Kearney & Gentile, 2003; Latash et al., 2002). For example, young preschoolers with DS (Kearney & Gentile, 2003) as well as school-aged children with DS (Charlton et al., 1996) had slower speed of reaching and grasping movements compared to TD controls, along with greater difficulties in feedback-dependent motor control. However, slowing down could also be viewed as an adaptive strategy to improve movement accuracy and performance. Adults with DS were slower than typically developing adults while tapping a tablet with a stylus but had a similar accuracy rate of hitting the target (Lam et al., 2009). Furthermore, the trade-off between the speed of movement and accuracy was a function of task difficulty (i.e., size and distance). Similar research has not been done in children with DS, hence, it is unclear whether children with DS use slowing down as an adaptive strategy to improve task comprehension and/or performance. Slower processing and execution times could pose serious challenges on everyday functioning of children and have negative implications on the learning of new skills and behaviors. Therefore, we recommend future studies to further investigate slower initiation and slower movement rate in children with DS, as well as their implications on everyday functioning and acquisition of new skills.
Lastly, we would like to acknowledge the differences in family demographics in our study sample. Parental education years were significantly lower in the DS group compared to the TD group. Several studies examined the association of socioeconomic status (SES) such as parents’ education and income with the child’s development including cognitive and social-emotional development (Bradley & Corwyn, 2002). Specifically, low maternal/paternal education is associated with poor academic performance and lower IQ scores in their children, possibly due to lack of resources, stimulating environment, as well as differences in parenting styles and attitudes (Mercy & Steelman, 1982; Scarr & Weinberg, 1978). Research has also demonstrated less effective object exploration in infants residing in lower SES households compared to those living in higher-SES households (Tacke et al., 2015). However, if we follow the cut-offs for low and high SES in the above-mentioned studies, a majority of the DS families in our study sample could qualify for high SES. For example, Tacke et al. (2015) considered families with 2+ years of college education as high SES, and 11 out of the total 14 DS families in our sample had 12+ years of education and would thus fall under the high SES category (see Table 1 for average parent education years). We would recommend further investigation of family factors (e.g., parents’ occupation, education) and neighborhood factors (e.g., geographic location) on child’s exploration and manipulation of objects, especially focusing on intersectional factors that could exist for children with disabilities living in lower-SES households.