In the current research, we aimed to examine the effectiveness of the VWM-B program on verbal WM capacity, reading ability, and postural control in children with DD. In comparison with VWM-program, the VWM-B program showed superior performance on the verbal WM capacity, reading skills, and postural control after a short-term intervention.
In line with older evidence [43, 48, 78], participants in the control group showed an improved verbal WM capacity; however, participants in the intervention group had higher performance in the verbal WM following the intervention. Also, participants in the intervention group, in comparison with participants in the control group, showed higher improvement in reading ability (see Tables 2 and 3). The positive effects of the VWM-program on reading ability are in line with previous studies [25, 44, 48]. Correlation analysis in the current study indicated that the verbal WM capacity is associated with increased reading ability (see Table 4). Considering these results and the close link between the verbal WM capacity and reading ability [79], it seems that the improved verbal WM capacity probably facilitated the word and non-word reading ability via improved phonological awareness (tested by the WR and PD subtests of NEMA) [80, 81]. Also, improvement in the grapheme-phoneme decoding (tested by the NWR and CW subtests of NEMA) and phonemic awareness (tested by the WR and PD subtests of NEMA) may be related to an improvement in reading comprehension (tested by the TC subtest of NEMA) [81, 82]. It is concluded that the VWM-B program is more effective than the VWM-program to improve verbal WM capacity and reading skills.
The close link between WM capacity and attention has been confirmed by previous studies [83]. It has been reported that the WM modulates attention [67, 83, 84], and on the other hand, that attention promotes the encoding, maintaining, and manipulating of information in the WM [85, 86]. Despite these pieces of evidence, the SCWT outcomes in the present study were uncorrelated with verbal WM capacity (see Table 4). Decreasing the Stroop interference was correlated only with the WR subtest of NEMA. The significantly decreased Stroop interference in the intervention group participants is justified by the structure of SWCT. The word-naming step of the SCWT needs the subject to read the colored words [66]. It shows the subjects' reading rate and reflects their speech-motor problems [66, 69]. Children with DD usually spend further time to complete this component [66, 68]. Keeping this point in mind, the decreased Stroop interference in the current study probably resulted from the improvement in word reading ability (tested by the WR subtest of NEMA). This claim is supported by previous studies when they declared that decreased Stroop interference implies improvements in reading ability as well as selective attention [66–68].
Based on our best knowledge, the VWM-B is the only training program, which contains a mix of cognitive and balance-related performance simultaneously, which has been used in DD. Previous studies have also reported some positive effects for a combination of cognitive and physical training in other populations (see [87–89] for more information). Regarding the sequential nature of the process in a dual-task condition, the nervous system first prioritizes a task and then assigns further cognitive/attentional resources for the prioritized task. Therefore, the performance decreases on the non-priority task [90]. Furthermore, sufficient manipulating and maintaining information in the verbal WM is critical for increasing verbal WM capacity [91]. Keeping these points in mind, the featured maintenance step of the VWM-B program probably had an important role in efficiently improving the measured functions in children with DD (see Sect. 2.2.2 in the Method). We designed the maintenance step of the VWM-B program within two passive and active balance states of the subject. In the passive state, we designed the cognitive task as a prioritized task. In the active state, however, the balance was considered a prioritized task. Hence, we expected the balance-related movements would be automatized [73], and as a result, further resources assigned to the cognitive task [90].
The present study showed improvement in the intervention group participants' postural control after the intervention, which was perceived in both eyes open and closed conditions of CoP (see Tables 2 and 3). The older evidence revealed that there is no significant difference between the upright standing postural control (eyes open) of the dyslexic and non-dyslexic children [92]. However, children with DD have weaker postural control when they use visual information to perform an activity (actions often are complex or dual-task) [34, 92]. The reason is supposedly insufficient coupling of the visual inputs and postural sway while performing an activity [92]. The improved postural control in the eyes-open condition demonstrated that coupling visual information and body sway were probably improved, and the intervention group participants could assign sensory information to produce purposeful actions [92]. In other words, these participants showed higher performance in using non-visual information to maintain postural control, and benefitted from the visual information to perform purposeful actions. The improved postural control in the eyes-closed condition implies that the intervention group participants probably benefited from vestibular and/or proprioceptive information and were less dependent on visual inputs to maintain postural control [93]. It is concluded that the motor strategies relating to balance control were automatized [73, 94–96], and further neural resources were allocated to the cognitive task following the intervention with the VWM-B program [90, 94, 95]. In the present study, there were no significant changes in the CoP measures of the control group participants. Furthermore, the improved postural control was correlated with verbal WM capacity and reading ability in the intervention group participants. Therefore, we conclude that the higher performance in the measured cognitive functions of participants in the intervention group probably stemmed from the automatized balance-related movements.
Almost all human activity needs trial-and-error (supervised) learning, which is a sub-type of procedural learning [97]. The cerebellum is a central structure in human brain circuits, and it is a crucial point that only the cerebellum has a hub circuitry to support supervised learning [97]. It implies that if this type of learning is required (e.g., in reading), it is necessary to involve the cerebellum as part of the circuit, along with the other parts of the brain involved in reading [97] (e.g., cortical regions of perisylvian [6] and prefrontal [23] involved in phonological processing and verbal WM, respectively). The corticocerebellar circuits involve loops from the cortex to the cerebellum to thalamic nuclei and back to the cortex [98]. Insufficient skill automatization due to impaired cerebellar function leads to problems in reading, though via different cerebellar circuits [99]. Considering the findings of the current study for CoP, it seems that the VWM-B program caused changes in the activation of cerebellum circuitry. Some regions of the cerebellum may be activated in this dual-task performance [36] and could integrate motor and cognitive networks and adjust these networks to be more efficient for performing the dual-task properly [36].
Although it needs future neuroimaging studies to adequately investigate the changes in the activation of the cerebellar circuits after treatment with the VWM-B program, previous neuroimaging studies have confirmed the role of the cerebellum in verbal WM, reading, balance, and complex actions [100, 101]. For example, a loop between the right VI and crus I lobules of the cerebellum and Broca's region of the left frontal lobe activates during articulatory rehearsal and verbal WM tasks [100, 102]. Activation of the right VI and crus I lobules of the cerebellum provide internal motor sequences for the phonological content of words [101]. Also, it has been reported that the loops between the bilateral cerebellar VI and VII lobules and the cerebral regions of the left inferior frontal lobe and the left inferior occipitotemporal lobe have a critical role in the reading network [103, 104]. Furthermore, researchers have recently discovered a novel topographic map in the cerebellar lobules of VI and VIIA, which shows the role of these lobules in complex motor tasks [100]. Therefore, the authors suggest considering these cerebellar lobules in future neuroimaging studies (by treatment with the VWM-B program).
A brief review of the literature indicates that the causal link between the balance deficits and reading problems is under controversy [105–108]. Also, there are limited studies that investigate balance training effects on children with DD. For example, Goulème et al. demonstrated the effect of balance training only on postural control [28]; however, Reynolds et al. reported the positive effects of the exercise-based treatment on balance, dexterity, eye movement control, and cognitive skills underlying literacy [109]. Whereas Rack et al. [110], following a commentary on the Reynolds et al. study [109, 110], have not confirmed the reported results. Overall, there were no sufficient balance training methods to improve the balance and literacy in the children with DD. Therefore, the present study has introduced a newly designed training program that has positive effects on the balance and reading-related cognitive functions in children with DD, concurrently.
This study has some limitations. Various differences in quality in the educational services may be observed between different districts in Tehran as a metropolis. Regarding the socio-economic status of participants as an inclusion criterion, study participants were recruited from the public primary schools located in District 20, Tehran, Iran. Therefore, recruitment did not include students of private schools located in this region because of possible different educational services. Moreover, the present study investigated only the short-term effects of the VWM-B program, and its long-term effectiveness needs to be followed-up in the future. We suggest investigating the effectiveness of the VWM-B program on attention using suitable measurements such as eye-tracking studies to investigate visual attention [111] and eye-movement changes, especially fixation [112] as an indicator for improving attention in DD.