The present study revealed that the susceptibility to interferences with an additional concurrent task was different between hand and tongue depending on the additional task. In short, distinct effects of an additional concurrent task between the hand and tongue were observed mostly on motor performance that required a rapid reaction or movement, whereas little distinction was apparent between the effects on the hand and tongue in motor performance that was performed slowly or steadily. Discussion will focus on these major findings.
During rapid discrete movement (Session 1), the concurrent motor task (HT) negatively influenced the speed-related motor performance, i.e., reaction time (Figure 3) and completion time (Figure 3), in both the hand and tongue. These negative influences were apparently greater in the hand than in the tongue resulting in significantly greater values during the concurrent motor task compared with the concurrent cognitive task only in the hand. The absence of a significant change in the CV of hand reaction time (Figure 3) may be due to this large increase in the mean reaction time (note CV = standard deviation divided by mean). The amount of increase in reaction time in the hand with a concurrent tongue task ( ~ ×2) was much greater than that with a concurrent motor task by a contralateral hand or foot ( ~ 10%) in the literature [20–23]. The greater influence of concurrent hand-tongue task on the speed-related motor performance in the hand than in the tongue was also supported by the significantly reduced speed only in the hand during rapid continuous movement with the concurrent motor task (Part 1 in Session 2) (Figure 3). It is likely that greater attentional resources were distributed to the tongue than hand task. The results suggested that, for rapid movement, motor performance is less susceptible in tongue than hand to the interference from the concurrent activity of the other motor modality.
The effect of the concurrent cognitive task was in contrast to that of concurrent motor task. In rapid discrete movements (Session 1), the reaction time and completion time increased with a concurrent cognitive task in both hand and tongue. However, the significant reduction in motor correctness was found only in tongue (Figure 3), and the cognitive correctness was reduced only in TC (Table 3). These results indicated that the accuracy of reaction movement tasks and cognitive tasks were more susceptible in tongue than hand to the interference due to the addition of a concurrent cognitive task. This indication in rapid movement appears to hold true whether the movement is discrete or continuous. In rapid continuous movements (Part 1 in Session 2), the concurrent cognitive task significantly reduced speed only in the tongue (Figure 4) without an influence on motor correctness (Figure 4). The absence of a significant difference in cognitive correctness between tongue-cognitive and hand-cognitive tasks (Table 2) implied the greater susceptibility in the tongue to interference from the concurrent cognitive task despite the assumed similar amount of available attentional resources in both the tongue and hand. These results suggested that rapid motor performance is more susceptible in the tongue than in the hand to the interference from a concurrent cognitive task.
The current findings highlight the influence of hand and tongue interaction for performing concurrent tasks. Experimental conditions were set for the confounds of a laboratory setting to provide a baseline for this line of research. Potential extrapolations can be made to individuals who might need assistive technology to perform tasks. The findings of decreased task performance during concurrent tasks for motor-motor and motor-cognitive tasks of the hand and walking [16–18, 24] are further extended to the tongue and hand in the present study.
Whereas no report can be found in the literature on goal-directed motor performance of the tongue with respect to the interference from a concurrent task, comparisons to available studies on the neighboring lip motor performance during speech may provide some insights. The greater influence from a concurrent cognitive than a hand task to the speed of tongue movement was contrary to the previous findings on lip movement [25, 26]. In both previous studies, velocity of lip movement during speech was reduced with a concurrent hand but not cognitive task. In these studies, the hand task was to manipulate nuts, bolts, and washers without time constraints  or to move a mouse and click on a moving object on the computer monitor as often as possible . The cognitive task was to count backwards from 100 by sevens  or to solve two-digit math subtraction problems . The current hand and cognitive tasks are practically similar to those in  rather than . Although the potential influence of differences in task details cannot be ruled out, the current findings implied that the susceptibility to interference from concurrent hand and cognitive tasks during rapid movement may be different between the tongue and lip despite of their neighboring existence.
For slow or steady movement, the current study found that an additional task influences the hand and tongue similarly with regard to the presence of interference. This is in contrast to the rapid movement described above. When subjects performed a slow steady movement (Session 2, Part 2), a significant effect of an additional concurrent task was found in the same comparison pairs across hand and tongue. In both the hand and tongue, variability and accuracy of displacement increased with an additional motor task using the other motor modality (HT in Figure 6). With an additional cognitive task, the unvarying variability and accuracy of displacement in the slow or steady movement was in contrast to degraded motor performance in rapid movements. In slow finger force control, no or little interference from a concurrent cognitive task has been demonstrated [17, 27]. The current results extended this finding in the hand to the tongue and indicated that strategies for performing the slow waveform tracking were less susceptible to interference from an additional cognitive task compared with those for rapid movements in both the hand and tongue. Degradation of task performance with an additional concurrent task occurs most likely because the attentional resources allocated to one or both tasks is reduced when the attentional resources needed to perform two tasks concurrently exceeds the total capacity . Indeed, the absence of a significant increase in variability and accuracy with the concurrent cognitive task (HC or TC) accompanied a reduced cognitive correctness in the tongue-cognitive task compared with the hand-cognitive task. These findings implied that a greater amount of attentional resources were naturally allocated to the tongue task than the hand task.
In general, motor performance was degraded with increased level of task difficulty in both hand and tongue with a couple of exceptions. Tongue speed was faster in Level 2 (i.e., continuous horizontal displacement) than Level 1 (i.e., continuous protrusion and retraction) in rapid continuous (Table 4; Session 2, Part 1) but not discrete (Table 2; Session 1) movement. Considering the reduced tongue correctness at Level 2 compared with Level 1 during rapid continuous movement (Table 4 the results suggested that the tongue can move more rapidly in the horizontal direction but the accuracy is compromised compared with the protrusion/retraction direction during continuous movement. Further studies are warranted to examine the potential factors that may influence these differences in tongue movement speed (e.g., speed of retraction, speed of change in direction) depending on the direction and continuousness of the movement. The greater CV of time interval at Level 1 compared with Levels 2 and 3 (Figure 5) would be due to the absence of a physical stopper (tooth or key) in Level 1. The increased CV of time interval during the concurrent motor task compared with other tasks at Level 3 implied that this task was challenging for both hand and tongue.
The current study focused on comparing the presence of significant main effects of an additional concurrent task on hand and tongue motor performance. Statistical comparison of the absolute values of the dependent variables between the hand and tongue performance was not conducted because the measurement technique and the task details were not regarded comparable enough to allow for the standard statistical comparisons in a strict sense. Nonetheless, every effort was provided to make the measurement technique and task as similar as possible within the constrained technical and practical allowance. Despite these constraints, the current study revealed novel and important uniqueness of dual task performance that involves tongue motor control: the susceptibility to interference from a concurrent task is different between hand and tongue depending on the type of a concurrent task (i.e. motor or cognitive). In practice, it would be advisable that the task in which rapid reaction is critical should be allocated to the tongue in a situation where both tongue and hand are concurrently used. In a situation with concurrent allocation of rapid motor tasks between the tongue and hand, the task in which the reaction time is crucial should be allocated to the tongue rather than the hand. In a situation with concurrent cognitive and rapid motor tasks, that kind of task should be allocated to the hand rather than the tongue. This way, the findings would help the potential users to practically decide the allocation of TDS depending on their specific needs and conditions that might involve dual task.
Tongue movement is controlled with coordinated activation of bilateral muscles that are directly innervated from the brain stem with the cranial nerves (hypoglossal and vagus nerves). The finger movement is controlled with unilateral forearm and hand muscles that are innervated with the ulnar, median, and radial nerves through the spinal cord. The shorter length of the nerves from the brain stem to the tongue muscles compared with the hand and forearm muscles may in part contribute to the smaller susceptibility to dual-task interference in the tongue than hand during the reaction task and the rapid motor task. Compared with finger movement, even simple tongue movement requires a greater engagement of larger area and volume of brain structures, including greater activation of the bilateral postcentral gyrus, supplementary motor area, and anterior cingulate cortex . Supplementary motor area and anterior cingulated cortex are involved in the planning, initiation, and execution of movement as well as the allocation of attention [30–33]. The greater effect of a concurrent tongue than hand task on cognitive correctness may be related to the greater engagement of these brain structures during tongue movement.