The purpose of this study was to determine the extent to which PFC activity, as measured by the NIRS, reflects the cognitive and physical demands of smartphone use during walking. In the present study, the degree of PFC activation during smartphone use while walking did not differ between young and older adults. However, our results suggest that the influence of PFC activation on cognitive and physical dual-task costs differed across age groups. In young adults, the left PFC inhibited inappropriate action and the right PFC stabilized gait performance during smartphone use while walking. In older adults, a less-lateralized PFC activity pattern suppressed the deterioration of gait performance, but this PFC activation resulted in impairment of the cognitive task. These results suggest that lateralization of motor and cognitive tasks aids in efficient task completion during a complex action such as using a smartphone while walking.

Dual-task cost differs across age groups

Our study showed that cognitive task performance during smartphone use while walking was impaired in older not but young adults. Smartphone task prolonged step time in the older group and decreased acceleration magnitude in both age groups. In addition to basic differences in physical and cognitive abilities, these findings might result from lower visual ability and/or unfamiliarity with smartphone use in older adults. Moreover, we found that the dual-task costs for physical and cognitive performance were more severe in the older group than in the young-adult group. There is clear evidence of dual-task cost, indicating that cognitive-motor interference can cause deterioration of one or both tasks [5, 6]. A far more consistent pattern of results can be found in older adults, where it is commonly reported that there is a reduction in walking speed and cognitive performance under dual-task walking. In line with these data, our study showed that smartphone use while walking prolonged step time and reduced acceleration magnitude while impairing cognitive performance in older subjects. These results are consistent with previous reports showing that gait stability was prioritized in older adults with low postural control ability during dual-task walking [5, 10]. In addition to the effect of postural control ability, cognitive function, in particular, attention and executive function, has been linked to dual-task cost [5, 9]. It is generally considered that overall cognitive function decreases with aging, especially executive functions such as monitoring and attention [17, 18]. Several studies have reported that lower measures of executive function are associated with increasing deterioration of gait performance during dual-task walking [9, 35, 36]. Therefore, it is likely that our observed increase in dual-task cost in older adults resulted from the decline of executive function in this group.

On the other hand, young adults could play a smartphone game while walking without deterioration of cognitive performance, although they reduced their magnitude of acceleration. These results indicate that the young adults allocated more attention to the cognitive task compared with older adults, but that they could also pay attention to gait stability. Recent work has proposed that cognitive tasks may be prioritized depending on postural control ability, self-awareness, and task complexity [8]. In this model, healthy individuals who have sufficient postural control can elect to prioritize a cognitive task over gait stability during dual tasks. Our data from smartphone use while walking partly support this hypothesis; however, young adults could appropriately allocate attention to two simultaneously performed tasks.

PFC activity reflects cognitive and physical demands in smartphone use while walking

Previous studies have shown that a dual task increased oxy-Hb levels in PFC regions relative to a single task, but it remains controversial whether the PFC is more strongly activated in young or older adults during the performance of dual tasks [13, 19]. In the present study, time series measurements of oxy-Hb values during dual task performance were not significantly different between young and older adults. This discrepancy may be related to differences between previous studies and the present study in task design and methods of analysis of the NIRS data. On the other hand, we found that the influence of PFC activation on dual-task cost during smartphone use while walking differed across age groups. In young adults, left PFC activation reduced dual-task cost on the rate of mistake responses, and right PFC activation increased dual-task cost on acceleration magnitude during smartphone use while walking. These results suggest that left PFC activity may inhibit inappropriate action and right PFC activity be involved in a more conservative basic gait pattern for gait stability involving a reduced magnitude of acceleration. A previous study suggested that sufficient postural control ability and self-awareness allow healthy participants initially to allocate more attention toward the cognitive task than toward gait stability [8]. However, considering the correlation between PFC activation and dual-task cost, young adults might divide their attention between the cognitive task and gait stability equally.

In contrast, PFC activation in older adults correlated only with dual-task cost on gait, without effects on cognitive performance. In aging states, deterioration of postural control may cause alterations in balance and postural responses [37]. As a result, it becomes difficult to devote the same attention to cognitive performance. In fact, our results showed that older adults exhibited the more conservative gait pattern, reducing their walking speed and acceleration magnitude that was concomitant with impaired cognitive performance during smartphone use while walking. Therefore, we predicted that PFC activation would induce a conservative basic gait pattern as we had observed in young adults. However, in contrast to our expectation, PFC activation negatively correlated with dual-task cost on gait performance in healthy older adults. These results indicated that PFC activation might suppress the dual-task cost on gait performance to prioritize physical demand during smartphone use while walking. Of course, older adults might have felt difficulty in reducing walking speed and acceleration magnitude during smartphone game playing while walking, because they were less familiar with smartphones than young adults were. However, these differences in the correlation of PFC activity with gait function between young and older adults might also be explained from the monitoring system point of view. A previous paper suggests that the PFC contributes to monitoring of self-performance [38]. Therefore, the PFC might have to strongly activate to monitor gait performance and properly judge risk when dual-task cost is small in older adults. This may represent a compensatory mechanism by the PFC to ensure gait stability. As a result, the PFC in our older group had no capacity to cope with cognitive function, unlike in the young adults. In summary, the right PFC in young adults induced a dual-task cost that compensated for gait stability, but the left and middle PFCs in older adults suppressed the dual-task cost on gait performance and/or monitored gait instability during smartphone game playing while walking.

As described above, the roles of the individual sides of PFC in cognitive and physical tasks were clearly different in young adults. These results might be consistent with prior findings of PFC lateralization in executive function [39–41]. In general, lateralization is thought to allow each hemisphere to process information without interference by the contralateral hemisphere [42, 43]. Several studies have suggested that the speed of transcallosal conduction is limited in larger brains, which implies that the transfer and integration of information between the hemispheres through the corpus callosum require more time and energy in humans [44, 45]. Therefore, it may be more efficient for smartphone use while walking that each hemisphere works independently. In contrast, the hemispheric asymmetry reduction in older adults (HAROLD) model has been generally proposed as a theory of neural compensation for declining cognitive function during aging [46]. The HAROLD model posits that as age reduces the capacity for neuronal processing in each hemisphere, the hemispheres are required to work together bilaterally to solve a given task. In the present study, the dual-task cost of gait performance in older adults correlated with left and middle PFC activation, but not with right PFC activation. Thus, these activation patterns are not a typical HAROLD phenomenon, but are consistent with this concept, as the PFC might be required to activate more widely to focus on gait performance during smartphone use while walking in older adults.

Limitations and future studies

Unlike in young adults, PFC activity in healthy older adults was unable to sufficiently prolong step time and reduce acceleration magnitude to prioritize gait stability. Although this difference might result from less habituation to smartphone use as discussed above, this failure to enforce a posture-first strategy by the PFC might contribute to the fundamental factor that some older adults and patients with neurological disease inappropriately use a posture-second strategy [8, 47]. In this model, individuals are unable to properly judge the risk of their actions and inadvertently exacerbate their fall risk in dual-task situations. Therefore, elucidating the association between PFC activation and dual-task walking among patients with neurological diseases such as stroke and Parkinson’s disease may reveal the mechanism of posture-second strategy and fall risk. There is growing evidence that dual-task training improves executive function and the ability to divide attention more effectively than either physical or cognitive training alone [48, 49]. Further study is required to elucidate a more detailed mechanism of prioritization, to reveal whether change in PFC activity after dual-task training will influence dual-task cost. Furthermore, dual-task training might lead to better dual tasking by ameliorating the reduced lateralization of PFC activity seen in older adults.

There are several limitations of this study to be considered when interpreting these results. First, it is important to address potential confounds related to cognitive-motor interference and NIRS activation resulting from our design using a smartphone. In order to exclude the possibility that the gait pattern was altered because of decreased availability of visual information about the walker’s surroundings rather than because of increased cognitive demands, participants were instructed to keep their faces turned the smartphone screen while both walking alone and dual-task walking. However, this instruction itself might give priority to the cognitive over the physical demand. Therefore, to investigate a more precise mechanism for prioritization during smartphone use while walking, we should compare the data under dual-task instructions such as “focus on smartphone use,” “focus on walking,” and “focus on either task over the other by free decision.” Dual tasks using other cognitive tasks, such as verbal fluency or calculation, should also be used to clarify the cognitive and physical prioritization in the absence of visual instruction. Additionally, the lack of adjustment of the smartphone task difficulty for each subject may have caused the high level of variability in NIRS data seen here; this may have impaired our ability to uncover differences in NIRS data with regard to age and site.

Another limitation of the interpretation of these results is issues with evaluation parameters. This study did not assess executive function directly. We evaluated step time and acceleration magnitude as our gait measures, not but gait stability per se. Furthermore, we evaluated only the PFC activity using the wearable NIRS. Future studies will be needed for a more detailed evaluation of gait parameters and activity in other brain regions using multi-channel NIRS to understand the mechanism of prioritization between cognitive and physical demands during smartphone use while walking. Moreover, we did not directly compare NIRS data of smartphone use while walking with that of single tasks such as smartphone use or walking alone. In the future study, we should set sufficient intervals of walking alone between dual-task conditions for excluding pre- and post-activation factors in the analysis of the single-task condition. For precisely comparing NIRS data between single-task and dual-task conditions, it also might be desirable to design the study protocol to treat each condition (smartphone use alone, walking alone, and dual task) as a task and standing as a control.

Lastly, aging-related changes in functional hemodynamics might not be associated with changes in neural processing per se, but could rather be a consequence of neurodegeneration and cortical atrophy with aging, affecting NIRS sensitivity. The path length of near infrared light and the NIRS sensitivity are dependent on the scalp-to-cortex distance [32, 33]. To circumvent these issues, the NIRS data from each channel of each participant were normalized by linear transformation. However, future studies should address the impact of anatomical differences due to cortical atrophy, frontal sinus and skull thickness by using imaging [32, 33]. Moreover, it is necessary to monitor extra cortical physiological response such as blood pressure, heart rate and skin blood flow, which influence the NIRS measurements [50, 51].