We examined PFC activation by measuring changes in the concentration of Oxy-Hb levels in the PFC using NIRS during a dual-task condition and compared this with measured activation during two different single-tasks (calculation and stepping). The results demonstrated that: 1) Oxy-Hb values were significantly increased during both the calculation single-task and the combined dual-task; 2) during the dual-task, the older adults demonstrated significantly higher Oxy-Hb values than the younger adults, and 3) the increased activation (higher Oxy-Hb values) associated with dual-task performance in the older participants persisted 10 seconds beyond the completion of the task itself. Also, there was a significant negative correlation between Oxy-Hb values during the dual-task and TMT-B scores. There was also a significant negative correlation between the proportion of correct calculations made during the dual-task and TMT-B scores.
Increasing Oxy-Hb values has been demonstrated to be related to increases in cerebral blood volume in response to neuronal activation . Measured Oxy-Hb was significantly increased during both the dual-task and the calculation single-task compared to the stepping single-task. As expected, increased Oxy-Hb was not observed in the PFC during the stepping single-task because this is a simple motor task which does not require activation in the PFC. It is possible that the stepping single-task might increase Oxy-Hb in motor-related areas of the brain. However, this was not measured during our study. Both the calculation single-task and the combined dual-task require PCF activation and our findings support the results reported in previous studies [21, 22, 43, 44]. It has been suggested that actively increasing PFC activation in aging individuals might help to prevent or delay age-related changes to the brain [10, 13]. Therefore, a regular schedule of dual-task training for older individuals might mitigate the age-related decline of cognitive function often observed in older adults.
Time series measurements of Oxy-Hb values in both the stepping and calculating single-tasks demonstrated no significant difference between younger and older participants. This suggests that PFC activities in response to either single-task did not differ across age groups. However, significantly different temporal patterns of Oxy-Hb values were observed between the two age groups when performing the combined dual-task. In the older group, Oxy-Hb levels gradually increased and this increase was sustained until the completion of the dual-task. Among younger participants, Oxy-Hb levels increased rapidly at the onset of the dual-task and decreased substantially prior to task completion. Reuter-Lorenz and Cappell  have demonstrated overactivation in lateral and inferior PFC during higher levels of task demand in older individuals. Our results support previous reports that dual-task activity is associated with increased brain activation due to higher cognitive load in older participants. The results of this study, which indicated that older participants had higher PFC activation during the dual-task period, correspond with those reported by Reuter-Lorenz and Cappell  and suggest that performing a dual-task might impose a higher cognitive load in the older individuals compared with the younger participants. During dual-task performance, the number of steps counted by older participants was significantly decreased compared to the number of steps counted during performance of the stepping single-task. However, among older subjects, neither the number of calculations, nor the proportion performed correctly differed significantly during the dual-task from those observed during single-task performance. From our observations during the dual-task, it appears that older participants may turn their attention to the execution of the calculation task at the expense of the stepping task while young subjects are able to maintain appropriate attention to both calculation and stepping. This finding suggests that older participants required increased PFC activation in order to maintain attention to both the stepping and calculating tasks. It is possible that this requirement for increased PFC activation is related to decreased brain function associated with aging.
The comparison of single-task performance between young and older participant demonstrated a significant different in the proportion of calculations performed correctly, but no significant different in the total number of calculations performed or in the total number of steps. Also, there was no significant difference in task-related Oxy-Hb levels across age groups for each single-task (calculation and stepping). These results suggest that the calculation task was more difficult for the older participants compared with the younger group even though PFC activation was not significantly different across the two age groups during the calculation task. Schroeter  indicated that aging is generally associated with decreases in the hemodynamic response within the frontal association cortex during cognitive tasks. However, Reuter-Lorenz and Cappell  suggested older adult demonstrated overactivation in PFC during a particularly challenging cognitive task. Our results are in line with those reported by Reuter-Lorenz and Cappell , in that older participants had increasing PFC activation during the higher cognitive load imposed by the dual-task. It also seems likely that the calculating/stepping dual-task used in this study was not sufficiently challenging to be affected by the decreased activation observed in older adults exposed to highly complex tasks observed by Callicott et al. .
We also analyzed the relationship between brain activity during dual-task performance and participant executive function and attentional function, as measured by the TMT-B. Oxy-Hb values were negatively correlated with the number of errors in TMT-B. Previous studies suggested that PFC volume decreases with age  and that cerebral blood volume decreases with dementia . Therefore, the relationship we observed between lower PFC Oxy-Hb values and a greater number of errors on the TMT-B test may reflect a decrease in brain function due to aging-related cognitive decline or incipient dementia. Moreover, our result demonstrated a significant negative correlation between dual-task performance and TMT-B scores among the older study participants. Montero-Odasso et al.  have demonstrated that dual-task performance correlates with TMT-B performance. Successful dual-task performance requires attentional function and working memory. Decreases in these functions have been associated with decreased PFC activation in older adults.
There were some limitations in this study. First, because of limitations imposed by the measurement environment, this study used stepping in a chair as a physical activity task. Second, previous study has shown that elderly people with dementia demonstrate reduced brain activation during a cognitive task compared with individuals without dementia . All participants in this study, both older and younger, were cognitively intact so it is not possible to extrapolate these results to populations of older adults with pre-existing changes in cognitive ability. Finally, while our study might suggest that structured dual-task activity facilitates PFC activation in older adults, future study will need to clarify whether dual-task training can lead to facilitated PFC activation in older adults enabling them to maintain cognitive function.