In our previous report, we highlighted the usefulness of evaluating autonomic nervous function for assessing acute and chronic fatigue and depression [4–8]. In this clinical trial, we focused on the relationship between physiological brain function and autonomic nervous system activity, exploring the correlation between autonomic nervous function and learning abilities such as attention and memory.
In the KPT, participants were asked to circle Japanese vowels in a 400-character story written in Hiragana. The KPS was used to evaluate attention, representing the percentage of correct answers [8]. For the memory test, comprehension and memory were evaluated using 10 descriptive questions about the content of the story. To improve memory test results, participants needed to engage in both reading and selecting words while comprehending and memorizing the text's content. Proper allocation of brain function is essential for these activities.
Significant changes in autonomic nervous system activity were observed during the KPT. All autonomic nervous function indices significantly decreased between intervals A and B (Fig. 2), indicating significant changes in autonomic nervous system activity during this learning task.
During the pre-test resting period with eyes open, log LF, log HF, and log TP were found to be positively correlated with the KPS, which includes attention and content scores, indicating comprehension and memory abilities.
However, we did not observe any correlation between log LF/HF, which represents the balance between the sympathetic and parasympathetic nervous systems, and KPS. Instead, we observed a negative correlation between log LF/HF and content scores. These results suggest that higher autonomic nervous system activity during rest leads to increased attention; specifically, when the parasympathetic nervous system activity is higher, comprehension and memory abilities are in a heightened state.
Additionally, during the interval B, where participants were performing the KPT, the KPS showed a positive correlation with log HF and a negative correlation with log LF/HF. This suggests that even during the interval B, where autonomic nervous system activity decreases, maintaining parasympathetic nervous system activity and having a lower autonomic nerve balance lead to increased attention (Fig. 2).
To achieve high scores in memory tests, it is necessary to perform the KPT during the interval B while comprehending and memorizing the given sentences. The relationship between the results of the memory test during the interval B and autonomic nervous function showed a positive correlation with log LF, log HF, and log TP, and a negative correlation with log LF/HF, indicating that higher parasympathetic nervous system activity and lower autonomic nerve balance, represented by the LF/HF ratio, are suitable for comprehension and memorization tasks.
To accurately recall information memorized during the interval B and answer the memory test during the interval C, it is imperative for the brain functions associated with attention and memory to be actively involved. The relationship between autonomic nervous function during the interval C and memory test results showed a positive correlation with log LF, log HF, and log TP and a negative correlation with log LF/HF, similar to the interval B. In a previous study on monkeys, autonomic nervous function was examined during a simple discrimination learning task and a delayed matching task (DMTS). The study found that HF increased during the DMTS, which is a particularly difficult task [17]. Furthermore, Hansen et al. (2003) demonstrated that individuals with higher autonomic nervous system activity perform better in attention and working memory tasks among healthy adults and that parasympathetic nervous system activity specifically plays a crucial role in these cognitive processes [18]. These findings suggest that higher parasympathetic nervous system activity also benefits efficient memory recall.
Furthermore, analysis of the relationship between changes in autonomic nervous system activity and content scores revealed a significantly negative correlation between the changes in log LF and log HF between intervals B and C and the content scores. As shown in Fig. 1, log LF and log HF decreased significantly in the interval B for all participants and showed significant recovery in the interval C. However, it was also revealed that higher content scores were associated with less or decreased changes in log LF and log HF in the interval C, even when log LF and log HF increased in many participants.
The above results suggest that autonomic nervous system activity, particularly parasympathetic nervous system activity, plays a crucial role in learning activities such as attention, comprehension, memory, and recall. The use of frequency analysis for evaluating autonomic nervous system function can serve as an objective indicator of learning status.
Moreover, in the 21st-century society, where work-style reform is being pursued, even desk work such as administrative tasks require cognitive functions such as understanding, memory, and recall. To improve work efficiency, it is necessary to maintain and effectively utilize the abilities of employees. Autonomic nervous system function evaluation can also serve as an objective indicator in such situations. It is possible to use the evaluation of autonomic nervous system function during rest and changes in autonomic nervous system function during study or work as an objective indicator in efforts to improve study or work efficiency.
In the analysis of autonomic nervous function using frequency analysis, it has been widely recognized that HF reflects parasympathetic nervous activity [12, 13, 14], whereas the LF/HF ratio represents sympathetic activity [16]. However, recent criticisms have emerged suggesting that the LF/HF ratio may not accurately represent the balance between sympathetic and parasympathetic nervous system activity. Dr. Billman suggested that the LF component of HRV does not provide an index of cardiac sympathetic drive but instead reflects a complex and not easily discernible mix of sympathetic, parasympathetic, and other unidentified factors, with parasympathetic factors accounting for the largest portion of the variability in this frequency range [19]. As a result of these criticisms, clinical studies that evaluate autonomic nervous system function using frequency analysis primarily rely on the HF index, avoiding using the LF or LF/HF ratio in clinical studies.
However, we believe that the significance of LF, TP and LF/HF analysis has not been lost in frequency analysis. We have previously reported that LF, TP, and LF/HF, along with HF, are deeply involved in the state of fatigue. In this clinical trial, we confirmed that indicators such as LF, TP, and LF/HF ratio, in addition to HF, are useful in evaluating concentration and memory. Ginsberg et al. in a study on autonomic nervous system function analysis using frequency domain methods, reported that while the common usage of LF/HF ratio is not meant to simply explain the autonomic control of heart rhythm, overall, LF, HF, and LF/HF ratio are valuable indices for investigating autonomic nervous system function [20].
Recently, we conducted a study using autonomic nervous function evaluation to predict accident risks in truck drivers and to utilize it for accident prevention. We have identified the following correlations: In on-road driving, the activation of sympathetic nerve activity and inhibition of parasympathetic nerve activity increased the rear-end collision risk index in each quantile [21]. The rear-end collision risk index of the shift-day was positively correlated with the sympathetic nerve activity index of the post-shift condition on the previous day [22].
These results demonstrate that indicators such as LF, TP, and LF/HF ratio, in addition to HF, are useful for evaluating autonomic nervous system activity using frequency analysis.
We believe that such autonomic nervous system function evaluation can serve as an objective indicator for disease prevention, health maintenance, and improvement, as well as for improving learning performance and work efficiency.