Initially, we speculated that the volumes of different GM regions are positively correlated with the scores of different DSB categories because brain atrophy causes both mental and physical declines resulting in poor DSBs [22–24]. Nevertheless, this study revealed a regularly positive or negative correlation of each GM region with DSBs without exception. Ten of 56 GM regions were positively correlated with DSBs, while 8 GM regions were negatively correlated (Table 4). This finding suggests the existence of a brain mechanism for DSBs in which two GM regions with positive or negative correlations are uniformly organized regardless of the categories. If the positive or negative volumetric reactions reflect the hyperactivity or hypoactivity of neuronal functions between GM regions, the neuronal network for DSBs may contain the interaction between positive and negative responses in the complex space of the brain, in other words, a complex of two opposite directions, i.e., dual mechanics such as Yin and Yang. In the next step, a functional MRI (fMRI) study is planned to elucidate the neuronal connectivity regulating DSBs [25].
Considering the overall results, the DSB score of the female participants was lower than those of the male participants. Further verification using large samples is necessary because of the established gender difference in brain structures [26].
The number of significantly correlated GM regions varies by DSB categories: 11 GM regions for speeding (DSB2), 8 for visual search behavior (DSB1), 4 for steering (DSB6), 3 for signaling (DSB3), 1 for positioning (DSB5), and none for vehicle stability (DSB4). Thus, positioning and vehicle stability of DSBs, which are naturally essential for safe driving, were extremely low in correlation numbers. In this study, a participant drives a car with an official instructor sitting next to him/her, and the nervous situation may have largely affected DSB5 and DSB4 compared with other categories of DSBs. Most participants carefully drove approximately 20 km/h (data not shown). This slow speed or careful driving behaviors may especially correlate with none or only one of the GM regions for DSB4 or DSB5, respectively. The advantage of this study is the investigation using six categories of DSBs when driving an actual motor vehicle on a closed circuit. However, the experimental condition is largely different from free driving in residential areas. This difference may have degaussed the variation in the evaluation of DSBs. Further validation with free driving in residential areas is necessary.
The number of significantly correlated DSB categories varies by GM regions: 5 categories for the left postcentral gyrus, 2 for the right angular and parahippocampal gyrus and the left temporal, angular, and supramarginal gyrus, and 1 for 12 GM regions. Thus, the left postcentral gyrus may play an important role in the sufficient enforcement of DSBs because of the higher frequency although it was negatively correlated with DSBs and belongs to the somatosensory cortex unrelated to motor function [27]. Nevertheless, two previous studies have shown completely different results: the study by the Toyota Central Institute reported the supplemental motor area [16] and another by Keio University revealed the four GM regions, namely, the left superior part of the precentral sulcus, left sulcus intermedius primus, right orbital part of the inferior frontal gyrus, and right superior frontal sulcus [17]. Thus far, no studies except for the two are targeting the relationship between regional GM volumes and driving performances. Therefore, research outcomes should be evaluated carefully because the correlated brain regions may largely change depending on the experimental conditions, kinds of motor vehicles, driving on a closed-circuit course or free driving, driving locations, and methods of DSB evaluation.
The correlated GM regions with right-left symmetry were temporal, middle frontal, precentral, angular, and supramarginal gyrus (Table 4). Interestingly, right-left symmetry has a combination of positive and negative responses, that is, the middle frontal, precentral, and angular gyrus were positively or negatively correlated in the right or left cerebral hemisphere, respectively. On the other hand, the temporal and supramarginal gyrus were positively or negatively correlated in the left or right cerebral hemispheres, respectively. The results indicated that associated fibers between the right and left hemispheres may regulate the dual mechanics in brain functions for DSBs
Non-invasive brain-imaging techniques such as near-infrared spectroscopy (NIRS) and electroencephalography (EEG) can allow measurements of brain activations within actual motor vehicles but remain challenging in the reduction of motion and electric noises [28, 29]. In this study, volumetric data obtained by MRI were used for analyses. Recently, an fMRI study showed that sensorimotor areas increase their activities after changing the direction of a virtual car on a monitor on the bed of an MRI scanner [30]. If fMRI data can be additionally utilized, it will be possible to reproducibly identify brain regions involved in DSBs from both structural and functional data of the brain although MRI cannot measure in real-time on roads. Older drivers who may drive dangerously could be identified with MRI in advance when renewing their driving license soon. Brain atrophy is largely dependent on lifestyles, smoking, drinking alcohol, less exercise, and lack of sleep [31]. The improvement of lifestyles may not only affect the whole brain but also the cerebral regions and lead to the upregulation of driving performances.
The results of this study should be interpreted with caution because of several limitations. First, the number of participants may be relatively small. However, the number of participants in the two previous studies was even smaller: one study included 39 older participants [16], and the other enrolled 32 [17]. Second, the feasibility of the research design involving both actual driving evaluation and MRI examination may be difficult. Third, this study was conducted on a closed-circuit course under the supervision of an instructor. The condition must affect DSBs in comparison with free driving on general roads. Finally, all participants were over 70 years old. The relationship between the brain and driving performance may be universal at all ages. To validate the results, we would like to evaluate DSBs through MRI examinations for middle-aged and young drivers.