The present study aimed to examine effects of tDCS, and hence, of the induced manipulation of DLPFC activity on driving behavior in a customary driving simulator. As a main result, we found that anodal tDCS evoked less risky driving behavior while cathodal tDCS did not significantly influence the driving style. With respect to anodal stimulation, behavioral differences were found in four variables ("driving speed", "distance to driver ahead", "number of speed violations", "revolutions per minute") measuring different aspects of driving behavior. While the "distance to driver ahead" was larger after anodal tDCS as compared to the baseline measurement, the "number of speed violations", the "driving speed" and the "revolutions per minute" were reduced. These strong behavioral changes are evident despite the fact that the participants were not aware of their change in behavior.
The crucial association between functions mediated by the prefrontal cortex and risk-taking driving behavior found in this study is in line with previous findings about the prefrontal cortex [1
]. It is remarkable that a complex behavior such as driving a car can be directly influenced by an external modulation of the cortical excitability. Our main result is consistent with earlier research focusing on the external modulation of DLPFC activity and its effects on risk-taking behavior in situations less closely related to risk-taking in everyday life [5
]. Knoch et al., for example, showed that low-frequency rTMS applied over the DLPFC evoked more risky behavioral choices in a standard gambling paradigm [5
]. The authors did not, however, study the effect of DLPFC excitation (as evoked by high-frequency rTMS) in their study, which would have induced similar effects in the stimulated tissue as anodal tDCS. Furthermore, Fecteau et al. reported a reduction of risk-taking behavior in different task paradigms following tDCS applied over the DLPFC [6
]. In their studies, the two electrodes were mounted to overly the left and right DLPFC areas – an electrode configuration that allowed the simultaneous stimulation of both hemispheres. Depending on the task, behavioral effects were evident only after right anodal/left cathodal stimulation [24
] or after both, right anodal/left cathodal and left anodal/right cathodal DLPFC stimulation [6
]. Using EEG combined with the estimation of intracerebral sources of brain activation, a recent study of our group uncovered less activation in the right-sided DLPFC during speeded and impulsive driving [25
]. This finding is in close correspondence with the study by Clark et al. [4
] reporting the same lateralization effects in patients with brain lesions in the context of risk behavior and with the later study of Fecteau et al. [24
]. The present study, however, did not replicate this lateralization effect and, thus, rather supports the earlier study of Fecteau et al. [6
]. Considering the data currently available, we have to conclude that the issue of functional DLPFC lateralization in the context of risk-taking behavior is not entirely understood – studies comparing simultaneous stimulation protocols as used by Fecteau et al. with stimulation protocols where the reference electrode is positioned on a functionally ineffective position would contribute to clarify this issue.
The propensity of risk-taking behavior has been assumed being linked to the openness to drug experiences and the general vulnerability for pathological addictive behavior. Several previous studies demonstrated that noninvasive stimulation of the frontal cortex lessens the craving for typical drugs such as nicotine [26
] or cocaine [28
], hence suppressing the need to initiate reward-related behavior. In a broader sense, this effect corresponds to a more deliberate style of behavior and is consistent with the main result of the present study.
One may argue that the lowering of the driving speed can be explained by a stimulation-induced decline of attention. However, since anodal tDCS stimulation is known to enhance neural excitability in cortical regions underlying the stimulation electrode, it seems unlikely to us that anodal stimulation reduces attentional capacities (if anodal tDCS should modulate attention we would anticipate increased and not decreased attentional functionality). It can be further speculated about general effects of boredom or tiredness after the stimulation break; however, this effect should be the same after anodal and cathodal stimulation and can therefore not explain the observed difference between stimulation conditions. In addition, the increased carefulness of driving that we observed following anodal tDCS was not only characterized by a generally slower driving speed but also by a reduced number of speed violations, by reduced revolutions per minute and by an enlarged distance to the car driving ahead. In our opinion this combination of effects points to a more careful driving style rather than to enhanced tiredness.
Cathodal stimulation did not lead to a significant alteration of driving behavior. The reasons for this lacking effect are difficult to explain, thus we can only offer more or less speculative explanations. Given that functional lateralization of the DLPFC is an unsolved issue, it may well be that the unilaterally evoked hyperpolarization is simply not strong enough to induce a clear behavioral effect. The hemisphere not stimulated may be equipped to solely prevent the individual from showing more risky behavior – a mechanism that would be reasonable from an evolutionary perspective. Alternatively, it may be argued that the mere notice of receiving electrical stimulation (perceived as a slight itching at the beginning of the stimulation) leads to more careful driving that is independent of the stimulation condition. This effect may have counteracted a potential enhancement of a risky driving style induced by cathodal stimulation and on the other hand it may have facilitated the behavioral effect of anodal tDCS.
There is one methodological limitation of the tDCS technique that should be addressed. Given the electrode size of 35 cm2
, it is obvious that the spatial resolution is low. Furthermore, when using this technique, one has to deal with remote effects. Since the brain is a heavily wired system, current spread from the stimulated region to neighboring and interconnected regions is most likely. Remote effects have been proven in studies combining transcranial brain stimulation (TMS, tDCS) with PET or fMRI [29
]. With respect to the present study, it may be that the stimulation of the DLPFC triggered a co-activation of other regions in the frontal lobe such as the ventromedial or orbitofrontal cortex, which may have influenced task performance after stimulation. Since we do not know the real extent of the tDCS effect, we cannot disentangle precisely the neurophysiological underpinnings and the associated psychological processes. Although several methodological problems of tDCS are unsolved so far, there are several studies available supporting the precision and usability of tDCS [31
]. Uy and Ridding, for example, showed a specific increase of cortical excitability for the First Dorsal Interosseous (FDI) muscle after anodal tDCS while the Abductor Digiti Minimi (ADM) and the Flexor Carpi Ulnaris (FCU) were not affected. However, it was not in the scope of this study to clarify the exact neuroanatomical source of the effect. Further studies combining tDCS with neuroimaging methods are needed to address this issue properly.
It is important to highlight the significance of the present findings for the explanation of aggressive and risky driving behavior, especially in adolescents. The stimulation of the DLPFC influenced driving behavior. Exactly this site of the brain matures late in adolescents and might be the cause of deviant driving styles. Regardless of the high behavioral complexity in this paradigm, we found striking results with high external validity and direct transferability in everyday life. Moreover, the feasibility of external manipulation of the brain, even on complex behaviors, opens different possibilities for neural rehabilitation.