Given the assumption of a link between dopamine and mood or affect, we also explored whether the changes in performance were accompanied by changes in mood. This did not seem to be the case, however. For one, an (2 × 3) ANOVA of the NA score of the PANAS-S before and after the three sessions did not reveal any reliable effect, s > .3. The same ANOVA of the PA PANAS-S scores produced a significant effect of time point, F(1, 23) = 11.07, p = 0.003 (indicating a slight reduction of positive mood from 29.9 to 28.3), but this effect did not interact with auditory condition, p > .18. The analysis of the affect grid data also found no indication for condition-specific effects on average pleasure, F(2, 46) < 1, or arousal, F(2, 46) < 1 scores.
The aim of this study was to investigate whether binaural beats, a form of cognitive entrainment, affect people’s creative performance, and whether such impact might be mediated by the individual striatal dopamine level, as assessed by means of EBR. The outcome provides a straightforward picture.
First, we found no evidence for any influence of binaural beats on convergent thinking, while divergent thinking was systematically affected depending on base-line EBR. This supports the assumption that convergent thinking, and other kinds of highly constrained top-down search processes, rely more on the frontal part of the frontal-striatal interaction constituting cognitive control (in the sense of Cools and d’Esposito, 2009), while divergent thinking, and other forms of mental flexibility, lean more towards the striatal part (Akbari Chermahini and Hommel, 2010; Hommel, 2012). Moreover, the observation of a differential effect on one of the two kinds of creative performance reinforces claims that human creativity is not a unitary function but consists of multiple components (Wallas, 1926; Guilford, 1967; Nijstad et al., 2010).
Second, we could not find any difference between the Alpha and the Gamma condition—both had the same kind and the same degree of impact on divergent thinking. This suggests that binaural beats do not so much trigger or facilitate a particular neural synchronization processes but rather support neuronal phase locking in general (Kuwada et al., 1979). For instance, they might impose some temporal structure on neural processes and thereby reduce cortical noise (Karino et al., 2006), which again may make task-specific processes that rely on neural communication and/or synchronization more reliable. In which frequency this temporal structure is operating might be less relevant.
Third, our findings clearly suggest that binaural beats do not represent a suitable all-round tool for cognitive enhancement. While participants with lower EBRs (20 blinks per min or lower) showed clear beat-induced benefits in divergent thinking, binaural beats impaired the performance of individuals with higher EBRs (20 blinks per min or higher; see Figure 2). As suspected, this suggests that beat-induced cognitive enhancement depends on the individual striatal dopamine level—an observation that parallels Akbari Chermahini and Hommel’s (2012) finding of equally selective mood effects on divergent thinking.
There are at least two possible, not mutually exclusive explanations for this observation. First, there is evidence that lower-than-average EBR levels are associated with less effective performance in divergent-thinking tasks, especially regarding flexibility (Akbari Chermahini and Hommel, 2010). Even though this difference just missed the significance criterion in our study (in the control condition, the flexibility scores of the low and high EBR groups were 10.58 and 12.83, respectively, p = .08), individuals with rather low striatal dopamine levels might have more room for improvement and are, thus, more sensitive to cognitive-enhancement procedures. For instance, it might be that binaural beats induce, or increase the size of phasic dopamine bursts, which might have a stronger impact in individuals with a relatively low tonic dopamine level. Individuals with a more suitable dopamine level may not need these extra or extra-sized bursts and may end up with more than optimal cortical noise. This would also suggest that EBRs mainly reflect tonic dopamine activity in the striatum, but this lies outside the scope of the current study and, thus, remains speculation for now.
Second, it might be that binaural beats do not operate directly on the individual dopamine level, be it tonic or phasic. Note that we did not find any systematic, beat-induced mood effects. To the degree that changes in dopamine levels are accompanied by changes in mood (Akbari Chermahini and Hommel, 2012), this might suggest that binaural beats facilitated or enabled processes that compensate for the individual lack of striatal dopamine. For instance, it might be that dopamine is functional in driving neural synchronization (Schnitzler and Gross, 2005). If so, a relatively low level of striatal dopamine may thus make it more difficult to set up synchronized neural states, and this difficulty may somehow be overcome through other, compensatory processes that are induced or facilitated by binaural beats. As speculated earlier, binaural beats may increase the temporal structure of idling neural activities and thereby reduce cortical noise, which again might facilitate setting up synchronized states. Again, it is conceivable that individuals with more optimal dopamine levels do not need, or may even be impaired by this alternative way to create the necessary synchronized states.
Irrespective of which of these two scenarios will turn out to be more realistic, it is clear that binaural beats do not represent a one-size-fits-all enhancement technique. They can be effective in enhancing brainstorm-like creative thinking in individuals with low striatal dopamine levels, but they can at the same time impair performance in exactly the same kind of task in others. On the one hand, this calls for more care in the propagation of binaural beats as a cognitive-enhancement method and a better understanding of the underlying neural and cognitive mechanisms. On the other hand, however, it also implies that previous failures to find positive effects of binaural beats on cognitive performance need not be taken as evidence against the efficiency of the manipulation. In fact, careful selection of individuals involving a systematic evaluation of their cognitive control profiles is likely to yield evidence of cognitive enhancement, even under conditions that proved ineffective by previous research.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.