I am happy with the revised manuscript. Thank you for addressing the comments.

The authors have responded very thoroughly to my comments and I am happy to recommend IPA. Good luck!

The current research addresses the gap in understanding the causal role of dopamine in mind wandering. The authors aim to utilize tDCS to modulate dopamine levels. The research plan and hypotheses presented are well crafted. The chosen behavioral paradigm and sample size align with the researchers' previous work on mind wandering and tDCS. The criteria for significant results are clearly stated.

I have two comments regarding the required sample size for specific hypotheses:

1. If the primary focus of the research is the interaction between tDCS and dopamine on mind wandering, it might be worth considering omitting the manipulation of tDCS dosage and instead increasing the sample size to 60 per group. A recognized issue in tDCS research is the lack of replicability. Improving statistical power through a simpler design and a larger sample size could potentially enhance result stability.
2. The interpretation of null results for the hypothesis "Are relevant individual traits balanced between conditions?" should refrain from concluding the absence of an impact of personal traits in the sample. The sample size per group might be insufficient to explore relationships between traits and mind wandering adequately. Research on clinical samples has demonstrated the prevalence of ADHD in mind wandering among children (Frick et al., 2019, doi: 10.1111/bjc.12241). Previous studies on ADHD-like traits in community samples have involved >100 subjects to yield relevant results (Vatansever et al., 2018, doi: 10.1017/S0033291718003598; Franklin et al., 2017, doi: 10.1177/1087054714543494). I recommend revising the implications of the null results in this context.

I think this is an interesting and important study that is on a cool topic. I have never reviewed a registered report before so hopefully this will be what is expected. 

My main concern with this study is that it focuses on a single task. Multiple studies have shown that patterns of ongoing thought that are unrelated to the external envioronment vary in terms of their occurence in terms of how hard the external task was. In work from my lab, we have found that in a simple task the occurence of off task states are much more prevalent than in a more demanding task (e.g. Konisih et al., 2017, Cognition; Turnbull et al., 2018, Neuroimage; Turnbull et al., 2019, Nature Communications) and there are countless other examples in the literature. In this context, cognitive control plays an important role in regulating the tendency for people their thoughts in line with the task ( a process we call context regulation of thought). In the brain this is linked to the ventral attention system (As defined by Yeo and colleagues, 2011) and in particular the left dorso lateral prefrontal cortex. In our work we found that dLPFC suppress off task thought when tasks are hard but facilitates it when they are easier (Turnbull et al., 2019).

My concern with the proposed study is that with a task with only one level of difficulty, it will be very hard to properly characterise the role that dopamine plays in regulating thought, especially given the u-shaped relationships that the authors argues underpin the neurpharmaological effects. A simple remedy would be to vary the demands of the task in which spontaneous thought happens, in particular using one task in which the level of dynamic thought is equivalent to what is seen under no task condition, and a second where the experience is reduced. If the study used this design then it would mean that the possibility that dopamine can facilitate both dynamic thought and task focused thought can be seperated (i.e. it should facilitate dynamic thought in the simple task and suppress it in the harder task). If this is not possible, then it would need to be the case that the authors can explicitly caveat the effect, however, it would be a shame to not take advantage of this simple design change to address such a cool and important question.

Signed

Jonathan Smallwood