I found this to be an especially well-written and comprehensive review, with most arguments presented in a thorough and informative manner. The conclusions regarding the next steps are logical and well-supported by the evidence provided. However, there are a few minor areas where the review could be strengthened.

• The descriptions of ECAS and ALS-FTD-Q and their roles in identifying non-motor deficits are clear and informative. However, could the authors expand on their discussion by considering some of the limitations of these tests? How might factors such as retention rates in longitudinal studies and the potential effects of repeat administration impact the outcomes and our understanding of cognitive and behavioral changes over time?

• The section on pathological staging of TDP-43 was especially informative. I appreciate the discussion on the associations between TDP-43 burden and cognitive impairment, particularly in the more advanced stages and specific brain regions. However, could the authors further explore the potential for cognitive and behavioural deficits to emerge before motor symptoms in some patients? This might suggest that the motor cortex is not always the initial site of pathological spread. Given the lack of clear clinicopathological correlations and the complexity of TDP-43 dissemination, it would be valuable to consider whether these early cognitive changes might indicate alternative pathways or patterns of disease progression that deviate from the traditional motor-first model (or that are TDP-43 dependent).

• Adding to the comment above, I also noted that not all ALS patients present with TDP-43 pathology (e.g., SOD1, FUS cases), yet some of these patients still experience cognitive and behavioural changes, as noted in the section on genetics. This suggests that TDP-43 pathology may not be the only, or even the direct, cause of these non-motor symptoms. Could the authors comment on other proposed mechanisms that might underlie cognitive and behavioural deficits in the absence of TDP-43 pathology or its spread?

• I understand that the section on genetics is not meant to be a comprehensive overview of the genetic causes of ALS and FTD. However, I believe it would benefit from a discussion of additional risk genes shared between ALS and FTD, including TBK1 mutations. This addition would provide a more complete picture of the genetic factors contributing to the cognitive and behavioural changes observed in these patients.

• My understanding is that cognitive impairment in PLS is less frequently reported compared to ALS, but it does occur. Studies have shown that a subset of PLS patients can exhibit cognitive deficits, particularly in executive functions, attention, and language. These deficits are usually milder and less widespread than those observed in ALS. There are also some reports of behavioural changes in PLS, though these are also less common and less severe than in ALS. Given that ALS is the predominant form of MND, it is understandably the focus of this discussion. However, I believe it would be valuable to briefly consider non-ALS MNDs, particularly those that are UMN dominant, such as PLS. The slower progression of PLS, along with its relative sparing of LMNs, could offer unique insights into the relationship between motor and cognitive decline. For instance, examining cognitive and behavioural changes in PLS may help to highlight how these deficits can develop and progress even in the absence of clear LMN involvement, supporting the notion of widespread involvement of extra-motor areas.