Reviewer #1 (Public Review):
King et al. provide an interesting reanalysis of existing fMRI data with a novel functional connectivity modeling approach. Three connectivity models accounting for the relationship between cortical and cerebellar regions are compared, each representing a hypothesis. Evidence is presented that - contrary to a prominent theoretical account in the literature - cortical connectivity converges on cerebellar regions, such that the cerebellum likely integrates information from the cortex (rather than forming parallel loops with the cortex). If true, this would have large implications for understanding the likely computational role of the cerebellum in influencing cortical functions. Further, this paper provides a unique and potentially groundbreaking set of methods for testing alternate connectivity hypotheses in the human brain. However, it appears that insufficient details were provided to properly evaluate these methods and their implications, as described below.
Strengths:<br />
• Use of a large task battery performed by every participant, increasing confidence in the generality of the results across a variety of cognitive functions.<br />
• Multiple regression was used to reduce the chance of confounding (false connections driven by a third region) in the functional connectivity estimates.<br />
• A focus on the function and connectivity of the cerebellum is important, given that it is clearly essential for a wide variety of cognitive processes but is studied much less often than the cortex.<br />
• The focus on clear connectivity-based hypotheses and clear descriptions of what would be expected in the results if different hypotheses were true.<br />
• Generalization of models to a completely held-out dataset further increases confidence in the generalizability of the models.
Concerns:<br />
• The main conclusion of the paper (including in the title) involves a directional inference, and yet it is notoriously difficult to make directional inferences with fMRI. The term "input" into the cerebellum is repeatedly used to describe the prediction of cerebellar activity based on cortical activity, and yet the cerebellum is known to form loops with the cortex. With the slow temporal resolution of fMRI it is typically unclear what is the "input" versus the "output" in the kinds of predictions used in the present study. Critically, this may mean that a cerebellar region could receive input from a single cortical region (i.e., the alternate hypothesis supposedly ruled out by the present study), then output to multiple cortical regions, likely resulting (using the fMRI-based approach used here) in a faulty inference that convergent signals from cortex drove the results. On pg. 4 it is stated: "We chose this direction of prediction, as the cerebellar BOLD signal overwhelmingly reflects mossy-fiber input, with minimal contribution from cerebellar output neurons, the Purkinje cells (Mathiesen et al., 2000; Thomsen et al., 2004)." First, it would be good to know how certain this is in 2022, given the older references and ongoing progress in understanding the relationship between neuronal activity and the BOLD signal (e.g., Drew 2019). Second, given that it's likely that activity in the mossy-fiber inputs has an impact on Purkinje cell outputs, and that some cortical activity supposedly reflects cerebellar output, it is possible that FC could also reflect the opposite direction (cerebellumcortex). It would seem important to consider these possibilities in the interpretation of the results.<br />
• It would be helpful to have more details included in the "Connectivity Models" sub-section of the Methods section. The GLM-based connectivity approach is highly non-standard, such that more details on the logic behind it and any validation of the approach would be helpful. More specifically, it would be helpful to have clarity on how this form of functional connectivity relates to more standard forms, such as Pearson correlation and perhaps less standard multiple regression (or partial correlation) approaches. If I understand this approach correctly, each cortical parcel's time series is modulated (up or down) using that parcel's task-evoked beta weights, then "normalized" by the standard deviation of that parcel's time series, with the resulting time series then used in a multiple regression model to explain variance in a given cerebellar voxel's time series. It would be helpful if each of these steps were better explained and justified. For example, it is unclear what modulation of the cortical parcel time series by task-related beta weights does to the functional connectivity estimates, and thus how they should be interpreted.<br />
• It appears that task-related functional connectivity is used in the present study, and yet the potential for task-evoked activations to distort such connectivity estimates does not appear to be accounted for (Norman-Haignere et al. 2012; Cole et al. 2019). For example, voxel A may respond to just the left hemifield of visual space while voxel B may respond to just the right hemifield of visual space, yet their correlation will be inflated due to task-evoked activity for any centrally presented visual stimuli. There are multiple methods for accounting for the confounding effect of task-evoked activations, none of which appear to be applied here. For example, the following publications include some options for reducing this confounding bias: (Cole et al. 2019; Norman-Haignere et al. 2012; Ito et al. 2020; Rissman, Gazzaley, and D'Esposito 2004; Al-Aidroos, Said, and Turk-Browne 2012). If this concern does not apply in the current context it would be important to explain/show why.<br />
• It is stated (pg. 21): "To reduce the influence of these noise correlations, we used a "crossed" approach to train the models: The cerebellar time series for the first session was predicted by the cortical time series from the second session, and vice-versa (see Figure 1). This procedure effectively negates the influence of noise processes, given that noise processes are uncorrelated across sessions." However, this does not appear to be strictly true, given that the task design (parts of which repeat across sessions) could interact with sources of noise. For example, task instruction cues (regardless of the specific task) likely increase arousal, which likely increases breathing and heart rates known to impact global fMRI BOLD signals. The current approach likely reduces the impact of noise relative to other approaches, but such strong certainty that noise processes are uncorrelated across sessions appears to be unwarranted.<br />
• It appears possible that the sparse cerebellar model does worse simply because there are fewer predictors than the alternate models. It would be helpful to verify that the methods used, such as cross-validation, rule out (or at least reduce the chance) that this result is a trivial consequence of just having a different number of predictors across the tested models. It appears that the "model recovery" simulations may rule this out, but it is unclear how these simulations were conducted. Additional details in the Methods section would be important for evaluating this portion of the study.
References:
Al-Aidroos, Naseem, Christopher P. Said, and Nicholas B. Turk-Browne. 2012. "Top-down Attention Switches Coupling between Low-Level and High-Level Areas of Human Visual Cortex." Proceedings of the National Academy of Sciences of the United States of America 109 (36): 14675-80.<br />
Cole, Michael W., Takuya Ito, Douglas Schultz, Ravi Mill, Richard Chen, and Carrisa Cocuzza. 2019. "Task Activations Produce Spurious but Systematic Inflation of Task Functional Connectivity Estimates." NeuroImage 189 (April): 1-18.<br />
Drew, Patrick J. 2019. "Vascular and Neural Basis of the BOLD Signal." Current Opinion in Neurobiology 58 (October): 61-69.<br />
Ito, Takuya, Scott L. Brincat, Markus Siegel, Ravi D. Mill, Biyu J. He, Earl K. Miller, Horacio G. Rotstein, and Michael W. Cole. 2020. "Task-Evoked Activity Quenches Neural Correlations and Variability in Large-Scale Brain Systems." PLoS Computational Biology. https://doi.org/10.1101/560730.<br />
Norman-Haignere, S. V., G. McCarthy, M. M. Chun, and N. B. Turk-Browne. 2012. "Category-Selective Background Connectivity in Ventral Visual Cortex." Cerebral Cortex 22 (2): 391-402.<br />
Rissman, Jesse, Adam Gazzaley, and Mark D'Esposito. 2004. "Measuring Functional Connectivity during Distinct Stages of a Cognitive Task." NeuroImage 23 (2): 752-63.