Comments and ControversiesTowards a statistical test for functional connectivity dynamics
Section snippets
Towards a statistical test
Leonardi and Van De Ville's 1/fmin recommendation corresponds to the smallest window length for which the sliding window covariance between two identical sinusoids is constant across all shifts in the window position; or in other words, the smallest window length for which there are absolutely no spurious fluctuations in covariance over time (see Fig. 1A). However, from a statistical viewpoint, some level of spurious fluctuations can be tolerated, particularly in the presence of system noise.
To
Limitations of the sinusoidal model
While Leonardi and Van De Ville's sinusoidal model is analytically tractable, it does not capture the BOLD signal's 1/f spectral distribution. The 1/f characteristic implies that covariance estimates are dominated by the lowest resolvable frequency, since it is this frequency that is of greatest amplitude. Dynamics arising from higher frequency BOLD fluctuations might therefore be overshadowed when using single-resolution approaches, or when modeling the BOLD signal as a sinusoid. In this
The need for generative null models
Regardless of the choice of window length, it is important to disambiguate fluctuations in connectivity dynamics of a neural origin from spurious dynamics arising from scanner drift, head movement (Van Dijk et al., 2012), variations in the respiratory volume/rate and cardiac rate (Chang et al., 2013) and non-stationarity in the fMRI data itself due to sleep, for example (Tagliazucchi and Laufs, 2014). Our test for stationary is indifferent to the origins of connectivity dynamics: The null
Conclusion
We concur with Leonardi and Van De Ville on the importance of recognizing and eliminating spurious connectivity dynamics due to inappropriate window lengths. From a statistical viewpoint, we suggest that their 1/fmin recommendation provides a good rule of thumb, but may be overly conservative in moderate SNR conditions. We contend statistical testing and appropriate surrogate data is crucial in this respect. We also contend that if dynamic fluctuations in connectivity are confirmed with
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