Scaling data dramatically affects ALFF

[tsalo]

Summary

Just to be clear, this documents a problem with the ALFF metric, not with XCP-D.

ALFF is dependent on the scale of the data, so the scaling step I added in #1020 dramatically changes the ALFF outputs. Unfortunately, it's not just a change in the ALFF scale, since some testing by @joellebagautdinova shows that the ALFF maps produced from scaled data don't show any of the spatial patterns that ALFF maps produced from unscaled data do.

The spatial patterns in the unscaled (i.e., original) ALFF maps are very similar to the mean BOLD maps from the preprocessed data (and probably the intercept image from the denoising step), so ALFF itself may be problematic, but it's also clear that the scaled ALFF looks bad.

Additional details

• xcp_d version: 0.6.1 (unreleased)
• Docker version:
• Singularity version:

[tsalo]

The scaling difference is handled by #1033, but my concerns about ALFF remain unresolved.

[tsalo]

Should we do what other ALFF implementations have done and scale by the mean ALFF value in the brain?