review output: max motion displacement calculation

Hey AFNI gurus,

I am looking at some data with two runs of a task, one right after another. They are processed together in the same call. At the moment, I am curious about how much motion happen, on average, over all runs, in addition to the max displacement. In examining the out_ss_review text file (or generated table), the average looks good but max displacement is a bit confusing.

For max displacement, it appears to be calculated from the concatenation of the motion parameters, ignoring run seperation. To me, this seems to cause an issue when the subject moves between runs. If they do that, then the max displacement reflects that shift - not max displacement during data collection.

For example, in my two runs of data the subject moves ~0.1mm during each run, but between them they shifted by 4mm, and thus, 4mm is reported as max displacement even though that did not occur while the scanner was running.

For my purposes, I am more curious about the max displacement during a run - rather than how much they shifted. Is this the intended behavior or perhaps I am misunderstanding something?


The max displacement is indeed intended to include movement between runs, as the purpose is to decide whether the subject moved enough to cause notable distortion effects.

  • rick

Good to know - the other summary metrics of motion get the center stage then! Thanks for the clarification.

For the record, ‘max censored displacement’ is one of 3 motion metrics that we are using for subject dropping criteria, along with ‘censor fraction’ and ‘average censored motion’. We recently used a command like this to generate the subject drop list:                                          \
        -outlier_sep space                                      \
        -report_outliers 'censor fraction' GE 0.15              \
        -report_outliers 'average censored motion' GE 0.1       \
        -report_outliers 'max censored displacement' GE 8       \
        -infiles sub*/s*.results/*.txt

This is part of our effort to post sets of scripts for complete analyses, from datasets to final group results. This one is not yet part of that documentation, but it is under AFNI_demos/AFNI_pamenc if you are curious. That needs some organization, too…

  • rick