Hi All,
In case anyone in the future finds this thread and is interested in the answer, I found an old thread from 2014 that addresses the difference between global correlation and ICC:

https://www.nitrc.org/forum/forum.php?th...

The former is an average and the latter is the root mean square, and the important point is that these methods handle anticorrelations differently. I refer you to the above link for more details. (The link discusses this in the context of using local correlation with a kernel size of Inf with an older version of conn, which I believe is the same as using global correlation in the current version.)

Emily