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MCFLIRT question: Granted that we use the -refvol default of the middle volume as the reference, and call that volume N. The MCFLIRT web page explains how volume N+1 is registered to N via a search for the best fit. We're not able to determine something about how N+2 (or any N+x) is registered. A simple alternative is that the search for each of the N=? volumes is relative to N. Downsides of that are that (1) it won't handle a gradual motion that may be small from volume to volume but accrues too large more distant in time from N and (2) that accrual problem will be worse for longer recording sessions. The MCFLIRT web page says that the N+1 adjustment is used as an estimate for adjusting N+2. Our question is: what is meant by "estimate"? Read literally, the web page could be understood to mean that the N+1 adjustment is applied to N+2 (i.e., no further search or optimization is done for N+2). That would mean that the only motion that can be corrected is that which occurs from N to N+1 or from N to N-1 (surely not the case). Or, "search" could mean "starting point" for an optimization for N+2. A second alternative is that each N+x+1 volume is registered by comparison to the N+x volume, with the final registration applied to N+x+1 being the cumulative or net registration of all volumes N+1 to N+x. A concrete implication of this choice is: if one sets a 2 mm criterion for maximum allowed motion, in the first alternative that 2 mm is the difference allowed between the N+x volume and the N (middle) volume, whereas in the second alternative the 2 mm is the difference allowed between adjacent volumes (thus, the change from N to N+x could be almost as large as 2*x mm). Thanks in advance.