Hello again! I apologize for the barrage of novice questions about ICA. 

I see from re-reading posts about group analyses that I should apparently 
either be: 
1. concatenating all my data from each group into one 4D file 
("the current version of melodic does not perform group ICA - for now  
you could try concatenating all data into one 4d file and run  
standard melodic on this file.  After estimating the spatial maps and time  
courses you can split up the latter into subject-specific chunks and  
feed these timecourses into some meta-analysis.)"

or 

2. using the information in the melodic_IC file, which is the unthresholded 
z-map data. ("Once individual Z-maps have been generated it is a question 
of *how* you combine these. . . you'll need to model those variance 
components separately. There is no fundamental difference in the required 
statistical procedures between ICA and general linear 
modelling"; "unthresholded Z-maps (the maps contained in melodic_IC)") 

This leaves me with two questions:
1. If I concatenate all the data from one group of subjects into one 4D 
file, I do not understand how to then split the time course into subject-
specific chunks. It seems like when you put everyone into one file, you are 
treating your group of subjects as an individual. How do you recover 
individual subject information from this in order to account for between-
subjects variance?

2. If I choose to evaluate components from the melodic_IC file, how do I 
find individual components in this file? It appears to be just one file, 
unlike the separate files for each component in the stats folder. I looked 
in the header, and that looked basically the same as the headers for 
individual components in the stats folder.

I also have a couple of questions relating to a mistake I made.
Let me preface my questions with the fact that I am rather abysmal at math 
and I am playing with this on my own. It seems to me that if you can create 
a spatially normalized z-map of a given component, you should be able to 
determine your component of interest for each subject, and then perform 
group statistics on these z-maps in a regular GLM analysis.

I did not realize that the z-maps in the stats folder output by melodic 
were all thresholded. I picked one component for each person and entered 
these into two-sample t-test in SPM. If this is already giving you a 
headache, all apologies. 

When I did this, I found that the first component for each person was 
different than the other components. In the header of the first component, 
aux_file=none, and the cal_min and cal_max values varied. In all the other 
components, aux_file=render1, cal_min=-0.95, and cal_max=1.05. All the 
other variables in the header were the same. Why are these variables 
different in the first component? What is the aux_file?

Secondly, I have run ICA multiple times on one subject. I got a different 
number of components, so I ran the analysis again, this time without even 
closing the window, just hitting go again after the first analysis finished 
(in case I had mistakenly changed something between analyses). This time I 
got the same number of components, but the components were different. Why 
is this? If I am inputting the same data with the same settings, shouldn't 
the output be the same? 

I cannot thank the list, and Christian in particular, enough for any 
insights you can offer. Happy New Years!

Christina