Thanks Alexander, Michael and Elliot for the reply, I will see the AAL atlas and the  maxprobmap more deeply. I am in the beggining in fmri in the hippocampus for 
epilepsy patients, so, I will have time to explore more and more in working with roi analysis.

Best wishes from Antioquia

On Sun, Feb 20, 2011 at 10:35 PM, G Elliott Wimmer <[log in to unmask]> wrote:

Regarding the AAL template (which is most likely what you have already seen in the WFU pickatlas), I too have noticed that the AAL hippocampus definition tries to only cover grey matter. This exclusion leads to gaps and holes in the mask that I think you are referring to.

Given the anatomical variability between subjects in the hippocampus, it appears to me that the AAL mask attempts to be far too precise; moreover, it is not left-right symmetric.  In the past, I have used AFNI's 'draw dataset' function to correct these problems with the AAL mask and used the resulting modified mask to define ROIs.

Alternatively, I've seen probabilistic anatomical masks that I think were used in FSL, and that may be worth checking out.

Best Regards,

G Elliott Wimmer

Dept. of Psychology
Columbia University