Hi again,
Here's the output when trying to get the length of the spm.mat:

>> load SPM.mat
SPM.Sess.U
length(SPM.Sess.U)

ans =

1x4 struct array with fields:
    name
    ons
    dur
    P
    dt
    u
    pst


ans =

1x4 struct array with fields:
    name
    ons
    dur
    P
    dt
    u
    pst


ans =

1x4 struct array with fields:
    name
    ons
    dur
    P
    dt
    u
    pst

??? Error using ==> length
Too many input arguments.