It sounds like you are somewhat confused by the different DCM
variants. DCM for induced responses does not distinguish between
forward and backward connections, just between linear and nonlinear.
You should usually have some well-defined model (or a series of
models) to specify which are which. See Chen et al. NeuroImage,
45(2):453-462, 2009 and Chen et al. J Neurosci. 23;30(25):8393-9 for
SPM and DCM cannot handle variable length trials. Also since DCM for
induced responses models the average response your must take a set of
trials that when averaged will yield something meaningful. So I
suggest that you select a set of trials with comparable durations and
cut them to the same length. Usually for induced responses your trials
will be longer than 200-300ms especially for areas high in the
hierarchy. But you can just look at your averaged TF data and select
the trial boundaries so that the interesting features will be
included. The input should be more or less at the time when the
activity starts departing from baseline.
On Mon, Aug 8, 2011 at 10:18 PM, liang wang <[log in to unmask]> wrote:
> Hi DCM experts,
> I would like to use DCM to investigate information flow direction across
> several attention regions (mainly focusing on frontal parietal attention
> network). The regions selected are ventral medial frontal gyrus (vMFG),
> middle frontal gyrus (MFG), dorsal premotor motor cortex (dPMC),
> intraparietal sulcus (IPS) and inferior parietal lobe (IPL). I have a couple
> of questions for specifying the connections.
> 1) to my understanding, all the regions are high-level regions. Is that
> possible that I just select those regions if I have a hypothesis about the
> connection direction. I am not sure whether or not it is necessary to
> specify forward (bottom-up) and backward (top-down) connections in this
> situation (or leave them alone). The reason for no visual region is that the
> electrodes did not cover that area.
> 2) I want to focus on induced responses (not evoked response). Could I set
> the start window, like starting at 200ms? and chane 60ms onset (default
> setup for neural response to my understanding) to 0?
> 3) because of variable stimuli duration for my experiment, is it possible to
> make each trail have different data length? what is the minimum (restricted)
> data length for DCM analysis? (because sometime one of trails has very short
> duration, like 300ms).
> 4) (the last question may be not related to this topic) could you give me
> some advice how to specify forward or backward connection. Let me say if I
> have several regions including thalamus and few cortical regions. For an
> attention task, thalamus is an important area, partly because this area
> projects a lot of anatomical connections to posterior part of the brain.
> However, it is hard to simply specify it as low- or high-level region. To
> me, a simple solution is trying each possible model and compare those model
> based on BMC tool (too much time will be taken). Maybe there is alternative
> ways to easy achieve that. thanks.
> Please let me know if you think some steps are important for this type of
> analysis that I need pay more attention on.
> Liang Wang, PhD
> Neuroscience of Attention and Perception Laboratory
> Princeton Neuroscience Institute
> Princeton University
> Princeton, NJ, 08540