Hi Sophia,
There are many different scenarios that could explain this asymmetry besides crossing fibres. One that immediately jumps to my mind is that there might be a major white matter tract running right next to the subcortical regions. Streamlines seeded from the cortex would tend to remain within this white matter tract and hence miss the subcortical regions, while streamlines seeded from the subcortical region would have no problem to enter this white matter tract and from there travel to the cortex. However, there are many other scenarios possible.
One thing you might want to look at to investigate this further is to investigate where the streamlines from the cortex tend to deviate from the WM tract connecting the subcortical and cortical regions. You could do this by overlaying a map of the WM tract (made by having one region as seed and the other as waypoint) with the map of all streamlines seeded from your cortical ROI.
Best wishes,
Michiel
> On 6 Jan 2018, at 05:40, Sophie Huang <[log in to unmask]> wrote:
>
> Greetings,
>
> I often find that when I do tractography between a subcortical region and a cortical region, the connectivity is much lower when I use the cortical region as seed and subcortical region as target versus the other way around. Is this due to more crossing fibers near the cortex or other reasons?
>
> Any insights will be greatly appreciated.
>
> Thanks,
> Sophia
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