I have a question regarding how to set up modulatory effects from a region to the connectivity between two other regions. Although I'm working on a working memory task dataset, there are some similarities between my design and the motion/attention design tutorial provided in SPM12 manual, I'm going to use the tutorial design to quickly illustrate my question:

In the tutorial, there are 3 regions (V1, V5, and SPC) and 3 task conditions (Photic, Motion, and Attention). The provided models indicate Motion effect on the V1-V5 connectivity and the Attention effect on V5-SPC connectivity. If I want to models the effects of attention so that Attention is an input into SPC and SPC modulates the connectivity between V1 and V5 (i.e., attention indirectly influences connections between V1-V5 through SPC), how would I go about setting up this model? In SPM, the specification of models only allows effects of Motion/Attention on regions and connections but not effects of a region on connections. Thus, I'm unclear how to proceed. Could anyone kindly help me with this setup?

Thank you!

TL