How does glial cell activity influence fMRI, EEG signals? Glial cells comprise about 80% of the cells in the human brain. But physiological models of EEG and fMRI often relate only to neuronal activity.

How do glial cells affect large-scale imaging modalities?

Also, does glial activity artifact dye imaging and local field potentials? 

Are microscale recordings more sensitive to artifacts due to glial activity than macroscale recordings?
I'm only going to attempt to answer a small part of your question: how does glial activation affect the blood oxygen level-dependent (BOLD) response measured by fMRI?

Schulz et al. (2012) were able to investigate the role of glial signaling on fMRI activity by simultaneously measuring neuronal responses with invasive optical imaging and fMRI. They found that glial activation prolonged the BOLD response, as shown in this figure:



Here's how they interpret this result:


  Although these glial responses did not initiate vessel dilations, they were associated with prolonged BOLD signals, suggesting a possible contribution to persistent vascular responses.


In other words: they hypothesize that the activity of the glial cells prolongs the dilation of the blood vessels, which is detected as a prolonged BOLD response.