  Vacuum polarization mediated by quark loops is susceptible to external
electromagnetic fields as well as to the QCD vacuum structure. Employing the
stochastic vacuum model, we calculate the modification of the one-loop
Euler-Heisenberg effective action due to stochastic color fields with the
Fock-Schwinger technique. Our results indicate nonperturbative light quark
contributions of the same order of magnitude as the usual QED terms. Various
theoretical and experimental implications are discussed in this progress
report.
