  An excellent description of both spin-independent and spin-dependent quark
distributions and structure functions has been obtained with a modified
Nambu-Jona-Lasinio model, which is free of unphysical thresholds for nucleon
decay into quarks - hence incorporating an important aspect of confinement. We
utilize this model to investigate nuclear medium modifications to structure
functions and find that we are readily able to reproduce both nuclear matter
saturation and the experimental F^A_2N / F_2N ratio, that is, the EMC effect.
Applying this framework to determine g^A_1p, we find that the ratio g^A_1p /
g_1p differs significantly from 1, with the quenching caused by the nuclear
medium being about twice that of the spin-independent case. This represents an
exciting result, which if confirmed experimentally, will reveal much about the
quark structure of nuclear matter.
