  We derive simple relations between the polarized and unpolarized valence
quark distributions in a light-cone SU(6) quark-spectator model for nucleons.
The explicit x-dependent Wigner rotation effect for the light-flavor quarks is
calculated. It is shown that the mass difference between the scalar and vector
spectators could reproduce the up and down valence quark asymmetry that
accounts for the observed ratio $F_2^{n}/F_2^{p}$. The proton, neutron, and
deuteron polarization asymmetries, $A_1^p$, $A_1^n$, and $A_1^d$, are in
agreement with the available data by taking into account the Wigner rotation
effect. The calculated $x$-dependent helicity distributions of the up and down
valence quarks are compared with the recent results from semi-inclusive hadron
asymmetries in polarized deep inelastic scattering by the Spin Muon
Collaboration.
