  Through an impulse approximation analysis of single spin Sivers and Collins
asymmetries in the Bjorken limit, the possibility to extract the quark
transverse-momentum distributions in the neutron from semi-inclusive deep
inelastic electron scattering off polarized 3He is illustrated. The analysis is
generalized to finite momentum transfers in a light-front Poincare' covariant
framework, defining the light-front spin-dependent spectral function of a J=1/2
system. The definition of the light-front spin-dependent spectral function for
constituent quarks in the nucleon allows us to show that, within the
light-front dynamics, only three of the six leading twist T-even
transverse-momentum distributions are independent.
