  Carrier dynamics in amorphous a-Si$_{1-x}$RE$_{x}$ (RE=Gd, Y) films has been
studied in the doping regime close to the metal-insulator transition by means
of infrared spectroscopy. Optical constants throughout the entire intra-gap
region ($\hbar \omega <$ 1 eV) have been found to be anomalously sensitive to
changes of temperature and/or magnetic field. The observed behavior is
consistent with the model of hopping transport where the interaction of
carriers with both the lattice and large core spin of Gd ions is taken into
account.
