  We report $^{17}$O(2,3) and $^{63}$Cu(2) spin-lattice relaxation rates and
the $^{17}$O(2,3) spin-spin relaxation rate in different magnetic fields in
YBa$_2$Cu$_3$O$_7$ near $T_c$. Together these measurements enable us to test
the magnetic field dependence of the pseudogap effect on the spin
susceptibility in different regions of the Brillouin zone using the known form
factors for different nuclei as filters. Thus, we study the momentum dispersion
of the pseudogap behavior. We find that near the antiferromagnetic wave vector
the pseudogap is insensitive to magnetic fields up to 15 T. In the remaining
region, away from the $(\pi,\pi)$ point, the pseudogap shows a magnetic field
dependence at fields less then 10 T. The first result is indicative of the
opening of a spin-pseudogap that suppresses antiferromagnetic correlations
below a temperature $T^*$; whereas, the second result shows the effect of
pairing fluctuations on the spin susceptibility as a precursory effect of
superconductivity.
