  Knowledge of the derivative of the topological susceptibility at zero
momentum is important for assessing the validity of the Witten-Veneziano
formula for the eta' mass, and likewise for the resolution of the EMC proton
spin problem. We investigate the momentum dependence of the topological
susceptibility and its derivative at zero momentum using overlap fermions in
quenched lattice QCD simulations. We expose the role of the low-lying Dirac
eigenmodes for the topological charge density, and find a negative value for
the derivative. While the sign of the derivative is consistent with the QCD sum
rule for pure Yang-Mills theory, the absolute value is overestimated if the
contribution from higher eigenmodes is ignored.
