  The real part of the in-plane optical self-energy data in underdoped
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) and ortho II
YBa$_{2}$Cu$_{3}$O$_{6.5}$ contains new and important information on the
pseudogap. Using a theoretical model approach we find that the density of state
lost below the pseudogap $\Delta_{pg}$ is accompanied with a pileup just above
this energy. The pileup along with a sharp mode in the bosonic spectral
function leads to an unusually rapid increase in the optical scattering rate
and a characteristically sloped peak in the real part of the optical
self-energy. These features are not found in optimally doped and overdoped
samples and represent a clearest signature so far of the opening of a pseudogap
in the in-plane optical conductivity.
