  We report on results of our theoretical study of the in-plane infrared
conductivity of the high-Tc cuprate superconductors using the model where
charged planar quasiparticles are coupled to spin fluctuations. The
computations include both the renormalization of the quasiparticles and the
corresponding modification of the current-current vertex function (vertex
correction), which ensures gauge invariance of the theory and local charge
conservation in the system. The incorporation of the vertex corrections leads
to an increase of the total intraband optical spectral weight (SW) at finite
frequencies, a SW transfer from far infrared to mid infrared, a significant
reduction of the SW of the superconducting condensate, and an amplification of
characteristic features in the superconducting state spectra of the inverse
scattering rate 1/tau. We also discuss the role of selfconsistency and propose
a new interpretation of a kink occurring in the experimental low temperature
spectra of 1/tau around 1000cm^{-1}.
