  We study the complex scalar loop corrections to the boundary-boundary gauge
two point function in pure AdS space in Poincare coordinates, in the presence
of a boundary quadratic perturbation to the scalar. These perturbations
correspond to double trace perturbations in the dual CFT and modify the
boundary conditions of the bulk scalars in AdS. We find that, in addition to
the usual UV divergences, the 1-loop calculation suffers from a divergence
originating in the limit as the loop vertices approach the AdS horizon. We show
that this type of divergence is independent of the boundary coupling, and
making use of which we extract the finite relative variation of the imaginary
part of the loop via Cutkosky rules as the boundary perturbation varies.
Applying our methods to compute the effects of a time-dependent impurity to the
conductivities using the replica trick in AdS/CFT, we find that generally an
IR-relevant disorder reduces the conductivity and that in the extreme low
frequency limit the correction due to the impurities overwhelms the planar CFT
result even though it is supposedly $1/N^2$ suppressed. Comments on the effect
of time-independent impurity in such a system are presented.
