Comment on: Role of inelastic tunneling through the insulating barrier   in scanning-tunneling-microscope experiments on cuprate superconductors
  In a recent letter [1] Pilgram, Rice and Sigrist (PRS) propose to perform oxygen isotope dependent conductivity measurements along the c-axis of the cuprate superconductor BSSCO. In Raman isotope data for vibrations along the c-axis from 1988 [2], and in site selective susceptibility experiments determining Tc, no isotope effects were observed for oxygen ions in the insulating layer [3]. Based on these earlier not by PRS quoted experiments, we argue that the suggestion made in Ref. 1 is not worth to be undertaken, and if it is carried through, no effect will be seen. From the meanwhile 12 years old site selective data [3] the observed isotope effects stem to nearly 100% from oxygen ions in the CuO2-planes. These results evidence polaronic behavior in these planes [4]. 
A. R. Bishop, S. D. Conradson, A. Bussmann-Holder, O. Dolgov, H.   Keller, R. Khasanov, K. A. Mueller, V. Z. Kresin, H. Kami...