  It is shown that a correlation between the positions of the $c$-axis
longitudinal optic ($LO_c$) phonons and ``notch''-like structures in the
$a$-$b$ plane conductivity of high-$T_c$ superconductors results from
phonon-mediated interaction between electrons in different layers. It is found
that the relative size of the notches depends on
$\lambda_{ph}(\Omega_{ph}/\gamma_{ph})$, where $\lambda_{ph}$, $\Omega_{ph}$
and $\gamma_{ph}$ are the effective coupling strength, the frequency and the
width of the optical phonon which is responsible for the notch. Even for
$\lambda_{ph}\approx 0.01$ the effect can be large if the phonon is very sharp.
