  Signatures of strong coupling effects in cuprate high-$T_{c}$ superconductors
have been authenticated through a variety of spectroscopic probes. However, the
microscopic nature of relevant excitations has not been agreed upon. Here we
report on magneto-optical studies of the CuO$_{2}$ plane carrier dynamics in a
prototypical high-$T_{c}$ superconductor YBa$%_{2} $Cu$_{3}$O$_{y}$ (YBCO).
Infrared data are directly compared with earlier inelastic neutron scattering
results by Dai \textit{et al}. [Nature (London) \textbf{406}, 965 (2000)]
revealing a characteristic depression of the magnetic resonance in H $\parallel
$ \textit{c} field less than 7 T. This analysis has allowed us to critically
assess the role of magnetic degrees of freedom in producing strong coupling
effects for YBCO system.
