Signatures of strong coupling effects in cuprate high-Tc 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 CuO2 plane carrier dynamics in a prototypical high-Tc superconductor YBa2Cu3Oy (YBCO). Infrared data are directly compared with earlier inelastic neutron scattering results by Dai [Nature (London) 406, 965 (2000)] revealing a characteristic depression of the magnetic resonance in Hc field less than 7T. This analysis has allowed us to critically assess the role of magnetic degrees of freedom in producing strong-coupling effects for YBCO system. © 2005 The American Physical Society.