Gated spontaneous emission and four-wave-mixing signals ar expressed using a mixed time-frequency representation of the fields (Wigner spectrograms) and of the material response functions. Well-separated and overlapping pulses are described using two-sided (noncausal) and one-sided (causal) spectrograms, respectively. Pump-probe and fluorescence spectra are recast in an anologous form which facilitates the direct comparison of the underlying microscopic dynamics. (C) 1997 American Institute of Physics.