Optical sensitivity to the shape of the IMF. Black, orange, and purple lines indicate the age sensitivity of the TiO₂ absorption feature for a Salpeter IMF, a bottom-heavy single-power-law IMF, and a bottom-heavy broken-power-law IMF, respectively. For old stellar populations, such as those harbored by massive quiescent galaxies, single-power-law IMF parametrizations are indistinguishable from broken-power-law IMF parametrizations because the stellar-mass range probed along the IMF is rather narrow. For young stellar populations, however, different model assumptions on the IMF shape lead to dramatically different model predictions, highlighting the potential use of line-strength indices in constraining the shape of the IMF. Predictions are shown at the native resolution of the MILES models (2.51 Å).