Top: Response functions to temperature (left), LOS velocity (middle), and magnetic field strength (right) of the Mg I 5173 Å and Fe I 6173 Å lines computed for V_R 0 with DeSIRe, using the model resulting from the NLTE inversions displayed in Fig. 9. All the cases are normalized to the maximum of the RF of Stokes I to changes in the temperature. Green dotted lines delimit a spectral window of ±100 mÅ around the core of the Mg line, while black dotted lines enclose the core of the Fe line in the spectral range ±56 mÅ. Bottom: Average of the unsigned RFs within the spectral windows containing the core of the Mg line (green) and the core of the Fe line (black) lines as defined in the upper plots. The vertical dashed lines are placed at the optical depths of maximum average response in each case.