Response functions to changes in the atmospheric parameters. From left to right, RFs to changes in temperature, line-of-sight velocity, and the three components of the magnetic field vector in polar coordinates. Each curve corresponds to the maximum value for the four Stokes parameters and all the computed wavelengths at each optical depth for the Si I 10827 Å spectral line. Blue and red designate the results from the LTE and NLTE solutions, respectively. RFs are normalised to the maximum value of the corresponding NLTE solution.