Finite-wavelength tomography of the Earth or the Sun critically relies on models of the sensitivity of seismic travel times to localized heterogeneities, as evidenced by the controversial discovery of plumes in the Earth's mantle. We have used time-distance helioseismology to directly measure the spatial sensitivity of surface-gravity wave travel times to magnetic perturbations. The data strongly speak in favour of 'banana-doughnut' theory according to which body-wave travel times are sensitive to the wave speed in a broad region surrounding the geometrical ray path.