Features of the second-order visual filters sensitive to the spatial frequency modulations

We investigated selectivity of 2nd order visual filters sensitive to the spatial frequency (SF) modulations to orientation and to SF of modulation. We carried out psychophysical experiment using masking paradigm. It was found that 2nd order filters are selective to SF of modulation (with bandwidth +1.5 octaves), and do not show any selectivity to orientation of modulation. We suppose receptive fields of 2nd order mechanisms have concentric form.