  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.
