Modulation transfer functions of the human visual system revealed by magnetic field measurements

Properties of a neural source of magnetic field localized in the occipital lobe was measured in a steady-state paradigm using contrast reversing gratings. Comparisons with scalp potentials provided evidence that the evoked field was associated with intracellular currents. Its modulation transfer functions were similar to the analogous functions for the scalp potential and the detection of a grating. Moreover, the amplitude of the evoked field was linearly related to the potential amplitude and their phases were nearly identical. An analysis of the results in terms of theoretical relations between evoked field and potential led us to conclude that these two measures may yield a similar characterization of the source when one dipolar source predominantly gives rise to both measures, but they may yield complementary information when multiple sources contribute to the measures.