Direction-specific adaptation of magnetic responses to motion onset

We investigated the direction-specificity of motion adaptation, by recording magnetic responses evoked by motion onsets under both adapted and control conditions. The inter-stimulus interval was equated between the conditions to precisely evaluate the effect of motion adaptation itself. The onset stimuli at 1.5, 3.0 or 6.0 deg/s moved in the same direction or in the opposite direction to an adaptation stimulus at 3.0 deg/s. The perceived velocity of each test stimulus was measured in separate sessions. The most prominent peak (M2) of evoked responses appeared around 200-300 ms after motion onsets, and the dipoles were mainly estimated in the temporo-occipital area. Adaptation largely affected both perceived velocities and the M2 amplitudes. The M2 amplitudes were decreased by adaptation for both directions of test stimuli, and the decreases were significantly larger for the test stimuli in the adapted direction (49-63% of control condition) than for the test stimuli in the opposite direction (17-27% of control condition). The present study, for the first time, found that magnetic responses evoked by motion onsets reflect the activities of neurons that have direction-specificity.