Psychophysical Evidence for Spatiotemporal Tuning in Human Motion Sensing Receptive Fields

According to current models of motion detection, cortical motion sensors are tuned in both space and time to create spatiotemporally-oriented receptive fields. Motion direction is encoded by summing activity across sensors tuned to the same direction, and subtracting the outputs of sensors tuned to different directions. A psychophysical adaptation experiment tested for (i) subtractive interactions between sensors tuned to different directions and (ii) spatiotemporal tuning in motion sensing receptive fields. Participants viewed a counter-phase stimulus containing superimposed saw-tooth gratings moving in opposite directions. The contrast of one grating (pedestal) was fixed, while the contrast of the other (test) was varied to establish a motion null (no net apparent motion in the counter-phase). After adapting to a single grating drifting in the same direction as the test component, more test contrast was required to achieve a null relative to baseline. After adapting to a grating drifting in the opposite direction to the test component, less contrast was required to achieve a null. When both adapting and test gratings were counter-phase gratings, a small degree of test contrast threshold elevation was found which depended on the spatiotemporal phases of adapting and test components, consistent with spatiotemporally tuned motion sensor receptive fields.