Neural correlates of non-retinotopic motion integration
Under normal viewing conditions, due to the motion of objects and to eye movements, the retinotopic representation of the environment constantly changes. Yet we perceive the world as stable, and we easily keep track of moving objects. Here, we investigated the neural correlates of non-retinotopic motion integration using high-density EEG. We used a Ternus-Pikler display to establish either a retinotopic or non-retinotopic frame of reference. Three disks were presented for 250 ms followed by an ISI of 150 ms. The disks then reappeared either at the same location (retinotopic reference frame), or shifted sideways (non-retinotopic reference frame). After another ISI, the sequence started over again. In the middle disk, a dot was either changing positions across frames in a rotating fashion, or stayed in the same position. Every 5th to 9th frame, the dot started or stopped rotating, and observers reported this with a button-press. We found higher EEG responses for rotating than static dots. This effect occurred rather late (>200 ms), i.e. after basic stimulus encoding (P1 component). Importantly, these results hold for both the retinotopic and the non-retinotopic conditions, indicating that the encoding of rotation does not depend on reference frame. In line with this, reference frame effects were observed at earlier latencies and did not interact with rotation effects. Electrical source imaging showed that the underlying neural processing of this non-retinotopic effect seems to be located partially in extrastriate visual areas.