The neural correlates of non-retinotopic motion integration as revealed by high-density EEG
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 notch was either changing positions across frames in a rotating fashion, or stayed in the same position. Every 5th to 9th frame, the notch started or stopped rotating, and observers had to report this with a button-press. GFP analysis revealed a stronger response for rotating than static notches. This effect appears around 200 ms after frame onset when a change from static to rotation has just occurred, but later in the sequence it is instead present already after circa 100 ms. 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, only very minor reference frame effects were observed and these did not interact with the rotation effects. Electrical source imaging showed that the underlying neural processing of the rotation activity seems to be located partially in the right middle temporal gyrus.