The Fate of Visible Features of Invisible Elements

To investigate the integration of features, we have developed a paradigm in which an element is rendered invisible by visual masking. Still, the features of the element are visible as part of other display elements presented at different locations and times (sequential metacontrast). In this sense, we can “transport” features non-retinotopically across space and time. The features of the invisible element integrate with features of other elements if and only if the elements belong to the same spatio-temporal group. The mechanisms of this kind of feature integration seem to be quite different from classical mechanisms proposed for feature binding. We propose that feature processing, binding, and integration occur concurrently during processes that group elements into wholes

How are non-retinotopic motion signals integrated? -A high-density EEG study

Objects moving in the visual scene cause retinal displacements that are not the result of motor commands and thus cannot be accounted for by efference copies. Yet, we easily keep track of moving objects even without following them with our gaze. Here, we investigated the neural correlates of non-retinotopic motion integration using high-density EEG. We presented three disks that either flickered at the same location (retinotopic reference frame) or moved left-right in apparent motion, creating a non-retinotopic reference frame in which the features of the disks are integrated across retinal positions. In one disk, a notch was either changing positions across frames in a rotating fashion, or stayed in the same position. The notch then started or stopped rotating after a random number of frames. We found stronger EEG responses for rotating than for static notches. In the novel state (first frame of rotating or static), this effect occurs in the N2 peak and resembles a motion-onset detection signal. Inverse solutions point to the right middle temporal gyrus as the underlying source. Importantly, these results hold for both the retinotopic and the non-retinotopic reference frames, indicating that the rotation encoding is independent of reference frame

EEG Correlates of Relative Motion Encoding

A large portion of the visual cortex is organized retinotopically, but perception is usually non-retinotopic. For example, a reflector on the spoke of a bicycle wheel appears to move on a circular or prolate cycloidal orbit as the bicycle moves forward, while in fact it traces out a curtate cycloidal trajectory. The moving bicycle serves as a non-retinotopic reference system to which the motion of the reflector is anchored. To study the neural correlates of non-retinotopic motion processing, we used the Ternus-Pikler display, where retinotopic processing in a stationary reference system is contrasted against non-retinotopic processing in a moving one. Using high-density EEG, we found similar brain responses for both retinotopic and non-retinotopic rotational apparent motion from the earliest evoked peak (around 120 ms) and throughout the rest of the visual processing, but only minor correlates of the motion of the reference system itself (mainly around 100-120 ms). We suggest that the visual system efficiently discounts the motion of the reference system from early on, allowing a largely reference system independent encoding of the motion of object parts

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