Two movies below present demonstrations of mobile crowding -- crowding that is specific to the flankers that move with the target as opposed to flankers that surround the target in retinotopic coordinates. See the bottom of the page for the ECVP abstract for this work. The alternation rate in our demonstration is quite low (3 Hz) so the crowding you see in the first movie may not be very pronounced (this depends on the individual). The experiments showed that the strongest crowding is seen at higher rates (4 to 5 Hz or higher). In the first movie, you should fixate the center and attend to one location (our subjects attended to the bottom location) and report whether the target (3rd letter down) is a normal or left-right reversed letter (reversed here). Notice that the target on each arm is different but they all have the same version (all reversed here) so that even if you notice a different target, in this demonstation movie you will have the right answer. Click on the play button on the bottom left to start the movie.

Next we add a guide that jumps from location to location, timed so that it lands on each arm when only the target is there. In addition, we move the whole array one position on each frame so the same letter is present all the time where you are attending. Fixate again and follow the light sector with attention as it moves, noticing whether the target is normal or left-right reversed (reversed here). If you are like our subjects, you may find this moving target easier to see than the stationary one, suggesting that crowding is non-retinotopic: it depends on the flankers that move with the target, not those that surround it in retintopic coordinates.

Sixteen radial arms of 5 letters each extend from fixation -- the first and last two letters on each arm are distractors while the middle letter is the target. The display has two counterphasing frames: in one, even arms show only the targets without distractors and odd arms show only the distractors without the targets; in the other frame, odd arms show distractors and even arms, the targets. The two frames alternate rapidly so that each flickering target is flanked by flickering distractors. However, when a guide (a faint radial sector enclosing one arm) moves from arm to arm in step with the counterphase alternation, the arm within the guide contains ONLY the target and no flankers. Targets are presented either in normal or mirror reversed orientation. Subjects fixate the center of the array and report the orientation of the target. RESULTS: Without the guide and with attention directed to one fixed location, there is substantial interference from the flanking letters. When following the guide with attention, however, crowding is greatly reduced. CONCLUSION: Crowding is generated by the flankers that move with the moving target and not by the flankers that surround it locally in retinotopic coordinates.