Our ability to track multiple objects at once appears to be limited by both speed and crowding. Crowding occurs when objects move too close together, and can no longer be reliably distinguished by your attentional marking system. In this paper, we suggest that this crowding limit is also the root cause of the speed limit. Surprisingly, as objects move faster, they may not actually be harder to track.
Instead, faster objects are more likely to experience 'close calls' with other objects, increasing their confusability. Imagine an 8-second trial where your tracked targets move at 4 degrees per second, and experience on average 4 'close calls'. If you double the speed of the objects, you would expect 8 'close calls' on average.
So how can we separately test raw speed, and the effect of speed on crowding, as explanations for the impairment caused by fast moving objects? Crowding scales with size, but speed does not. If an MOT display is scaled to be 4 times bigger, crowding is roughly the same, but the speed of each object across the retina has increased by 400%. We tested speed limits on such small and large displays, and found increasing the display size by 400% did not affect performance, suggesting that our 'speed' limits actually had little to do with retinal speed, and instead reflected increased 'close calls' per second.
We now know of no existing evidence for a speed limit on multiple object tracking. We speculate that if you can track one object at speed X
(no cheating, you have to be able to see it), you can track an unlimited number of objects at speed X!
(Of course, this is practically hard to observe because it's normally difficult to get rid of the confounding increase in crowding with higher speeds.)
You can try this effect for yourself below. Try the slow and fast speeds with the small displays. Keep your eyes on the center dot, and track the objects that start as blue. They will turn blue again at the end so that you can check your answer. You'll see that slow is not so bad, but fast is a bit too hard. if both are too hard, try tracking just two dots (one from the left and one from the right)
By the way, you may think the 'fast' condition is too hard because the objects are hard to see. But keep your eyes on the dot again and try to track just one object. No problem, right? So there's no visual acuity problem. Instead, trying to attend to many objects at once causes an attentional acuity problem
Now try the same scene-relative speeds in the large displays (3x larger). The large movies are simply scaled versions of the small ones, so the objects actually move 3x as fast across your retina. Nevertheless, you'll probably still find that the small & large 'slow' movies are of equal (easier) difficultly, even though the objects move 3x as fast in the larger version.
You'll also probably still find that the small & large 'fast' movies are of equal (harder) difficultly, even though the objects move 3x as fast in the larger version.
To put it another way, you had a lot of trouble with the small 'fast' display, but the large 'slow' display was easier. Well, objects in the large 'slow' display were actually moving faster across your retina!