Well, this IS the Mind Tavern, and Jon says we can sometimes divert from the main thread, so pardon me while I digress....
Uncle, that effect of which you spoke is an effect of the reptilian portion of our brainstem, where it is MOVEMENT that attracts our attention. Many reptilian animals do not see stationary objects unless they are themselves moving, perhaps because they don't have the brainpower to integrate a still image. But the ability to perceive movement of spots (perhaps partly hidden by natural camouflage) and integrate it into a complete image is a survival ability.
The "balloon expansion" image is because your optical cortex is trying to create a complete image where it only has a partial one (the fragments of burning materials) and it suggests a surface when in fact it really IS just a bunch of points. In this case, it is a "virtual" surface. The visual art style called pointillism (an image suggested by a series of dots) is based on that idea. So is the art style of tromp l'oeil (French; "trick of the eye") in which something that is actually 2-dimensional nevertheless appears to have that 3rd dimension.
The visual cortex of the human brain is not one of the smaller structures. However, seeing an aerial pyrotechnic at dusk or nighttime gives you an essentially dark background where there are no competing images. That air burst probably would not have the exact same effect during broad daylight. But where there is no stationary context, that effect you mentioned doesn't have any complex competition. So you can see it.
Motion pictures are another example of brain/image integration. Based on many experiments, anything that produces at least 24 distinct images per second will produce the effect of a continuously moving image even though the individual images are stationary. Early TVs could reach this frame rate reasonably well. With modern digital circuits and higher transmission frequencies, we can get higher frame rates and in fact we know that the brain can see things at a faster level than the integrated scene that we are watching. We can see things hidden behind the higher speeds. So-called subliminal advertising was banned precisely because of that. Some of the most advanced computer games, using dedicated video co-processors, can EASILY reach frame rates of 60/sec and some go higher than that even for complex scenes.
Oddly enough, the human brain also does that kind of "mental integration" with sounds. As an organist approaching my instrument from a technical background, I learned about the physics of musical sounds. Every musical sound we hear is comprised of a mixture of harmonics from a base frequency, the lowest frequency makeable by the thing being played at the moment. All other sounds are harmonics (base frequency times an integer) blended with the 1st harmonic or base frequency. Usually the base frequency is loudest and the other harmonics diminish in proportional loudness. Electronic devices can produce a sound with no extra harmonics, but in the world of instruments, the closest to that pure sound comes from flutes and pan-pipes. Violins and complex percussion instruments are on the other end, with the most harmonics in the mix.
With sufficiently complex keyboards, you can also build a tone that DOESN'T contain the frequency that should be the base harmonic, but if you have nearly equal amounts of 2nd and 3rd harmonic, your mind "hears" the missing harmonic anyway. That is why even a cheap speaker will give you some appreciation of bass notes that the speaker actually cannot produce with any power.
This trick was well known in pre-electronic organs that had pipe-based voicing. Some pedal voices were intentionally "built" from that 2nd/3rd harmonic combination to give the impression of deeper (and therefore longer) pipes than were actually present. In theater and church organs, that kind of voice is often called a "Mixture." Other pipe voices were built using a trick of "open" vs. "closed" pipes that forced a either vibratory node or an anti-node to form at the far end of the pipe from its "throat" (the part where the sound usually is generated). You can still hear that in some electronic organs built for churches or mimicing theater organs because they will have voice names such as "Open Diapason" or "Tibia Clausa" (a closed-pipe flute-like sound). The "closed" sound is often apparently deeper in pitch because of that 2nd/3rd harmonic trick.