Why does a moving laser leave a perceived trail in your vision? Is this due to a chemical reaction in the retina? A secondary effect in the brain?

I don't believe i'm following exactly what you are saying, but you can't see a laser unless its bouncing off of something (dust particles) in the air and travels at the speed of light so you might have better eyes than my own but I can't really watch things as they move at the speed of light

Are you talking about a persistence of vision effect?

Your eye produces photosensitive chemicals, to detect light. These chemicals are constantly destroyed by light and replaced. An excessively bright light will use them up quickly, leaving a temporary shortage of them, so that normal light viewed as less bright in the affected cells, for a while. This will result in a slight negative image that lasts a while.

 

Is that what you are talking about?

Your eye produces photosensitive chemicals, to detect light. These chemicals are constantly destroyed by light and replaced. An excessively bright light will use them up quickly, leaving a temporary shortage of them, so that normal light viewed as less bright in the affected cells, for a while. This will result in a slight negative image that lasts a while.

 

Is that what you are talking about?

 

That happens as well.

 

I'm talking about if you point a laser pointer at a wall and make little loops with it. The laser appears to leave a trail in your vision. Part of that trail does consist of a negative image, yes, but another part appears just as bright as the laser itself.

http://en.wikipedia.org/wiki/Persistence_of_vision

 

I don't know if that's the whole explanation, but it's definitely part of it.