Probably not helpful in the context of market research for video content, but for me, it's when I hear the lee laugh differently to any other laugh that I've heard them produce before...
I recall a time with a very ticklish ex-girlfriend, where she had succumbed to this rhythmic, one-pitched cackle... I'd never heard that laugh before , or again, from her... I think that was the most I'd ever pushed her limits, although at the time I wasn't knowingly doing anything different/more torturous to the normal ticklings I'd give her.
In general, I much prefer varied laughter rather than continuous laughter that doesn't change at all and certainly rather than screaming/shouting, which I don't really like at all.
I like giggling/gasping/periodic yelps and silent laughing or hissing type laughter you might get when someone is trying to resist laughing at all.
Me too. 
