Does your baby smile when he/she laughs?

My little girl (almost five months) finally laughed last night, she's laughed before but only shortly, but last night she laughed hysterically, I'm not sure at what, but the strange thing is that she doesn't smile when she laughs. Maybe a bit after she's done laughing, but besides that she has sort of a straight face and I'm positive it's a laugh. It's very strange. Has anyone else noticed this in their baby?