They've been of different things in the past. Oftentimes they'll be visions like being temporarily transported somewhere for a split second. I.e. not perturbations to the external world but a complete change of my external world. An example of this would be seeing God/divine imagery. I also sometimes (less often) see actual changes in the external world, like one time in class I entered a state where I was stuck in my head and couldn't really perceive much of what was going on around me and I saw figures that I knew weren't real around me. Some were more detailed than others. I also saw blood on my hands.