I've noticed this too. It's like there are two (or more?) distinct "layers" or dimensions in my mind; one where hallucinations reside and another where my visualization (which includes day dreaming) resides. Often to go to bed at night I day dream (ironic) and sometimes I've gotten distracted or my thoughts are interrupted by hallucinations (simple colors or geometric patterns). The two were existing simultaneously so clearly there are separate processes of the brain at work there.