The common understanding of perception is that it is achieved by means of stable internal representations. There are experimental paradigms that challenge this view. Research has shown that perception is strongly linked to attentional resources and attentional demand. The phenomenon of 'change blindness' has been taken to demonstrate this. The simplest form of this paradigm is that which you may have experienced as a child: comparing two apparently identical pictures to try to identify differences between them. If our recognition of the world is not as detailed as we think it is our perceptions may be 'virtual'. (for more discussion on this phenomenon and its implications see Alva Noe (2006) 'Action and Perception' MIT Press
University of Idahos' 'GoCognitive' website has a neat demonstration of Change Blindness that you can participate in. Go try it!
Via Marinella De Simone