 Recently such a view of the nature of representational content has been explored by Raftopoulos and Müller (2006). They suggest that what makes the content of a representation nonconceptual is precisely the fact that its content is insulated from other knowledge structures available to the person. Indeed, according to Raftopoulos and Müller, “the existence of cognitively impenetrable mechanisms is a necessary and sufficient condition for nonconceptual content” (Raftopoulos and Müller 2006, p. 190). They review evidence to the effect that various early stages of visual processing, e.g., those responsible for attending to, and tracking, objects in space and time, operate in purely bottom-up fashion. As such, they argue, the representations underlying such capacities are produced in a conceptually unmediated way and hence their content is nonconceptual. Thus, they say, “[t]he content of the states of zero-crossings is nonconceptual … because that content is retrieved bottom-up from a visual scene. To justify the nonconceptual character of the contents of either phenomenal and subdoxastic stages [sic] … it suffices to invoke the fact that these contents are retrieved from visual scenes in conceptually unmediated ways” (Raftopoulos and Müller 2006, p. 215).