Many theorists touch upon important themes of Marxist aesthetics without strictly being Marxist aestheticians, Joel Kovel, for instance, has extended the concepts of Marxian ecology which deeply implicates aesthetics. He is also a part of the struggle to bridge the space between Marx and Freud, which has Marxist aesthetics as a central concern.[5] Current themes within the field include research on the effect of mass-produced industrial materials on the sensed environment, such as paints and colors.[6] A strong current within the field involves linguistics and semiotics, and arguments over structuralism and post-structuralism, modernism and post-modernism, as well as feminist theory.