<P> David Deutsch addresses Johnson's objection to idealism in The Fabric of Reality when he states that, if we judge the value of our mental images of the world by the quality and quantity of the sense data that they can explain, then the most valuable mental image--or theory--that we currently have is that the world has a real independent existence and that humans have successfully evolved by building up and adapting patterns of mental images to explain it . This is an important idea in scientific thought . </P> <P> Critics of scientific realism ask how the inner perception of mental images actually occurs . This is sometimes called the "homunculus problem" (see also the mind's eye). The problem is similar to asking how the images you see on a computer screen exist in the memory of the computer . To scientific materialism, mental images and the perception of them must be brain - states . According to critics, scientific realists cannot explain where the images and their perceiver exist in the brain . To use the analogy of the computer screen, these critics argue that cognitive science and psychology have been unsuccessful in identifying either the component in the brain (i.e., "hardware") or the mental processes that store these images (i.e. "software"). </P> <P> Cognitive psychologists and (later) cognitive neuroscientists have empirically tested some of the philosophical questions related to whether and how the human brain uses mental imagery in cognition . </P> <P> One theory of the mind that was examined in these experiments was the "brain as serial computer" philosophical metaphor of the 1970s . Psychologist Zenon Pylyshyn theorized that the human mind processes mental images by decomposing them into an underlying mathematical proposition . Roger Shepard and Jacqueline Metzler challenged that view by presenting subjects with 2D line drawings of groups of 3D block "objects" and asking them to determine whether that "object" is the same as a second figure, some of which rotations of the first "object". Shepard and Metzler proposed that if we decomposed and then mentally re-imaged the objects into basic mathematical propositions, as the then - dominant view of cognition "as a serial digital computer" assumed, then it would be expected that the time it took to determine whether the object is the same or not would be independent of how much the object had been rotated . Shepard and Metzler found the opposite: a linear relationship between the degree of rotation in the mental imagery task and the time it took participants to reach their answer . </P>

What part of the brain is responsible for visualization