European comics studies began with Töpffer's theories of his own work in the 1840s, which emphasized panel transitions and the visual–verbal combination. No further progress was made until the 1970s. Pierre Fresnault-Deruelle then took a semiotics approach to the study of comics, analyzing text–image relations, page-level image relations, and image discontinuities, or what Scott McCloud later dubbed "closure". In 1987, Henri Vanlier introduced the term multicadre, or "multiframe", to refer to the comics a page as a semantic unit. By the 1990s, theorists such as Benoît Peeters and Thierry Groensteen turned attention to artists' poïetic creative choices. Thierry Smolderen and Harry Morgan have held relativistic views of the definition of comics, a medium that has taken various, equally valid forms over its history. Morgan sees comics as a subset of "les littératures dessinées" (or "drawn literatures"). French theory has come to give special attention to the page, in distinction from American theories such as McCloud's which focus on panel-to-panel transitions. Since the mid-2000s, Neil Cohn has begun analyzing how comics are understood using tools from cognitive science, extending beyond theory by using actual psychological and neuroscience experiments. This work has argued that sequential images and page layouts both use separate rule-bound "grammars" to be understood that extend beyond panel-to-panel transitions and categorical distinctions of types of layouts, and that the brain's comprehension of comics is similar to comprehending other domains, such as language and music.