The Imagery Debate began when Pylyshyn (1973) challenged Kosslyn's results of time delay in "imaging travel" and subsequent explanation by Kosslyn of imagery using a mechanism of spacial representation. Pylyshyn disagreed with Kosslyn that spacial representation was actually happening in imagery. Pylyshyn proposed that the underlying mechanism in imagery is propositional, meaning the relationship can be represented with symbols (language) rather than depicted by a picture or spacial representation.
Denis, M., & Kosslyn, S. M. (1999). Scanning visual mental images: A window on the mind. Cahiers de Psychologie Cognitive/Current Psychology of Cognition, 18(4), 409-465.