McLuhan saw the 'structuralism' of the modern age, that is, the interpretation of forms (figures) in terms of their underlying structure (ground), as an effect of the 'field' perception imposed by electronic communications technology.
Though schooled in gestalt theory by I.A. Richards, his mentor at Cambridge University, McLuhan in fact did not embrace the gestalt method until much later.
McLuhan's 'structural' analysis is built around the concepts of 'visual' and 'acoustic' space, where 'acoustic space' designates oral (and electronic) culture, while 'visual space' designates a society founded upon the phonetic alphabet and the printing press.
Where Harold Innis, McLuhan's antecedent in communications theory, speculates about the structural effects of technologies on communication over space and time, McLuhan shows himself to be more interested in the structural effects of media upon the psyche, though he nods to Innis (and to Siegfried Giedion, author of Space, Time and Architecture) by conceptualizing the psyche in spatial terms.