Closure, or 'Subjective Completion', means that the sensorium is apt to 'fill in' perceptions where they are not directly stimulated by the environment.
The concept of Subjective Completion (later renamed by McLuhan as 'Sensory Closure' and in Understanding Media merely called 'closure', in the manner of the gestalt psychologists) is taken from E.H. Gombrich's Art and Illusion, and from one of the books that influenced Gombrich, Adolf von Hildebrand's The Problem of Form in the Figurative Arts (1893).
McLuhan connects the concept of 'closure' with that of 'projection' in analytical psychology;1 Richard Cavell connects it also with the concept of 'empathy' (Einfühlung), to be found in Heinrich Wölfflin's Principles of Art History (1915).2
In the 'Report on Project in Understanding New Media' (1960), upon which Understanding Media (1964) is based, McLuhan produces a number of 'charts' intended to show 'the dynamic symmetries and contours of the media'.3
The charts introduce four categories: 'SI' (Structural Impact, later renamed 'Sensory Impact'); 'SC' (Subjective Completion, later renamed 'Sensory Closure'); 'LD' (Low Definition) and 'HD' (High Definition).
McLuhan explains that SI 'refers to sensory impressions as they affect the beholder or audience' while SC 'concerns the effect of this impression as it is processed by the audience', and that 'Our SC depends much on previous SI ... The beholder must collaborate in creating the illusions of space, as of time.'4
The charts presented in the NAEB Report depict the SI and SC for the media of speech, writing, print, prints (art), press, photography, telegraph, telephone, phonograph, film (movies), radio and television.
McLuhan says that a medium that is auditory in High Definition, such as the radio, prompts a Subjective Completion (or SC) which is visual and tactile in High Definition; conversely, a film, which is visual in High Definition, prompts a Subjective Completion which is auditory and tactile in High Definition.
A Low Definition medium, meanwhile, such as the telephone, which McLuhan says is auditory, prompts a Low Definition Subjective Completion in the other senses.
McLuhan says that a Low Definition medium evokes the greatest degree of involvement or participation, while High Definition media have a 'hypnotic' effect.
Describing 'speech' as 'technologically a very poor medium', McLuhan says that speech promotes involvement 'precisely because it does such a poor job of communication'.5
He attributes the pervasive 'tactility' of TV to the fact that it, too, is 'low in information' yet 'high in contour, sculptural and tactile values', explaining that these qualities 'elicit the highest degree of participation response of any medium we have'.6
McLuhan says that the movie (along with the photograph) is a high-definition 'reversal' of print technology, and 'utterly unlike telegraph, radio, and TV'.7
Film has a High Definition Sensory Impact (with the corresponding High Definition Sensory Closure) while TV has a Low Definition Sensory Impact (with the corresponding Low Definition Sensory Closure); both, he says, are tactile in their Sensory Closure, but TV is more tactile than film because with TV 'it is the SI itself which bombards us with tactile values'.8
Despite the fact that film is 'technically superior' to TV, McLuhan says that TV, like speech, is more agreeable to 'dialogue', for 'dialogue reaches a higher intensity as information is withheld'.9