Bion first used the term "contact-barrier," which he borrows from Freud, in Learning from Experience (1962, p.17):
I shall now transfer all that I have said about the establishment of conscious and unconscious and a barrier between them to a supposed entity, that I designate a 'contact-barrier'; Freud used this term to describe the neurophysiological entity subsequently known as a synapse. In conformity with this my statement that the man has to 'dream' a current emotional experience whether it occurs in sleep or in walking life is reformulated thus: The man's alpha-function whether in sleeping or waking transforms the sense-impressions related to an emotional experience, into alpha-elements, which cohere as they proliferate to form the contact-barrier. This contact-barrier, thus continuously in process of formation, marks the point of contact and separation between conscious and unconscious elements and originates the distinction between them. The nature of the contact barrier will depend on the nature of the supply of alpha-elements and on the manner of their relationship to each other.
This contact-barrier, like a number of other concepts in psychoanalysis, can be seen as a structural concept, an area between conscious and unconscious, or as a function, a constant transformation of beta-into alpha-elements. It would result in what Freud saw as a permeable repressive barrier, in which the conscious and unconscious are in constant symbolic communication. Bion also compares it to a continuous dream in interaction with conscious rational experience. An alpha-element contact-barrier gives emotional meaning, and resonance in communication with an external object, where the contact-barrier may be transformed into an impermeable beta-screen, and the internal communication between the conscious and unconscious is blocked.
See also: Alpha-elements; Beta-elements; Beta-screen; Love-Hate-Knowledge (L/H/K links); Physical pain/psychic pain; Psychosomatic limit/boundary; "Project for a Scientific Psychology, A".
Bion, Wilfred R. (1962). Learning from experience. London: Heinemann; New York: Basic Books.
——. (1963). Elements of psycho-analysis. London: Heinemann.