as a result of the experience of satisfaction, a facilitation comes about between two mnemic images and the nuclear neurones which are cathected in the state of urgency.
(1950: 318)
The two “mnemic images” are of the reflex movement (the “motor image” in the last-but-one quote) and of the satisfying object (the breast). The third item in the facilitation, the nuclear neurones, is the part of the nervous system, which receives stimulus from the interior of the body; in this case, the hunger/thirst that was assuaged at the breast. The facilitation which is created between the two images and the nuclear neurones has a particular purpose. When, subsequently, an object is discovered in perception which coincides with the mnemic image of the original satisfying object, the facilitation is there ready to enable flow of quantity to the motor image, “firing” the action represented by that image (sucking on the breast), obtaining satisfaction and dissipating the stimulus.
So, instead of facilitations as such somehow representing memory, we suddenly have memories (mnemic images) linked by facilitations with one another and, most fancifully, with neurones. Freud specifies that the three-way facilitation arising from the experience of satisfaction comes about in a special way. He explains:
there is a basic law of association by simultaneity … which is the basis of all connections between ψ neurones. We find that consciousness – that is, the quantitative cathexis of a ψ neurone, α – passes over to another, β, if α and β have at some time been simultaneously cathected from φ (or elsewhere).
(1950: 319)
By this mechanism the nuclear neurones, the image of the object and the motor image are linked together by facilitation just because they were “simultaneously cathected” in the experience of satisfaction. The ψ-neurones are the core of the nervous system (or mental apparatus or system of elements – take your pick), including the nuclear neurones, while the φ-neuronesare the fringe of the system, which receives excitation from the external world. The definition, which Freud gives here of