I think that the « mirroring » (what Zizek calls the « uncanny resemblance ») between the quantum level and that of human subjectivity is more than a simple formal homology and less than an ontological grounding. As a working hypothesis I call this mirroring a type of « formal causality ».
Johnston’s argument is that Zizek’s use of quantum mechanics to explain the emergence of free subjectivity starts at a level that is too far from the phenomenon that it is trying to explain and would require a long series of “bridge” theories before getting to the level of the human subject. It would thus be more economical to begin with biology and brain science, which occupy levels just adjacent to the human subject and which (similarly to quantum physics) premise an ontological incompleteness of nature.
Johnston’s objection depends on what I call “the argument from distance”, which itself depends on the standard view of science as edified on the stratification of the levels of emergence (or of reduction, depending on which direction you take, moving up or down the levels). This is what has been called the “layer cake” model of explanation and reduction.
Zizek’s idea is that on the layer cake model the quantum level appears “distant” from the human level, with many other intervening levels, but that from a formal view they are quite close. This means that for him the layer cake model is not always the best or most useful way to envisage the relation between different ontic domains.
Zizek gives primacy to the quantum model not because it is the most fundamental level following the the descending line of reductions and of efficient causality, but because it is the most “deconstructed” model, and thus formally closer to human subjectivity. The sort of causality that Zizek is emphasising here is a formal causality, where the “highest” (or most distant) abstractions are inscribed in the real itself. In other words, Zizek is arguing for a realist interpretation of quantum concepts.
This formal analogy between quantum physics and subjectivity means that the formal causality is operative not only at the “base” or sub-microscopic level but equally at every succeeding level. Real emergence from one level to another, that cannot be explained by reduction to lower levels, is only possible because of the ontological incompleteness that is best described by quantum mechanics (at the present moment, for Zizek this notion of the « best » scientific description is an empirical question).
Note: I am indebted to a discussion with Cadell Last for helping me to clarify my ideas on this question.