What should we do? Given the constraints on gathering data, how are we supposed to determine the NCC? Even if we suppose that Flohr's theory is correct, how can we prove that NMDA computations screen off everything else?
The short answer is that we should turn to the pragmatic aspects of explanation and the various explanatory heuristics that science has adopted (for better or ill) over time. Wimsatt (1984) argues that we should relax our notion of screening off. In actual science, with real-world constraints, we perform a cost-benefit analysis so that we would say A "effectively" screens off B if adding B to our explanation increases our understanding of the effect by only a small amount and B is difficult or expensive to procure. Determining whether some variable screens offanother is partially a pragmatic decision. In all explanations, some otherwise relevant events are set aside as not being significant enough to warrant including. Not all causal influences are created equal, and we need worry about only the most obvious in our explanations and research.
Which NCC we choose will be at least partially determined by which gives us the most bang for the buck, as it were. This concession to the social pressures on science goes a bit of the way toward solving our problem. We need not worry about the conscious experiences of creatures with brains very different from our own, since we have little to no access to their mental lives—what is it like to be a bat—and such data would be very difficult to get.
But it does not solve our central concern, the vertical question of which event, at which level of organization, in the human brain is most closely associated with consciousness. Data that separate NMDA receptor computations from their quantum effects or the formation of cell assemblies are, at least for the moment, impossible to get for intact brains. Is determining the NCC an insurmountable problem, too?
We have one explanatory move left to us, and that is to turn to previously accepted explanatory heuristics in science: Simpler is better; consiliance and parsimony are preferred; and so forth. These set the standards for ideal explanations. The best explanation of some phenomenon is one that is very simple to model mathematically, contains few variables, and dovetails nicely with previously accepted theories. These sorts of explanatory goals inform scientists' hypothesizing. No one is going to propose, much less have accepted, a complicated theory if there is a simpler one available.
In biology and neuroscience, there has been a distinct bias toward reductionism: the assumption is that the smaller the unit of analysis, the better, for the more fundamental processes occur at the lower levels of organization. With this bias in place, we should say that the NCC is most likely the smallest unit we can discern which covaries with consciousness. In this case, someone like Flohr or Hammeroff would be right: The neural correlate of consciousness is probably something like receptor computations or quantum effects.
Recently, however, this propensity toward "smallism" has come under fire. (I take the term "smallism" from Wilson 1999.) With the increasing popularity of large-scale dynamical systems explanations of brain phenomena, we are losing our unspoken agreement that the real stuff occurs down below and the surface appearances are mere reflections of the underlying causal interactions. We are finding champions of "lar-gism'' at every turn. They disdain the small as irrelevant data, and seek true understanding in the large-scale patterns that emerge out of the mess of tiny interactions, each of which is insignificant when considered alone. A largist would claim that the complex cell assemblies are the true NCC, whereas the microtubules and NMDA receptors merely support the assemblies.
Which way should we jump? Which explanatory bias should we adopt in consciousness studies? Unfortunately, the answer is not forthcoming at the moment. We are caught in an odd time; a war over explanatory biases in the biological sciences is being fought in journals and laboratory hallways around the world. Some neurobiological explanations are reductionistic; others are not. And neither type has the upper hand at the moment with respect to explanatory power, funding decisions, centrality in the profession, and the like (see Hardcastle 1999).
Hence, we cannot say with any certainty how we should understand the NCC. We are simply going to have to wait and see which side wins. Whether we should be smallists or largists in our biological explanations of mental events is currently undetermined. For now, we have only educated guesses, personal declarations of faith, and a plethora of individual research programs. But much basic research remains to be done and, more important for our concerns, our fundamental theoretical scaffolding remains to be constructed. For now, the NCC remains a truly Hard Problem with no solution in sight.
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