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An existence proof for a good alternative is potentially far more persuasive than the best critique of quantum-mechanical spirits and little-man-inside reasoning, so let me shift gears here. If we take consciousness to be important for Popper's "solution of problems of the non-routine kind," then shaping up a good-to-excellent ("quality") course of action in thought is a key aspect of consciousness, one that goes well beyond mere awareness or shifting attention. We no longer have to take it on faith that there are mechanisms capable of recursively bootstrapping random novelties into something of quality. For the last 160 years, there has been an existence proof, the Darwinian process (Calvin 1996b, 1997). The way in which quality is achieved using this process has long occupied the best minds in evolutionary biology (see, for example, Maynard Smith and Szathmáry 1995). And the slow evolution of species, on the time scale of millennia, is no longer the only example: the immune response is now known to be another Darwinian process, operating on the time scale of days to weeks as a better and better antibody is shaped up in response to the challenge of a novel antigen. For decades, computer science has used a solution-finding procedure, called the genetic algorithm (Holland 1992), that mimics an expanded version of the Darwinian process on a time scale limited only by computer size and speed. It would be surprising if the brain did not make some use of this fundamental principle for bootstrapping quality. Can this same well-known process operate quickly enough in the brain, on the time scale of thought and action? Can it account for much of what we call "consciousness"? I undertook to answer such questions in one of my 1996 books, The Cerebral Code, which analyzes the recurrent excitatory circuitry of mammalian neocortex. I showed that this widespread wiring principle was capable of running the six essential features of the classical Darwinian process: a pattern (spatiotemporal firing pattern of a Hebbian cell-assembly, in this case) that copies with occasional variation, where populations of the variants compete for a limited work space, their relative success biased by a multi faceted environment (both memorized and real-time, in this case), and with further variations centered on the more successful of the current generation (Darwin's inheritance principle). This full-fledged Darwinian process is what is associated with the recursive shaping up of quality; it should not be confused with mere selective survival of a single pattern and other "sparse sets" that utilize only a few of the "six essentials" (Calvin 1997). The best way of demonstrating what I consider an appropriate level of explanation, that of circuits involving stamp-to-postcard-sized areas of cerebral cortex (we have enough neocortex to fill four sheets of typing ), is to take the reader through an example. It's not the only possible example, just the one due to Darwin, but I expect that competing theories will also come to occupy the same level of explanation, that of emergent properties in the shifting dynamics of substantial areas of cortex. After I explain enough of the circuitry so that the reader can imagine how a Darwinian process could operate in neocortex, I will briefly return to Quantum Consciousness, trains of thought, and the explanatory coverage needed for a theory of mind. The Brain's Darwinian Circuitry The cortical circuitry that makes a full-fledged Darwinian process possible is not an obscure feature known only to a few neuroanatomists: it is easily the most prominent wiring principle seen in cerebral cortex, that of the patterned recurrent excitatory connections between neighboring pyramidal neurons in the top layers of neocortex. It has just taken a while to realize one of the implications of it, an emergent property of the circuit not possessed by any of the individual elements: synchronized triangular arrays of pyramidal neurons, with nodes about 0.5 mm apart, is what you expect to observe, some of the time. Each pyramidal neuron has an axon that branches nearly 10,000 times. Some travel through the white matter but most of the branches never leave the cortical layers, terminating in a synapse within a millimeter or so. The axon travels sideways to excite other cortical neurons, mostly other pyramidal neurons. The deep-layer (V and VI) pyramidal neurons have such sideways axons that remain within the cortical layers, some terminating nearby and others more distantly.It's the wiring seen (e.g., Lund et al 1993) in the branching of the axon of the superficial-layer pyramidal neurons (layers II and III), however, that is so striking. Their terminations are patterned: their axons are like express trains that skip a long series of intermediate stops, concentrating their synaptic outputs in zones about 0.5 mm apart. That's what makes synchronized triangular arrays likely to form on occasion. Though the 0.5 mm spacing is similar to that of macrocolumns (say, the orientation columns of visual cortex) and though it may be related to helping organize such macrocolumns during development (Calvin 1995), the analogies to macrocolumns (as we currently know them) are limited. What's intriguing is their relation to minicolumns (orientation columns are the best known examples), which are vertical columns of about a hundred neurons that are organized around a dendritic bundle in the manner of stalks of celery; the distance between neighboring bundles is about 0.023-0.031 mm (in monkey; twice that in cat: Peters and Yilmaz 1993). The nearest outputs of a superficial pyramidal neuron's axon are not 0.5 mm away but to immediate neighbors. Cells within a minicolumn tend to share interests, as in elongated visual stimuli of a particular orientation, and this local recurrent excitation is one of the reasons. Synchrony on a Small Scale One consequence of the express-train axon is that cells 0.5 mm apart will tend to talk to one another: they will recurrently excite. While a chasing-their-tails loop is one possibility if synaptic strengths are quite high, even weak synaptic strengths have an important consequence: entrainment. Since 1665, when the Dutch physicist Christiaan Huygens noticed that pendulum clocks on the same shelf synchronized their ticks within a half hour, much additional work has been done on entrainment. A dramatic example from the Phillippines was ed in Science by Hugh Smith in 1935: Imagine a tree thirty-five to forty feet high, apparently with a firefly on every leaf, and all the fireflies flashing in perfect unison at a rate of about three times in two seconds, the tree being in complete darkness between flashes. Imagine a tenth of a mile of river front with an unbroken line of mangrove trees with fireflies on every leaf flashing in synchronization, the insects on the trees at the ends of the line acting in perfect unison with those between. Then, if one’s imagination is sufficiently vivid, he may form some conception of this amazing spectacle. Dueling Choirs for Concept Competition Now imagine dueling choirs, abutting hexagonal mosaics singing different tunes, trying to recruit members at the expense of the other. Along the battlefront, there are hexagons that have both tunes superimposed, just as in a symphonic work. If the combination resonates well with the local neural network, we might speak of harmony, just as we do for the major and minor scales. Borderline superpositions (as well as the more extensive ones that can be created by long corticocortical bundles) illustrate a powerful recombination principle, a way of doing associative memories that can represent relationships with the same 0.5 mm hexagonal code space as used for objects. Another lesson of levels is that mechanisms that suffice at one level may prove to be shaky foundations, that other ways of doing the same thing are more extensible. Hexagonal codes are a much better foundation for superstructures (such as coding for analogies) than are the better-known associative memory mechanisms at the level of synaptic mechanisms for classical conditioning. This isn't the place for showing the many implications of a cerebral coding scheme based on the spatiotemporal firing pattern within one of the recurrent-excitation-defined hexagons, a book-length project that I tackled in The Cerebral Code. But with the notion of hexagonal mosaics that transiently compete for space in association cortex, you can now appreciate how a Darwinian process could operate in association cortex via the spatiotemporal patterns copying themselves sideways: Like the classical examples of a full-fledged Darwinian process, there is a pattern that is copied (indeed, what is reliably copied defined the hexagonal-shaped spatiotemporal pattern), variations occur (dropouts, off-focus nodes, superpositions), populations of the variant patterns compete for a work space, their relative success is biased by a multifaceted environment (current sensory as well as resonances with memorized patterns), and the more successful of the current patterns tend to produce more of the next round of variants (Darwin's inheritance principle is implemented because bigger mosaics have more perimeter, and the perimeter is where dropouts and off-focus nodes can escape the standardization enforced by six surrounding nodes all firing at the same time). Unlike the generation times spanning days to decades of the usual Darwinian examples, cortex operates on a time scale of milliseconds to seconds, though its operations are biased by memories that span far longer time scales. Within seconds to minutes, neocortex ought to be capable of implementing all of the classical means of accelerating the rate of evolution (systematic recombination, parcellation, rapid "climate change," and refilling empty niches).()，英语论文题目，英语毕业论文 |
