The only biography I could find. From a list of great Canadian scientists (the tag is to the start of the H's, you'll have to scroll down a screen or two).

  "The problem was this, Licklider had explained: Through a microscope, most of the brain appears to be a study in chaos, with each nerve cell sending out thousands of random filaments that connect it willy-nilly to thousands of other nerve cells. And yet this densely interconnected network is obviously not random. A healthy brain produces perception, thought, and action quite coherently. Moreover, the brain is obviously not static. It refines and adapts its behavior through experience. It learns. The question is, How?

  "Three years earlier, in 1949, Hebb had published his answer in a book entitled The Organization of Behavior. His fundamental idea was to assume that the brain is constantly making subtle changes in the "synapses," the points of connection where nerve impulses make the leap from one cell to the next. This assumption was a bold move on Hebb's part, since at the time he had no evidence for it whatsoever. But having made it, he argued that these synaptic changes were in fact the basis of all learning and memoiry. A sensory impulse coming in from the eyes, for example, would leave its trace on the nerual network by strengthening all the synapses that lay along its path. Much the same thing would happen with impusles coming in from the ears or from the ears or from mental activity elsewhere in the brain itself. And as a result, said Hebb, a network that started out at random would rapidly organize itself. Experience would accumulate through a kin of positive feedback: the strong, frequently used synapses would grow stronger, while the weak, seldom-used synapses would atrophy. The favored synapses would eventually become so strong that the memories would be locked in. These memories, in turn, would tend to be widely distributed over the brain, with each one corresponding to a complex pattern of synapses involving thousands or millions of neurons. (Hebb was one of the first to describe such distributed memories as "connectionist.")

  "But there was more. In his lecture, Licklider went on to explain Hebb's second assumption: that the selective strengthening of the synapses would cause the brain to organize itself into "cell assemblies"-subsets of several thousand neurons in which circulating nerve impulses would reinforce themselves and continue to circulate. Hebb considered these cell assemblies to be the brain's basic building blocks of information. Each one would correspond to a tone, a flash of light, or a fragment of an idea. And yet these assemblies would not be physically distinct. Indeed, they would overlap, with any given neuron belonging to several of them. And because of that, activating one assembly would inevitably lead to the activation of others, so that one assembly would inevitably lead to the activation of others, so that these fundamental building blocks would quickly organize themselves into larger concepts and more complex behaviors. The cell assemblies, in short, would be the fundamental quanta of thought."

  "What about the computer? Does it think, and if so does that make it conscious? For the present at least, the answer is that it does not think in the sense that human beings and other mammals think and so is not conscious in the same sense. Furthemore, the mammalian brain is enormously more complex than any present computer, not only in the number of functional elements but also in its connections, the individual neuron frequently having synaptic connection with upwards of a thousand others. What I propose is that we have here a parallel with the physicist's concept of a critical mass. Consciousness, that is, depends on a critical degree of complexity of neural action. Probably also it requires the kind ofr pattern of complexity characteristic of the structure of mammalian cortex. The individual neuron then is not conscious, nor any small group of interacting neurons. Consciousness appeared in evolution when thought became possible, and ther is no evidence of thought in lower animals, even those with quite extensive nervous systems. It probably exists in birds such as the crow, but it has not atually been demonstrated except in mammals. In them the cortex is well developed, but vestigial or absent in other animal forms.

  "The argument then is that a computer built on the plan of the mammalian brain, and of a complexity at least equal to that of the brain of the laboratory rat, might be conscious- given the same capacity to learn and a suitable early experience. This is unlikely, but conceivable"

In this passage, Hebb has graciously given me the most concise and elegant statement of my major: that consciousness is a direct product of the neural complexity of the brain (and implicitly, the complexity of their environment: the evidence for requiring "suitable early experience" can be found both in the deficient mylenization of under-stimulated rats and in the absence of language in so-called "feral" children). I think Gerald Edelman has done more to further this program than anyone else, resolving that complexity of connections to "reentrant circuits" of multiply-overlapping aggregates of neurons (circuits defined by their firing in tandem, say, all at forty hertz). The major modification, as I see it, that Hebb's original theory needed to undergo, was that these complex neural circuits are themselves adaptively arrived at, through the feedback mechanisms of experience. Edelman's Darwin machines (up to Darwin III, I believe) are attempts to build robots whose "cognitive maps" are adaptively-built response sets to certain patterns of experience. Too many artificial intelligence (AI) and artificial life (alife) programs have tried to "unwrap the problem" (in John Holland's colorful phrase) by putting their hypotheses about how consciousness works into the programs from the outset. In AI, this amounts to having your would-be Turing machine regurgitate canned phrases without trying to have the program "understand" the question or comment, and generate a "live" response (a gut-level reaction, as it were). In alife, "unwrapping" begins by looking at high-level organisms or even entire ecosystems and having your genetic algorithms mimic their gross behaviors from the outset, rather than letting generations of the program adapt to their changing environments and seeing if organization and speciation emerge through natural selection.

But I digress. Back to Hebb, our Galileo of the mind (if Freud was our Ptolemy!).

From The Organization of Behavior, a brief outline of Hebb's connectionism:

  "Any frequently repeated, particular stimulation will lead to the slow development of a "cell assembly," a diffuse structure comprising cells in the cortex and diencephalon (and also, perhaps, in the basal ganglia of the cerebrum), capable of acting briefly as a closed system, delivering facilitation to other systems and usually having a specific motor facilitation. A series of such events constitutes a "phase sequence"-- the thought process*. Each assembly action may be aroused by a preceding assembly, by a sensory even, or--normally--by both. The central facilitation from one of these activites on the next is the prototype of "attention." The theory of proposes that in this central facilitation, and its varied relationship to sensory processes, lies the answer to an issue that is made inescapable by Humphrey's (1940) penetrating review of the problem of the direction of thought.

  "The kind of cortical organization discussed in the preceding paragraph is what is regarded as essential to adult waking behavior. It is proposed also that there is an alternate, "intrinsic" organization, occuring in sleep and in infancy, which consists of hypersynchrony in the firing of cortical cells. But besides these two forms of cortical organization there may be disorganization. It is assumed thatthe assembly depends completely on a very delicate timing which might be distrubed by metabolic changes as well as by sensory events that do not accord with the pre-existent central processes. When this is transient, it is called emotional disturbance; when chronic, neurosis or psychosis.

    "The theory is evidently a form of connectionism, one of the switchboard variety**, though it does not deal in direct connections between afferent and efferent pathways: not an 'S-R' psycholgy, if R means muscular response. The connections serve rather to establish autonomous central activities, which then are the basis of further learning. In accordance with modern physiological ideas, the theory also utilizes local field processes and gradients, following the lead particularly of Marshall and Talbot (1942). It does not, further, make any single nerve cell or pahtway essential to any habit or perception. Modern physiology has presented psychology with new opportunities for the synthesis of divergent theories and previously unrelated data, and it is my intent to take advantage of these opportunities as I can."

*AMAZINGLY similar to Edelman's notion of a global mapping (of circuits) as the basis of consciousness. Was this guy a visionary, or what? **Earlier in this chapter, Hebb put all brain-based theories of the mind into two broad categories: those that looked for complex switchboard connections (before the phone company used computers to route calls) between tangled webs of neurons and those that looked to some kind of physical field, calling the brain an essentially homogenized medium.