HOW DOES ONE explain the power of the human brain compared to the brain of, say, a grizzly bear or a rhinoceros, or any other living creature? Traditionally, neuroscientists have pointed to the expansion of the human cerebral cortex, particularly the frontal lobes. Yet, closer examination of this claim uncovers a series of paradoxes.

First, no cell, neural circuit, neurotransmitter or receptor is unique to the human cerebral cortex. A similar situation prevails in regard to brain functioning. The properties of the nerve impulse are similar if not identical in a squid axon, a rat nerve and a neuron in the human cerebral cortex.

A comparison of genetic endowments, or brain weight, or size isn't much help either: the total amount of DNA doesn't vary by more than 10 percent in the monkey, ox, chimpanzee and man. And several living animal species possess bigger and heavier brains than humans.

If brain weight is combined with body weight the human brain comes off somewhat better (one fortieth compared to, say, one ten-thousandth in the blue whale) but still lags behind the humble marmoset or ferret whose brain weight is one twelfth of body weight.

In Neuronal Man, already a best seller in Europe, Jean-Pierre Changeux, a neurobiologist at the Coll of the molecular laboratory at the Institut Pasteur in Paris, explores the fascinating question of how the human brain, similar in so many ways to the brains of less developed species, is able to accomplish so much more.

Changeux suggests that the complexity of the human brain is dependnt upon the vast number of synapses (connections) between brain cells. In the cerebral cortex alone, a person counting 1,000 synapses per second would require between 3,000 and 30,000 years to count them all. According to Changeux, these synaptic connections are established or fall by the wayside according to how frequently they're used. Those synapses which are in frequent use tend to endure (are "stabilized") while others are eliminated via "Darwinian selection." Since this process extends over a lifetime, the human brain is best understood as a dynamic structure which evolves over many years. This is in stark contrast to the brains of other creatures which display an initial period of vigorous synaptic activity followed by little change over the remainder of the lifespan.

In the human brain, according to Changeux, "interactions with the environment contribute to the formation of a more and more complex neuronal organization. . . . One of the advantages of the evolution divergence that led to Homo sapiens is, of course, the increased capacity of the brain to adapt to its environment, with an accompanying increase in the ability to form and combine mental objects." In other words, man's environment has a tremendous influence on the way the human brain works and how it has evolved.

Based on his theory of "selective stabilization," Changeux suggests that it is time for a revision of some of our ideas about the relationship between the mental, or spiritual, and the physical. And what he finds is that our brains are best understood as physical mechanisms. "Man," he proclaims, "no longer has a need for the 'Spirit'; it is enough for him to be Neuronal Man."

In support of this thesis, Changeux points to the new technology of PET scanning (positron emission tomography) whereby radioactive isotopes can be localized and displayed in living color in those areas of the brain most active at a given moment. Listening to music, a professional musician, for instance, isplays a distinct color coded pattern on his or her PET scan which differs significantly from the musically unsophisticated listener.

With additional advances in technique, it's likely, Changeux believes, that positron images will make possible for us to one day see the concrete picture of a subject's thought processes. He believes that this will enable neuroscientists to "destroy the barriers that separate the neural from the mental and construct a bridge, however fragile, allowing us to cross from one to the other."

When this ambitious undertaking has been accomplished, there will no longer be "justification for a split between mental and neuronal activity. What is the point," Changeux asks "of speaking of a 'mind' or 'spirit'?"

Changeux presents his stridently materialistic view with verve, conviction and an admirable lucidity. Neuronal Man should appeal to a wide audience on this side of the Atlantic. Overall, he accomplishes his purpose ("to inform and, if possible, interest readers in the neurosciences"). He does so, however, by omitting or downplaying several other important facts about the brain.

For one thing, each brain is biologically unique and "specific to the particular environment in which it has developed." It's unlikely, therefore, that a PET scan, however sophisticated, will ever be capable of capturing the subtleties of neuronal activity in a particular brain. Reading this paragraph, for instance, results in an activation of different brain areas in different readers according to their interests and backgrounds. Neither PET scans nor any technology presently conceivable would be capable of distinguishing this degree of uniqueness. Furthermore, to a great extent biological individuality is no less striking and impenetrable than the "vitalist" or "spiritualist" models of the past which Changeux inveighs against with such vigor.

In addition, would the emergence of a neurophysiological explanation of human mental activity (Neuronal Man) represent an advance in self-understanding over theories of the past which employed philosophical, religious or, more recently, psychological concepts? Changeux clearly feels that this transition is necessary and beneficial albeit dangerous. "Any research that directly or indirectly challenges the immaterial nature of the soul, threatens faith and risks being burned at the stake."

Such melodramatics aside, it's likely that Neuronal Man will be no more insightful than his less scientifically sophisticated predecessors. Indeed, a purely neurobiological explanation of ourselves contains a Catch-22 which Changeux elects not to introduce into his discussion.

Everything that can be said about the brain is itself the product of the brain and therefore limited by the constraints under which all human brains must operate. "Men judge things according to the organization of their brain," as Benedict D. Spiut it in 1677 in Ethics.

Our logic, reasoning, and conclusions, therefore, are as determined by our brain's structure and function as the final shape of a nest is determined by the brain of the robin which constructed it.

If we are truly entering the age of Neuronal Man, as Changeux suggests, we may find that although we've altered our vocabulary and our focus and have replaced philosophers with neurophysiologists, we still have a long way to go when it comes to the goal of comprehending mankind's greatest koan: self-understanding.