Hybrid analog/digital computing technologies and the human mind/brain

[Judith Rosen <***>]

Another interesting find:

FROM: http://brain.web-us.com/brain/digital_brain_the_extraordinary_.htm
The Digital Brain
Individual brain neurons contribute
to the brain 's unmatched complexity

The computing power  inside your skull vastly exceeds that of any supercomputer in the world. But for the past  half century, neuroscientists have generally supposed that the process by which the brain  achieves its phenomenal performance is fundamentally similar to the way electronic  computers work. According to the conventional view, thinking takes place through the  aggregate action of billions of simple elements--cells called neurons--that are wired up in an extremely complicated way. Supercomputers are likewise built of millions of  interlinked simple switches, consisting of transistors on silicon chips by companies such  as Intel.

Recent research is forcing a  re-evaluation of this standard model. Individual neurons, it turns out, are not so simple  after all: experiments have shown that they can actually perform surprisingly complex  calculations and register fine discriminations. It is even possible that networks of  interacting molecules within an individual neuron might perform specific computations, Christof Koch of the California Institute of  Technology reported in the January 16, 1997 issue of Nature.  The organ of thought is looking far more complex than scientists believed just a few years  ago.

Koch's conclusions are based on studies of the precise electrical behavior of cells in  the brain. Neurons conduct signals in the form of tiny electrical impulses, known as spikes. Messages travel from  one neuron to another as pulses of chemicals that are released at specialized junctions,  or synapses; there are trillions of such junctions in the human brain. How and when  synapses relay messages between neurons is crucially important in controlling mental  activity. Moreover, neuroscientists believe that learning occurs through a change in the  strength of certain synaptic connections. A frequently-used synapse becomes stronger,  whereas an infrequently used one may grow weaker over time.

Researchers have long understood the basic division of function in the neuron. One set  of branch-like extensions from the cell bears incoming synapses; another set of branches,  usually located at the end of a long threadlike extension, processes outgoing messages. In  the traditional view of the neuron, which goes back to experiments conducted in the 1940s,  the cell functions as a fairly simple on-off switch. A spike would be initiated in a  neuron if the total amount of stimulation at all the incoming excitatory synapses exceeded  some critical level. (Conversely, if the neuron received enough inhibitory synaptic  signals, it would stop producing spikes.)

Yet as Koch observes, scientists have discovered that neurons actually have numerous  electrically-active components in the incoming branches. These active components, which  include the NMDA receptor, a  protein that spans the neuronal membrane, modify the effect of incoming messages. For  example, the active components ensure that spikes received at synapses that are adjacent  to one another carry more weight than spikes received at widely-separated synapses.  Computer simulations show that active elements probably multiply the influence of  adjacent synapses, rather than merely adding them together as the traditional neurologists  had supposed. This finding adds a layer of complication to the picture of how the brain  works.

And the story gets still more involved. Koch notes that the conventional idea that the  timing of individual spikes is unimportant turns out to be quite wrong. Researchers had  generally supposed that the representation of information in the brain depends essentially  on the overall rate of firing of the neurons. But experiments over the past few  years have shown conclusively that some cells in monkeys' brains can adjust the  intervals between spikes in increments as little as one hundredth of as second. Moreover,  the temporal patterns of spike activity across different neurons is sometimes controlled  with an even finer accuracy of about one thousandth of a second. Contrary to the common  wisdom, "the brain appears to care a great deal about timing," Koch says.


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BioTheory: An electronic journal of general science based on the Relational (Rosennean) Complexity Paradigm