[From Bill Powers (941024.1105 MDT)]
Bruce Buchanan (941024.00.10 EDT)--
My interpretation of the *question* is: how does activity of nervous
tissue relate to thinking, what are the structures involved and how do
they function? This may have a bearing upon the characteristics of
models of the processes involved. The point is that the new findings
introduce more possibilities for complex multidimensional functions.
I can give a general answer but not a specific one. In your comments of
yesterday you mentioned that neural activity involves "brief" events,
neural impulses and jolts of chemicals entering neural cell bodies. I
think this is looking too closely at the processes, like trying to
explain an electronic circuit in terms of individual electron
trajectories.
I can make more sense of the system by thinking in terms of neural
frequencies (impulses per second) and chemical concentrations that are
adjusted upward and downward by changing the frequency with which
neurotransmitters are sent across synaptic junctions. Inside the
receiving cell body, concentrations rise and fall as a consequence of
continuing parallel streams of incoming neurotransmitters, both
excitatory and inhibitory, entering the cell in various locations and
contributing to the net concentrations around the axon hillock, where
outgoing impulses are generated. Most neurons do not respond to single
impulses; rather, the chemical concentrations near the axon hillock
adjust electric potentials and thus govern the rate of firing of the
neuron. This doesn't work for all types of neurons, but it's a useful
general picture.
With that basic picture, we can see neural activity as involving
continuing streams of impulses going from one location to another, and
continuously-variable chemical concentrations inside neurons which make
the frequency of the output stream of impulses (one output per neuron)
depend on the frequencies of multiple input streams. Each neuron thus
acts as an analog computer, not a digital computer.
My mental model of the system identifies these ongoing activities as the
objects of awareness. Since I don't know what awareness is or how it
works, this is strictly a working hypothesis. My image comes from analog
computing, a branch of computing that has unfortunately become almost a
lost art. In an analog computer the computations are done without
benefit of symbols. The signals themselves are the variables which show
the results of the computations, and the computations are physical
processes which have the same form as the physical processes being
modeled (they are analogs of the modeled physical processes).
To model the response of a mass to a force, for example, we represent
the magnitude of the force by one voltage and the velocity of the mass
by another voltage. The first voltage enters a computing element where
it is converted into a current running into a capacitor; the voltage on
the capacitor is the integral of the input current in amperes, divided
by the capacitance in farads. Thus the output voltage on the capacitor,
the second voltage, is the time-integral of the input voltage. The
voltages are related just as velocity and force are related, with
capacitance corresponding to mass.
Now if the input voltage is made to vary as a force is observed to vary,
the output voltage will at all times have a magnitude that corresponds
to the velocity of the mass. We can predict what the velocity of the
real mass will be if we apply a force to it that varies in any complex
way with time, just by applying a corresponding voltage to the input of
this little analog computer and recording the changes in output voltage.
When I use an analog computer, I must use a voltmeter or an oscilloscope
to observe the inputs to the computation and the outputs from it. I must
apply the proper conversion factors so I know, for example, that an
output voltage of 1.5 volts means a velocity of 30 meters per second,
and an input voltage of 2.0 volts means an applied force of 1.0 newtons
of force, with the capacitance in farads corresponding to the mass in
kilograms.
The important fact about using an analog computer is that the observer
knows about the results of the computation only by receiving information
about the behavior of the voltages. Even if the observer is not
observing, the computation is still going on.
So this is my image of the relationship of the aware human observer to
the operation of brain functions, at least at the lower levels. The
analog computations go on continuously, input signals being converted
into output signals according to the forms of the analog computing
functions that apply. The output signals become inputs to other analog
computations. And all these signals become the world that the Observer
experiences, just like the user of an analog computer who is applying
voltmeters to the wires to see what the computer is doing.
As to "The point is that the new findings introduce more possibilities
for complex multidimensional functions", we don't have to look far to
find the possibilities of as much complexity as we can grasp. Each
neuron is an analog computer, and the individual computers can be hooked
up in any possible way to do anything you can define. In a way we're
like a kid who is poking around a huge electronics lab. The kid picks up
a resistor and asks "What does this thing do?" The electronic designer
is hard-pressed to answer. From one point of view it just determines how
much current will flow when a given voltage is applied to it. From
another, it fixes the gain of an operational amplifier, or the time-
constant of a filter, or the output of a voltage divider, or limits the
heat dissipation in another component -- the list is endless. What it
does depends on what it's used for, and its relationship to all the
other components in the whole system. By itself it doesn't do anything
interesting.
So what do the astrocytes do? Anything you can imagine, depending on how
they're connected to each other and to the main neural system. Could
they have to do with memory? With consciousness? With life support for
the neurons? With reorganization? Sure, any of the above and anything
else you can think up. By themselves, they probably don't "do" anything,
any more than a resistor does.
This reminds me, by the way, of how some people talk about
neurotranmitters. You can tell that they have never had much to do with
system design. A neurotransmitter is just a way of connecting one
variable to another. The fact that neurotransmitters of a certain type
are found in certain places in the brain doesn't tell us anything about
function -- it just tells us that there is some neurotransmitter there.
Instead of worrying about what the neurotransmitter "does", we should be
looking at how it participates in the circuits in a certain place. It's
like noticing that some parts of an electronic circuit use electrons to
carry signals, and others use holes, except that in the brain there are
a dozen or so types of message-carriers. The fact that you find a lot of
electrons flowing in a given part of a circuit doesn't mean that the
electrons explain what that part of the circuit does, or "cause" the
kind of behavior we see. The electrons in an audio amplifier are
certainly necessary for the amplifier to work, but the connections, the
organizations of circuit relationships, tell us what the circuit _does_.
The same circuit could be designed to use holes as the signal carriers,
and it would still do the same thing. I imagine that the brain uses an
assortment of neurotransmitters mainly as a substitute for insulation --
to keep signals from one system from leaking into another system and
interfering with its operation.
···
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"Thinking" is such a broad term that it would be hard to make any
general statement about its relation to nerve function. Do you mean
running sentences through your mind? Mathematical formulas? Estimating
how a swatch of color will go with the living-room decor? Wondering
whether your egg salad sandwich is swarming with Salmonella? Considering
the opening chords of Grieg's piano concerto? The term thinking covers
just about any covert activity in consciousness, and I don't think it
can be associated with any one kind of brain function. I've tried to
substitute semi-technical terms for general words like thinking, so we
can at least attempt to characterize the kind of functions that are
going on.
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Best,
Bill P.