[From Rick Marken (2003.07.03.01015)]
Marc Abrams (2003.07.03.0039)--
> Rick Marken (2003.07.02.2000)--
> The Figure shows that the perceptual signal in this is carried by sensory
nerve impulses.
"THE" perceptual signal? One perception = one signal?,
Yes, one perceptual variable = one signal.
Do you actually mean _one_ signal?
Yes.
What are 'nerve impulses'?
The traveling change in voltage polarity across the cell membrane of a neuron that
is known as a "spike" or action potential. PCT assumes that a neural signal
(perceptual, reference, error) is the average spike rate.
What happened to the chemical ions in the neurons? Do they play a part in
synaptic actions and neuronal communication? Exactly where in B:CP should I
find these answers?
I think that's a level of detail that is unnecessary. The PCT model is at the
level of circuit analysis in electronics. To build a functioning circuit you don't
need to know the details of the atomic structure of the conductor (though its
certainly nice to know it) just its resistance and capacitance. In PCT, you don't
have to know the ionic basis neural impulse conduction (though its certainly nice
to know it), just that these impulses are generated at differential rates
depending on the level of input stimulation to the cell.
> The reference signal is carried by efferent nerve impulse,
Did you know that many neurons are _bi-directional_? How do you know the
'reference signal' ( again, only one signal? ) is efferent?
Neurons that function as reference signal carriers must carry their impulses away
from the central nervous system. Some neurons may carry impulses bidirectionally.
That's certainly interesting and it may have implications for how control loops
are actually architected in the nervous system. But it doesn't lead me to revise
my assumption that the nervous system is a perceptual signal controller
Where does this 'reference' signal come from?
Reference signals exist at many levels in the nervous system. The reference
signals for the spinal motor loops described on p. 83 are presumably carried by
neurons that descend through the spinal cord from connections in the cerebellum;
the references signals for the control loops in the cerebellum are presumably
carried by neurons that descend from the next level up in the nervous system, the
cerebellar cortex.
Don't tell me from the level above.
Oops.
Again you do not mention the
chemical component in intercellular communication.
I think this is an unnecessary level of detail. What's relevant to PCT is that
there be a way for the nervous system to carry signals of varying magnitude from
one place to another. Clearly, there is a way. Exactly how it is done is
interesting but not of crucial functional significance to the model.
Where do you think the 'electrical impulses' come from?
As far as I can recall, they are carried by a change in potential across the cell
wall of the neuron, the change at one point being caused by the electrical force
caused by the change at the previous point along the cell, all this being stated
by electrical stimulation at the "input" to the neuron, which is typically the
dendrites, though I believe there are cells sans dendrites so the input to these
cells must be the cell body itself.
>the reference efferent neurons in this case descending down the spinal
cord.
Where did they originate? These of course being 'motor neurons' right?
As I said, they originate in higher levels of the nervous system. They are (from a
PCT perspective) mistakening called motor (or efferent) neurons because they were
thought to directly cause motor output. In fact, all efferent neurons except
those directly connected to the environment, as in the spinal motor loop, are
probably reference neurons, specifying the intended firing rates of sensory input
neurons.
A ganglia is nothing more then a cyst. What is a 'motor ganglia'.
It's a collection of cells bodies of "efferent" (what we would call reference)
neurons. So they are likely to be the anatomical site of a collection of
comparators.
There is a
part of the brain known as the basal ganglion, and in the visual cortex
there are ganglion cells, but I never heard of a 'motor ganglia'.
There are motor ganglia but they are apparently distributed throughout the body,
not just near the spinal cord.
The cell
body does nothing. The cell is a highly specialized and compartmentalized
entity and the cell body simply holds the parts of the cell. The only way
that cells speak to each other is through ionic channels. Most neurons do
act independently. They 'act' in 'patterns'. Might I suggest _I of the
Vortex_ by Rodolfo Linas 2001 MIT Press. Dr. Llinas is the Chairman of the
Physiology and Neuroscience Department at NYU's Medical School here in NYC.
Either that or look for the Cliff notes in your nearest book store.
I'm willing to take a look if you can convince me that what I learn will affect my
understanding of the functional nature of the perceptual control system model and
how it explains purposeful behavior. What does the fact that nerve cells "act in
patterns" have to do with the fact that a person balancing a broom on their hand,
say, is controlling perceptions of torque, velocity and so on.
Huh? You mean _all_ sensory inputs are 'inhibitory' and _all_ motor outpouts
are 'excitatory'?
Not necessarily. It can work either way as long as negative feedback around the
loop is preserved.
How can a synapse be efferent?
The synapse is not efferent; the neuron is efferent. The synaptic connection of
efferent neuron to "comparator" cell body can inhibit (prevent the comparator cell
from propagating an impulse) or excite (facilitate propagation).
A synapse is nothing more
then a cleft between two neurons. And finally, what is a 'firing rate'?
The inverse of the time between neural impulses (spikes). Dag does a rather nice
vocal imitation of variations in neural firing rates. Give him a call and treat
yourself.
What
gets 'fired' from one cell to another? This is real bad. Not only is it
totally wrong, but it makes the rest of the model, which has some
_excellent_ properties, look like ^%#&^%.
My admittedly simplistic understanding of neural conduction is that impulses
(spikes) carried down the axon of one neuron (A) lead, via synaptic connection, to
activation of an impulse that goes down the axon of the next neuron (B). So a
neural impulse in neuron B is "fired" by a neural impulse in neuron A. What is so
"real bad" about that?
On and on you go. What the hell is an 'error' signal? Is it a special type
of coded transmission between cells? How would you know an 'error' signal
from a 'reference' signal?
An error signal is a signal carried by a comparator neuron, representing the
difference between the firing rates of a perceptual and reference neuron.
According to PCT, there is no coding involved in the transmission of any neural
signals. It's the magnitude of the neural signal (in terms of neural current,
impulses/sec) that matters. You can, in principle, tell an error signal from a
reference signal in terms of function or in terms of the anatomy of the neurons
carrying the signals.
How does the receptor 'sense' the weight?
I don't know the exact mechanics of sensor function. But I don't think the Golgi
tendon receptors sense weight; just tension in the tendon. Weight would be sensed
at a higher level
> Tension on the Golgi receptor causes neural firing in the sensory nerve,
> the rate of firing being a perceptual representation of the tension on the
>tendon.
This sounds 'wonderful'. Where is the feedback and control? I just see
motor output. What if the receptor 'senses' wrong? How does the 'Golgi
Receptor' adjust?
The feedback control is through the muscle to the Golgi receptor. The variable
under feedback control is (of course) the perceptual signal coming out of the
Golgi receptor representing tendon tension. The control loop keeps perceived
tension at the reference. If the reference is constant, muscle tension will vary
to compensate for any load variations to keep tendon tension constant. What is
controlled is what the sensor senses. Depending on how what the receptor senses
is wrong, the system either won't work (if the perceptual signal is the inverse of
the tension, say) or will work poorly (if the sensor resolution is poor).
> A more thorough picture of the parts of the brain and spinal cord that are
> thought to be involved in three lower levels of the hierarchy is shown on
> p. 117, Figure 9.1.
Very nice, but not quite accurate.
I don't suppose you would consider explaining _why_ it's inaccurate?
> The first order control systems are in the spinal cord, the second order
> systems are in the cerebellum and the third order systems are in
> the cerebellar cortex.
Says who?
It's a guess based on the neuroanatomy.
Outdated, and old ideas about the brain had it divided
'functionally', that is, certain parts of the brain performed certain
functions.
OK. Let's say all my ideas about brain functionality are outdated. How should this
affect my understanding of purposeful behavior? Should I change my ideas about the
nature of perception? The nature of control? About the idea that behavior is the
control of perceptual variables?
How about giving just one, specific example of how new findings about how the
brain works leads to a specific change in the PCT model.
Best regards
Rick
···
--
Richard S. Marken, Ph.D.
Senior Behavioral Scientist
The RAND Corporation
PO Box 2138
1700 Main Street
Santa Monica, CA 90407-2138
Tel: 310-393-0411 x7971
Fax: 310-451-7018
E-mail: rmarken@rand.org