Judging and Deciding
Some take the view that the DNA creates the brain because in the past
this neural net generated motor acts that enabled the organism to
survive. Others say, "No! It is the cell, as a whole, that creates
using the DNA as blueprint. We should ask ourselves, "Is mental life
a byproduct? Or is just the expression of a normally functioning
nervous system?" Either way, we assert that the soul (mind) has no
causal powers. That neural action is complete, in and of itself.
A challenge, for one who would explain brain action in this light, is
to show how neurons can judge and decide without the help of a soul
(mind). We would have a material world, a world of neurons responding
only to afferent pulses. Descartes postulated a soul (mind) with causal
powers. He saw the pineal body as the nexus through which the soul
(mind) exercised its will. The Princess Elizabeth denied him. She said,
"And I admit it would be easier for me to concede matter and
extension to the soul, than the capacity of moving a body and of being
moved, to an immaterial being."
Most men today should probably opt for Descartes' position. The
running debate over free will evidences this. Men want a ghost in the
machine to explain their beautiful thoughts.
We want a mechanical brain, an agglomeration of neurons. Each neuron
accepting weighted afferent pulses according to its personal history.
How they are weighted is another story, presently we are only concerned
with judging and deciding.
The thalamic reticular nucleus holds a unique position in the brain.
All the efferents of the neurons that comprise it are inhibitive, and
they course only to other thalamic nuclei. All the axons that link the
thalamus to the cortex and sub cortical nuclei, and the reciprocal
axons that pass the other way, establish synaptic branches en passant.
The thalamic reticular nucleus stands ready to halt any and all
relaying in the entire thalamus. It is in a position to halt any
sensory input on its way to the neocortex, thus allowing, thus allowing
the neocortex to associate (think). It is in a position to halt any
motor program on its way to the premotor and motor cortex, thus
allowing the controller-initiator-central pattern generator to proffer
another, and different, motor program.
The axons that pass through the TRN, and are active in the current
association in the neocortex, and those from the central pattern
generator that is active, fire on synapses in the TRN. Any of these
synapses that were active in a past bad outcome involving the present
association and motor program are heavily weighted. If present
conditions suggest a bad outcome (based on past experience), the motor
program will be halted. The central pattern generators are free to
suggest other motor programs. Some other central pattern generator will
fire and the process will be repeated. We call this judging. The
thalamic reticular nucleus needs no controlling soul (mind), the
percentage of bad synapses control it.
Language is such a succession of motor programs.
The neurons of the thalamic reticular nucleus are also enervated by the
reticular formation in the brainstem. It acts to inhibit the TRN, and
thus to disinhibit the thalamic nuclei. If the reticular formation, the
original pre-vertebrate brain, is sufficiently exercised it will fire
and the motor program will proceed. There may be trouble, but there
will be action. This is the mammal's way of getting Buridan's Ass
off dead center.
Whichever way (a suitable motor program or an exercised reticular
formation) the thalamic reticular nucleus is inhibited. We have
decided. We need no ghost in our machine.