Lately I’ve been researching feedback controllers in the brain,
and I had this idea about how neurons could implement them.
Background: The purpose of a closed-loop controller (aka feedback controller) is to bring a measurable quantity (the sensory input) to some desired target value (the setpoint) by commanding a motor system. Inside of every feedback controller is a subtraction, when it compares the setpoint with the sensory input to determine which direction to go towards. A defining feature of closed loop controllers is the subtraction and how that’s implemented. There are many different ways that the brain could implement subtraction, and this post proposes yet another.
Hypothesis: Dendrites can implement a simple comparison operator based on the fact that inhibition can block excitatory NMDA-events from reaching the soma, but only if the inhibition is located between the soma and the NMDA-event. If the inhibition is further down the dendrite than the NMDA-event then it won’t block the NMDA-event.
Example: NMDA-Event does NOT reach soma:
vvv Inhibition
Soma O================================== Dendrite Tip
^^^ Excitation
Example: NMDA-Event DOES reach soma:
vvv Inhibition
Soma O================================== Dendrite Tip
^^^ Excitation
The dendrite is arranged like a number line, with each segment along its length detecting one number with both excitatory and inhibitory synapses. The excitatory synapses represent the setpoint and the inhibitory synapses represent the sensory feedback. In order to detect both positive and negative results, you need two cells with their number lines arranged in opposite directions.
I do not know it could learn this arrangement of synapses.