Artificial Consciousness/Neural Correlates/Neural Models/Depolarization vs Partial Depolarization
Depolarization vs Partial Depolarization
[edit | edit source]Although it was originally thought that it was only when cells actually fired that they transmitted neuro-transmitter across the synaptic gap, scientists have noted that in many cases the wave-form generated at the recieving neuron is sensitive to the area under the graph, not just the number of spikes in the waveform. This approach suggests that the amount of Neuro-transmitter secreted increases with the voltage, and even partial depolarization influences the network. While early network models were satisfied with binary values that were 1 when the neuron fired, today, in order to capture things like Habitutation we need to model the neuron with at least an integer value, if not a floating point value.
If this is the correct approach, then what firing actually does is clip the high voltages off the top of the neural output, and replace them with multiple spikes that may reach a higher voltage than the graph would otherwise, but quickly drop to lower voltages keeping the net voltage within a comfortable range for the cell. In essence what this does is protect the cell membrane while expressing a range of function that exceeds the dielectric strength of the cell membrane. The net effect is to express the lower voltage, without sustaining it at a level that will damage the cell.
This means that the more often a cell fires in a set period of time, the less healthy it might be, because of loss of membrane coherence. Scientists however have noted that in some cases the cells firing is almost constant, creating a train of signals instead of a single spike. In this case it is less important that the cell fire, than that it sustain a particular voltage over time. As a result, scientists have started to suggest that in fact, the neurons seem to compete at pre-firing voltages, and only selected neurons actually are allowed to fire. When arrays of micro-contacts are embedded in the brain, it is easily seen that in fact neurons especially in the Cerebral cortex, are clustered into neural groups where one neuron tends to express the activity of the group as a whole and the other neurons in the group are suppressed. However over time, the neural group may change its active neuron without changing the actual activity of the group as a whole, this allows neurons to rest if they have become fatigued. The idea that a group of neurons might fire as a single unit is sometimes called Center Surround, in that it is assumed that the neuron that fires is the exemplar, or centroid of the group, and that the surrounding neurons are suppressed.
While we are not sure of the exact mechanism of the center surround, we suspect it has something to do with the layer six pyramidal neurons, and with cerebral columns, especially in the left hemisphere of the brain. Some scientists have noted that the right hemisphere does not act in this manner, but as yet no mechanism has been offered to explain why. It should be noted that firing is important to the production of tag proteins that are sequestered by the synapse, and therefore that limiting the number of neurons that fire at a particular point, has the effect of narrowing the number of neurons that remember a particular memory. Some have suggested that this means that the right hemisphere is better at modeling minor variations in a concept while the Left hemisphere is better at making memories seem discrete and separate. The center surround effect might make the difference between these two different operating styles.