When NeuroBytes are first powered up, they default to Integrate-and-Fire mode (IAF). In this mode, the LED indicates the current membrane potential of the simulator. Absent external stimulus, NeuroBytes stay at a membrane potential of zero and the LED remains pure green. This point is known as the resting membrane potential. As the membrane potential increases, the LED fades from green to yellow, orange, and red, as the action potential threshold is approached. Once this point is exceeded, the LED flashes bright white to indicate a firing event. Conversely, when the NeuroBytes board is hyperpolarized (i.e. the membrane potential is below the resting membrane potential level), the LED starts to turn blue:
Membrane potential is modified via dendritic stimulation. Each dendrite has a specific weighting constant, and depending on the type of input cable (excitatory or inhibitory), will either increase or decrease the membrane potential by the magnitude of this constant, shown as a percentage of the board's action potential threshold:
As long as the stimulus is maintained, the NeuroBytes board holds its new elevated (or depressed, in the case of inhibition) membrane potential. When the stimulus is removed, the membrane potential exponentially decays back to its resting membrane potential:
If the stimulus causes the membrane potential to push past the action potential threshold, or if multiple stimuli arrive quickly enough to sum above the threshold prior to decaying, the NeuroBytes board will fire:
A firing event causes the NeuroBytes board to send a brief pulse through its axon to any devices connected downstream:
After a NeuroBytes board fires, its membrane potential briefly dips below zero, entering a hyperpolarized state known as the refractory period. This can be seen in both videos above; immediately after the white flash, the LED turns blue. During this period the board is less sensitive to dendritic stimulation.
The concepts above are best illustrated in the time domain, where the y-axis represents membrane potential:
If sufficient NeuroBytes are connected in a line, the last action potential will occur after the first board finishes its refractory period. The user can then feed the network back onto itself, creating an infinite loop that will run until power is removed: