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. 2018 May 28;9:346. doi: 10.3389/fneur.2018.00346

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Copyright © 2018 Otero-Millan, Optican, Macknik and Martinez-Conde.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Schmitt trigger. (A) Positive feedback loop. (B) Hysteresis function corresponding to (A). When the system is in state A, the input needs to reach the value T to switch to state B. When in state B, the input needs to reach −T to switch to state A. (C) Effect of hysteresis. Top, example of fluctuating input. Middle, changes of state for a simple threshold (red). Bottom, unwanted switches (in middle panel) can be reduced by having hysteresis with two thresholds (green). (D) The same positive feedback loop as in (A) formed by double inhibition through brain stem neurons. Because burst neurons (BNs) inhibit omnipause neuron (OPN) which in turn inhibit BNs, the result is net positive feedback loop. (E) Further inputs can modulate the behavior of the system. Examples include biasing the threshold or using an additional negative feedback loop to bring the system back to the original state after a switch (that is, ending the saccade).