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. 2016 Jan 19;9:85. doi: 10.3389/fncir.2015.00085

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Copyright © 2016 Frémaux and Gerstner.

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) or licensor 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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Schematic of eligibility traces. (A) Joint activity (top) of two neurons (red-filled circles mark active neurons, open circles inactive neurons) leads, a few moments later (bottom), to the raising of a “flag” at synapses connecting those two neurons. These synapses are now eligible for change. The activity of other neurons (filled pink circles) does not interfere with the flag, which persists over a short time. (B) Axonal branches of dopaminergic neurons are shown in blue. If a neuromodulatory signal (green arrows, top) arrives, the synapses with raised flags undergo plasticity (top: synapses of medium strength before phasic neuromodulatory signal; bottom: synapses after change). Bottom: In a model where the amplitude of STDP is amplified by the neuromodulator, a synapse becomes stronger (bold arrow) in case of earlier pre-before-post spike timing and weaker (dashed arrow) in case of reversed timing. Synapses remain stable thereafter.