# stdp_nn_symm_synapse – Synapse type for spike-timing dependent plasticity with symmetric nearest-neighbour spike pairing scheme¶

## Description¶

stdp_nn_symm_synapse is a connector to create synapses with spike time dependent plasticity with the symmetric nearest-neighbour spike pairing scheme 1.

When a presynaptic spike occurs, it is taken into account in the depression part of the STDP weight change rule with the nearest preceding postsynaptic one, and when a postsynaptic spike occurs, it is accounted in the facilitation rule with the nearest preceding presynaptic one (instead of pairing with all spikes, like in stdp_synapse). For a clear illustration of this scheme see fig. 7A in 2.

The pairs exactly coinciding (so that presynaptic_spike == postsynaptic_spike + dendritic_delay), leading to zero delta_t, are discarded. In this case the concerned pre/postsynaptic spike is paired with the second latest preceding post/presynaptic one (for example, pre=={10 ms; 20 ms} and post=={20 ms} will result in a potentiation pair 20-to-10).

The implementation involves two additional variables - presynaptic and postsynaptic traces 2. The presynaptic trace decays exponentially over time with the time constant tau_plus and increases to 1 on a pre-spike occurrence. The postsynaptic trace (implemented on the postsynaptic neuron side) decays with the time constant tau_minus and increases to 1 on a post-spike occurrence.

## Parameters¶

tau_plus |
ms |
Time constant of STDP window, potentiation (tau_minus defined in postsynaptic neuron) |

lambda |
real |
Step size |

alpha |
real |
Asymmetry parameter (scales depressing increments as alpha*lambda) |

mu_plus |
real |
Weight dependence exponent, potentiation |

mu_minus |
real |
Weight dependence exponent, depression |

Wmax |
real |
Maximum allowed weight |

## Transmits¶

SpikeEvent