# 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.

Warning

This synaptic plasticity rule does not take precise spike timing into account. When calculating the weight update, the precise spike time part of the timestamp is ignored.

## 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