stdp_nn_restr_synapse – Synapse type for spike-timing dependent plasticity with restricted symmetric nearest-neighbour spike pairing scheme
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Description
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``stdp_nn_restr_synapse`` is a connector to create synapses with spike time
dependent plasticity with the restricted symmetric nearest-neighbour spike
pairing scheme (fig. 7C in [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, but only if the latter occurred not earlier than the previous presynaptic
one. When a postsynaptic spike occurs, it is accounted in the facilitation
rule with the nearest preceding presynaptic one, but only if the latter
occurred not earlier than the previous postsynaptic one. So, a spike can
participate neither in two depression pairs nor in two potentiation pairs.

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 relies on an additional variable - the postsynaptic
eligibility trace [1]_ (implemented on the postsynaptic neuron side). It
decays exponentially with the time constant ``tau_minus`` and increases to 1 on
a post-spike occurrence (instead of increasing by 1 as in ``stdp_synapse``).

.. warning::

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

Parameters
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========= =======  ======================================================
 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
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SpikeEvent

References
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.. [1] Morrison A., Diesmann M., and Gerstner W. (2008) Phenomenological
       models of synaptic plasticity based on spike timing,
       Biol. Cybern. 98, 459--478

See also
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:doc:`Synapse <index_synapse>`, :doc:`Spike-Timing-Dependent Plasticity <index_spike-timing-dependent plasticity>`

Examples using this model
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.. listexamples:: stdp_nn_restr_synapse