parrot_neuron – Neuron that repeats incoming spikes
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Description
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The parrot neuron simply emits one spike for every incoming spike.
An important application is to provide identical poisson spike
trains to a group of neurons. The ``poisson_generator`` sends a different
spike train to each of its target neurons. By connecting one
``poisson_generator`` to a ``parrot_neuron`` and then that ``parrot_neuron`` to
a group of neurons, all target neurons will receive the same poisson
spike train.

Remarks
.......

- Weights of connections *to* the ``parrot_neuron`` are ignored.
- Weights on connections *from* the ``parrot_neuron`` are handled as usual.
- Delays are honored on incoming and outgoing connections.

Only spikes arriving on connections to port 0 will be repeated.
Connections onto port 1 will be accepted, but spikes incoming
through port 1 will be ignored. This allows setting exact pre-
and postsynaptic spike times for STDP protocols by connecting
two parrot neurons spiking at desired times by, e.g., a
`stdp_synapse` onto port 1 on the postsynaptic parrot neuron.

Receives
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SpikeEvent

Sends
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SpikeEvent