Issue with triplet STDP triplet synapse model

[clinssen]

There is a regression in the STDP triplet synapse model, as is visible in the tutorial notebook on the current master branch.

The regression bug should be fixed, and assertions have to be added to the notebook to prevent this issue from happening again.

Everything seems to work fine if the following line:

double w_ = S_.w + S_.tr_r1 * (P_.A2_plus + P_.A3_plus * start->get_tr_o2__for_stdp_triplet_nn_synapse_nestml());

is replaced by

double w_ = S_.w + S_.tr_r1 * (P_.A2_plus + P_.A3_plus * ((post_neuron_t*)(__target))->get_tr_o2__for_stdp_triplet_nn_synapse_nestml(t_hist_entry_ms));

Thus, the value of tr_o2 that is being put into the postsynaptic spiking history buffer (via set_spiketime()), and which is here read out again by start->get_tr_o2_...(), is ostensibly incorrect.

This issue could actually be related to getting the trace value just-before, versus just-after the update due to the arriving spike.

I added some debug plots to the notebook, please see https://github.com/clinssen/nestml/blob/triplet_stdp_fix_wip/doc/tutorials/triplet_stdp_synapse/triplet_stdp_synapse.ipynb

Notice how in general, the NESTML trace values (in blue) are tracking the reference trace values (in orange) quite well.

Perhaps if we get the value always just-after the spike update, especially in the nearest-neighbour case, we are always setting tr_o1 and tr_o2 to 1, just before writing them back to the history. So we are only writing 1s to the history in that case.

See the following lines of generated code in set_spiketime():

/**
* print extra on-emit statements transferred from synapse
**/
S_.tr_o1__for_stdp_triplet_nn_synapse_nestml = 1;
S_.tr_o2__for_stdp_triplet_nn_synapse_nestml = 1;

/**
* push back history
**/

last_spike_ = t_sp_ms;
history_.push_back( histentry__iaf_psc_delta_neuron_nestml__with_stdp_triplet_nn_synapse_nestml( last_spike_
, 0 // access counter
, get_tr_o1__for_stdp_triplet_nn_synapse_nestml()
, get_tr_o2__for_stdp_triplet_nn_synapse_nestml()
) );