16 Dec
2024
16 Dec
'24
11:14 a.m.
Hi Tom,
Thanks for your input on this! I was just looking at the code too. It seems that the order
of updates (first update adaptation variables, then send spike, versus first send spike,
then update adaptation variables) was changed, in order to prevent the adaptation from
kicking in for the very first spike to ever go through the synapse. See
https://github.com/nest/nest-simulator/pull/1672/.) Notice that t_lastspike is initialised
to zero, rather than some "special" value like -1. It is possible that this fix
was erroneous, in that it ends up affecting not just the first-ever spike, but also
subsequent spikes. I am investigating a bit more, but if you are convinced that the
current behaviour is the correct one, then I can stop investigating.
Much obliged!
Charl
On Mon, Dec 16, 2024, at 11:01, Tom Tetzlaff wrote:
Dear Pavel,
I can confirm you observation, and I share your surprise. Intuitively,
one would expect that the synaptic state should return to the initial
state after the long period of silence. However, after consulting the
source papers (todyks_synapse: doi:10.1523/JNEUROSCI.20-01-j0003.2000,
tsodyks2_synapse: doi:10.1016/S0893-6080(01)00144-7) and looking at the
NEST code, I conclude that what the example shows is indeed the intended
behaviour.
A key difference between tsodyks_synapse and todyks2_synapse is the
order of update steps: upon arrival of the presynaptic spike,
tsodyks_synapse first updates the synapse state (x,y,z,u; see lines
258-277 in tsodyks_synapse.h), and then reports the new weight (line
285
in tsodyks_synapse.h). In tsodyks2_synapse, this is reversed: first,
the
synaptic weight is computed based on the current (i.e., the old)
synapse
state (line 236 in tsodyks2_synapse.h), and then the synapse state is
updated (lines 243,244 in tsodyks2_synapse.h). When the first spike of
the second burst of presynaptic spikes arrives in the example
(https://nest-simulator.readthedocs.io/en/stable/auto_examples/evaluate_tsod…),
the recovery value of tsodyks2_synapse still is where it was at the end
of the first burst of spikes (before the break). This is exactly how
the
model is defined in (Maass and Markram, 2002;
doi:10.1016/S0893-6080(01)00144-7; see equations (1-5)). It needs one
more spike to really "recover".
I hope this clarifies things a bit.
Best
Tom
On 12.12.24 12:05, Pavel Haman wrote:
Hello,
I have followed the example script comparing `tsodyks_synapse` and `tsodyks2_synapse`
models, which can be found here:
https://nest-simulator.readthedocs.io/en/stable/auto_examples/evaluate_tsod…
Using the parameter set for depression instead of facilitation I found that
`tsodyks2_synapse` model leads to weird peaks of synaptic currents (at least I would not
expect that for short term depression) - during the second stimulation the weight
rescaling seems to be shifted by one spike compared to tsodyks_synapse (as if parameter
"u" would be updated after rescaling the weight instead of before).
Does anybody know why is there this difference between the two synapse models?
Thank you.
All the best,
PH
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--
---
Dr. Tom Tetzlaff
Institute for Advanced Simulation (IAS-6)
Computational and Systems Neuroscience
Jülich Research Centre
Jülich, Germany
https://www.fz-juelich.de/profile/tetzlaff_t
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