17 Dec
2024
17 Dec
'24
12:22 p.m.
Hello Tom and Charl,
thank you for your time, explanations and replies. You confirmed what I
thought was happening.
I quickly checked the paper (Maass and Markram, 2002;
doi:10.1016/S0893-6080(01)00144-7) and equations (1-5).
I think that the model should not update weights with delay of one spike,
since $\Delta t_n$ in the equations is actually $t_{n+1}-t_{n}$, i.e.
referring to the new spike (it is explained in the sentence before
equations), not to older spikes. (I believe here could be the issue, since
interpreting $\Delta t_n$, as I would naturally do, as $t_{n}-t_{n-1}$
leads to this one spike delay of updates)
Effective weight for spike $n+1$, $A_{n+1}$ is given as
$A_{n+1}= A u_{n+1} R_{n+1}$
and $u_{n+1}$ depends on the times of the new spike $t_{n+1}$ and previous
spike $t_{n}$
I will check Charl's PR later this week.
All the best,
Pavel
po 16. 12. 2024 v 22:19 odesílatel Charl Linssen <nest-users(a)turingbirds.com>
napsal:
A little follow-up: I made a pull request with a
suggested fix for this
issue: https://github.com/nest/nest-simulator/pull/3384
Tom, Pavel, would you mind having a look to see if this addresses the
issue?
Again, much obliged!
Charl
On Mon, Dec 16, 2024, at 11:14, Charl Linssen wrote:
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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