Quoting the Programmer's Reference documentation of Exp2Syn, this mechanism
produces a synaptic current with alpha function like conductance (if tau1/tau2 is appoximately 1) defined by
i = G * (v - e)
Implicit in the documentation is the fact that a single activation of this mechanism produces a synaptic current given by i = G*(V-e) where G is the product of the weight associated with the event and a normalized biexponential function (a function that is the difference between decaying exponentials with time constants tau1 and tau2, multiplied by a scale factor so that its maximum is always 1).
For events with weight > 0, all is well and good--the synaptic conductance is always >= 0.
Events with weight < 0 produce results that are not physically realizable, because current flow would oppose the electrochemical gradient.
The moral of this story is: make sure that the NetCons that target event-driven conductance change synapses, such as ExpSyn and Exp2Syn, have positive weights.
sanjaybioe wrote:
1) When syn1.e = -60 and weight is 0.05, effect is inhibitory
2) When syn1.e = -60 and weight is -0.05 (negative weight), effect is still inhibitory
3) When syn1.e = -60 and weight is 0.5 (10 times the initial weight), effect is inhibitory
4) When syn1.e = -60 and weight is -0.5 (negative), effect is excitatory
Why are the outputs different in conditions 2) and 4)?
My guess would be that the negative conductance in case 2 is counteracted by local net positive conductances that are sufficiently large to nullify its effect. This being a completely hypothetical case--even more hypothetical than the usual computational model--I am disinclined to pursue the issue.
It would be good if you could give little detail of what actually the weight term in this context implies.
In addition to my brief discussion above, the source code for ExpSyn and Exp2Syn would probably answer a lot of questions--examine expsyn.mod and exp2syn.mod in c:\nrnxx\src\nrnoc (MSWin) (UNIX/Linux/OS X users should get the nrn...tar.gz file, expand it, and look in nrn-x.x/src/nrnoc/).
Because general notion is if weight is negative, the effect is invariably Inhibitory.
Quite true in the context of artificial neural nets. In biology, however, the effect of opening synaptic channels depends on whether the electrochemical gradient of the permeant ionic species favors net influx or efflux of positive charge.
I could not clearly understand the reply you had posted for Kane's query.
I'll have to re-read it to see where it might lack clarity.