parameters for modifiable synapses in integrate and fire networks
The conductance and transmission probability are treated independently and are only changed if the Gmodif and Pmodif variables are true, respectively. If decay is set, then the weights decay to their rest values on a timescale tau-dec.
For the conductance, the initial value is G-0, maximum value Gmax and rest value G-rest. The latter is only used if the decay option is set. For the probability, the initial value is Pprop with minimum and maximum values 0 and 1.
Synaptic weights are updated each time the pre-synaptic (pre)-cell fires. The update algorithm uses the current time Tnow, the time of the last spike, if any, on the pre-cell Tlast and the time of the first spike, if any, on the post-synaptic (post)-cell since Tpre.
Updating is effected by looking up the quantity Trel in the Profile pointed to by GmodProf or PmodProf, for conductance and probability respectively, where Trel is either Tpre - Tlast or Tlast - Tnow according to whether the post cell did or did not fire since the last firing of the pre cell.
Thus, if the post cell did fire after the last spike in the pre-cell, Trel is positive, and the positive part of the Profile is used. Typically one might have a Profile which is zero for a small after zero then positive for a window then zero again. The first zero part implements a conductance delay: if the post cell fires so soon after the pre-cell that the signal could not have reached it there should be no synaptic modification.
If the post-cell did not fire, that is the last spike in the pre-cell failed to elicit a spike in the post-cell before the pre-cell fired again, then Trel is negative and the negative part of the Profile is used. Again one might put a zero stretch between time 0 and small negative values, and then a negative stretch from there to large negative values. This would have the effect of penalizing all propagation failures, except when the pre-spikes are so close together that they couldn't be expected to elicit spikes in the post-cell.
The quantity delta found from interpolating the Profile for the time Trel is interpreted differently for the conductance and probability. For the conductance, the quantity f = -1 + 2 / [1 + exp -delta] is evaluated and then for positive f, G -> G + f * [Gmax - G] while for negative f, G -> G - f * G. The effect is that small delta are treated like linear increments or decrements in the conductance, but for large delta the changes saturate at mapping G to Gmax or zero. For the probability delta is applied directly on the log-odds scale. That is, the quantity p/[1-p] is changed by delta.
Finally, if in the network a meta-Profile is attached to the synaptic link, then delta is first multiplied by the value of that Profile at the current time. Note also that in the ProfileEditor new Profiles have the start point fixed at 0 by default. To make Profiles over negative time ranges first click the free button then the point at zero to allow the start time to be varied freely.