|
Load synapse1.ccm show input events show plasticity law show potentials run the model |
Each synapse population has two inputs labelled "afferent spike" and "back prop spike". The first is the normal input from the pre-synaptic axon. The second is the mechanism by which as synapse is told that the post-synaptic cell has fired. This is necessary for synapse models, such as the SynapsePopulation component, which do not use the post-synaptic potential directly.
The example shows a somewhat artificial situation where both inputs to the synapse are driven externally. That is, an external signal tells the synapse that the post-synaptic cell fired, irrespective of whether the cell did indeed fire. This causes the synapse to undergo spike timing dependent plasticity just as if the post-synaptic cell had fired.
The event sequence contains has six events on two channels which come in three pairs, channel 1 lagging channel zero by a few milliseconds. These spikes go through a splitter and then into the two ports of each of the synapses. In the upper synapse, the earlier spike goes to the afferent port and the later one to the back prop port. In the lower case, the connections are reversed. The last ingredient of the model is a plasticity rule specified by the G_ModRule selector in the synapse properties panel. This has a typical form with potentiation caused by pre-then-post conjunctions and de-potentiation caused by post-then-pre conjunctions. The interpretation of the profile is that the x-axis gives the time from the "pre" spike to the "post" spike, and the y axis gives the percentage modification (which is implausibly large here for illustration). Thus, the positive part of the x axis is for pre-then-post pairings and the negative part for when the "post" spike precedes the "pre" spike.
After running the model, you will see two membrane potential traces each with three synaptic events. The growing ones come from the synapse where the afferent spikes precede the back propagating spikes. The decreasing ones come from the other synapse.
|
Load syplast3.ccm show potentials run the model |
This model extends the one above by including modulation of transmission and plasticity of the synapses. The manually supplied spike sequence has been replaced by a regular spike generator and a delay, so each spike takes two different routes, arriving 5 ms later by the delayed routes. As before, in the upper cell, the first spike goes to the afferent input and the second is treated as a back-propagationg spike. The situation is reversed in the lower cell. The result is that one set of synapses is potentiated, and one depressed.
The blue vector input at the top and upper left of the synapse populations are for modulation of transmission and plasticity respectively. They expect values between 0 and 1, and use these to scale the amplitude and plasticity respectively. In the model, transmission is modulated for the upper cell, and plasticity for the lower cell, both by 2Hz sinusoids. In the results, this shows up as a a series of bumps in the envelope of the membrane potential for the depressed synapse (transmission modulation). The envelope of the potentiated synapse, which undergoes plasticity modulation, shows a series of sloping steps: the synaptic strength increases fastest when the modulation factor is nearest 1, corresponding to the peaks in the envelope of the depressed synapse.