Thursday, July 5, 2012

Spike-Timing Dependent Plasticity

Markram, H. Lubke, J. Frotscher, M. Sakmann, B. (1997) Regulation of Synaptic Efficacy by Coincidence of Postsynaptic APs and EPSPs.  Science 275, 213.

Bi, G. Poo, M. (1998) Synaptic Modifications in Cultured Hippocampal Neurons: Dependence on Spike Timing, Synaptic Strength, and Postsynaptic Cell Type. The Journal of Neuroscience 18(24): 10464-10472.

These were the first two papers that showed STDP experimentally. STDP is a learning rule that is dependent on the precise temporal order of spiking in the pre- and post-synaptic cells. Bi, Poo show it nicely:
Basically, if the spikes are temporally causal (pre before post) then the synapse will get stronger. If they are acausal (pre after post) they get weaker. Here's how Markram showed it:

There's extensive work on the mechanisms of STDP. The primary mechanism is an NMDA channel, I'll explain that in more detail later. The signal is based on Calcium - excess calcium causes a chemical cascade that recruits or removes AMPA receptors to the synapse.

These papers naively think about the neuron as a single compartment. There is much more work about how STDP is really dependent on the dendritic spike - this is a big calcium signal. Eliciting an action-potential (or burst which is sometimes necessary to get this effect), will influence the dendrites and make a dendritic spike more likely.  So the learning rule is not really based on pre-post spikes, but whether the dendrite spikes.

There are a lot of other factors that can modulate this learning rule - frequency, synaptic strength, chemical modulators. Dopamine is an especially interesting one as it has been implicated in reinforcement learning and has been shown to be able to modulate STDP.