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Spike timing dependent plasticity (STDP) is a general term for functional changes in neurons and at synapses that are sensitive to the timing of action potentials in connected neurons. The phrase 'STDP' typically refers to increases or decreases in the efficacy of synaptic transmission (known as synaptic plasticity), though it can also refer to other functional changes, such as altered dendritic integration. STDP can result from presynaptic spikes preceding postsynaptic spikes (known as pre-post spiking) and postsynaptic spikes preceding presynaptic spikes (known as post-pre spiking). The timing sensitivities are on the order of milliseconds. Usually, pre-post spiking causes long-term potentiation (LTP) of the synapse, and post-pre spiking causes long-term depression (LTD).
The first experiments that specifically examined the effect of millisecond relative timing of pre and postsynaptic action potentials was carried out by W. B. Levy and O. Steward in 1983. However, STDP was not established at the cellular level until Henry Markram, in Bert Sakmann's laboratory, used dual patch clamping techniques to repetitively activate pre-synaptic neurons 10 milliseconds before post-synaptic target neurons, and found the strength of the synapse increased. When the activation order was reversed so that the pre-synaptic neuron was activated 10 milliseconds after its post-synaptic target neuron, the strength of the pre-to-post synaptic connection decreased. Further work, first by Li Zhang and Mu Ming Poo in 1998, mapped the entire time course relating pre- and post-synaptic activity and synaptic change, to show that in their preparation synapses that are activated within 5-40 ms before a postsynaptic spike are strengthened, and those that are activated within a similar time window after the spike are weakened. This phenomenon has been observed in various other preparations, with some variation in the time-window relevant for plasticity. Several reasons for timing-dependent plasticity have been suggested. For example, STDP might operate as a learning rule that maximizes the mutual information between inputs and outputs of simple networks.
From Hebbian Rule to STDP
According to the Hebbian Rule synapses increase their efficacy if the synapse persistently causes the postsynaptic target neuron to generate action potentials. An often used but not entirely accurate simplification is those who fire together, wire together. With recent advancements in technology we can more precisely measure the spike timing of neurons. As it turns out, the synaptic connection between two neurons is more likely to strengthen if the presynaptic neuron fires off shortly before the postsynaptic neuron. Revisiting, the Hebbian rule, we can tweak it to accommodate the new model. Synapses increase their efficacy if the presynaptic neuron is activated momentarily before the postsynaptic neuron is activated. OR Synapses in which the pre-synaptic input fired before the postsynaptic cell get stronger; in the inverse situation, the synapse gets weaker.
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