Spike cohen wiki8/19/2023 LTD in a cerebellum-like structure in the electric fish was also discovered in 1997 to be tightly spike timing-dependent, but in this case pre-leading-post spike order drove LTD ( Bell et al., 1997), similar to anti-Hebbian LTD at the parallel fiber-Purkinje cell synapse in mammalian cerebellum. This is termed ‘Hebbian’ STDP because it strengthens synaptic inputs that lead (and therefore contribute to) postsynaptic firing, and depresses inputs that are uncorrelated with postsynaptic spikes. Plasticity requires multiple (typically, 60–100) pre-post spike pairs. In canonical STDP, LTP occurs when presynaptic spikes (and associated EPSPs) lead postsynaptic spikes by up to ~20 ms, and LTD occurs when postsynaptic spikes lead presynaptic spikes and EPSPs by up to 20–100 ms, with a sharp (1–5 ms) transition between LTP and LTD ( Markram et al., 1997 Bi and Poo, 1998 Celikel et al., 2004) ( Fig. This dependence was characterized in detail by Bi and Poo (1998) and named “spike timing-dependent plasticity” (STDP) by Song et al. controlled pre- and postsynaptic spike timing using dual whole-cell recording, and discovered that the sign and magnitude of LTP and LTD indeed depended on the order and timing of pre- and postsynaptic spikes on the 10-ms time scale ( Markram et al., 1997). Precise timing- and order-dependent plasticity was predicted by Gerstner (1996) to explain development of phase locking in sound localization. While most early studies suggested a correlation requirement of about ± 100 ms for plasticity ( Baranyi and Fehér, 1981 Gustafsson et al., 1987), a few studies noted an effect of spike order, with LTP occurring when presynaptic inputs led or were synchronous with postsynaptic spikes (evoked by a second pathway or by current injection), and LTD occurring when presynaptic input followed postsynaptic spikes ( Levy and Steward, 1983 Debanne et al., 1994 Debanne et al., 1997). This reflects the molecular properties of postsynaptic NMDA receptors, which provide calcium to trigger LTP and LTD. ![]() The critical requirement at most synapses was found to be temporally correlated presynaptic spiking and postsynaptic depolarization, with strong depolarization leading to LTP, and weaker, more sustained depolarization leading to LTD ( Wigström et al., 1986 Lisman, 1989 Artola et al., 1990). Early work showed that high-frequency presynaptic firing drove LTP, while low-frequency firing drove LTD (e.g., Bliss and Lømo, 1973). Understanding the rules governing LTP and LTD induction is essential for understanding their function. It is now clear that associative synapse strengthening and weakening are implemented at many synapses by long-term potentiation (LTP) and depression (LTD). ![]() Others amended this idea to include weakening of ineffective synapses ( Stent, 1973 von der Malsburg, 1973 Sejnowski, 1977 Bienenstock et al., 1982). In his famous implementation of this rule, Hebb proposed that when cell A reliably contributes to spiking of postsynaptic cell B, the functional strength of the synapse from A to B is increased ( Hebb, 1949). This type of plasticity has been proposed as a basis for learning and memory since the late 19 th century ( James, 1890). In associative synaptic plasticity, simultaneous or rapid sequential activation of two synaptically connected neurons leads to a change in the strength of synapses between them.
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