Heterosynaptic phenomena are events that occur at the junctions (synapses) that form connections between neurons. A heterosynaptic phenomenon is one that involves interactions between separate synapses or groups of synapses. This term is often used in relation to long-term depression and long-term potentiation of neuronal firing. For example, heterosynaptic long-term depression occurs when neural activity involving one of a neuron's synapses causes the strength (the ability to transduce input) of another of the neuron's synapses to decrease.
Experimentally, Fitzsimonds et al. found in cultured hippocampal neurons that the induction of LTD is also accompanied by back propagation of depression in the dendrite tree of the presynaptic neuron. Further, depression also propagates laterally in the pre- and postsynaptic neuron. Similar results hold for the propagation of LTP. For a review of experimental results see Bi et al. Emmert-Streib introduced a learning rule for neural networks based on heterosynaptic plasticity.