Synaptic mechanisms for long-term depression.

Neural networks that store information at synapses rely on mechanisms for increasing and decreasing synaptic strengths. In 1973 Bliss and Lramo reported that stimulating the perforant pathway projecting to the hippocam pus with a high ,frequency tetanus resulted in a rapid and persistent elevation. of the synaptic efficacy [ 11. This form of long-term potent&ion (LTP) lasts for many hours in slice preparations of the hippocampus and for many days in vivo. At some synapses in the hippocampus LTP can be induced by stimulating a test input only a.few times if the stimulation is accompanied by strong postsynaptic depolarization [2,3]. An increase in synaptic efficacy under these conditions, as suggested by Hebb [4], is termed hebbian [51. One of the most thoroughly explored uses of the simple hebb rule is in the formation of associations between patterns of neural activity [6]. Modiiications of the simple hebb rule to include long-term depression of synaptic strength can improve the ef53cienc-y and performance of associative recall in neural network models [7,8].