Synchronization, decrease in strength of pattern, illusory contours, and neon-color effect

The data are reviewed concerning synaptic plasticity of the hippocampal monosynaptic pathways evoked by afferent activation which simulates activity of neurons in theta-rhythm. Activation with short high-frequency bursts applied with 5 Hz frequency effectively induces long-term potentiation. In contrast, activation with single pulses at 5 Hz causes the depotentiation or even long-term depression of the activated synapses. "In-phase" activation of two afferent pathways in the theta-rhythm induces long-term potentiation, while the "out-of-phase" activation induces long-term depression of a "weak" pathway. A train of 30-50 pulses at 5 Hz evokes heterosynaptic short-term depression, i.e., a suppression of all synaptic inputs for 1 min. So-called "hidden" plasticity (the enhancement of the effect of delayed activation in producing long-term depression) is also effectively evoked by the theta-like activatory patterns. Therefore, practically all known types of synaptic plasticity can be effectively evoked by the afferent activation which reproduce the pattern of the hippocampal theta-rhythm. These phenomena can underlie the theta-rhythm participation in learning and memory.