The Depression Transfer Function

over time but instead depends partly on the elapsed time between individual input events. The biophysical mechanisms underlying this shortterm synaptic plasticity are numerous and complex (29). As a result of such variety, different synapses supplied with the same temporal pattern of inputs may respond very differently. Thus a synapse does not simply transmit impulses from the presynaptic to the postsynaptic neuron; it applies a transfer function by which interevent interval is transformed into voltage amplitude. Over the past few years, researchers have identified many neural circuits in which successful performance depends on the specific transfer function found at a component synapse (for reviews, see Refs. 1, 13, 21, 22, 33, and 36). Two forms of shortterm plasticity—facilitation and depression—have distinct influences on the synaptic transfer function (30, 31). Facilitation is an increase in synaptic strength with each successive input event, whereas depression is a decrease. Theoretical analysis has demonstrated their respective contributions toward temporal integration and differentiation of synaptic inputs, resulting in enhanced sensitivity to input intensity and timing, respectively (31). Here we review recently discovered uses for temporal discrimination by depressing synapses. Computational tasks performed by depressing synapses in simple feedforward circuits include signaling of average input rate and its derivative, the rate of change of input rate (2, 4, 8, 30, 34), gain control (2, 10, 30), coincidence detection (10, 14), and input rate filtering (3, 5, 6, 9, 11, 24), and in oscillatory networks the setting of phase offset (8, 16) and cycle period (20, 28) and the regulation of spontaneous oscillatory activity (27, 32). Organism behaviors dependent on these computations include sensory tasks such as habituation (4), sound localization (10, 14), and sensory input selection (5–9, 24, 33), as well as rhythmic motor tasks such as mammalian intestinal peristalsis (28) and the crustacean pyloric rhythm (20). The effect of depression on the neuronal inputoutput transformation cannot be predicted by any general rules, because it depends on both the kinetics of depression and the context. The context includes activity patterns of individual inputs and the input population as well as intrinsic properties of the postsynaptic membrane. The versatility of synaptic depression as a timing device helps explain its ubiquity in the nervous system.