Electrically evoked behaviors: axons and synapses mapped with collision tests.

The properties of many axon bundles mediating electrically evoked responses have been described using double-pulse methods in behaving animals. The directly stimulated axons whose activation leads to the behavior are defined by their refractory periods, conduction velocities and trajectories in those studies. In this review, new collision effects (asymmetric collision) are described that locate synapses mediating turning and startle responses. The direction, time and reliability of transmission are determined by the direction, time and strength of the asymmetry. In systems mediating turning, five axon bundles have been localized with symmetric collision effects and evidence for two synapses has been provided with asymmetric collision effects. In systems mediating startle responses, three fast, reliable synapses have been located with asymmetric collision. Asymmetric collision effects between a loud acoustic stimulus and a single electrical pulse in hindbrain sites define the timing and location of the acoustic volley that produces acoustic startle. Therefore, when synapses are strong, circuit diagrams can be constructed by use of collision tests in behaving animals.