Fig. 1. A, Locations of bipolar stimulating (filled) and recording (open) electrodes. Typically the stimulating electrodes were placed ∼200–300 μm caudal of the recording area. The recording pipette was advanced through layers 1–5 of the dorsal torus semicircularis. Drawings were adapted from Carr and Maler (1986).CC, Corpus cerebellum; EGp, eminentia granularis posterior; ELL, electrosensory lateral line lobe; EO, efferent octavolateral nucleus;LL, lateral lemniscus; MLF, medial longitudinal fasciculus; OT, optic tectum;PEd, praeeminentialis dorsalis; RF, reticular formation; TSd, dorsal torus semicircularis;VA, valvula of cerebellum. B, Ampullary sensory stimuli. Top, Middle,Traces are 5 and 20 Hz “bursts,” respectively, each 1 sec in duration. Bottom, An ongoing 20 Hz sensory stimulus was interrupted by a gap (4–200 msec in this study).C, Tuberous sensory stimuli. These stimuli were generated by adding two sinusoidal signals of high frequency (see Materials and Methods). The amplitude envelope of these beating stimuli mirrored the temporal structure of the ampullary stimuli.D, Temporal patterns of LL stimulation.Top, Middle, A pair of pulses [35 msec interpulse interval (IPI)] preceded, by 510 msec, and a second pair followed a train of pulses (tetanus). The delay in the presentation of the post-tetanus pair of pulses ranged from 25 to >600 msec. Within each tetanus, trains of pulses were delivered at rates of 5 Hz (top) or 20 Hz (middle). The 5 Hz tetanus consisted of five trains of pulses per second, each train having nine pulses presented at 100 Hz (10 msec IPI). The 20 Hz tetanus consisted of 20 pairs of pulses (10 msec IPI; 40 msec between pairs), which generally elicited PSPs that were similar to those in response to 20 Hz sensory stimulation. PSPs in response to the pairs of pulses before and after the tetanus were compared to determine whether synaptic depression or loss of facilitation was responsible for PSP depression during the tetanus. Bottom, To test whether PSP depression, induced by the 20 Hz tetanus, attenuated responses to longer duration trains, the first pulse of the pretetanus and post-tetanus pair was replaced with one to two trains that had 9–10 pulses, delivered at 100 Hz.