Conditioning-related changes in the simple-spike responses to the CS, and US-evoked complex-spike firings in a PC. Average responses of ten consecutive trials in a habituation session (A) and an acquisition session (B), respectively. Relative heartbeat frequencies relative to the average heartbeat frequencies in the 3 s before the CS are shown. Rastergrams of complex-spike timings are also shown at the bottom. CR: conditioned response; UR: unconditioned response. * denotes a pause in simple-spike firing after the US presentation. The arrowhead indicates the timing of the US presentation. Bin width = 100 ms. C. Representative recordings at the end of the CS in habituation (a) and acquisition (b) sessions. In acquisition session (b), complex spikes (dots) were evoked by US with relatively short latencies. CS: conditioned stimulus; US: unconditioned stimulus. A, B and C were obtained from the same preparation.