Peristimulus time histograms of simple spikes and heartbeat frequencies in the habituation, acquisition (last ten trials), and extinction sessions. Average responses of ten consecutive trials in each session are shown. Relative heartbeat frequencies in relation to average heartbeat frequency in the 2 s before the CS are shown. Rastergrams of simple-spike timings are also shown below the frequency plots. Horizontal bars indicate periods of CS presentations. Arrowheads indicate the timing of the US presentations. + and * denote significantly higher and lower frequencies relative to the highest or lowest bin in the 2 s before the CS, respectively (Wilcoxon signed rank test, p < 0.05). CS: conditioned stimulus; US: unconditioned stimulus; s: seconds.