Simultaneous recordings of PC activities and heartbeats during the conditioning procedure. A. The eighth trial in the habituation session. B. The 21st trial in the acquisition session. In this case, firing frequency of simple spikes was decreased during the first two seconds of the CS presentation in the 21st trial in the acquisition session (B). PC activities and heartbeats are shown in upper traces and lower traces, respectively. Dots indicate complex spikes. Horizontal bars indicate 5.1 s periods of CS presentations. The arrow indicates the timing of the US presentation. C. Superimposed recordings of simple (blue) and complex (red) spikes that were observed in A.