A, a 0.5 s stimulus at 20, 50, 100 or 200 Hz was embedded into constant 10 Hz background firing (80 s after onset of the background activity). The release probability (top left), the RRVP occupancy (top right) and the EPSC amplitudes (bottom) are shown. B, EPSC amplitudes for a 0.5 s stimulus at 100 Hz, embedded in background activity at 5, 10, 20 and 50 Hz (top: absolute EPSC amplitudes; bottom: EPSC amplitudes during stimulation (left) and recovery (right), normalized to the steady-state value during background activity). The inset (bottom right) shows the recovery curves normalized to the last EPSC amplitude during stimulation. Stimulation periods are indicated by the black bars, and background firing by grey bars.