Figure 15:A: PSTH for a representative stimulus-locking unit responding to the 70-ms-interval pulse train (top) and 15-ms-interval pulse train (bottom). In general, 70-ms pulse intervals led to most potent stimulus locking (region ß; ref., Figure 6A-1); 15-ms pulse interval led solely to onset responses (region d). B: The number of discharges involved in individual discharge clusters as shown in (A) (open circle: 70-ms interval train; filled circle: 15-ms interval train). The square represents the sum of discharges evoked by the initial two pulses in the 70-ms interval train. C: Population data (mean ± SD) for (B). The number of discharges was normalized to that involved in the 1st discharge cluster [= control]). At 70 ms intervals (in region ß) , the number of discharges decreased with a progression of the pulse train, consistently with “frequency-dependent depression”. At 15 ms intervals (in region d), the onset response was contributed to by the initial 2.4 ± 0.7 pulses and contained 2.1 ± 0.4 of normalized discharge number. Such number of discharges is much larger than the sum of discharges that were evoked even by the initial three pulses in the 70-ms interval train (square). This suggests that when several consecutive clicks fall within a few tenths of millisecond, there is a synergy of impact for evoking more discharges, consistently with “paired-pulse facilitation.” (Modified from Sakai et al. [1])