Figure 8.

Spike generation sharpened tuning and increased phase lead (reduced latency) of the afferent signal. A, B, Current-clamp records from a striolar double calyx (P7, 27°C) during sinusoidal stimulation of one hair bundle. In sequence from the top are the ±600 nm stimulus, (A.1) EPSPs averaged from three sweeps obtained without spikes from a resting potential of −71 mV, (A.2) a single trace showing spiking from the same cell (which began when the cell spontaneously depolarized to −66 mV), (A.3) 14 sequential sweeps. Strong phase-locking produced near coincidence of spikes in different sweeps. The 20 Hz sinusoidal burst evoked the most spikes. B, Two sweeps of the 20 Hz response at high temporal resolution, showing both spikes and EPSPs. As shown by arrows, spikes often precede the peaks of EPSPs (i.e., spikes phase-lead EPSPs at 20 Hz). C, Gain and phase of spikes/cycle (gray circles) and spikes/second (black circles) referenced to bundle displacement expressed in micrometers. Averaged from five calyces, including the calyx in A. The number of spikes per cycle is more analogous to the sizes of RPs, EPSCs, and EPSPs plotted in Figures 3C and 6C; the number of spikes per second can be compared with in vivo afferent data. D, Transfer functions for spikes/second referenced to EPSPs for the quantal data in C (solid black) and for the nonquantal (NQ) data in Figure 5B (open brown triangles). By comparing spike rate with EPSPs, these plots show the additional tuning at the spike-generation stage; failure to trigger spikes at outlying frequencies significantly sharpens the spike-tuning curve. Bottom, Phase difference between spikes and EPSPs (spike phase minus EPSP phase), with EPSP phase measured in two ways for the quantal data: from the f₀ FFT component of EPSPs (black circles) or from the voltage peak (cyan circles; see B, arrows). Below 30 Hz, spikes had a substantial phase lead in either case, but the lead was smaller when peak EPSP was measured. For the nonquantal calyx in Figure 5B (open brown triangles), spikes held a phase lead relative to the f₀ component of nonquantal postsynaptic potentials up to higher frequencies (100 Hz).