A, each ‘run’ of an experiment was divided into three parts: (1) a control period, where tones were presented without efferent stimulation; (2) a test period, where tones were paired with electrical stimulation of MOC efferents; and (3) a recovery period, where tones were again presented without efferent stimulation. The detailed timing of the two experimental paradigms is shown in panels B and C. B, in the single-tone paradigm, responses were averaged across multiple presentations of a 160 ms-long tone pip, and efferent stimulation was delivered in 100 ms-long shockbursts. C, in the multiple-tone paradigm, responses were averaged across multiple presentations of an acoustic stimulus consisting of two batches of four 50 ms-long tone pips, and efferent stimulation was delivered in 300 ms-long shockbursts. Individual shock bursts were timed to produce minimal effects near the start of each acoustic stimulus, and to produce maximal effects near the end of each stimulus.

A-D, BM responses to CF tones (19 kHz, 35 dB SPL) at the times indicated above E and G. Shading in A-D marks the early (○) and late (×) analysis windows. Pulse trains above B and C illustrate the fine timing patterns of the efferent stimulation. Responses have been high-pass filtered (f_c = 15 kHz) to clarify illustration of the fast and slow effects. E and F, variations in BM response amplitudes (E) and phases (F) across the period of the run. Shading indicates the overall period of efferent stimulation (the test period; cf. ). Slow effects of the efferent stimulation are manifest as changes in the responses in the early analysis windows (○) across time (i.e. as differences from the control, or ‘baseline’ responses). Fast effects are manifest as differences between the responses in the early (○) and late (×) analysis windows during the test period. G and H, derived amplitude and phase changes attributed to the fast (▵) and slow (•) effects. Experiment GP16, run 2.

BM vibration amplitude (A-C) and phase (D-F) changes attributed to fast (▵) and slow (•) effects are shown for three runs of the single-tone paradigm (cf. ). Stimulus details and reference vibration levels for 0 dB amplitude changes are indicated at the top. Shading indicates the overall period of efferent stimulation (the test period; cf. ). G-I, amplitudes of simultaneously recorded CAPs, illustrating the slow effects of the efferent stimulation on the auditory nerve.

The relative strengths of the fast (▵) and slow (•) amplitude changes (A-D) and phase changes (E-H) vary systematically with frequency. The tones were presented in a pseudo-randomly interleaved manner (19, 14, 21 and 4 kHz data correspond to tones 1–4 in ). Shading indicates the overall period of efferent stimulation (the test period; cf. ). Stimulus details and reference vibration levels are indicated at the top. Experiment GP27, run 4.