Use of stimulus-frequency otoacoustic emissions to investigate efferent and cochlear contributions to temporal overshoot

Abstract

Behavioral threshold for a tone burst presented in a long-duration noise masker decreases as the onset of the tone burst is delayed relative to masker onset. The threshold difference between detection of early- and late-onset tone bursts is called overshoot. Although the underlying mechanisms are unclear, one hypothesis is that overshoot occurs due to efferent suppression of cochlear nonlinearity [von Klitzing, R., and Kohlrausch, A. (1994). J. Acoust. Soc. Am. 95, 2192-2201]. This hypothesis was tested by using overshoot conditions to elicit stimulus-frequency otoacoustic emissions (SFOAEs), which provide a physiological measure of cochlear nonlinearity. SFOAE and behavioral thresholds were estimated using a modified maximum-likelihood yes-no procedure. The masker was a 400-ms "frozen" notched noise. The signal was a 20-ms, 4-kHz tone burst presented at 1 or 200 ms after the noise onset. Behavioral overshoot results replicated previous studies, but no overshoot was observed in SFOAE thresholds. This suggests that either efferent suppression of cochlear nonlinearity is not involved in overshoot, or a SFOAE threshold estimation procedure based on stimuli similar to those used to study behavioral overshoot is not sensitive enough to measure the effect.

Figure 1

This figure shows the threshold shifts that are predicted from the von Klitzing and Kohlrausch (1994) model. The modeled BM I∕O functions (with arbitrary normalization) are plotted as a function of stimulus level at 1 ms after the noise onset (solid lines) and 200 ms after the noise onset (dashed line). The simulated noise sound pressure spectrum level is listed in each panel (as noise level) and represented on each plot by a triangle at this input level. The noise level decreases in 5-dB steps from 60 dB (top panel) to 40 dB (bottom panel). Each threshold is marked on each plot using a circle symbol and labeled with the appropriate threshold of the early (Θ₁) or late (Θ₂₀₀) burst. In this model, the SNR for threshold for the burst is assumed to be 8 dB above the noise level on the respective BM I∕O function (i.e., 8 dB difference between triangles and circles on the Y axis, or output). For example, in the middle panel, the noise level is 50 dB, so that the output level at threshold is 8 dB higher, or 58 dB, for the late burst (Θ₂₀₀). The early-burst threshold Θ₁ attains this output level at an input SPL of 74 dB. The overshoot is the difference in input SPL of the early burst to that of the late burst, or 16 dB in this example.

Figure 2

The modeled stimulus waveform responses in the ear canal (in arbitrary units) are shown for the conditions used to estimate SFOAE thresholds. Modeled responses for SFOAEs elicited by tone bursts masked by notched noise in the early condition are shown in the left column, and responses in the late condition are shown in the right column. The top panels show the modeled signal+gated noise stimulus waveform responses p₁, the middle panels show the tonal suppressor responses p₂, and the bottom panels show the sum p₁₂ of these responses.

Figure 3

The simulated signal response p₁ is plotted in arbitrary units as a function of time (in milliseconds) for early and late bursts in noise in the left and right panels, respectively. Each panel also shows the first and second windows, each 1024 samples (or 46 ms) in length, which were used for the signal-present and signal-absent detection conditions, respectively. Each window is drawn as a box.

Figure 4

Mean behavioral (gray lines) and SFOAE (black lines) thresholds in decibel HL as a function of the masker level, i.e., the noise sound pressure spectrum level (reference bandwidth of 1 Hz) for the tone burst presented in noise at 1 ms after the masker onset (solid lines) and 200 ms after the masker onset (dashed lines). The thresholds in quiet for each measure are plotted as detached symbols on the left side. Each error bar extends ±1 SE from its mean.

Figure 5

The group mean ±1 SE of the behavioral overshoot (dashed line) and SFOAE overshoot (solid line) calculated from the group responses in Fig. 4 are plotted as a function of the masker level, i.e., the noise sound pressure spectrum level (reference bandwidth 1 Hz).