Time course of suppression

Lower boundaries of two-tone suppression regions in the guinea pig

Lower boundaries of two-tone suppression regions were determined in single fibres of the guinea pig with a tracking algorithm as described by Schmiedt (1982). For a suppressee at CF having a level of 20 dB above the threshold of the tip, suppression at the high-frequency (hf) side of the FTC could almost always be found. With the method used, the percentage of fibres in which suppression could be found at the low-frequency (lf) side of the FTC decreased with decreasing CF. Moreover, the occurrence of lf-suppression decreased for lower suppressee levels for fibres with CF approximately 2-5 kHz. For each fibre the minimum level difference between lf-suppression boundary and tip threshold was larger than 20 dB, for the whole group of fibres the difference was 34 dB on average. The hf-suppression regions sometimes reached below the tip for fibres with CFs in the 4 kHz region. The frequency at the lowest level of the hf-suppression boundary, best suppression frequency or BSF, is related to the CF as: BSF = 0.55 + 1.13 CF. When the suppressee level increased, the lower boundary at the hf side shifted upwards with a rate greater than 1 dB/dB. On the whole the two-tone suppression data in the guinea pig agree with those found in other rodents.

Detuning of cochlear action potential tuning curves at high sound pressure levels: influence of temporal, spectral and intensity variables

Action potential (AP) tuning curves (TCs), generated by probe stimuli of 60-65 dB SPL with short rise and decay (r&d) times, are less sensitive (have elevated tip thresholds) and are detuned (the frequency is shifted away from that of the probe stimulus, towards a middle frequency of the audiogram). These effects are more pronounced with forward than with simultaneous masking. TCs generated by masking tonal and narrow band noise stimuli are nearly identical, even though the spectrum is much wider for the noise stimulus. Decreasing r&d time has the same effect on TCs generated from both noise and tonal stimuli, even when it only measurably increases the acoustic splatter of the latter. Detuning appears to be related to a temporal-intensity interaction.

Forward masking and unmasking of the offset cochlear compound action potential of the gerbil: comparison with suppression areas of the onset cochlear compound action potential

Simultaneous and forward maskers were used to generate 'onset' and 'offset' compound action potential tuning curves (TCs) in the gerbil. The simultaneously masked offset TC, generated in response to a 16 kHz, 65 dB SPL probe stimulus, is W-shaped, with a low frequency tip at 11 kHz, a high frequency tip at 20 kHz, and a peak which occurs at 16 kHz. The 16 kHz forward masked onset TC has a single tip which occurs at 11 kHz. Although it lacks the finely tuned peak and high frequency tip of the stimultaneously masked offset TC, its single tip is more finely tuned than the low frequency tip of the simultaneously masked offset TC. Normalizing these two TCs [(1977) J. Acoust. Soc. Am. 62, 1048-1051] produces a figure which resembles 11 kHz onset TCs with their nonoverlapping regions which are analogs of two-tone suppression (2TS). A similar pattern occurs when probe stimuli at frequencies from 13 to 24 kHz are used to generate offset TCs; i.e., the forward masked offset TC resembles an onset TC and normalizing it to a simultaneously masked offset TC produces areas which resemble analogs of 2TS. Unmasking of the forward masked onset TCs [(1979) Hear, Res. 1, 133-154] produces regions of 2TS which are very similar to those produced by unmasking the forward masked offset TC which is generated by a higher frequency tone. These regions of 2TS for the Offset TC, as determined by unmasking, are very similar to the analogs of 2TS described above.