Frequency Distribution of Two-Tone Suppression in Forward Masking
III. Level Effect of Suppressee
1985 Volume 28 Issue 2 Pages 143-155
Details
1985 Volume 28 Issue 2 Pages 143-155
Frequency distribution of two-tone suppression was measured in forward masking and a comparison of psychophysical and physiological results were examined. In each experiment, the masked threshold of 15ms probe tone caused by the addition of a 600ms second masker (M2) to a 600ms first masker (M1) was determined as a function of probe frequency. M1 was a narrow-band noise centered at 3kHz (F1) with 200Hz bandwidth. M2 was a narrow-band noise centered at 2.1kHz (F2) with 200Hz bandwidth. M1 and M2 were generated by band-pass filter with a slope of 215dB/oct. Probe tones were filtered by 192dB/oct band-pass filter with a bandwidth of two thirds of each critical band. The spectrum levels (N0) of maskers varied from-10 to 50dB SPL by 5 or 10dB steps. There was no silent interval between the maskers and probe tones in forward masking.
The results were as follows:
1) In all cases, suppression was maximum near or at F1. This was true regardless of levels of F1 and F2. This implied that suppression magnitude was not related to the frequency difference between travelling wave peaks for F1 and F2 on the basilar membrane.
2) The effect of level on suppression was nonlinear, i.e., suppression magnitude was seen to be a monotonically increasing function, suppression magnitude, however, decreased for M1 or M2 level higher than N0 40dB SPL. This implied that within the auditory-nerve fiber's dynamic range, suppression behavior is related to AGC mechanism.
3) Suppression in a small amount occurred for lower frequency suppressor M2, but large suppression magnitude occurred for higher frequency masker M1, even though the suppressor M2 intensity was weak in values of 10-30dB. This indicated that suppression was generated as a result of interaction and implied that suppression played some role in the encoding of steady-state vowels and of sharp frequency discrimination at auditory peripheral level.
These results agree with previously published reports on physiological suppression behavior and implied that suppression was generated as a result of bidirectional transduction that ocurred within hair cells and stereocilliary-tectorial structures, and that nonlinear transduction of suppression was attributed to the synapse mechanism.
