ASA 125th Meeting Ottawa 1993 May

5aPP4. Masking can explain the effects of notched noise on forward-masked intensity discrimination.

Fan-Gang Zeng

House Ear Inst., 2100 W. Third St., Los Angeles, CA 90057

Plack and Viemeister [J. Acoust. Soc. Am. 92, 1902--1910 (1992)] investigated the role of ``off-frequency'' listening in a nonmonotonic intensity discrimination function under forward masking, in which a midlevel hump was originally reported for 25-ms sinusoidal standards presented 100 ms after an intense masker [Zeng et al., Hear. Res. 55, 223--230 (1991)]. Plack and Viemeister used a notched noise to prevent off-frequency listening and found that the notched noise removed or severely reduced the midlevel hump. A problem with the use of a notched noise in their experiment is that the level of the notched noise increases as a function of the standard level, introducing on-frequency masking in the form of elevated thresholds. They measured intensity discrimination at sensation levels between 10 and 14 dB, independent of the standard level. In the present study, the experiment of Plack and Viemeister was replicated with a narrow-band (800--1200 Hz) noise presented simultaneously with the sinusoidal standards to produce threshold elevations similar to the equivalent notched noise conditions. Presumably, the narrow-band noise does not prevent off-frequency listening, but severely reduces the midlevel hump in a fashion similar to the notched noise. This result suggests that the effects of notched noise on forward-masked intensity discrimination are mainly due to the on-frequency masking. The possible role of excitatory and suppressive masking at the auditory nerve level will be discussed. [Work supported by NIH.]