Abstract:
A notched noise simultaneously presented with a standard tone reduces the midlevel hump in forward-masked intensity discrimination [Plack and Viemeister, J. Acoust. Soc. Am. 92, 1902--1910 (1992)]. This reduction is difficult to explain based on the assumption that the notched noise would force the listener to use high-threshold neurons within the on-frequency band and thus should produce an increase in the midlevel hump. However, a problem in the Plack and Viemeister study was that the notched noise level was increased as a function of the standard level. In the present study, thresholds of a 25-ms, 1-kHz tone were measured in a 125-ms notched noise and shown to grow nonlinearly as a function of the noise level. The slope of the masking function was less than 1 at low noise levels and was greater than 1 at high noise levels. Compared with data in the absence of notched noise, the notched noise produced a greater midlevel hump at low noise levels and reduced or removed the midlevel hump at high noise levels. These data suggest dual effects of the notched noise on intensity discrimination: (1) restriction of off-frequency listening at low levels, and (2) masking the on-frequency listening through two-tone suppression at high levels.