Gerald Kidd, Jr.
Christine R. Mason
Tanya L. Rohtla
Dept. of Commun. Disord., Boston Univ., Boston, MA 02215
Listeners learned to identify six tone patterns in continuous broadband noise. The patterns consisted of eight sequential 48-ms tones having frequencies that were: (1) constant, (2) rising, (3) falling, (4) alternating, (5) one-step up, or (6) one-step down. The frequency extent was 14% of the nominal center frequency. Masking level differences (MLDs) were measured in a 2I2AFC adaptive detection task by comparing thresholds for NoSo and NoS(pi) conditions. Identification level differences (ILDs) were measured in a 1I6AFC fixed-level identification task by comparing the psychometric functions for the two interaural conditions. Two center frequencies, 500 and 3000 Hz, were used. In addition, for ILDs, the center frequency was randomly roved over the 2500-Hz range. Average MLDs and ILDs were 12 dB at 500 Hz and 3 dB at 3000 Hz. However, in the roved condition, the psychometric function for the NoS(pi) condition was much shallower than for the NoSo condition, so ILDs decreased with increasing S/N level. This result is in good agreement with similar findings for speech [H. Levitt and L. R. Rabiner, J. Acoust. Soc. Am. 42, 601--608 (1967)]. The orderly relationship between detection and identification breaks down in certain conditions when an ``informational masker'' is used. [Work supported by NIH/NINDCD.]