THE INCORPORATION OF LEVEL AND LEVEL-INVARIANT CUES FOR THE DETECTIONOF A TONE ADDED TO NOISE

The potential contribution of level-dependent and level-invariant cues for the detection of a tone added to narrow bands of noise was assess ed in two experiments. In the first experiment, three masker bandwidth s, 40, 120, and 360 Hz, and three center frequencies, 600, 1800, and 5 400 Hz were tested. For the tone-in-noise detection task, the signal t o be detected was a tone with a frequency equal to the center frequenc y of the noise masker. The level of the added tone was adjusted so as to generate d' scores of approximately 2 in a two-alternative, forced- choice procedure. Then, the distributions of across-interval changes i n level were measured. The distribution of differences in level was ap plied to either the noise-alone or the tone-plus-noise stimuli, allowi ng the measurement of sensitivity to changes in level for a two-interv al, forced-choice intensity discrimination task. For one of the four o bservers, there was a good correspondence between the d' values obtain ed in the tone-in-noise task and the d' values obtained in the intensi ty discrimination task. For the other three observers, the discriminab ility of changes in intensity could not account for the detection of a tone added to noise. In order to estimate sensitivity to level-invari ant cues, the noise-alone and tone-plus-noise waveforms were scaled so as to present no reliable differences in level, and observers again d etected which stimulus contained the added tone. Normalization led to d' scores smaller than those obtained in the initial tone-in-noise dis crimination task, but performance levels did not fall to chance. For t hree of the four observers, the detection of a tone added to noise app eared to depend on both level and level-invariant cues. In a second ex periment, psychometric functions were obtained for the detection of a tone added to noise, for the detection of changes in level associated with the noise-alone and the tone-plus-noise stimuli, and for the dete ction of a tone added to noise using noise-alone and tone-plus-noise w aveforms with no reliable differences in level. The maskers were cente red at 1800 Hz, and bandwidths of 40 and 100 Hz were tested. Individua l differences in detection strategy were obtained. Two observers appea red to rely on changes in level, one observer appeared to rely on leve l-invariant cues, and the remaining four observers appeared to adopt a decision strategy that integrated level and level-invariant cues. The results suggest that (a) both level-dependent and level-invariant cue s are available to the observer for the detection of a tone added to n oise, and (b) different observers employ the cues in different ways.