Frequency discrimination procedure:

DLFs for pure tones were then measured, using all of the frequencies around the Fc used for the loudness-scaling task except the highest. In order to prevent subjects exploiting any residual loudness differences as the frequency changed in the DLF procedure, the level of each stimulus was varied by a random value selected from a uniform distribution ranging between -6 and +6 dB around the loudness matching value. The DLFs were measured using a three-interval, two-alternative forced-choice procedure with a two-down, one-up decision rule. On each trial, two of three successive observation intervals contained pure tones at a reference frequency; the remaining interval, which had a 50% chance of coming second and a 50% chance of coming third, contained a pure tone of a frequency that was variable but always higher than the reference frequency. The intervals were separated by 300 ms silent gaps. The subject’s task was to identify the interval that contained the highest tone. The difference between the reference and the variable frequency was initially set at 20%. After two consecutive correct responses, the difference in frequency was decreased, and was increased after each incorrect response. Ten reversals were obtained in each run. The frequency difference was varied by a factor of 2 until the fourth reversal occurred and by a factor of 1.41 thereafter. After each run, the geometric mean for the last six reversals was computed. The order of testing was determined by random selection from among the set of frequencies, spaced 1/8 octave apart, for which the absolute hearing threshold had been previously measured. Three runs were collected at each of these test frequencies.