Discrimination of Simultaneous Frequency and Level Changes in Electrical Stimuli

Frequency difference limens for sinusoidal electrical stimuli were measured at operationally defined equal-loudness points in a behaviorally trained monkey that was deafened and implanted in one ear. The equal-loudness points were defined as the levels at which the discrimination of a frequency change was minimal when frequency and level were varied simultaneously. To determine accurately these points, we varied the level in very fine steps (as small as 0.05 dB) above and below the estimated equal-loudness point. With this method we also determined precise equal-loudness contours and level difference limens. Frequency difference limens ranged from 7% at 100 Hz, 17 dB sensation level (SL) to about 30% at 100, 300, and 600 Hz, 7 to 9 dB SL. Level difference limens ranged from 0.4 to 1.9 dB. Slopes of the equal-loudness contours were 0 at 100 Hz, about 6 dB/octave at 300 Hz, and leveled off to about 2 dB/octave above 600 Hz.