Figure 2.

Psychophysical discrimination functions obtained from three groups of rats; relative lever pressing, recorded as a suppression ratio (y-axis) is plotted against test-stimulus sound level (x-axis). A suppression ratio of 0 reflects no lever presses, while a suppression ratio of 0.5 reflects lever pressing at baseline rate preceding the test stimulus. Both experimental groups (n = 8 each; filled square data points) were unilaterally exposed to band-limited noise (120 dB, SPL, octave band centered at 16 kHz) six months prior to testing. The unexposed controls (n = 8; unfilled circular data points) were trained and tested in parallel. Panel A shows the average of 5 sessions using 20 kHz test tones. A subset of exposed subjects suppressed significantly more to the 20 kHz stimuli. The statistical difference between the Exposed-with-tinnitus and Unexposed groups is shown in the inset. Suppression behavior (average of 5 sessions) of the same animals tested with broad band noise (BBN), diagnostic for free-field hearing but not tinnitus, is shown in panel B. Data points are group means averaged over 5 test sessions; error bars indicate the standard error of the mean. Significance levels were determined using a mixed analysis of variance (n = 8 per group). SPL, sound pressure level.