Hearing Loss Disrupts AM Processing Figure 1. Developmental CHL increases behavioral detection thresholds for slow but not fast sAM. (A) Detection thresholds during initial testing and for all subsequent testing days indicate gradual improvement (linear fitted lines) which did not differ across groups. (B) Behavioral detection curves for 5 Hz noise were averaged over the last 3 days of testing. (C) Thresholds derived from these curves showed that CHL animals had a small but highly significant increase in detection thresholds. (D) Improvement for 100 Hz sAM detection across testing days (linear fitted lines) did not differ across groups. (E) In contrast with slow sAM detection, behavioral curves for 100 Hz sAM detection overlapped in CTR and CHL animals, and (F) depth detection thresholds did not differ. Black = CTR; Orange = CHL. ** = p,.01. doi:10.1371/journal.pone.0041514.g001 CTR and CHL groups that were trained with 100 Hz sAM. Animals in the two groups showed equivalent improvement in performance over initial and subsequent testing days (Fig. 1D; linear fits for CTR: R = 20.38, p,.0001 and CHL: R = 20.18, p = 0.08; ANCOVA interaction measuring differences in slopes of fitted lines: F(7,7) = 1.3, p = 0.25). Yet in contrast to trained performance on 5 Hz sAM, CTR and CHL animals did not differ when trained and tested on 100 Hz sAM. As shown in Figure 1E, psychometric functions compiled over the last three days of each animal’s performance overlapped for the two groups, as did detection thresholds (Fig. 1F; CTR: 31.662.6%MD vs CHL: 27.461.9%MD; t(21) = 1.32, p = .20). The same equivalent performance held when comparing each animal’s best perfor- mance day (CTR: 24.961.7%MD vs CHL: 25.262.0%MD; t(21) = 0.1, p = .91). Therefore early-onset CHL did not impair detection of 100 Hz modulations. Sensation Level Alone does not Account for the Effect of CHL The increased detection thresholds for 5 Hz sAM (Fig. 1C) could be attributable to factors other than developmental experience. One possibility is that stimuli were not delivered at equivalent sensation levels. The two groups were tested with stimuli that differed by 35 dB (35 vs 70 dB SPL for CTR vs CHL, respectively). These sound levels were based on pilot experiments in which sAM task-specific thresholds were obtained (see Methods). It was not possible to obtain task-specific thresholds from each animal prior to sAM detection testing for two reasons: 1) animals needed to be very practiced to yield accurate level thresholds and 2) presenting difficult stimuli (e.g., low sound level) early in training can extinguish the learned behavior. Thus, it was only after obtaining sAM detection thresholds, that we determined each animal’s task-specific sound level threshold for detection of a 100% MD signal. The between group difference for task-specific thresholds (Fig. 2A; CTR: 12.060.7, n = 11 vs CHL: 48.561.1 dB SPL, n = 10, t(19) = 28.5 p,.0001; three animals stopped per- forming reliably prior to acquiring level thresholds) was consistent with the levels chosen for assessment of sAM detection. These measurements revealed that individual animals were tested from 16 to 26 dB above their task-specific threshold (Fig. 2B, x-axis). However, the stimulus level relative to threshold did not predict sAM detection ability (Fig. 2B). There was no significant correlation between stimulus level (relative to hearing threshold) and best sAM detection threshold for either CTR (R2 = 0.05 p = .53) or CHL animals (R2 = 0.08, p = .45). It is also essential to demonstrate that even the quietest portion of the modulated signal (the trough) was fully audible to both groups. Although we did not collect absolute threshold detection levels for the noise carrier, they could not be higher than the task-specific threshold, so we use these values here as PLoS ONE | www.plosone.org 5 July 2012 | Volume 7 | Issue 7 | e41514