Hearing Loss Disrupts AM Processing Figure 2. Differences in 5 Hz sAM detection thresholds are not attributable to hearing level, measured behaviorally or neurally. (A) Task-specific behavioral thresholds (the sound level required for detection of a fully-modulated sAM) were predictably increased in CHL animals, by ,35 dB, indicating the effectiveness of the CHL manipulation. (B) The sAM stimulus level for each animal was the amount above task-specific threshold at which its behavior was tested. In neither CTRs nor CHLs was there a correlation between this stimulus level and sAM detection threshold, indicating that performance differences are not attributable to stimulus level differences. (C) Neural ABR thresholds measured across frequency (left) and for clicks (right) were predictably increased in CHL animals by ,40 dB. (D) Consistent with the lack of correlation with behavioral hearing thresholds, in neither CTRs nor CHLs was there a correlation between neural hearing thresholds and sAM detection thresholds. Black = CTR; Orange = CHL. doi:10.1371/journal.pone.0041514.g002 an audibility indicator. Across the range of sAM thresholds for each group, the trough dB SPL ranged from 32 to 34 dB SPL in CTRs and 66 to 68 dB SPL in CHLs, both well above the task-specific thresholds shown in Figure 2A. Furthermore, the difference for each animal between trough level at sAM best threshold and task-specific threshold did not differ across the groups (CTR: 21.360.9, n = 11 vs CHL: 19.361.0 dB, n = 10, t(19) = 1.4 p = 0.17), indicating that the audibility of the trough did not account for the group sAM threshold difference. Therefore, behavioral hearing thresholds did not account for poorer sAM detection in CHL animals. Another measure of hearing threshold is the auditory brainstem response (ABR). After behavioral testing with 5 Hz sAM was completed, ABRs were measured for a subset of CTR and CHL animals (Figure 2C). In response to clicks, ABR thresholds were 37.5 dB higher in CHL vs CTR animals; averaged across a range of tone frequencies, ABR thresholds were 47 dB higher. However, as was the case for the behaviorally-measured hearing thresholds, these did not predict sAM detection ability. Figure 2D illustrates the lack of correlation between ABR click threshold and best sAM detection threshold for CTR (R2 = 0.02 p = .78) and CHL animals (R2 = 0.24, p = .32). Thus ABR-based hearing thresholds did not account for poorer sAM detection in CHL animals. Since we acquired both behavioral and ABR-based hearing thresholds in a subset of animals, we were able to directly compare these measures. Of note is that within either group, there was no significant correlation between behavioral and ABR measures of threshold (CTR: R2 = 0.005 p = .90, CHL: R2 = 0.008 p = .86). Training and Performance Variables do not Account for the Effect of CHL The amount of practice or cognitive factors (e.g., motivation, attention) could have contributed to differences in sAM detection threshold. To determine whether CTR and CHL animals were equally proficient at the task, we compared sensitivity for the largest MD (100%) at two time points: during initial testing and over the course of training. Figure 3A (x-axis) shows that during the final day of training, CHL animals actually achieved better competence on the basic task (training performance for CTR animals: 69.560.09% vs CHL animals: 79.060.07%; t(22) = 3.3, p = .003). Furthermore, during the initial testing, d9 detection levels for 100%MD did not differ between groups (CTR: 2.360.2 vs CHL: 2.260.2, t(22) = 0.3, p = .79; Fig. 3B (x-axis)). Neither the proxy for performance during training (Fig. 3A), nor the proxy for proficiency during initial testing (Fig. 3B) were correlated with best detection thresholds for either group (Training CTR: R2 = 0.004 p = .86, CHL: R2 = 0.001 p = .93; Initial Testing CTR: R2 = 0.05 p = .49, CHL: R2 = 0.008 p = .79). This suggests that the higher 5 Hz detection thresholds in CHL animals could not be attributed to differences in training or the ability to perform the task. It is possible that the sAM detection threshold differences were related to a decline in attention to the task. In behavioral studies, a common proxy for attention is the false alarm rate (in this study, the rate at which animals broke contact with the spout during safe trials). Figure 3C shows that the average false alarms across sAM depths over all days was quite low and, more importantly, did not differ between groups (CTR: 0.07560.009 vs CHL: 0.09060.012, t(22) = 1.0, p = .32). The same was true for average false alarms over the last three days of performance (CTR: 0.07060.011 vs CHL: 0.09860.014, t(22) = 1.6, p = .12). A decline in general motivation could also have contributed to poorer performance by CHL animals. To assess this, we asked whether CHL animals performed fewer trials per session, as compared to CTRs. Figure 3D indicates that this was not the case: there was no difference in the number of trials performed by the two groups averaged over all days of repeated testing (CTR: 33.862.0 vs CHL: 35.863.1, t(22) = 0.54, p = .60). Additionally there was no relationship between number of trials performed and best detection threshold for either group (CTR: R2 = 0.12 p = .27, CHL: R2 = 0.04 p = .53). PLoS ONE | www.plosone.org 6 July 2012 | Volume 7 | Issue 7 | e41514