Hearing Loss Disrupts AM Processing Figure 4. Limited effects of CHL on cortical responses to static tones. Cells were analyzed irrespective of responses to sAM, thus include a range of BMFs. (A) Cells were sampled across an equivalent frequency range, indicated by similar distributions of BF across groups. Rate-level functions were taken at each neuron’s BF. There were no group differences in (B) dynamic range, (C) the proportion of monotonic versus non- monotonic cells, or (D) the first-spike latencies when cells were driven most strongly, measured at the RLF peak. (E) As expected in the CHL animals, the level threshold across cells was higher than in CTRs. Group differences emerged based on firing rate: both (F) the firing rate at RLF threshold and (G) the maximal firing rate of the RLF were lower in CHL animals. Black = CTR; Orange = CHL. *** = p,.001; (*) = p = .05. doi:10.1371/journal.pone.0041514.g004 At a qualitative level, a visible envelope response emerged at a shallower depth for the population of CTR cells vs CHL cells (Fig. 5B, arrow). A group difference in response magnitude across the lower range of depths is most relevant to the behavioral difference in detection thresholds (Fig. 1C). The mean values for all three measures across MDs ranging from 5%–40% are shown in Figure 5C. The hearing loss group responses over this range of depths were less sensitive than those of controls when measured by power, the measure which best represents the envelope response shape (VS: F(1,1) = 1.5 p = .21, FR: F(1,1) = 0.7 p = .39, PMF: F(1,1) = 4.3 p = .03). In general, cortical responses to sAM mimic the shape of the signal envelope, although there are a wide variety of periodic discharge patterns [36,46]. The power measure is relatively robust to this response diversity. Therefore, we chose to use power when comparing behavioral and neural detection sensitivity across separate animals. Psychometric and Neurometric Measures Correlate with Hearing Status A major goal of the study was to compare neural and behavioral sensitivity. Therefore we transformed the 5 Hz sAM-evoked spike trains into d9 curves for individual neurons, a form that could be compared directly to the behavioral data. The response for each MD was compared to the control response, i.e., to the smallest MD presented (5% MD). We defined d9 = 1 as threshold for detection of the signal to match the behavioral definition of threshold. Individual d9 curves based on the power measure are shown in Figure 6A. To focus on threshold performance, each curve displays only the portion that crossed threshold (horizontal dashed line) and is truncated at the peak d9 value attained, up to a maximum of d9 = 4. Neurons which never reached threshold at any MD are shown in gray. Fewer neurons with very sensitive thresholds (,30%, vertical dashed line) were observed in CHL animals (35%) compared with CTRs (52%). Figure 6B shows that the distribution of power-based d9 thresholds is significantly higher for CHL animals (t(115) = 2.1 p = .04). The practiced behavioral thresholds for CTR and CHL groups from Figure 1 are replotted in Figure 6B over the neural thresholds. The vertical shaded bars show the region of CTR thresholds that does not overlap with CHL thresholds. While there was a large overlap, individual values extended to lower values for CTR animals and to higher values for CHLs. Behaviorally, practiced CTR and CHL animals differ in performance by ,4% MD (measured either as best day or mean of last 3 days performance). The magnitude of this behavioral difference is reflected in neurons that crossed threshold, as the average neural threshold across groups differs by ,3% (CTR 30.963.2% MD vs CHL: 33.863.8% MD). Notably, none of the PLoS ONE | www.plosone.org 8 July 2012 | Volume 7 | Issue 7 | e41514