Hearing Loss Disrupts AM Processing Figure 5. CHL alters cortical responses to sAM. (A) Modulation transfer functions (measuring responses across modulation frequency) differed across CTRs and CHLs for response measures that include strength of firing: firing rate (middle) and power at the stimulus MF (bottom). They did not differ for the response measure of synchrony: vector strength (top). Unlike at slow MFs, at 100 Hz sAM the response measures overlapped across the groups. (B) Population period histograms for those cells with a BMF of 5 Hz. Histograms were normalized for number of cells in each group, and are shown for several sAM modulation depths. The envelope shape of the sAM is increasingly apparent in the histograms at larger depths, for both groups. Qualitatively, in the CTR population a visible envelope shape emerges at a lower depth (10%), and the magnitude of the envelope is larger at higher depths. (C) Neural responses over the range of depths where behavioral thresholds differ were stronger for CTRs when measured by power (bottom), the measure that best reflects envelope shape. Black = CTR; Orange = CHL. *** = p,.001; * = p#.05. doi:10.1371/journal.pone.0041514.g005 animals achieved thresholds as low as those of the best neurons, implying that neither group takes full advantage of the most sensitive cortical cells. The neural d9 is a measure of within-cell sensitivity, which assumes that individual neurons are capable of signal detection. An alternative neurometric relies on the convergent neural architecture of a downstream neuron that sums across a pool of cells, ‘‘detecting’’ a signal when the summed response is larger than that elicited by the background signal (e.g., an unmodulated sound). This convergent architecture does not require individual cells to be good detectors, as detection can emerge across a population of cells. We tested whether this architecture would predict hearing loss-induced differences in sAM detection by applying this pooling neurometric to cortical responses based on the power measure. We chose to use power because this measure includes both firing rate and periodicity information and therefore represents the envelope shape of the response. This is particularly important for lower MFs where cortical neuron discharge rate co- varies with AM stimuli with idiosyncratic phase-specific responses [36,46]. Also, the depth function based on power revealed a CHL- induced neural difference (Fig. 5C). For each MD, the power response was summed across all cells, and compared with the summed response magnitude to background (5% MD), where a higher sum was considered a correct detection. Figure 6C top illustrates the summed values from each depth that are compared with the lowest depth: each depth must be only just larger than baseline (represented by the dashed lines) to yield correct detection. The summed functions approximate the mean power functions in Figure 5C. A gradual increase in power is converted by pooling into a sharp rise in detection performance (Fig. 6C bottom). At lower depths, CHL cells did not distinguish sAM from background as well as CTR cells. Thus whether assessing the distribution of d9 thresholds or a pooled neurometric, there is less information available in cortex to animals reared with hearing loss. Discussion Developmental auditory deprivation induces anatomical and physiological effects that persist in adult animals, suggesting that perception should also be affected. To test this idea, we examined the correlation between behavioral and neural measures of sAM detection in animals reared with CHL. When tested with a conditioned avoidance procedure, adult animals reared with CHL displayed significantly poorer detection thresholds for a slow (5 Hz) modulation, as compared with CTR animals. However, detection thresholds for a fast (100 Hz) modulation were equivalent (Fig. 1). This performance difference could not be attributed to overall differences in hearing level, task performance at the largest depths, strategy, or proxies for attention and motivation (Figs. 2, 3). Since our testing regimen challenges PLoS ONE | www.plosone.org 9 July 2012 | Volume 7 | Issue 7 | e41514