Hearing Loss Disrupts AM Processing Figure 3. Differences in 5 Hz sAM detection thresholds are not attributable to measures of training or attention. (A) Higher sAM detection thresholds in CHLs were not due to better training, as CHLs reached slightly better compentence during training than CTRs, and criterion performance for detection of a fully-modulated sAM did not correlate with sAM detection thresholds for either group. (B) Proficiency with the task during initial testing, measured by d9 levels for fully-modulated sAM, did not explain differences in sAM detection threshold. (C) False alarm rates, a measure of attention, were not different across groups. (D) The number of trials per session, a measure of motivation, also did not explain differences in sAM detection threshold. Black = CTR; Orange = CHL. doi:10.1371/journal.pone.0041514.g003 Auditory Cortical Responses to Static Tones in Awake CTR and CHL Animals The behavioral findings suggest that early-onset bilateral auditory deprivation leads to a perceptual deficit in slow amplitude modulation detection. Since in vitro slice studies demonstrate that this form of moderate hearing loss affects synaptic function in cortex (e.g. [10]), we examined the effects on single ACx neuron response properties and asked whether these properties were correlated with behavior. However, the measures shown here do not, in themselves, imply that the ACx is the only locus of change. Single-unit recordings were obtained from primary ACx of awake untrained adult animals with treatments that paralleled the behavioral groups: 15 control animals and 8 animals with bilateral CHL induced at P10. The control cells include those presented in a previous study along with additional control neurons. The best frequency (BF) was determined manually, and rate-level functions (RLFs) were collected using 200 ms tone pips at BF for 122 CTR and 67 CHL neurons. The distribution of BFs did not differ between groups (Fig. 4A; t(187) = 0.37, p = 0.71), indicating that we sampled from a similar distribution of cells across the ACx tonotopic axis. Hearing loss did not affect cortical neurons’ dynamic range (Fig. 4B; t(187) = 1.0, p = 0.3) or the ratio of monotonic to non-monotonic cells (Fig. 4C; x2 = 0.43, p = 0.51). The first spike latency to tone pips that elicited the most spikes (i.e., pips at the peak of the RLF) did not differ across groups (Fig. 4D: CTR: 43.162.0 ms vs CHL: 47.463.2 ms, t(187) = 1.2 p = .23). As expected, absolute thresholds were signif- icantly higher in CHL neurons (Fig. 4E; CTR: 20.161.9 dB SPL vs CHL: 49.161.7 dB SPL, t(187) = 9.8 p,.0001). Finally, tone pips evoked higher firing rates in CTR versus CHL neurons; this was marginally significant at threshold levels (Fig. 4F; t(187) = 1.9 p = .057) and significant at peak levels of the RLF (Fig. 4G; t(187) = 3.8 p = .0002). Developmental Hearing Loss-induced Changes in Cortical sAM Coding The behavioral measures (Fig. 1) indicate that, for animals with developmental CHL, sAM detection is poorer at a slow modulation frequency (5 Hz), but is unaltered at a faster MF (100 Hz). We examined whether behavioral sensitivity levels were reflected in the sensitivity of ACx neurons. We recorded from 64 CTR and 53 CHL ACx neurons in untrained awake gerbils, and obtained responses to sAM tones at each neuron’s BF, presenting MFs ranging from 1 to 100 Hz to obtain a modulation transfer function (MTF). Responses were quantified based on firing rate, vector strength, and power at the sAM modulation frequency (see Methods). Firing rate ignores temporal properties, while vector strength measures phase-locking to the MF (e.g., high values signify discharge at a particular phase of the modulation cycle). Power provides the magnitude of the spiking response at the MF independent of phase. Thus, power incorporates both firing rate (response magnitude) and firing pattern information, and is well suited to quantify the neural responses that reflect the shape of the sAM envelope. Figure 5A depicts MTFs using the mean values across cells for each of the three measures. A similar form was apparent for each measure: cortical responses to sAM have a low- or band-pass form. When comparing CTR and CHL responses, vector strength displayed equivalent phase-locking across all MFs (Fig. 5A top panel; F(1,1) = 0.83 p = .36). In contrast, firing rate and power measures showed stronger responses in CTR cells than in CHL cells (FR: F(1,1) = 8.9 p = .003, PMF: F(1,1) = 2.9 p = .05). Consis- tent with behavioral observations (Fig. 1), CHL responses were less robust at slower MFs, but equivalent to CTR responses at faster MFs (Fig. 5A bottom two panels) for measures that include response magnitude but not pure phase-locking. To assess whether CHL reduced modulation depth sensitivity in ACx neurons, we identified the subset of ACx neurons with a best MF of 5 Hz (27 CTR and 36 CHL), and obtained responses to sAM tones at each neuron’s BF. The population response was quantified and compared for the two groups. Modulation period histograms were generated for each modulation depth, then summed across cells and normalized by cell number to create pooled MPHs for both groups (Figure 5B). A periodic shape was apparent in the temporal pattern of discharge of each population histogram, and the magnitude of the response increased with MD. PLoS ONE | www.plosone.org 7 July 2012 | Volume 7 | Issue 7 | e41514