Figure 1.

Neural system identification analyses of single-unit AN fiber responses reveal a significant loss of tonotopicity after NIHL for both TFS and ENV coding. A, Wiener–kernel analyses of AN fiber responses to Gaussian noise used to study coding of TFS and ENV cues. The first-order Wiener kernel (h₁) is the mean stimulus waveform before a spike. The second-order kernel (h₂) is related to the spectrotemporal receptive field (fft[h₂]) or mean stimulus spectrogram before a spike. h₁ identifies the frequency tuning of TFS coding. The first eigenvector of h₂ identifies the dominant frequency tuning of ENV coding; that is, the carrier-frequency band driving the fiber's time-varying response to ENV fluctuations. BF of suprathreshold tuning is given in kilohertz for both TFS (black) and ENV (red) coding. B, Effects of NIHL on AN fiber coding of TFS and ENV. Pure tone tuning curves (left) were obtained from a control fiber (top) and two fibers with similar CF but different severity of NIHL. Hearing loss increases moving from top to bottom. CF of the threshold tuning curve is given in kHz near the tip. Threshold (Th) and TTR are given in dB SPL and dB, respectively. The right column shows the frequency tuning of TFS coding and ENV coding obtained from Wiener–kernel analyses. BF of suprathreshold TFS/ENV coding is given in kHz near the peak of each curve (n.s. indicates nonsignificant coding amplitude <3 SDs above the noise floor). The control fiber encodes only ENV structure near CF. Mild NIHL introduces pathological coding of low-frequency TFS. Moderate NIHL shifts coding of both TFS and ENV to low frequency (data from Henry et al., 2016).