Figure 3.

Histological and physiological assessment of the degree of de-efferentation. A, Mean survival of MOC vs LOC terminals at 45 weeks of age. For each ear, confocal image stacks (e.g., Fig. 1C–F) were obtained from 8 cochlear locations (see B). MOC and LOC innervation was quantified in each stack as described in Materials and Methods and then averaged into three regions: apex (5.6–8.0 kHz), middle (11.3–32 kHz), and base (45–64 kHz). Regional values in each case are expressed as “survival” by normalizing to regional mean values for young (8 week) Controls (n = 6). Cochlear regions with <75% survival of both LOC and MOC terminals (dashed lines) were classified as OC Lesion (n = 44 surgical cases). Each region from each case generates one point in A and was considered independently for this and all subsequent analyses. B, The degree of de-efferentation in OC Lesion regions was relatively uniform throughout the cochlear spiral. Group mean data (±SEM) are normalized as described in A. Differences between the OC Lesion group and age-matched controls were highly significant at all frequencies (p ≪ 0.001). C, To measure MOC effects in vivo, DPOAEs were measured before, during, and after a 70 s train of shocks to the OC bundle (gray box). Response amplitudes are normalized to preshock values, and the size of MOC suppression is defined as shown for one sample run. Mean MOC effects (±SEM) are shown as a function of stimulus frequency (f₂) for OC Lesion regions (defined in A) compared with control (n = 14). Intergroup differences were significant at the p ≪ 0.01 level for test frequencies at 11.3, 16, 22.6 and 45.2 kHz. D, The size of the MOC effect in individual cases, shown here for f₂ = 22.6 kHz, correlates (r = 0.76) with survival of MOC innervation in the appropriate cochlear region of the same ear. For D, data are shown from controls and all surgical cases where DPOAEs were robust enough to record shock-evoked MOC effects (n = 41).