The masking level difference in chinchilla auditory cortex. Effects of inner hair cell loss

The purpose of the present study was to investigate responses from the unanesthetized chinchilla auditory cortex (AC) to conditions producing a masking level difference (MLD) in perceptual studies, both before and after inner hair cell (IHC) loss caused by carboplatin. Tungsten electrodes were chronically implanted in the right AC (active) and anterior cranium (common) in six adult chinchillas. Following a recovery period, AC responses were obtained from the unanesthetized animal placed in a passive restraint. Toneburst input/output functions were obtained. Tonebursts (500 Hz) ranged from 0 to 80 dB pSPL, in 10 dB steps, and were presented to the left ear, the right ear, binaural (in-phase) and binaural (out-of-phase). For the MLD series, responses to 70 dB pSPL, 500 Hz tonebursts and a continuous broadband noise (40-90 dB SPL, in 5 dB steps) were studied. Three MLD versus non-MLD conditions were obtained: SpiN0/S0N0, SLN0/SLNL, and SRN0/SRNR. Following baseline data collection, each animal was given 75 mg/kg carboplatin intraperitoneally. Four to five weeks later, the electrophysiology protocol was followed again. At 5 weeks post-carboplatin, the animals were sacrificed, the cochleas were harvested, and cochleograms (hair cell loss across cochlear place) were constructed. For all conditions, response latencies increased and amplitudes decreased with decreasing toneburst level and increasing level of masking noise. Masked AC response thresholds were higher (better) for the MLD conditions than their respective non-MLD conditions. AC response latencies across masking noise level did not appear to vary systematically across conditions. Under some conditions, the MLD conditions showed larger amplitudes than their respective non-MLD conditions for equivalent masker levels. Post-carboplatin, cochleograms showed moderate IHC loss (on average, approximately 40% loss in apex) with minimal outer hair cell loss. The differences in MLD versus non-MLD conditions in terms of masked threshold and response amplitude were often reduced post-carboplatin.