engleski

Neural tuning measured with forward-masked compound action potentials

Frequency selectivity is one of the most fundamental cochlear properties (Robles and Ruggero, 2001). Recent behavioral and otoacoustic measurements suggest that it is sharper in humans than in laboratory animals (Shera et al., 2002), but this is disputed based on comparisons of behavioral and electrophysiological measurements across species (Ruggero and Temchin, 2005). There are no direct neural recordings from human single auditory nerve fibers, but gross potentials may enable tighter comparisons across species. We search a protocol that is minimally invasive and gives good correspondence with single fiber selectivity and can be applied to humans. Oxenham and Shera (2003) argued that forward masking of a brief signal probe is more suitable to study frequency selectivity than simultaneous masking, because it is less confounded by cochlear nonlinearities. We combined the forward masking paradigm of the latter study with compound action potential (CAP) recordings at the round window of chinchillas and cats - two species for which extensive single fiber data are available. Brief probe tones were presented at a level fixed at 10-15 dB above the unmasked CAP threshold, and the level of a notched noise masker causing a criterion reduction in CAP amplitude (typically 50%) was determined. By varying notch width, Q10 values (ratios of signal frequency to 10 dB notch bandwidth) were obtained. We found better agreement between Q10 values of forward-masked CAPs and single fibers than previous studies using simultaneous masking. Q10 values increased with probe frequency ; straddled the values for single fibers ; and were higher for cat than for chinchilla. These results suggest that forward-masked CAPs may enable electrophysiological assessment of human cochlear frequency selectivity. Supported by grants from FWO and BOF (Flanders, Belgium) and a BOF visiting professorship and FONDECYT to RL.

auditory neuroscience

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