This research uses psychophysical measures of electrical hearing to investigate limitations in auditory analysis that may affect the perception of speech and other sounds through a cochlear implant. Work in the current proposal focuses on the spatial resolution needed to resolve spectral acoustic cues. Cochlear implants encode stimulus frequency by electrode position along the cochlear duct, with the assumption that different electrodes excite unique populations of auditory nerve fibers. This assumption may be violated in the case of reduced neural survival; thus, individual differences in spatial resolution may explain much of the variability in speech recognition observed among cochlear implant users. Psychoacoustic experiments proposed in this project aim to characterize individual differences in spatial resolution, to compare different measures of spatial resolution, and to determine which measures of spatial resolution predict speech perception in quiet and in noise. Forward-masked spatial tuning curves (fmSTCs) obtained with a fixed-level probe will provide measures of spatial selectivity across electrodes, and evaluation of the tip locations of fmSTCs for different probe electrodes will provide an estimate of neural distribution patterns, or neurotopicity. Measures of spatial resolution obtained from fmSTCs will be compared to measures of tonotopicity (electrode pitch magnitude estimation and electrode pitch ranking), and other measures of spatial resolution that may be clinically practicable (across-channel gap detection and phase dependent threshold shift). The general hypothesis is that measures obtained at stimulus levels near threshold will provide evidence of the distribution of surviving auditory nerve fibers and of spatial selectivity, whereas measures performed at suprathreshold amplitudes will reflect the spatial resolution capabilities of large regions of auditory nerve fibers, but will provide less information about nerve-fiber distributions or spatial selectivity. The effects of stimulus level, pulse rate and electrode configuration will be assessed for selected measures of spatial resolution. Measures of spatial resolution will be related to measures of speech recognition in quiet and noise, including measures of spectral and temporal cue perception. Findings should enhance our understanding of spatial phenomena in electrical hearing and lead to improved strategies for mapping speech processors in individual patients.
