Electrically-evoked auditory brainstem responses (EABRs) will be recorded in implanted human subjects and rats for several purposes: to develop clinical methods to evaluate patients and aid in the adjustment of processors, to study the relationships between the EABR, speech reception and the condition of the peripheral auditory system, and to test models developed in Project 4 to improve our understanding of the mechanisms of electrical stimulation. The first set of experiments will examine the feasibility of estimating psychophysical thresholds using EABR threshold measurements, primarily for the purpose of setting the adjustable parameters of implant processors. These EABR threshold data, together with latency measures for EABR growth functions obtained at the same time, will also be used to test a model electrical nerve stimulation. Another series of experiments will concentrate on the information obtained from wave I of the EABR, the auditory nerve compound action potential. The focus of these efforts will be first to develop methods to measure wave I in the human and then to determine the relationships among waves I, III and V and the subjects' speech recognition scores measured in Project 1. Our primary purpose here is to determine if the EABR can reveal both peripheral and central factors related to speech perception with cochlear implants, as our experiments to data suggest. In addition, we will assess in rats the possibility that wave I growth functions can provide useful information about the distribution of surviving spiral ganglion cells in Rosenthal's canal. The final series of experiments will measure the non-simultaneous interactions between electrodes using the EABR. Experiments in humans will relate the amount of interaction to speech recognition and to comparable interactions measured psychophysically in the same subjects in Project 1. EABR experiments in rats will relate electrode interactions of the same kind to the distribution of surviving spiral ganglion cells. Both the human and rat studies will look to single-unit studies of the auditory nerve in Project 5 for verification of the neural models we employ to interpret the interactions we describe at a more molar level.
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