DETERMINANTS OF SINGLE-CELL RESPONSIVENESS

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Project III investigates ways that spatial and temporal properties of intracochlear electrical stimuli define responses of first- and second- order auditory neurons. Recordings of auditory nerve fiber and anteroventral cochlear nucleus bushy cell activity will be obtained acutely in normal-hearing cats and in monaurally deafened cats with varying degrees of spiral ganglion survival. Neural responses will be analyzed quantitatively using two multi-channel intracochlear electrode designs and various electrical field-coupling configurations. Experiments are subdivided into two major classes. The first class investigates effects of intracochlear current field geometry and summation on single-cell responses. In this, we will use an experimental 11-contact electrode to determine the effects of varying field coupling configurations on neural response threshold and rate of growth. We will also examine channel interactions produced by stimulating two electrode channels simultaneously. Late in the award period, similar experiments will be done using advanced electrodes developed in Project I. The second class of experiments utilizes the same 11-contact array to investigate temporal, dynamic and stochastic aspects of single-cell response. Effects of varying the temporal structure of stationary and time-varying waveforms will be determined for pulsatile and sinusoidal signals, and the effects of neural refractoriness and discharge history will be examined using non-simultaneous masking paradigms. For both experimental classes, physiological data taken from deafened animals will be related to anatomic information about patterns of spiral ganglion survival and the detailed morphology of single cells. Another component of Project III is the development of lumped-element and stochastic node models that will be used to predict single-cell responses to electrical stimuli and the anatomic dependencies of neural activity. The lumped-element models anticipate distributions of electrical potential within and around spiral ganglion cells, given a specific current field orientation and strength. The stochastic models use descriptions of neural node biophysics and morphology to predict neural response threshold, rate of response growth, and the temporal dispersion of the response. Clinically relevant applications of the data collected in Project III include (1) defining the electrode configurations and stimulating waveforms that generate optimal responses, (2) exploring possibilities for exerting control over the spatial extent and temporal fine structure of the neural response, and (3) correlating neural response properties with detailed anatomic information on spiral ganglion survival.