The effect of electrode configuration and duration of deafness on threshold and selectivity of responses to intracochlear electrical stimulation

This report examines the effects of intracochlear electrode configuration a nd mode of stimulation (bipolar or monopolar) on neural threshold and spati al selectivity in the inferior colliculus (IC) of the cat. Single and multi unit IC recordings were made in three groups of animals; acutely deafened a dults (controls), neonatally deafened animals studied at 6 to 18 months of age and neonatally deafened cats studied at 2.5 to 6.5 years. Response thre sholds were plotted versus IC depth to measure the spatial distribution of responses. The response selectivity for each stimulating configuration was defined as the width of the resulting spatial tuning curve (STC) measured a t 6 dB above threshold. Spiral ganglion cell (SG) survival was examined his tologically in all neonatally deafened animals and correlated with physiolo gical results. Animals studied at less than 1.5 years had SG densities of 2 3.5%-64.4% of normal (mean= 42.7%) while animals studied at greater than 2. 5 years had densities of 5.1%-18.3% of normal (mean=9.9%). Electrophysiolog ical results include the following. (1) Monopolar thresholds were 7-8 dB lo wer than bipolar thresholds in the same animals. (2) Varying the configurat ion of bipolar contacts (measured as radial, offset radial and longitudinal pairs) did not systematically affect IC threshold in either controls or sh ort-term neonatally deafened animals. In contrast, the long-term neonatally deafened animals showed a difference in threshold with each configuration. (3) The spatial distributions (Q(6dB)) Of responses to bipolar stimulation were approximately 40% more restricted than those for monopolar stimulatio n. (4) The spatial selectivity of neonatally deafened animals studied at ag es up to 1.5 years was equal to that of control animals with normal auditor y experience. However, selectivity was degraded in the older animals. (5) S electivity was decreased in some animals with the longitudinal bipolar conf iguration and multiple response peaks were seen in several cases using this stimulus configuration. (C) 2001 Acoustical Society of America.