Neuronal Responses in Cat Primary Auditory Cortex to Electrical Cochlear Stimulation: II. Repetition Rate Coding. Schreiner, Christoph E. and Marcia W. Raggio. Coleman Memorial Laboratory, W.M. Keck Center for Integrative Neuroscience, Department of Otolaryngology, University of California at San Francisco, San Francisco, CA 94143-0732, Department of Communication Disorders and Sciences, California, State University at San Francisco, San Francisco, CA 94132.
APStracts 2:0303N, 1995.
SUMMARY AND CONCLUSIONS
1) Responses of neurons in primary auditory cortex (AI) of the barbiturate anesthetized adult cat were studied using cochlear stimulation with electrical and acoustic stimuli. Neuronal responses to acoustic stimulation with brief biphasic clicks of the ear ipsilateral to the studied cortical hemisphere were compared to those evoked by electrical stimulation of the contralateral cochlea with brief biphasic electrical pulses delivered via a feline cochlear prosthesis. The contralateral ear was deafened immediately prior to implantation of the cochlear prosthesis. The feline cochlear prosthesis consisted of four bipolar electrode pairs and was placed in the scala tympani. Two bipolar electrode conditions were used for stimulation: one near radial pair with electrode spacing of 0.25 to 0.5 mm, and one longitudinal pair with electrode spacing of approximately 6 mm. 2) The firing rates obtained from single and multiple neuron recordings were measured as a function of stimulus repetition rate of electrical and acoustic pulses. From period histograms over a recording interval of 1000ms, the driven firing rate to repetition rates from 2 to 38Hz was obtained and repetition rate transfer functions (RRTF) were constructed. The RRTFs were characterized as lowpass or bandpass filters and several descriptors were obtained, such as the repetition rate producing the highest driven activity, high and low cut-off frequencies 6dB below maximum firing rate, and maximum firing rate. 3) For a given neuron, the main characteristics of cortical RRTFs obtained with electrical and acoustic cochlear stimulation were quite similar. However, some small but statistically significant differences in the best repetition rate, cut-off frequencies, and maximum firing rate could be observed between the different stimulation modes. The proportion of bandpass RRTFs was larger for electrical stimulation (57%) than for acoustic stimulation (41%). The high cut-off frequencies for electrical stimulation were slightly but consistently higher than for acoustic RRTFs ofthe same neuron and the maximum firing rate for electricalstimulation was significantly higher than that evoked by ipsilateral acoustic stimulation. 4) The entrainment of cortical neurons to electrical and acoustic pulses was determined and entrainment profiles were constructed. For a given neuron, electrical entrainment profiles showed higher cut-off frequencies than with acoustic stimulation when judged with a fixed entrainment criterion of 0.25 spikes per event. The maximum entrainment seen for electrical stimulation was about 20% higher than seen for the same neuron with acoustic stimulation. 5) Correlation analysis of repetition coding and latency parameters revealed several relationships between these response aspects. Most prominent among them was a significant correlation between measures of the response latency and estimates of the ability to follow temporal repetitions for acoustic as well as electrical conditions. 6) Parametric and comparative evaluations of cortical responses to acoustic and electrical cochlear stimulation support the conclusion that the temporal resolution seen in cortical neurons is largely a consequence of central processing mechanisms based on cell and circuit properties and to a lesser degree a consequence of particular spatial and temporal peripheral excitation patterns. The slightly higher temporal resolution found for the electrical stimulation modes suggest that the temporally highly coherent electrical stimulation appears to engage, in a more effective manner, the excitatory/inhibitory mechanisms contributing to the response in AI than acoustic click stimulation with less temporal coherence. 7 ) These results demonstrate that electrical peripheral stimulation is a useful tool for the characterization of physiological response properties of auditory neurons, and the mechanisms contributing to the generation of spectral and temporal receptive field attributes, such as the interplay of central inhibitory and excitatory processes. In addition, these findings provide important baseline information regarding mechanisms contributing to the creation of perception with electrical stimulation and are crucial for studies of the effects of the chronic use of cochlear prostheses on the functional organization of the auditory cortex. 

Received 6 September 1994; accepted in final form 11 October 1995.
APS Manuscript Number J563-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95