Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation. I. Intensity dependence of firing rate and response latency

Abstract

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. Acoustic stimulation of the ear ipsilateral to the studied cortical hemisphere with brief biphasic clicks was compared with electrical stimulation of the contralateral cochlea with brief biphasic electrical pulses delivered via a feline cochlear prosthesis. The contralateral ear was deafened immediately before 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 approximately 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 intensity for single electrical and acoustic pulses. Resulting rate/level functions were characterized by a fast growing low-level segment and a more slowly growing, saturating, or decreasing high-level segment. The slopes of these two segments as well as the stimulus level and firing rate at the juncture of these two segments (the transition point) provide a complete characterization of the response magnitude behavior as a function of stimulus intensity. 3. The main characteristics of rate/level functions obtained with electrical and acoustic cochlear stimulation were quite similar. However, for any given neuron, differences in the primary growth behavior, such as monotonic or nonmonotonic growth, could be observed between the different stimulation modes. 4. Response latencies from single- and multiple-neuron recordings were obtained as a function of stimulus intensity for electrical and acoustic pulses. Resulting latency/level functions were characterized by a rapidly decreasing low-level segment and a more slowly decreasing high-level segment. The slopes of these two segments as well as the stimulus level and response latency at the juncture of these two segments (the transition point) provide a complete characterization of the response latency behavior as a function of stimulus intensity. Transition point levels for the rate/level function and the latency/level were nearly identical. 5. The characteristic latency behavior for each neuronal response was found to be very similar for acoustic and electrical stimulation. Correlation analysis revealed a close relationship between latency parameters of the two electrical stimulation conditions, a weaker relationship between the longitudinal electrical and the acoustic conditions, and the weakest relationship between the radial electrical and acoustic conditions. 6. Correlation analysis for rate and latency parameters revealed several relationships between these response aspects.(ABSTRACT TRUNCATED AT 400 WORDS)