Evoked responses in the auditory cortex of the congenitally deaf white cat following electrical stimulation of the cochlea [Abstract]

Abstract

Knowledge of the functional status of central auditory structures is important when estimating possible benefits of cochlear implantation in the congenitally deaf. Thus far the performance of prelingually deaf adults following cochlear implantation has been disappointing. We have used congenitally deaf cats as a model for prelingual deafness. These animals are deaf from an early age as shown by longitudinal recordings of auditory brainstem responses. They were studied as adults (age 2 years). Under general anaesthesia the cochleae were electrically stimulated using the NUCLEUS-22 banded scala tympani electrode array. Recordings were made from the contralateral auditory cortex and inferior colliculus. Gross potentials, together with multi� and single-unit activities were recorded. Here we confine ourselves to gross potential recordings from the auditory cortex. The skull was opened over the auditory area and the cortex photographed. A computer-controlled 3-axes microdrive provided precise and reproducible positioning of the monopolar recording electrode. Gross potentials were evoked by electrical stimulation of the auditory nerve using bipolar electrodes 1/2, 7/8 or 1/8 (electrode 1 being the most apical). These potentials were recorded from both the cortical surface and at depths of up to 4 mm, amplified and band pass filtered (10 Hz to 10 kHz). The stimuli (0.2 ms biphasic pulses) evoked middle latency responses (10 - 20 ms) over the primary and secondary auditory areas. Thresholds were lowest using electrodes 1/8 (-24 dB re. 1 mApp). Narrower electrode configurations (1/2 and 7/8) were up to 15 dB less effective. The potentials evoked were mono-, bi- or triphasic in shape, depending on recording site. We observed little evidence of tonotopic cortical mapping of stimulation site (1/2 vs. 7/8). If present at all, potentials were considerably smaller when the recording electrode was placed outside the auditory areas. Moreover, threshold currents were far higher (40 dB). It is concluded that the auditory cortex of congenitally deaf animals receives specific information via stimulation of the auditory nerve.