A NEUROPHYSIOLOGICAL STUDY ON COCHLEAR IMPLANTATION BY OPTICAL IMAGING

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To observe the spatio-temporal representation of cochlear electrical stimulation in the auditory cortex, we applied the optical imaging technique in the guinea pig. The cortical activity induced by acoustic and cochlear electrical stimulation were recorded with a 12*12 photodiode array and a voltage-sensitive dye. For electrical stimulation wire electrodes were inserted into the scala tympani and the scala vestibuli of the cochlea. Bipolar stimulation with single electrical pulses to the cochlea induced two active spots separated in the auditory cortical fields, anterior and dorsocaudal, which are tonotopically organized. Such active spots were transiently localized in the cortex also in acoustic stimulation. However, the active spots induced by electrical stimulation moved transversally across isofrequency contours in the auditory cortex and spread broadly throughout the cortex with time. The magnitude of responses as a function of electrical intensity showed a very narrow dynamic range. In addition, the response latencies were almost constant for various stimulus intensities. These results were significantly different from those for normal sound stimulation. This suggests that electrical stimulation bypasses the sensory transducer mechanism, and activates directly the nerve terminals in the cochlea.