OPTICAL IMAGING OF CORTICAL ACTIVITY-INDUCED BY INTRACOCHLEAR ELECTRICAL-STIMULATION

To observe the spatio-temporal representation of cochlear electrical s timulation in the auditory cortex, we applied the optical imaging tech nique in the guinea pig. The cortical activity induced by acoustic and cochlear electrical stimulation were recorded with a 12x12 photodiode array and a voltage-sensitive dye. For electrical stimulation wire el ectrodes were inserted into the scala tympani and the scala vestibuli of the cochlea. Bipolar stimulation with single electrical pulses to t he 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 aco ustic stimulation. However, the active spots induced by electrical sti mulation moved transversally across isofrequency contours in the audit ory cortex and spread broadly throughout the cortex with time. The mag nitude of responses as a function of electrical intensity showed a ver y narrow dynamic range. In addition, the response latencies were almos t constant for various stimulus intensities. These results were signif icantly different from those for normal sound stimulation. This sugges ts that electrical stimulation bypasses the transducer mechanism, and activates directly the nerve terminals in the cochlea.