In the guinea pig the effects of electrical stimulation of the inner ear were measured by recording the evoked potentials at the auditory cortex. The cortical evoked responses to electrical stimuli greatly resembled those resulting from auditory stimuli in the same ears. The similarity was in wave shape, latency, duration, sharpness of thresholds, etc. It was also possible to produce evoked cortical responses when the electrical stimuli were delivered to ears suffering from severe acoustic trauma. These ears were so traumatized by acoustic over-stimulation that they were totally unresponsive to sound. In addition, it was possible to produce electrically evoked cortical responses in ears suffering from drug induced damage. The damage was of sufficient long standing as to produce extensive degeneration in the spiral ganglion. The damage was verified histologically and was severe enough to produce a 90% loss of the spiral ganglion cells. Similar electrical stimulation has been carried out in one human subject with normal hearing. Using sinusoidal electrical currents of approximately 10–100 microamperes, hearing sensations were produced only for stimulus frequencies between 4,000 Hz and 10,000 Hz. The intent had been to place a chronic electrode on the round window membrane of a normal human ear. When the same electrode was used to record the alternating current (a.c.) cochlear potential the resulting data were so insensitive as to suggest that the electrode placement had been less than ideal. Exploration revealed the electrode to have been on the floor of the fossula of the cochlea fenestra. A repeat procedure, with improved visualization, located a new chronic electrode on the round window membrane. Recordings and stimulations are currently in progress with that electrode.
Primary auditory cortical responses to electrical stimulation of the thalamus