ELECTROMAGNETIC STIMULATION OF THE AUDITORY-SYSTEM OF DEAF PATIENTS

Electromagnetically induced auditory perception was investigated in 18 deaf patients who were candidates for cochlear implants. In the extra cranial magnetic stimulation (EMS) procedure, patients were stimulated with time-varying magnetic field brief pulses from a coil positioned at the i) auricle, ii) the mastoid, and iii) the temporal lobe area. E MS elicited auditory sensations in 26 ears (of 14 patients/subjects). The lowest threshold of auditory sensation (TAS) was found to be at th e 20% EMS level, with a range of 20-50% of the maximum level (2.0 Tesl a), and approximately equal sensitivity in each coil position. Eleven of the subjects hearing EMS-induced sound perceived changes in pitch w hile 6 heard ''clicks'' or clicks and tones. Spearman Rho correlation analysis showed a mild negative correlation between the EMS/TAS and th e pre-implant FFA, best tone threshold (BTT), and direct promontorial electrical stimulation (ES) thresholds at 250 Hz and 500 Hz. No correl ation was found between EMS or ES and performance on the pre-implant o r post-implant psychacoustic tests (MAC VIII or 3-Digit speech tests) or the measurements of the thickness of cutaneous and osseous tissue f rom the stimulation sites at the mastoid and ear canal to the cochlear and 8th nerve. A fair positive correlation was found between the EMS/ TAS and the post-implant (6 months) ES threshold when the electrodes a llocated the 500 Hz frequency range were stimulated. A mild positive c orrelation between the pre-cochlear-implant promontorial electrical st imulation (ES) at 250 Hz and the four frequency tone average (FFA: 0.5 , 1, 2, 4 kHz) was also found. Middle ear muscle reflex tests in 5 pat ients with EMS stimulation of the auricle, mastoid, and temporal lobe area contralateral to the probe ear did not yield interpretable respon ses. It was concluded that extracranial EMS in the area of the auricle , mastoid process, and temporal lobe evokes auditory sensations in dea f ears, and that this technique may have potential for use in clinical audiology, particularly with an increased stimulus frequency capacity (Hz), and as a possible noninvasive activator of surviving neural ele ments of the auditory system.