gms | German Medical Science

Using a forward masking paradigm the present study investigates the spectro-spatial interactions of combined electrical and acoustical stimulation (EAS) of the cochlea in inferior colliculus (IC) neurons.

Under general anesthesia normal hearing cats were implanted with a scala tympani electrode array. An earphone was sealed to the ipsilateral auditory meatus for acoustic stimulation. A 60-ms acoustic masker preceded a 20-ms electric probe (sinusoid). Masker and probe were systematically varied in intensity and frequency. Neural activity was recorded simultaneously at 16 sites along the tonotopic gradient of the contralateral IC using a multi-channel silicon probe.

At low intensities, electric probe frequencies activated primary IC locations that corresponded to the probe frequency. This activity was masked when the electric probe was preceded by acoustic stimuli of the same frequency.

At higher intensities, the electric probe activated secondary IC locations that corresponded to the cochlear site of the stimulating electrode(s). This activity was masked by acoustic frequencies that corresponded to the same cochlear site. Strength of masking was generally increased by increasing masker intensity.

The findings indicate that EAS produces complex spatial interactions in the central auditory system. The extent of these interactions is dependent on the intensities and spectral characteristics of both electric and acoustic stimulus components. The results also suggest that electric stimulation of the hearing cochlea evokes low-threshold acoustic-like electrophonic responses as well as high-threshold direct activation of the auditory nerve.

(Supported by NOHR, NIH N01 DC-2-1006, NIH N01 DC-3-1006 and MedEl)