Perceptual grouping of tone sequences in cochlear implant stimulation

The perceptual grouping of alternating tone sequences, also known as auditory stream segregation, has already been studied for normal hearing [1] and sensorineurally impaired listeners [2]. If the frequency separation between two tones A and B is less than a critical value, called the fission boundary, the two streams can not be heard as separate patterns any more. For normal hearing listeners, the frequency separation at the fission boundary amounts to about 100 cents (1/12 of one octave). Listeners with cochlear hearing loss, expected to exhibit generally larger frequency separations at the fission boundary because of broader auditory filters, showed either normal or very large values. This paper reports on preliminary measurements of the fission boundary in postlingually deafened subjects wearing multichannel cochlear implants. This kind of stimulation by-passes the peripheral hearing mechanisms by direct electrical stimulation of the auditory nerve. The present results indicate that fission in electrical stimulation occurs at about the same frequency distance between tones A and B as in normal hearing.