Encoding and decoding of auditory signals in relation to human speech and its application to human cochlear implants.
A description of the optimal electrode and stimulus configuration, based on mathematical model simulation of (1) the mechanical conduction and (2) the mechanoelectrical transduction of the middle and inner ear, will be presented. It is assumed that a sensorineurally deaf patient, whose auditory nerve is at least partially intact, will be able to hear and understand, when an activation pattern in specific groups of remaining fibers of the hearing nerve can be reproduced in a manner equivalent to normal hearing. The placement of 2 bipolar concentric electrodes in the modiolus of the basal and middle cochlear turn in a deaf patient has provided the opportunity for testing of electrical hearing thresholds, differential thresholds, and electrode impedance changes over a 5-month period, with direct access to the electrodes through a skin window. The stimulator was housed in a pocket case to provide easy use in a daily environment. Bipolar pulses of 0.02-1.0 ms were triggered either by zero crossings or by a speech envelope controlled oscillator. It was found that loudness should be encoded by pulse amplitude or duration rather than by pulse repetition rate, since the charge per pulse was the primary variable of loudness sensation. The periodicity hearing allowed for electrical differential thresholds equivalent to or lower than the acoustical ones in normal persons in the 80- to 150-Hz region. For the optimal coding of speech, a multichannel system of 4-10 electrodes each for different nerve fiber groups will be necessary.
D003054:Cochlear Implants; D003638:Deafness / Q000534:rehabilitation*; D004558:Electric Stimulation; D006309:Hearing / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D013067:Speech Perception / Q000502:physiology*; D000159:Vestibulocochlear Nerve / Q000502:physiology*