Operating ranges and intensity psychophysics for cochlear implants. Implications for speech processing strategies.

This report reviews human and monkey psychophysical data related to the detection and perception of intensity information by subjects with cochlear implants. The threshold contour for sinusoidal stimuli is characterized by a slope near zero at frequencies below about 100 Hz, a slope of 5 to 15 dB per octave at frequencies from about 100 Hz to between 0.2 and 2.0 kHz, and a slope of 4 dB per octave at higher frequencies. Equal loudness (or latency) contours follow the threshold contour at low intensities but change gradually as intensity is increased, assuming a shape that can be characterized by zero slope below 100 to 250 Hz and a 3-dB-per-octave slope at higher frequencies. Loudness growth functions and intensity difference limens are also dependent on stimulus frequency and intensity. These psychophysical data suggest that membrane characteristics and other factors impose marked alterations on incoming electrical stimuli, which must be considered carefully when developing speech encoding strategies.