Temporal modulation transfer functions in patients with cochlear implants

Thresholds for the detection of amplitude modulation were measured in cochlear implant patients as a function of modulation frequency. Three types of threshold measures were taken: detection of amplitude modulation, detection of low-frequency sinusoidal current waveforms, and detection of beats in two-tone complexes. The temporal modulation transfer function (TMTF), defined as the plot of modulation detection thresholds as a function of modulation frequency, show low-pass filter characteristics with similar cutoff frequencies for all three tasks. The similarity of these three measures suggests a common temporal mechanism. While modulation detection differs somewhat in normal-hearing and implanted listeners, both exhibit the same general characteristics. The TMTFs are low pass with a cutoff frequency near 70 Hz for normal-hearing listeners and near 140 Hz for implanted listeners. Patients with cochlear implants can best detect temporal modulation at modulation frequencies below 300 Hz, and are most sensitive to 80- to 100-Hz modulation. At high carrier levels many implant patients could detect smaller modulation amplitudes than normal-hearing listeners, a finding that is consistent with the smaller intensity DLs for some implanted listeners at high levels. These results demonstrate that, while implant listeners cannot discriminate steady-state, high-frequency stimuli, speech information might be conveyed by the envelope of the high-frequency components of speech.