Am J Audiol. 2002 Dec;11(2):124-7
The relative importance of amplitude, temporal, and spectral cues for cochlear implant processor design.
Shannon RV.
Speech understanding with cochlear implants has improved steadily over the last 25 years, and the success of implants has provided a powerful tool for understanding speech recognition in general. Comparing speech recognition in normal-hearing listeners and in cochlear-implant listeners has revealed many important lessons about the types of information necessary for good speech recognition--and some of the lessons are surprising. This paper presents a summary of speech perception research over the last 25 years with cochlear-implant and normal-hearing listeners. As long as the speech is audible, even the relatively severe amplitude distortion has only a mild effect on intelligibility. Temporal cues appear to be useful for speech intelligibility only up to about 20 Hz. Whereas temporal information above 20 Hz may contribute to improved quality, it contributes little to speech understanding. In contrast, the quantity and quality of spectral information appear to be critical for speech understanding. Only four spectral "channels" of information can produce good speech understanding, but more channels are required for difficult listening situations. Speech understanding is sensitive to the placement of spectral information along the cochlea. In prosthetic devices, in which the spectral information can be delivered to any cochlear location, it is critical to present spectral information to the normal acoustic tonotopic location for that information. If there is a shift or distortion of 2 to 3 mm between frequency and cochlear place, speech recognition is decreased dramatically.
Ear Hear. 2004 Apr;25(2):173-85.
Music perception with temporal cues in acoustic and electric hearing.
Kong YY, Cruz R, Jones JA, Zeng FG.
OBJECTIVE: The first specific aim of the present study is to compare the ability of normal-hearing and cochlear implant listeners to use temporal cues in three music perception tasks: tempo discrimination, rhythmic pattern identification, and melody identification. The second aim is to identify the relative contribution of temporal and spectral cues to melody recognition in acoustic and electric hearing. DESIGN: Both normal-hearing and cochlear implant listeners participated in the experiments. Tempo discrimination was measured in a two-interval forced-choice procedure in which subjects were asked to choose the faster tempo at four standard tempo conditions (60, 80, 100, and 120 beats per minute). For rhythmic pattern identification, seven different rhythmic patterns were created and subjects were asked to read and choose the musical notation displayed on the screen that corresponded to the rhythmic pattern presented. Melody identification was evaluated with two sets of 12 familiar melodies. One set contained both rhythm and melody information (rhythm condition), whereas the other set contained only melody information (no-rhythm condition). Melody stimuli were also processed to extract the slowly varying temporal envelope from 1, 2, 4, 8, 16, 32, and 64 frequency bands, to create cochlear implant simulations. Subjects listened to a melody and had to respond by choosing one of the 12 names corresponding to the melodies displayed on a computer screen. RESULTS: In tempo discrimination, the cochlear implant listeners performed similarly to the normal-hearing listeners with rate discrimination difference limens obtained at 4-6 beats per minute. In rhythmic pattern identification, the cochlear implant listeners performed 5-25 percentage points poorer than the normal-hearing listeners. The normal-hearing listeners achieved perfect scores in melody identification with and without the rhythmic cues. However, the cochlear implant listeners performed significantly poorer than the normal-hearing listeners in both rhythm and no-rhythm conditions. The simulation results from normal-hearing listeners showed a relatively high level of performance for all numbers of frequency bands in the rhythm condition but required as many as 32 bands in the no-rhythm condition. CONCLUSIONS: Cochlear-implant listeners performed normally in tempo discrimination, but significantly poorer than normal-hearing listeners in rhythmic pattern identification and melody recognition. While both temporal (rhythmic) and spectral (pitch) cues contribute to melody recognition, cochlear-implant listeners mostly relied on the rhythmic cues for melody recognition. Without the rhythmic cues, high spectral resolution with as many as 32 bands was needed for melody recognition for normal-hearing listeners. This result indicates that the present cochlear implants provide sufficient spectral cues to support speech recognition in quiet, but they are not adequate to support music perception. Increasing the number of functional channels and improved encoding of the fine structure information are necessary to improve music perception for cochlear implant listeners.
Cereb Cortex. 2006 Jan;16(1):31-6.
Dynamics of auditory plasticity after cochlear implantation: a longitudinal study.
Pantev C, Dinnesen A, Ross B, Wollbrink A, Knief A.
Human representational cortex may fundamentally alter its organization and (re)gain the capacity for auditory processing even when it is deprived of its input for more than two decades. Stimulus-evoked brain activity was recorded in post-lingual deaf patients after implantation of a cochlear prosthesis, which partly restored their hearing. During a 2 year follow-up study this activity revealed almost normal component configuration and was localized in the auditory cortex, demonstrating adequacy of the cochlear implant stimulation. Evoked brain activity increased over several months after the cochlear implant was turned on. This is taken as a measure of the temporal dynamics of plasticity of the human auditory system after implantation of cochlear prosthesis.