Background

      One of the central questions in the design of a cochlear implant is how many channels of stimulation are needed to achieve a high level of speech understanding.  One way to gain a purchase on the answer to this question is to first answer a different question, i.e., how many channels of stimulation are needed by normal-hearing listeners to achieve a high level of speech understanding.  The answer to this question sets a minimum criterion for the number of channels we should provide an implant patient. 

        Recently two groups of investigators have conducted experiments with adults to provide an answer to this question. To reach 90 % accuracy in quiet, four channels are necessary for simple sentences (Dorman et al., 1997; Shannon et al., 1995).  Five channels are necessary for more difficult sentences produced by multiple talkers (Loizou et al., 1999) and eight or more channels are necessary for monosyllablic words (Dorman et al., in press). 

      In the research reported here we extend our observations on the number of channels necessary to reach high levels of word understanding to young (3-5 year old) normal-hearing children.  Young children do not have the full complement of linguistic and cognitive skills enjoyed by an adult. As a consequence young children may differ from adults in how they respond to signals that have reduced spectral content.

     Speech was processed through a small number of channels and presented to normal-hearing children (3-5 year old) for identification. Examples of processed words are given in the table below:

Listening simulations 
 

To listen to the simulations, just click on the underlined words in the Table below and wait a few seconds until a small window pops up. If asked whether you want to open the tile or save it to a disk, select to open it.

How many channels?

How many independent channels are needed to achieve high levels of speech understanding? The results, with normal-hearing children as well as adults, are shown below in Figure 1. Twelve channels are needed for normal-hearing children to achieve a high (>90% correct) level of word recognition.
 

 

The results for 59 children with cochlear implants (from the Indiana University School of Medicine sample) are shown in the Figure above. Forty-five of the children were classified as “late implanted” (middle figure) and 14 of the children were classified as “early implanted” (right figure). The mean age at fitting for the late-implanted children was 5.4 years, and the mean age for the early-implanted children was 3.3 years. 
     The mean score for the late-implanted children was 44 % percent correct.  The mean score for the early-implanted children was 53 %.  These levels of performance fall between the levels of performance allowed by 6- and 8-channel processors for normal-hearing children and are closer to the level of performance allowed by a 6-channel processor than an 8-channel processor.

• Children need more channels than adults to reach a given level of performance on tests of word recognition, e.g., children need four more channels than adults to achieve 80-90% correct on word recognition. The differences in performance between adults and children are most likely due to differences in the ability to access lexical information from speech with degraded spectral cues. 
• The mean score for late-implanted children on the MLNT word test was 44 % correct.    The mean score for early-implanted children on the MLNT was 53 % correct.  These levels of performance fall between the levels of performance achieved by normal-hearing children listening to words processed through 6 and 8 channels and falls between the levels of performance achieved by normal-hearing adults listening to words processed through 4 and 6 channels. 
• The distribution of word  scores for the early and late-implanted children differ. Twenty two percent of the late-implanted children achieved scores below that allowed by a 6-channel processor.  None of the early-implanted children fell into this category.  The relative absence of very poor scores for the early-implanted children suggests a higher probability of good neural survival for this group of children.  In contrast, the late-implanted children may be characterized by variable neural survival.
• Children with implants, like adults with implants, receive only a small portion of the information potentially available from a signal processor with 20 or more channels. If signal processors/electrode arrays could provide implanted children with the equivalent of 8-12 functional channels of stimulation, then the tasks of word recognition and language acquisition would be significantly facilitated. 

Related Publication:
M. Dorman, P. Loizou, L. Kemp and K. Kirk. "Word recognition by children listening to speech processed into a small number of channels: Data from normal-hearing children and children with cochlear implants," submitted to Ear and Hearing, November 1999.