An Examination of Sources of Variability Across the Consonant-Nucleus-Consonant Test in Cochlear Implant Listeners

Abstract

The 10 consonant-nucleus-consonant (CNC) word lists are considered the gold standard in the testing of cochlear implant (CI) users. However, variance in scores across lists could degrade the sensitivity and reliability of them to identify deficits in speech perception. This study examined the relationship between variability in performance among lists and the lexical characteristics of the words. Data are from 28 adult CI users. Each subject was tested on all 10 CNC word lists. Data were analyzed in terms of lexical characteristics, lexical frequency, neighborhood density, bi-, and tri-phonemic probabilities. To determine whether individual performance variability across lists can be reduced, the standard set of 10 phonetically balanced 50-word lists was redistributed into a new set of lists using two sampling strategies: (a) balancing with respect to word lexical frequency or (b) selecting words with equal probability. The mean performance on the CNC lists varied from 53.1% to 62.4% correct. The average difference between the highest and lowest scores within individuals across the lists was 20.9% (from 12% to 28%). Lexical frequency and bi-phonemic probabilities were correlated with word recognition performance. The range of scores was not significantly reduced for all individuals when responses were simulated with 1,000 sets of redistributed lists, using both types of sampling methods. These results indicate that resampling of words does not affect the test-retest reliability and diagnostic value of the CNC word test.

Keywords: cochlear implants; lexical frequency; monosyllabic words; speech perception.

Figure 1.

Average and SD of performance across listeners (y axis) are plotted for each of the 10 CNC word lists, sorted by lowest to highest average score (x axis). (a) Includes data published in Skinner et al., 2006 and (b) includes both the Skinner data and the additional six subjects tested for the present study.

Figure 2.

a) Performance (y axis) for individual CI listeners (x axis), with each list indicated by color (legend). The subjects are sorted by mean score. (b) The normalized range of scores (y axis) for each subject and list. The subjects are sorted by the range of scores.

Figure 3.

In the top panel, the average performance for each word across all CI listeners (y axis) is plotted as a function of word lexical frequency (x axis). In the bottom panel, the average performance for each word across all CI listeners (y axis) is plotted as a function of the lexical neighborhood density for each word (x axis).

Figure 4.

Each panel represents data for one list and all CI listeners. Average performance for each word (y axis) is plotted as a function of lexical frequency (x axis). Titles indicate the list number, the average performance across listeners and the average lexical frequency. The panels are ordered from low to high average performance.

Figure 5.

The ranges of scores with the original and simulated CNC word lists (sorted by original score range as in Figure 2(b)). Blue and green bars indicate lexical frequency or random sampling, respectively. Red stars represent each subject’s range of scores on the original CNC word lists. The red bar at the far right shows the distribution of score ranges across subjects. Box plots represent the distribution of the data as follows; the median is represented by the arrowheads, the edges of the boxes represent the 25th and 75th percentiles, while the whiskers extend to the most extreme data points that are not considered outliers, and the outliers are plotted as + signs.