Sound Processing and Language in Children with Mild to Moderate Hearing Loss

Results

Aided and unaided sound processing abilities of children with MMHL

Figure 1 shows the performance of the MMHL group compared to controls on the behavioural psychophysical measures of sound processing when they were not wearing their hearing aids. In these assessments, a higher score means that children needed there to be a bigger difference between two sounds in order to tell the difference between them. That is, a higher score represents poorer performance. We found that children with MMHL showed poorer performance than normally hearing controls on all but one of the measures of sound processing while they were not wearing their hearing aids. These findings suggest that children with MMHL do have problems with discriminating a range of different sounds when they are not wearing their hearing aids.

Results1
Figure 1: Thresholds of the MMHL group and the control (CA) group on the behavioural psychophysics tasks. A higher percentage score represents poorer performance. The MMHL group obtained significantly higher (poorer) thresholds than controls on all of the tasks except AMD, where performance of the two groups was the same.

We then asked whether children with MMHL also showed difficulties in sound processing when they were wearing their hearing aids. We did this in two ways. First, we asked whether wearing hearing aids improved the sound processing skills of children with MMHL. We found that the wearing of hearing aids improved the performance of children with MMHL on 4 out of the 7 tasks. Second, we asked whether children with MMHL performed more similarly to normally hearing controls when they were wearing their hearing aids. We found that the wearing of hearing aids brought the performance of children with MMHL to within normal limits on 2 out of 6 tasks. Note that the performance of children with MMHL was already within normal limits without the use of hearing aids for the AMD task. These findings suggest that the wearing of hearing aids does improve the sound processing skills of children with MMHL. However, hearing aids do not appear to improve the processing of all sounds, and even where performance is improved, it is not always brought to within normal limits.

[Note that this does not mean that hearing aids are not working. Hearing aids are designed to amplify the sounds that we think are important for hearing and discriminating speech. The sounds that we used in our study were quite artificial and it is therefore not necessarily surprising that wearing hearing aids did not always improve performance.]

Passive sound processing abilities of children with MMHL

The results of Experiment 1 suggested that children with MMHL do have problems in processing sounds. However, poor performance on behavioural measures of sound processing can be caused by many factors, including lack of attention, memory etc. One possible explanation for these findings therefore is that children with MMHL performed more poorly on these tasks because much of their processing resources were going into listening to the sounds, leaving fewer resources available for discriminating them. We tested this idea by including a task that did not require any active listening.

A subset of the P1-N1-P2 ERP responses of children with MMHL and controls are shown in Figure 2. For the speech-like condition, children with MMHL showed ERPs that were less similar to those of controls, regardless of age. For the non-speech and speech conditions, only the older MMHL group showed ERPs that didn’t resemble those of controls.

Results2ICC_groups
Figure 2: P1-N1-P2 ERP responses of the MMHL and control groups for one scalp position for the speech condition.

One possible explanation of these findings is that children with MMHL might be delayed in their development of sound processing skills. To assess this, we compared the ERPs of the older MMHL group to those of younger controls. If children with MMHL were delayed in their development, we would expect their ERPs to resemble more closely those of younger controls than those of their age-mates. We did not find this. Instead, our results suggest that children with MMHL, particularly those aged 12 years+, show differences in their auditory ERP relative to controls, but that these differences reflect a deviant rather than a delayed pattern of development. We plan to follow up this finding to explore possible reasons why children with MMHL show different ERPs. We also intend to see whether we can improve the sound processing of children with MMHL through training.

Links between sound processing and language in children with MMHL

Our third aim was to find out whether the sound processing skills of children with MMHL might be linked to their language development. Specifically, we asked whether poor sound processing skills might be the cause of the poor language skills of some children with MMHL.

To answer this question, we first needed to know whether some children with MMHL did show poorer language skills than their normally hearing peers. Figure 3 shows the performance of the two groups on the range of language tasks used. For these tests, we have presented performance as z-scores, where normal performance falls at zero, and +1 represents performance 1 standard deviation above the mean, and -1 1 standard deviation below the mean. We found that, as a group, children with MMHL did perform slightly worse than controls on 4 out of 7 of the language assessments we used. However, three points are worth mentioning here. First, children with MMHL only performed more poorly than controls on the measures that involved some aspects of hearing (i.e. hearing a question or repeating back a word or a sentence). They performed normally on those assessments which did not involve hearing. Second, on all bar one of the language tests, children with MMHL on average scored a z-score of around 0. That is, their performance was within normal limits for their age. Rather, the performance of the controls was above average. However, third, there were considerable individual differences, and a significant percentage of children with MMHL did perform more poorly on the language assessments than we would have expected based on their age. This confirms the finding that whereas many children with MMHL acquire completely normal language abilities, some do not.

Results3

Figure 3: Z-scores of the MMHL group and the control (CA) group on language assessments. A higher score represents better performance. The MMHL group obtained significantly lower (poorer) thresholds than controls on the NEPSY, CELF vocabulary, TROG, and CELF sentence repetition tests.

In order to assess whether problems with sound processing caused the language difficulties of some children with MMHL, we did two things. First, we looked at whether poor sound processing and poor language always went together. Figure 4 shows the performance of the MMHL and control children on two measures. The x (horizontal) axis shows their performance on a derived measure of sound processing. Scores to the right of the dotted vertical line are those that fell in or below the bottom 5% of that of controls. The y (vertical) axis shows performance on a derived measure of language. Scores below the dotted horizontal line represent those that fell in or below the bottom 5% of those of controls. If poor sound processing and poor language always go together, then children who had poor sound processing should always have poor language, and vice versa (i.e. children who have good sound processing should also have good language). We can see from Figure 5 that we did not find this. In fact, about 18% of our sample showed poor sound processing but normal language skills (top right quadrant). This suggests that poor sound processing is not sufficient to cause poor language skills. Moreover, about 2% of our sample showed normal sound processing skills but nonetheless had poor language (bottom left quadrant). This suggests that poor sound processing is not necessary to cause poor language skills.

Results4
Figure 4: Performance of the MMHL and control children on a derived measure of sound processing (x axis) and on a derived measure of language processing (y axis). See text for details.

Finally, we asked whether other factors in addition to poor sound processing might contribute to the language difficulties seen in some children with MMHL. We did this by asking what factors, if any, discriminated between those children who had poor sound processing and poor language skills and those who had poor sound processing and good/normal language skills. Table 1 shows the differences between these groups. Children with MMHL and poor sound processing who had normal language did not differ from those with normal language on age, level (severity) of hearing loss, or the age at which their hearing loss was diagnosed. However, we found three differences between the groups. First, the group that showed poor language included a higher proportion of boys than girls. Second, the group who showed good language had significantly higher nonverbal IQ than the group with low language. This suggests that high nonverbal IQ may act as a protective factor in supporting language development in children with MMHL. It is important to note that the children with MMHL who had poor language did not have low IQ – only children who had IQ within the normal range were included in the study. Third, there was a trend for the group who showed poor language to have a higher likelihood of family history of language difficulties. This suggests that MMHL may act in conjunction with genetic predisposition to lead to language difficulties in some children.

Table 1: Descriptive statistics for the MMHL poor sound processing group, divided into those who showed poor performance on the language tests and those who showed normal/good performance on the language tests
  MMHL poor sound processing
MMHL poor sound processing Statistics
  Poor language (n=14)
Normal language (n=13)
 
Age (years)
11.73 10.87  
Gender (M:F)
8:6 1:12 ***
Level of hearing loss (dB)
48.33 43.63  
Age at diagnosis (months)
49.93 49.77  
Nonverbal IQ
52.00 59.08 **
Family history language problems 43% 15% *
*** Significant at p<.01
**Significant at p<.05
* Just missed significance (p<.059)