Do frequency discrimination deficits lead to specific language impairments?

Frequency discrimination deficits in children with specific language impairment (SLI) is a topic that has received little attention. Currently, there are five studies which attempt to determine if children with specific language impairment have difficulty discriminating frequency differences. These research studies reflect behavioral and electrophysiologic experiments which are all similar in approach, and they reveal highly similar findings which suggest significant deficits in frequency discrimination in children with SLI. To date, no research has shown contradictory evidence disproving the presence of such a relationship. Interestingly, textbooks on children with language disorders fail to mention any implication of a relationship between frequency discrimination and SLI. This research review highlights an important hypothesis of a probable auditory component to specific language impairment. Provided that frequency discrimination precedes the development of language and is elemental to communication, is it possible that the deficits play a role in specific language impairments in children? OVERVIEW OF THE LITERATURE The methodologies of the five studies looking at the relationship of frequency discrimination and specific language impairment are very similar. Mengler et al. used a psychoacoustic task involving the presentation of three tones, the second of which was randomly selected to be the same as either the first or third tone, and the child would identify the different tone by pressing a corresponding button.1 The study by Hill et al. used a highly similar method.2 Nickisch and Massinger used a two-tone comparison method of presenting an initial stimulus of 1000 Hz with an initial difference of 100 Hz, a range difference of 0.5-260 Hz, and a step width of 10, 5, or 1 Hz.3 The Davids et al. study is significantly different from the others, as it used evoked potentials (mismatch negativity). The mean ages of the children studied in this investigation ranged from 9 months to approximately 14 years of age.4 The results of the aforementioned studies are highly similar, as they each have found poorer frequency discrimination in children with SLI as compared to children in their respective control groups. The results of the study by Mengler et al. demonstrated that children with specific language impairment require a mean difference of 58 Hz in order to discriminate between tones. This value is significantly higher in comparison to the 25 Hz mean difference that the children in the control group needed in order to differentiate among frequencies. The study by Hill and colleagues found the mean frequency discrimination threshold in children with SLI to be 37.5 Hz, while the mean frequency discrimination threshold in children with average language development was 18.7 Hz. Of note, was the finding that the difference between the frequency discrimination thresholds in children with SLI and the frequency discrimination thresholds in children with average language development increases with age. McArthur and Bishop also discovered a significant coexistence of SLI and poor frequency discrimination.5 Additionally, this study found that children with SLI and poor frequency discrimination thresholds still have poor frequency discrimination thresholds upon reevaluation 1.5 years later. Nickisch and Massinger found similar deficits to be present with regard to frequency discrimination thresholds in their pediatric population with SLI. Perhaps most striking are the results of the Davids et al. study which demonstrate that children with SLI have an absence of mismatch negativity at the neural level in response to linguistic or nonlinguistic contrast. The results of this objective study indicate that frequency discrimination difficulties could be related to non-speech processing difficulties. IMPLICATIONS FOR TREATMENT AND RESEARCH The present studies evaluating the relationship of the presence of poor frequency discrimination in children with specific language impairment all indicate a significant coexistence. It is noteworthy that to date there has not been a single study showing normal frequency discrimination skills in children with specific language impairment. There are substantial differences in frequency discrimination skills in children with SLI in comparison to children with typically developing language. As the development of auditory skills precedes the development of language, children with poor frequency discrimination could be negatively impacted by the lack of ability to discriminate between sounds in early childhood. As a result, these children would not be able to discriminate speech information, which may be a contributing factor to a host of speech disorders. This association has the potential to significantly alter and improve treatment for those with specific language impairment, as frequency discrimination can be improved by training. Assuming that frequency discrimination deficits significantly impact the disorder of specific language impairment, the improvement in frequency discrimination skills would allow children with SLI to better discriminate between speech sounds. Therefore, training in frequency discrimination has the potential to improve the language skills in children with specific language impairment. Additional research is required to support this notion, and studies of this kind should be a priority. Frequency discrimination training is a viable auditory training procedure with considerable research in its support; hence, it seems critical that the information reported here regarding frequency discrimination and SLI not be ignored, but prioritized as an area of research with potentially profound clinical implications.

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