Development of working memory, speech perception and auditory temporal resolution in children with attention deficit hyperactivity disorder and language impairment

Abstract

Speech perception (SP), verbal working memory (WM) and auditory temporal resolution (ATR) have been studied in children with attention deficit hyperactivity disorder (ADHD) and language impairment (LI), as well as in reference groups of typically developed children. A computerised method was developed, in which discrimination of same or different pairs of stimuli was tested. In a functional Magnetic Resonance Imaging (fMRI) study a similar test was used to explore the neural correlate of the verbal WM. In addition the neurodevelopmental profiles of language impaired children have been investigated.

(i) Speech perception is deeply intertwined with language learning. If SP is not functioning, language learning will be impaired. SP has previously not been studied in children with neurodevelopmental disorders. To test SP, stimulus pairs with nine different consonant contrasts were used. Examination of LI children showed that they had reduced speech perception for three of the contrasts. However, it is uncertain whether this has negative consequences for language functions. Children with ADHD and children with ADHD + developmental coordination disorder (DCD) performed equally well as a control group. There was no apparent change of SP in typically developed children between five and eight years of age.

(ii) Verbal WM is important for the development of lexicon, language comprehension and expression both in typical children and in those with Ll. To test verbal WM with different memory loads, pairs of two to five syllable long non-words were used. A study of verbal WM in children with ADHD and ADHD+DCD, revealed that the latter group performed worse at higher memory loads than both the control group and the children with ADHD. The result was interpreted as a genuine WM problem since it could not be explained by group differences in IQ, hyperactivity or attention. An examination of LI children showed that their performance was worse than that of the controls for all memory loads. Yet, the LI group's performance was better than that reported in previous studies probably caused by differences in the methods. In typically developed children we found an improvement in verbal WM capacity between six and seven years of age.

(iii) A controversial hypothesis has suggested that poor ATR underlies specific Ll. To test ATR, stimulus pairs with two 20 ms long tones, 878Hz and 1350Hz, presented with five different inter stimulus intervals between 256 and 16 ms were used. In typically developed children we found weak associations between ATR and verbal WM and speech perception. In a second experiment, a weak association was found between ATR and gender and phonological awareness. The results did thus not indicate that ATR plays an important role for language functions in children. A possible developmental effect was found between five and six years of age. In Ll children we found no impairment in ATR, supporting that language problems are not related to ATR. In all experimental groups a high variability in ATR was observed.

(iv) The temporal sequence of cortical activations in a verbal working memory task was investigated with fMRI. Increased activation was found in the right pre-motor area when the subject was listening to the first stimulus of a pair. During rehearsal the right temporal cortex was activated in addition to the right pre-motor area. When the second stimulus in the pair was presented and a response decision was made, there were bilateral activations in the temporal cortex, ventro- and dorso-lateral prefrontal cortex and in the supplementary motor area. The findings indicate that the prefrontal cortex, and possibly the posterior parietal cortex, is more involved in response selection than in rehearsal in WM tasks. The lack of prefrontal activity during rehearsal is controversial and the experiment needs to be replicated before any definite conclusions can be drawn from the findings.

(v) We examined a broad spectrum of neurodevelopmental symptoms in a group of language impaired preschool children. Among children with moderate or severe LI, 90 % had additional functional deficits. In the motor assessment, deficits were found in 42% of the children and an additional 13% had isolated fine motor problems. The cognitive assessment showed that 12% of the LI group had an IQ < 71 and that 16% had an IQ of 72 to 85. Diagnostic levels in one or more of four standardised behaviour scales were found in 44% and when a milder degree of symptoms were measured 80% in the LI group were affected. The findings show that LI usually is accompanied by other dysfunctions, thereby demonstrating the need for multidisciplinary assessments of children with Ll.