Causality of LLI and APD

The proportion of children with SLI and APD with AP deficits9 was

significantly higher for most, although not all, the AP tests. Between 25-45% of the two clinical groups combined fell below this cut-off criterion in the tests where between-group significance was reached. It could be argued in the children with SLI/APD, that these AP deficits were the cause of the difficulties that led to their diagnosis of SLI or APD. The suggestion that poor auditory processing, specifically temporal processing, is the underlying cause of LLI as posited by Tallal and colleagues (1973; 2004) has been the centre of much debate over the last two decades. This theory was supported by a landmark study by (Wright et al., 1997) that showed impaired backward masking in SLI children compared to TD children. For poor auditory processing abilities to cause, rather than simply be associated with LLI (e.g. SLI or SRD), it has been argued that (i) AP deficits should be present in all children with LLI, (ii) conversely all children with normal language abilities should have normal auditory processing abilities, and (iii) there should be a relationship between AP abilities and language and literacy abilities (Bishop et al., 1999; Dawes and Bishop, 2009; Rosen, 2003). The same logic can also be applied to the AP deficits as a cause for APD.

9

Defined as more than 1.64 SD from the mean i.e. equivalent to the bottom 5% of the normal

171 The data in the current study do not support the first of these contentions as less than half the clinically referred children underperformed on AP tests compared to their mainstream (MS) school counterparts. Furthermore, there was a significant overlap between the MS and clinical groups shown for all the AP tests. This has also been shown in a number of other studies in children with SLI (Rosen et al., 2009) and APD (Dawes and Bishop, 2009; Rosen et al., 2010). Although Rosen et al. (2009) showed that a grammatical-SLI (G-SLI) group significantly underperformed against an age-matched control (CA) group, around half (8/14) of the G-SLI group were within normal limits (mean - 1 SD) for both backward and simultaneous masking tests, and 30% showed no auditory deficit at all. Similarly, a study of children with suspected APD identified on the basis of hearing and listening difficulties showed that whilst a significant proportion had AP deficits shown by poor performance on at least one of four auditory tasks, around one-third did not show any measureable auditory deficit (Rosen et al., 2010). Furthermore, whilst the cognitive

performance in the APD group was also poorer than the CA group for all three cognitive assessments (WISC NVIQ, BPVS and TROG), there was no

difference in cognitive measures for those who had, and those that did not have AP deficits.

The same was shown in another study of children with APD and dyslexia (Dawes et al., 2009). The children were diagnosed with APD on the basis of the SCAN and nonspeech AP tests. Both APD and dyslexic groups

underperformed the CA group on 3/4 auditory tasks (< mean - 1 SD; 40 Hz FM, 240 Hz FM and iterated rippled noise). Moreover, there were no

172 differences between the two clinical groups on any of the auditory tests. This was also shown for the SLI and APD groups in the present study. Of the dyslexic and APD groups, 42% and 46% were outside normal limits on two auditory tests respectively, and 21% and 36% outside normal limits for three or four tasks. Thus, less than half the clinical children demonstrated AP deficits, again similar to the present study. Conversely, 29% and 5% of the CA group underperformed on two and three/four tests respectively.

To address the third requisite for demonstrating that AP causes LLI and APD, it is necessary to know the relationship between AP deficits and language and reading. In the present study frequency discrimination was the only AP test that showed a significant, moderate, correlation with a marker for language skills (NEPSY) and reading (TOWRE) (r = -.33 to -.55) in both the MS and SLI/APD groups. The correlations remained significant even after accounting for NVIQ (MS: r = -.32, -.48; SLI/APD: r = -.41, -.44; p < .05, for language and reading respectively). When separating the sample into poorer and better FD performance using the cut-off criterion of 1.64 SD, the poorer FD

performers continued to show significantly poorer language (t (72) = 2.41, p = .019) and reading (t (73) = 2.56, p = .012). Following the argument by Rosen et al. (2010) that an absence of relationship between poorer and better AP performance with either cognition or language does not show a causal relationship, the results in the present study suggest that a causal relationship between FD abilities and language and reading skills remains a possibility. The present study also showed a relationship between FD and NVIQ and memory

173 in MS children and for NVIQ in the SLI/APD group (memory just missed significance levels at p = -.057).

The studies discussed here have paved the way for the general consensus today that whilst AP deficits do occur in children with LLI and are associated with certain aspects of phonological processing, AP deficits do not cause LLI. This was also the conclusion of a large study showing that AP does not contribute to academic difficulties (Watson et al., 2003; Watson and Kidd, 2009). It remains a possibility, however, from the findings from the present study that frequency discrimination, not reported in the other studies, may have some influence on language and literacy, and the same may be true for listening difficulties.

4.4.4 Auditory Processing in Relation to Speech Intelligibility and