This week's (9/27/95) PNAS has an article by Johannes Ziegler and colleagues (Université de Provence) titled, "Deficits in speech perception predict language learning impairment". The paper concerns specific language impairment, or SLI.
Readers may remember that a rare monogenic form of SLI is the disorder that led to the identification of the FOXP2 gene (OMIM). Later work showed that most SLI is not directly caused by FOXP2 variants, but apparently the adjacent cystic fibrosis gene (CFTR) does show a strong association with SLI. Interesting.
The current study investigated the behavioral correlates of SLI to see what might cause the language learning deficits. The abstract:
Specific language impairment (SLI) is one of the most common childhood disorders, affecting 7% of children. These children experience difficulties in understanding and producing spoken language despite normal intelligence, normal hearing, and normal opportunities to learn language. The causes of SLI are still hotly debated, ranging from nonlinguistic deficits in auditory perception to high-level deficits in grammar. Here, we show that children with SLI have poorer-than-normal consonant identification when measured in ecologically valid conditions of stationary or fluctuating masking noise. The deficits persisted even in comparison with a younger group of normally developing children who were matched for language skills. This finding points to a fundamental deficit. Information transmission of all phonetic features (voicing, place, and manner) was impaired, although the deficits were strongest for voicing (e.g., difference between/b/and/p/). Children with SLI experienced perfectly normal "release from masking" (better identification in fluctuating than in stationary noise), which indicates a central deficit in feature extraction rather than deficits in low-level, temporal, and spectral auditory capacities. We further showed that speech identification in noise predicted language impairment to a great extent within the group of children with SLI and across all participants. Previous research might have underestimated this important link, possibly because speech perception has typically been investigated in optimal listening conditions using non-speech material. The present study suggests that children with SLI learn language deviantly because they inefficiently extract and manipulate speech features, in particular, voicing. This result offers new directions for the fast diagnosis and remediation of SLI.
From an evolutionary perspective, a disorder that affects 7 percent of people is not a disorder; it is a normal variant. The 7 percent value comes from a study by Tomblin et al. (1997), who screened a sample of over 7000 monolingual English speaking kindergarteners, finding only small differences between boys and girls in the incidence of SLI. Such a high frequency raises a question: Why has this apparent problem persisted in human populations, when it would seem that language ability is very important to survival and successful integration into society?
Of course, more basic statistical answers must come before we worry too much about the evolutionary history of SLI. For one thing, is this actually a unique phenotypic variant, or is it merely the end of a continuous distribution of language-learning skill in children? And are all cases of SLI actually part of a single package of correlated symptoms, or is there a broader spectrum of different deficits in language learning that belong to the disorder?
The current paper by Ziegler et al. (2005) starts by looking at the actual behavioral correlates of SLI. The introduction to the paper gives some background on the issues:
The causes of SLI are still hotly debated. Current theories of SLI fall into two categories: those that attribute SLI to a specifically linguistic deficit and those that attribute SLI to general processing limitations (for a review, see ref. 5). Linguistic deficit theories typically assume that children with SLI have difficulty acquiring linguistic mechanisms, such as past tense rules or the grammatical principle of inflection (6, 7). Children with SLI are thought to be "stuck" at an early stage of grammatical development. Such a delay could actually reflect a general maturational delay of language and other cognitive systems (8, 9).
In contrast, general processing deficit theories assume that it is not the specific nature of the material that is important but rather how it is processed in the brain. Nonlinguistic deficits in either perception or memory are thought to be responsible for language disorder (1012). The most prominent theory of this kind, also called the fast temporal-processing deficit hypothesis, maintains that SLI is a consequence of a deficit in processing brief and/or rapidly changing auditory information and/or in remembering the temporal order of auditory information (1316). For example, Tallal and Piercy (13) found that some children with SLI have difficulty reporting the order of pairs of high- and low-frequency sounds when these sounds are brief in duration and presented rapidly. Such a deficit may underlie difficulties in perceiving grammatical forms (e.g., the or is), which are generally brief and unstressed (17).
The paper additionally presents possible criticisms of these views. The main idea of the research presented here is to test the hypothesis of auditory processing deficits by looking not at kids in sterile laboratory settings, but in normal listening conditions with background noise. The logic is that they might have trouble distinguishing speech sounds within complex aural environments.
The discussion fairly succinctly states the results:
The main findings of the present study can be summarized as follows. Under optimal listening conditions (silence), children with SLI showed only subtle speech perception deficits. However, under conditions of stationary noise and fluctuating noise, children with SLI showed substantial speech perception deficits. Note that conditions of fluctuating noise are not artificial; they are actually very representative of the kind of listening conditions that children will encounter in their daily life (in schools, for example). Thus, the present results raise the possibility that children with language learning disabilities have very serious problems with noise exclusion, which will certainly have tremendous consequences for normal phonological development. A similar proposal has recently been made with regard to visual (magnocellular) deficits that seem frequently associated with dyslexia (45). The authors showed that dyslexic children do not have visual (magnocellular) processing problems per se but rather problems of noise exclusion that become apparent in visual tasks using noisy displays. Noise exclusion could therefore be a very general problem responsible for poor phonological development of children with language learning problems and dyslexia.
To me, this is illuminating about the frequency of the disorder. If the problem is distinguishing sounds quickly and accurately in a complex aural environment, then it may be a problem that manifested much less, or possibly not at all, before people began living in crowds. The comparison with dyslexia may be quite relevant, since, of course, people didn't have to read through most of human existence either. This does leave the question of why the variation exists as it presently does. Are there other behaviors that weigh in a different direction from language learning? Is ADHD a pertinent analogy? Are there alternate strategies for learning langauge?
Probably the most important thing would be to get a better idea of the cross-cultural incidence of these traits. If SLI is really significantly associated with CFTR, there might well be significant variation among populations today. Just a glimpse into what child psychology can tell us about the evolution of culture.
Tomblin JB, Records NL, Buckwalter P, Zhang X, Smith E, O'Brien M. 1997. Prevalance of specific language impairment in kindergarten children. J Speech Lang Hear Res 40:1245-1260. PubMed
Ziegler JC, Pech-Georgel C, George F, Alario F-X, Lorenzi C. 2005. Deficits in speech perception predict language learning impairment. Proc Nat Acad Sci USA 102:14110-14115. Full text online