International Journal of Pediatric Otorhinolaryngology
The auditory basis of language impairments: temporal processing versus processing efficiency hypotheses
Introduction
Three to 10% of children who are otherwise unimpaired have difficulty learning language [1]. This condition is known as specific language impairment (SLI). In 1978, Zangwill [2] defined this condition as “slow, limited or otherwise faulty development of language in children who do not otherwise give evidence of gross neurological or psychiatric disability, and where the language difficulty is not secondary to deafness”. The aetiology of this condition is largely unknown. However, by definition, children with SLI have normal detection of pure tones, which is the conventional clinical assessment of hearing loss. Even so, a child may pass conventional audiological assessment but nevertheless process more complex sounds abnormally. Indeed, some authors have suggested that auditory processing deficits may cause SLI [3]. These claims have been termed the ‘temporal processing hypothesis’ [3]. Despite mounting evidence suggesting a link between SLI and auditory processing impairments, their role in the genesis of developmental language disorders remains controversial.
Section snippets
Language impairments and the auditory repetition task
Claims that language impairments are associated with deficits in auditory processing have been reported as early as 1965 [4]. However, the idea that auditory processing deficits cause SLI is most associated with Tallal and co-workers [3], [5]. An early study by Tallal and Piercy [5] investigated auditory processing in children with language impairments, using the so-called ‘Auditory Repetition Task’. The task involved listening to two brief tones. One tone was 100 Hz and the other one was 305 Hz.
SLI and tone-in-noise masking
More recently, Wright et al. [6] supported Tallal and co-worker’s claims through a demonstration that children with SLI have difficulty detecting a much simpler stimulus, brief tones presented with a masking noise (‘tone-in-noise masking’). Wright et al. found what appeared to be an even more robust effect than Tallal and co-workers’s, using more refined psychoacoustic techniques. In Wright et al.’s experiment, 8-year-old children sat in a soundproof room and tones and noises were played to the
Development of tone-in-noise masking
Temporal resolution has been defined as the minimum time interval within which different acoustic events can be distinguished [8]. One possible explanation for Wright et al.’s results was that temporal resolution was developmentally delayed in children with SLI. However, until recently, little was known about the normal development of this aspect of auditory processing. In 2001, we investigated development of temporal processing abilities in 6- and 8-year-old children and adults [9], using
Development of the auditory system
A wealth of anatomical, physiological and behavioural data suggests that aspects of the auditory system are immature at birth [10]. The difference between adult and children’s auditory processing abilities appears to arise from both structural and functional immaturities in the auditory periphery (peripheral to the auditory nerve), and from more central limitations on auditory perception [10], [11]. A few aspects of hearing, particularly those that depend almost exclusively upon cochlear
A model of temporal resolution
Although neural mechanisms underlying temporal resolution are not fully understood, they are thought to represent more central processing within the auditory system [13]. To understand further the nature of auditory processing deficits associated with language impairments in younger children, we analysed these data [14] in terms of a model of temporal resolution [15]. The model of temporal resolution, originally developed by Moore et al. [15], consists of four stages (Fig. 3A):
- (i)
First, a tone and
Analysis of psychophysical data in terms of the model
In terms of the model (Fig. 3), to suggest that a child with SLI has poor temporal resolution implies he has a widened temporal window. However, there is an alternative explanation for Wright et al.’s results [6]. Our analysis using the model suggests that poor processing efficiency can account for Wright et al.’s data, without changing the shape of the temporal window. Processing efficiency encompasses all factors, aside from temporal resolution, that may affect detection on a task, such as
A test of the competing hypotheses
A recent study in our laboratory [22] tested the competing temporal processing and processing efficiency hypotheses by measuring backward masking as a function of delays between the tone and the noise in children and adults. Twelve adults and 12 children, with normal reading and language skills, were tested using similar methods used in Wright et al.’s experiment. However, in this experiment, there were four different backward masking tasks, in which a silent interval was introduced between the
Conclusions
In this paper, we have argued that a deficit in auditory processing efficiency more completely describes a range of experimental data on language-impaired children than does a deficit in auditory temporal processing. However, the causal relationship between processing efficiency and language abilities remains unclear. Tallal et al. [23] hypothesised that auditory processing deficits might degrade the ability to perceive the brief elements of speech, and putatively cause language impairments.
Acknowledgements
We thank The Wellcome Trust and the Medical Research Council.
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