Neural correlates of phonological processing in speech sound disorder: A functional magnetic resonance imaging study

Abstract

Speech sound disorders (SSD) are the largest group of communication disorders observed in children. One explanation for these disorders is that children with SSD fail to form stable phonological representations when acquiring the speech sound system of their language due to poor phonological memory (PM). The goal of this study was to examine PM in individuals with histories of SSD employing functional MR imaging (fMRI). Participants were six right-handed adolescents with a history of early childhood SSD and seven right-handed matched controls with no history of speech and language disorders. We performed an fMRI study using an overt non-word repetition (NWR). Right lateralized hypoactivation in the inferior frontal gyrus and middle temporal gyrus was observed. The former suggests a deficit in the phonological processing loop supporting PM, while the later may indicate a deficit in speech perception. Both are cognitive processes involved in speech production. Bilateral hyperactivation observed in the pre and supplementary motor cortex, inferior parietal, supramarginal gyrus and cerebellum raised the possibility of compensatory increases in cognitive effort or reliance on the other components of the articulatory rehearsal network and phonologic store. These findings may be interpreted to support the hypothesis that individuals with SSD may have a deficit in PM and to suggest the involvement of compensatory mechanisms to counteract dysfunction of the normal network.

Introduction

Speech sound disorders (SSD) are the largest group of communication disorders observed in children requiring special education services (“IDEA,” Data updated as of August 3, 2009). Individuals with SSD have a reduced capacity to accurately and intelligibly produce the sounds of their native language (Peterson, McGrath, Smith, & Pennington, 2007) and often fail to apply linguistic rules for combining sounds to form words. Children with SSD have deficits on a number of phonologic tasks including phonological memory (PM) (Peterson et al., 2007) that may persist into adulthood (Kenney, Barac-Cikoja, Finnegan, Jeffries, & Ludlow, 2006). It is believed that individuals with SSD possess poorly formed and unstable underlying phonological representations that lead to speech sound errors (Pennington & Bishop, 2009). The goal of the present study was to use functional MRI (fMRI) to examine the neurological processing of individuals with a history of SSD during overt speech. We hypothesized that children with SSD fail to form stable phonological representations when acquiring the speech sound system of their language due to poor PM. While most adults with histories of SSD no longer present with overt speech sound errors, tasks such as NWR may reveal persistent PM deficits. Using functional imaging (fMRI), we expected to find neuroimaging evidence to support this supposition.

PM has been proposed as the component of short term memory that holds a temporary store of phonological information, a process believed to be essential to the formation of stable phonologic representations. The most widely accepted model of working memory is that introduced by Baddeley and colleagues (Baddeley, 1986). The model consists of several interacting components including a domain general control system referred to as the central executive and several modality specific maintenance subsystems (e.g. verbal and visuo-spatial). The central executive is posited to coordinate the activity of the maintenance subsystems and “to mediate the allocation of attention, the inhibition of task irrelevant processes and the coding of contextual and temporal order information associated with the representations held in memory” (Chein & Fiez, 2001, p. 1004). The verbal maintenance subsystem (e.g. verbal working memory) is referred to as the phonological loop and supports the short term maintenance of verbal information (Baddeley, 1986, Baddeley, 1992, Baddeley, 2003). The phonological loop is postulated to be composed of two components, a phonological store and an articulatory rehearsal process, which act in concert to enable representations of verbal material to be kept in an active state (Chen & Desmond, 2005).

Non-word repetition (NWR) tasks mimic the process of forming a phonologic representation for a new word. Multiple language processes are required to successfully perform NWR, including speech perception, phonological encoding, phonological memory, phonetic encoding (transforming the linguistic codes to articulatory codes), and articulation (Coady & Evans, 2008). “It requires a robust representation of underlying speech units, and sufficient memory both to temporarily store and operate on the novel phonological string” (Coady & Evans, 2008, p. 2). Gathercole and Baddeley and others have employed NWR to specifically measure PM (Gathercole and Baddeley, 1989, Gathercole and Baddeley, 1990, Graf Estes et al., 2007). These researchers have demonstrated significant correlations between NWR accuracy and other measures of PM such as digit span (Coady & Evans, 2008). Further evidence for the utility of NWR in tapping PM is provided by McGrath et al. (2007) who found that a NWR task loaded heavily on PM in children with SSD age 5–7 years. In addition Bishop, North, and Donlan (1996) also reported that deficits in non-word repetition persist into adolescence in individuals with a history of inherited language impairment even after the disorder has resolved.

Recent neuropsychological and neuroimaging studies have generated data that provide insight into the neural correlates of the phonological loop supporting PM. Collectively, the finding of these studies suggest that a network of areas typically associated with speech production, including the left dominant inferior frontal gyrus (Brodmann area [BA] 44/45), premotor area (lateral BA 6), supplementary motor area (medial BA 6), and bilateral (but right dominant) superior cerebellar hemisphere (Lobule V1/Crus I), are involved in the articulatory rehearsal system of the phonological loop (Chein and Fiez, 2001, Chen and Desmond, 2005). In contrast, the data suggest that the phonologic store resides in the left inferior parietal and supramarginal gyrus (BA 40) and that the right inferior cerebellum (VIIB) is also involved in this process (Chen & Desmond, 2005).

To date, the neural correlates of the behavioral deficits associated with SSD have not been investigated or validated by neuroimaging methods. We performed an fMRI study using an overt NWR task in a group of adolescents with a history of SSD and in a group of age matched typical speech and language (TSL) controls. Our objective was to examine group differences in neural activation consistent with the hypothesis that individuals with SSD may have a deficit in PM. We expected to observe functional differences between the two groups in brain regions known to support PM including the inferior frontal gyrus (IFG), premotor cortex, supplementary motor area, inferior parietal cortex, supramarginal gyrus and cerebellum.