The study of central auditory processing in stuttering children

Stuttering is an involuntary disruption in fl uency; it is characterized by abnormal frequency or duration of interruptions in the fl ow of speech, namely repetitions, prolongations, and blocks [2]. Although a variety of etiologies have been proposed to explain its etiology, the cause of stuttering is still unknown [2]. Many current models of stuttering incorporate atypical neurophysiology, genetic factors, a person’s environment, personality, learning ability, auditory processing, language processing, and the correlation of speech and stuttering [1,2]. Smith and Kelly [3] propose a nonlinear multifactorial model of stuttering, which incorporates the complex relationship of many factors that can infl uence stuttering and their compounded and interactive eff ects on the speech motor system. It is hypothesized that the contribution level of each factor determines distinctive behavior patterns that emerge among individuals.


Introduction
Stuttering is a developmental disorder aff ecting speech fl uency. It is present in 5% of preschool-age children and in 1% of the adult population [1].
Stuttering is an involuntary disruption in fl uency; it is characterized by abnormal frequency or duration of interruptions in the fl ow of speech, namely repetitions, prolongations, and blocks [2]. Although a variety of etiologies have been proposed to explain its etiology, the cause of stuttering is still unknown [2]. Many current models of stuttering incorporate atypical neurophysiology, genetic factors, a person's environment, personality, learning ability, auditory processing, language processing, and the correlation of speech and stuttering [1,2]. Smith and Kelly [3] propose a nonlinear multifactorial model of stuttering, which incorporates the complex relationship of many factors that can infl uence stuttering and their compounded and interactive eff ects on the speech motor system. It is hypothesized that the contribution level of each factor determines distinctive behavior patterns that emerge among individuals.
Central auditory processing (CAP) disorder is a condition involving listening diffi culties caused by impaired bottomup processing of sounds by the brain [4]. It is characterized by poor perception of both speech and nonspeech sounds, which is not attributable to intellectual problems or peripheral hearing loss. It commonly impacts listening, spoken language comprehension, and learning [5].
Th e present experiment focuses on one factor hypothesized to play a role in stuttering, which is auditory processing [1,2].

Aim
Th e aim of the study was to evaluate CAP in stuttering children and to compare the fi ndings with those of The study of central auditory processing in stuttering children Samir Asal a , Rania M. Abdou b

Background
There are evidences that indicate a relationship between auditory processing disorders and stuttering.

Aim
The aim of the study was to evaluate central auditory processing in stuttering children and to compare the ndings with those of normal uent children.

Patients and methods
Twenty stuttering school-age children of both sexes were included in the study. A control group included 20 age-matched and sex-matched nonstuttering children. Full informed consent from all participants was taken before initiating the study. All participants were subjected to the following central auditory processing tests: pitch pattern sequence test ( PPST), dichotic digit test ( DDT), speech in noise test (children version) ( SPIN), auditory fusion test revised ( AFT-R), and binaural masking level difference ( MLD) test. All patients were subjected to stuttering severity instrument III to assess their stuttering and its severity.

Results
The stuttering group scored signi cantly poorer in the PPST, DDT, and SPIN, whereas they scored similar to the control group in MLD and AFT-R. There was no correlation between the severity of stuttering and the performance on the central auditory processing tests.

Conclusion
Stuttering children have an intact brain stem integrity shown by the normal MLD and an intact right hemisphere as signi ed by the normal right and left ear difference in the DDT and by the improvement in the PPST on humming. Left hemisphere de cit appears in more complicated tasks such as PPST, DDT, and SPIN, but not in simple tasks such as AFT-R. We can conclude that the de cit is within the left cerebral hemisphere. normal fl uent children to detect the relationship between auditory processing and stuttering.

Patients and methods
Twenty stuttering school-age children of both sexes were included in the study. Another group of 20 nonstuttering age-matched and sex-matched children were included as control. Full informed consent from all participants was taken before initiating the study. All participants were subjected to complete history taking and to stuttering severity instrument III to assess their stuttering and its severity. Later, a selective CAP test battery was run on children to assess the diff erent auditory processing abilities; pitch pattern sequence test (PPST), dichotic digit test (DDT), speech in noise test (children version) (SPIN), and masking level diff erence (MLD) test were performed before their admission to speech therapy. Th e study was carried out during a time period of 6 months.

Statistical analysis
Statistical analysis was performed using statistical package for the social sciences ( SPSS, version 15; SPSS Inc., Chicago, Illinois, USA) software, and mean (X), SD, Student's t-test, and analysis of variance or F-test were computed.

Results
In this study, all students were of matched age and sex, with no signifi cant diff erence between both groups (Table 1). Th e stuttering group was found to score signifi cantly poorer in the DDT (  Table 2). Th ere was no statistically signifi cant diff erence in performance between both groups in the right ear-left ear DDT scores. Signifi cantly lower PPST results improved whenever the stutterers were asked to respond by humming ( Table 2). As for the auditory fusion test revised (AFT-R) (Fig. 4) and MLD (Fig. 5), both groups showed no signifi cant diff erences in performance across all the tested frequencies ( Table 2). Th e stuttering severity index III (SSI) showed that 30% of stutterers had mild stuttering, 40% had moderate Comparison between the studied groups according to dichotic digit test (DDT).

Figure 1
Comparison between the studied groups according to pitch pattern sequence test (PPST).

Figure 2
Comparison between the studied groups according to speech in noise test (children version) (SPIN). stuttering, and 30% had severe stuttering (Table 3 and Fig. 6). Th ere appeared to be no correlation between the degree of severity in stuttering and the performance on CAP tests (Table 4).

Discussion
In this study, school-aged stutterers showed a signifi cantly lower performance than their counter nonstutterers in DDT, PPST, and SPIN. Th e agematched performance of both groups shows that any discrepancy in performance is not attributable to the diff erences of neuromaturation, which is very important because of the continuous emphasis of the literature on the variability in performance with age [6][7][8].
MLD functionally provides a measure of binaural interaction. Anatomically, it assesses integrity at the level of the brain stem. It has been proposed that the MLD    Comparison between the studied groups according to auditory fusion test revised (AFT-R).

Figure 4
Comparison between the studied groups according to masking level difference (MLD).

Figure 5
Distribution of studied patients according to SSI-3 in the patients group. reliably tests the brain stem integrity and represents the brain stem ability to extract signal from background noise [6]. Th e nonsignifi cant diff erence of the mean performance in both tested conditions of the patients group relative to the control group signifi es a normal brain stem. Literature is confl icting in this matter, where studies have centered on the brain stem as a possible site of central auditory system dysfunction in stutterers. Depressed performance by stuttering patients on diff erent central auditory batteries has been reported by several investigators [9][10][11]. More recent studies show that functional stutterers problem is not within the brain stem as shown by our current study [12].
Th e dichotic digit ability depends on the integrity of the right and left hemispheres as well as on the interhemispheric transfer [13]. Th e patients group poor performance on the DDT individual as well as combined ear scores with no signifi cant diff erence in the right-left ear score assumes that the defi cit is within the left hemisphere.
PPST is a pitch-ordering task that is designed to examine a number of central auditory functions: temporal ordering, pattern recognition, auditory memory, and pitch discrimination [14,15]. Physiologically, temporal ordering task requires contour recognition, which occurs in the right hemisphere then transfer through the corpus callosum. Finally, linguistic labeling occurs in the left hemisphere [8]. Signifi cantly poor performance of the stuttering group relative to the control group, which signifi cantly improved on humming, further signifi es an intact right hemisphere. PPST ability with required verbal response is impaired in stutterers.
AFT-R is a test of temporal integrity at the level of the cortex designed to measure the temporal resolution [8]. Th e patients group performed normally within AFT-R task, which signifi es that a poor performance in temporal processing abilities is met whenever the task is complicated with further prerequisites of attention and auditory memory, as in PPST.
SPIN is a test of auditory closure ability and selective attention. Th e performance scores fi nding points out that the stuttering group may have such impaired abilities relative to normal fl uent individuals.
Th us, from the entire CAP test battery, we are able to conclude that the processing defi cit lies within the left hemisphere and is refl ected on the abilities testing that area. In fl uent speakers, the left language-dominant brain hemisphere is most active during speech and language tasks [16]. An important PET study [17] reported increased activation in the right hemisphere in a language task in developmental stutterers instead. Another PET study [18] confi rmed this result but added an important detail to the previous study where they found that activity in the left hemisphere was more active during the production of stuttered speech, whereas activation of the right hemisphere was more correlated with fl uent speech. Th us, the authors concluded that the primary dysfunction is located in the left hemisphere and that the hyperactivation of the right hemisphere might not be the cause of stuttering but rather a compensatory process [19]. Right hemisphere hyperactivation during fl uent speech has been more recently confi rmed with functional MRI [20].

Conclusion
In this study, we can conclude that the stuttering children have intact brain stem integrity and an intact right hemisphere as signifi ed by the normal right and left ear diff erence in the DDT and by improvement in the PPST on humming. Left hemisphere defi cit appears in more complicated tasks such as PPST, DDT, and SPIN but not in simple tasks such as AFT-R.

Recommendations
It is recommended from the previous fi ndings that CAP profi le of stuttering children should be assessed and the eff ect of CAP disorder rehabilitation program should be further studied.