EEG power and coherence in autistic spectrum disorder
Introduction
Autistic spectrum disorder (ASD) has been defined as a neurodevelopmental disorder with associated deficits in executive function, language, emotional, and social function (Rapin and Dunn, 2003; Belmonte et al., 2004; Hill, 2004; McAlonan et al., 2005). Increasing rates of prevalence have been reported for ASD. According to Blaxill (2004), the rates of ASD were reported to be <3 per 10,000 children in the 1970s and rose to >30 per 10,000 in the 1990s. The Centers for Disease Control and Prevention (CDC, 2006) summarized data from several studies on the prevalence rates for ASD ranging from 1 in 500 to 1 in 166, making it the sixth most common disability classification in the United States. In fact, their most recent report (CDC, 2007) suggests a prevalence of 1 in 150. The dramatic rise in the numbers of children classified as ASD highlights the need for further research to be conducted into this population.
A review of research on EEG screening for ASD found that seizures were common among 20–30% of individuals with autism. Epileptiform abnormalities were found in 10.3–72.4% of patients and subclinical anomalies in 6.1–31% (Kagan-Kushnir et al., 2005). While such problems are important in the diagnostic and screening process, Deonna and Roulet (2006) concluded that there is no evidence that autism can be attributed to an epileptic disorder.
Only a few studies have investigated the EEG of children with ASD, and each of these has used different paradigms. Ogawa et al. (1982) examined sleep EEGs of 28 normal children and 21 children with autism. Findings indicated higher levels of alpha bilaterally and frontally, with no hemispheric lateralization, in children with autism. Ogawa et al. (1982) concluded that children with autism were less responsive to external stimuli and that they had fewer active internal regulatory mechanisms than the control group.
Dawson et al. (1982) investigated EEG measures of hemispheric activation during four cognitive tasks (rote verbal memory task, verbal categories, block design, and copying designs) in 10 males with autism (ranging in age from 9.1 to 34.0 years), and 10 normal individuals matched for gender, age, handedness and family patterns of handedness. Seven of the individuals with autism had atypical patterns of cerebral lateralization, involving right-hemisphere dominance for both verbal and spatial functions. The reversal in lateralization indicated a lack of left-hemisphere specialization for linguistic functions. These findings suggested selective impairment of the left cerebral hemisphere. The group with autism showed stronger right-hemisphere dominance during verbal tasks than during spatial tasks. Stroganova et al., 2007a, Stroganova et al., 2007b have also recently shown a pattern of abnormal lateralization in autistics. Specifically, they hypothesized a diminished capacity of the right temporal cortex in the generation of EEG rhythms.
Cantor et al. (1986) conducted computerized EEG analyses of 11 children with autism between the ages of 4 and 12 years, in contrast to three groups of children: (1) 88 normal children, (2) a matched group of 18 mentally handicapped children, and (3) a group of 13 mental-age-matched normal toddlers. The findings indicated that children with autism had significantly greater coherence between hemispheres in the beta band than mentally handicapped, normal children, or toddlers. Children in the Autistic group had higher coherence in the alpha band than those in the normal group, and less inter- and intrahemispheric asymmetry than participants in the normal or mentally handicapped group. Amplitude asymmetries were noted for autistic children in the posterior-temporal, central, and occipital regions, with greater amplitude in the left than right hemisphere. Based on these findings, the researchers concluded that autism may be characterized by a maturational lag in cerebral functioning and a lack of cerebral differentiation (Cantor et al., 1986). Murias et al. (2006) conducted analyses of high-density EEG recordings in an eyes-closed resting state in 18 adult autistics compared to normal controls. The findings included both excessive and reduced coherence in the Autistic group. The ASD group also exhibited higher theta and beta 1 power than the controls. However, this was a study of adults, not children, so the comparability of results to a pediatric population are limited.
These studies suggest that atypical structural and functional neurobiological patterns are associated with the multiple symptoms present in this disorder. However, none of these studies has used an eyes-closed resting condition, the most common paradigm used in the investigation of EEG abnormalities among children with behavioral disorders. Further research is needed to characterize these neurophysiological profiles. The current study was designed to extend previous research, by investigating power and coherence differences between ASD and control subjects during an eyes-closed resting condition.
Section snippets
Subjects
Two groups of 20 subjects between the ages of 6 and 11 years old, with 14 boys and 6 girls in each group, participated in this study. Twenty patients were consecutively seen in clinical practice with a diagnosis of autistic spectrum disorder or autism based on DSM-IV criteria (APA, 1994). The Control group was drawn from previously published normative data (Clarke et al., 2001a). Subjects in each group were individually matched on age, using 1-year age bands to control for maturational changes
Results
There were no significant differences between the groups on age, gender, or IQ (Table 1).
Discussion
Previous research has found EEG power anomalies in autistics compared to controls (Cantor et al., 1986; Ogawa et al., 1982). While Ogawa et al. (1982) found elevated frontal alpha in autistics, Cantor et al. (1986) showed that autistic children had elevated power in frontotemporal regions, especially in the delta band. Murias et al. (2006) found autistic adults had greater relative theta (3–6 Hz) and beta1 (13–17 Hz) than controls. Only Cantor et al. (1986) investigated resting state power during
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