According to the criteria of the DSM-IV (American Psychiatric Association, 1994) and ICD-10 (World Health Organization, 1993), Asperger's disorder (AS) is a subtype of pervasive developmental disorders (PDDs). This disorder is defined as a syndrome in which the individuals meet the criteria for autism but have no history of delay in cognitive or language development. Recent studies indicated that the prevalence of PDD is higher than previously estimated. Fombonne (2003) estimated that the prevalence of PDD ranges from 30 to 60/10,000, while Kogan et al. (2009) indicated that the prevalence is as high as 110/10,000.
Persons with Asperger's disorder sometimes show repetitive maladaptive behaviors, stereotyped motor mannerisms, and rigid adherence to routines (Gaus, 2007). Clinical traits reported in studies of individuals with Asperger's disorder include superior systemizing (Wakabayashi et al., 2007), as well as problems organizing themselves in their environment (Happé, Booth, Charlton, & Hughes, 2006), understanding other people's feelings (Kaland, Smith, & Mortensen, 2007), and sympathizing with others (Baron-Cohen & Wheelwright, 2004). Kanai et al. (in press) reported high scores on the Schizotypal Personality Questionnaire (SPQ) and on the ‘Neuroticism’ and ‘Psychoticism’ scores of the Eysenck Personality Questionnaire (EPQ) in persons with Asperger's disorder. These clinical characteristics suggest the existence of neuropsychological dysfunction in persons with Asperger's disorder.
Some neuropsychological studies have shown impairment of executive function, including planning, flexibility and working memory in persons with PDD including Asperger's disorder (Corbett et al., 2009, Happé et al., 2006 and Ozonoff et al., 1991). In functional magnetic resonance imaging (fMRI) studies, individuals with PDD including Asperger's disorder have abnormalities of hemodynamic responses in the prefrontal cortex associated with spatial working memory (Luna et al., 2002 and Ring et al., 1999), motor inhibition (Schmitz, Rubia, Daly, Smith, & Williams, 2006), and visuomotor control (Müller, Kleinhans, Kemmotsu, Pierce, & Courchesne, 2003). In addition, PET study showed reduced dorsomedial prefrontal activation in adults with PDD (Happé, Ehlers, Fletcher, Frith, & Johansson, 1996). However, other reports have found no prefrontal impairment in persons with Asperger's disorder. In a study by Silk, Rinehart, Bradshaw, Tonge, and Egan (2006) in which a visual-spatial task similar to a Stroop-type task was employed, persons with Asperger's disorder performed similarly to age- and IQ-matched control subjects, even at higher levels of cognitive complexity.
Multi-channel near-infrared spectroscopy (NIRS), a recently developed functional neuroimaging technology, enables the non-invasive detection of spatiotemporal characteristics of brain function near the brain surface (Boas et al., 2004 and Strangman et al., 2002a). NIRS has enabled bedside measurement of the concentrations of oxygenated ([oxy-Hb]) and deoxygenated hemoglobin ([deoxy-Hb]) in capillary blood vessels. In contrast to other neuroimaging methodologies such as fMRI, PET, electroencephalography (EEG), and magnetoencephalography, NIRS can be performed in a restraint-free environment that is especially suitable for psychiatric patients. Indeed, NIRS has been used to assess brain function in several psychiatric disorders (Kameyama et al., 2006, Matsuo et al., 2003 and Suto et al., 2004).
Kuwabara, Kasai, Takizawa, Kawakubo, and Yamasue (2006) reported that prefrontal activation, assessed by 24-channel NIRS, during the letter fluency task was reduced in adults with PDD as compared with healthy adults. Using the same task, Kawakubo et al. (2009) reported similar results, although no significant difference was found between children with PDD and healthy children. However, these reports used only one activation task. In the present study, we used two types of verbal fluency tasks to investigate [oxy-Hb] changes in persons with Asperger's disorder.
