Autism spectrum disorder (ASD), a term including classic autism, pervasive developmental disorder-not otherwise specified (PDD-NOS), and Asperger syndrome, is a neurodevelopmental disorder characterized by impairments in language and communication, social deficits, and repetitive behaviors or intense interests (American Psychiatric Association, 1994
). Although deficits in language and communication are one of the core features of ASD, the specific nature of these impairments remains unclear. Complicating this, the entire range of language abilities occurs in ASD, including individuals who never develop language to those who perform normal or above on standardized language tests (for review, Tager-Flusberg et al., 2005
). Certain aspects of language, such as pragmatics are impaired in all individuals with ASD, regardless of functioning level (see Tager-Flusberg et al., 2001
). Other aspects of language, however, such as phonology, syntax, and semantics may not show impairment or may be impaired only in a subgroup (see Tager-Flusberg et al., 2001
Semantic processing in ASD has been studied in older adolescents (15 years or older) and adults using several behavioral paradigms. Long-term memory studies have suggested semantic processing differences. Toichi and Kamio (2002)
found semantic, compared to phonological or perceptual, processing of verbal items facilitated long-term memory in controls. In ASD, semantic facilitation did not occur, suggesting decreased use of semantic information. Mottron et al. (2001)
demonstrated that when oriented to the semantic features, both groups had better recall than when oriented to phonological features or no orientation. However, when controls were given semantic cues at retrieval, recall was better than with phonological cues, whereas in ASD, semantic and phonological cues had the same effect. This further supports decreased use of semantic information in ASD. Similarly, another study found that controls had better long-term memory and increased verbal associations to concrete than abstract nouns and a correlation between recall and number of associations, which was not found in ASD (Toichi & Kamio, 2003
) and implies different semantic processing in the ASD group.
Semantic priming has also been used to examine semantic functioning in ASD. In a study of ASD individuals without early language delay, Kamio et al. (2007)
showed semantic priming effects in controls, but not in ASD. Another priming study found no group differences, but found that when pictures, instead of words, were used as primes there was an increased priming effect in ASD, but not in controls (Kamio & Toichi, 2000
). Thus, in ASD, pictures may have an advantage over words in accessing meaning. Toichi and Kamio (2001)
also found no differences in semantic priming in ASD compared to controls. A correlation was found between priming performance with performance IQ and Raven's Progressive Matrices in ASD, suggesting the contribution of non-verbal factors and the possibility of different strategies being used.
Functional neuroimaging has been used to investigate the neurobiological substrates of language in ASD. The first studies were PET and fMRI studies that used passive paradigms to explore auditory language processing. These studies found decreased temporal activation and decreased left lateralization of activation in autism compared to controls (Boddaert et al., 2003
; Gervais et al., 2004
; Müller et al., 1998
Several studies have examined semantics or syntax using sentences in ASD adults. Müller and colleagues studied sentence production compared to sentence repetition. In one study, both groups had activation in the left inferior/middle frontal gyrus with left lateralization in the perisylvian region; controls also had activation in the left inferior temporal region (Müller et al., 1999
). In another study, there was decreased left BA 46 and left thalamus activation in the autistic group relative to controls (Müller et al., 1998
). Another group, focusing on syntactic processing during a visually presented sentence task, found increased posterior activation (left superior temporal gyrus) and reduced activation in frontal language areas (left inferior frontal gyrus) in autism compared to controls (Just et al., 2004
). There was also decreased functional connectivity between anterior and posterior language regions. The ASD's task performance was faster and less accurate than controls'. Similar to behavioral findings, Kana et al. (2006)
demonstrated activation in the ASD group in parietal and occipital imagery-related regions in low- and high-imagery sentences, whereas controls had more activation in these regions during high-relative to low-imagery sentences. For their task, participants indicated whether high- and low-imagery sentences were true or false and there were no group differences in performance.
Two fMRI studies focused on semantic processing of words in ASD and suggested atypical semantic processing or organization. Harris et al. (2006)
demonstrated less activation in Broca's area and increased middle temporal gyrus activation in ASD relative to controls. The ASD group also showed similar activation in Broca's to the semantic and perceptual task, whereas controls had activation in this region only during the semantic task. Subjects indicated whether a visually presented word was positive or negative compared with a case decision of the same words, with similar performance in both groups. Gaffrey et al. (2007)
had subjects determine whether visually presented words belonged to a given category and contrasted this with subjects indicating whether a specified letter occurred in letter strings. The control group was more accurate than the ASD group for the category task. The ASD group compared to controls had increased extrastriate visual cortex activation, which corresponded to increased errors on the semantic task. ASD individuals also had smaller activation clusters than controls in left inferior frontal regions, however, direct group comparisons did not reveal this difference.
The purpose of this study was to investigate semantic processing in adolescents with ASD and typically developing adolescents, utilizing fMRI. We used a visually presented response-naming task (Bookheimer et al., 1997
). Almost no previous semantic fMRI tasks in ASD have involved language production and to our knowledge, this is the only study that has used a semantic task involving semantic integration and word generation. We chose a task that each individual could easily and accurately perform so that differences in activation between the groups could not be attributed to differences in performance levels (Bookheimer, 2000
). fMRI semantic studies have been performed mainly in adults. To our knowledge, this is the first study to examine language activation patterns in ASD adolescents as young as 11 years old. Broca's and Wernicke's areas play key roles in semantic functions (for reviews, Bookheimer, 2002
; Foundas, 2001
; Vigneau et al., 2006
). In addition, functional neuroimaging studies in ASD have demonstrated differences in these regions during semantic tasks, with several studies reporting decreased Broca's activation (Gaffrey et al., 2007
; Harris et al., 2006
; Just et al., 2004
; Kana et al., 2006
) and some finding increased Wernicke's area activation (Harris et al., 2006
; Just et al., 2004
). We therefore hypothesized that ASD individuals would have decreased left frontal language activation and increased activation of left temporal language regions. These prior studies, however, did not involve language generation, which relies heavily on Broca's area; so alternatively, we might not expect to find group differences in Broca's activation with this task. Behavioral and imaging studies have suggested different semantic organization or strategies in semantic processing. Based on these findings, we predicted that the ASD group compared to typically developing adolescents would rely on different cortical areas during the semantic task.