contribution may lead to erroneous conclusions. These issues are dis- cussed in a comprehensive review of brain imaging studies of reading- related processes (Price, 1997). Similar difficulties are encountered in studies using complex language tasks. In the case of reading, for example, the images often show not less, but more activation in the brains of partici- pants with reading problems as compared with controls. Overactivation may be associated with the necessity of using more processing capacity for a task whose execution is less automatic. As with PET, so far there are only few fMRI studies with children.
Brain Activity in Children with SLI
The functional imaging techniques have been widely used in language studies in adults and have confirmed, overall, what was already known of left-hemispheric language dominance from neuropsychological and le- sion studies (Carr & Posner, 1995; Lukatela, Carello, Savic, & Turvey, 1986; Neville, Kutas, & Schmidt, 1982a, 1982b; Rugg, Kok, Barret, & Fischler, 1986; Zatorre et al., 1996). However, despite these data and their appeal for excellent localization of neural activation, the number of func- tional brain imaging studies with children with SLI is small. The studies reviewed in this chapter have mostly applied the rCBF techniques with xenon 133 inhalation and have assessed activity both during a rest-state and during language-related cognitive tasks. The blood flow differences between children with SLI and controls in rest-state measurements would reflect differences in baseline brain activity or nontask-related mental op- erations in brain regions of interest, in contrast to differences in cognitive processes measured during active tasks. The results from the few existing studies show some interesting parallels as well as some inconsistencies, which may be partly attributed to the heterogeneity in study populations.
In one of the first studies, Lou and colleagues (Lou, Henriksen, & Bruhn, 1984), using rCBF during rest-state, found brain regions, most of- ten symmetrically, with focal cerebral hypoperfusion indicating low met- abolic activity in all thirteen 6- to 15-year-old dysphasic children studied (11 of whom also had attention deficit disorder [ADD]). Only one dys- phasic child showed a slight structural abnormality. Participants with ver- bal dyspraxia had lower activity in the anterior perisylvian regions (Broca’s area), and one showed lower activity in posterior perisylvian ar- eas. Children with phonologic-syntactic dysphasia showed hypoperfu- sion in both anterior and posterior perisylvian regions and an individual with verbal auditory agnosia (word-deafness) only posteriorly. Denays and colleagues (1989) also found hypoperfusion during rest-state in a SPECT study of 5- to 16-year-old dysphasic children. In two children with verbal dyspraxia, this occurred in the inferior frontal gyrus of the left
hemisphere involving Broca’s area. The left temporo-parietal region was implicated in nine participants with both comprehension and expression- phonologic syntactic disorder. In addition, lower blood flow in the middle and superior regions of the right frontal lobe was also reported.
In a later study by Lou, Henriksen, and Bruhn (1990), also using rest- state SPECT, the results of their previous study were only partially repli- cated most probably because the diagnostic groups were somewhat dif- ferent. Asymmetric hypoperfusion (R > L) was found in the central perisylvian region in a group of four participants with decoding or lexical- semantic dysphasia and in the prefrontal area in three children with phonologic-syntactic dysphasia (but without any decoding or semantic difficulties). Recently, a reversed asymmetry (R> L), as compared with normal controls, was also found in eight 8- to 12-year-old boys with ex- pressive dysphasia in sensorimotor and auditory brain areas by Chiron and colleagues (1999) during rest-state with the same technique. In this study, boys with Duchenne muscular dystrophy (with reading disorders, but normal speech) also showed a similar asymmetry in the same areas and in Broca’s area. However, these findings seemed to be a result of higher levels of activation on the right, rather than from less activation in the left hemisphere as compared with controls.
A number of studies have not found such differences in children with SLI using rest-state SPECT. For example, Tzourio, Heim, Zilbovicius, Gerard, and Mazoyer (1994) did not find any hypoperfused areas in 8- to 10-year-old children with SLI (7 with expressive disorder, 7 with expres- sive-receptive dysphasia, and 6 with attention-deficit hyperactivity disor- der [ADHD]). However, when comparing rCBF values in relation to the whole cortex CBF during an active language related task, decreased blood flow levels were found. The dysphasic children (using pooled groups as no significant differences emerged) showed decreased activation in the left inferior parietal region during a phonemic discrimination task as com- pared with ADHD children. However, no comparable differences were found during a simple nonverbal auditory listening task. Moreover, ex- pressive and expressive-receptive groups had different left-to-right blood flow ratios for the given tasks: In the expressive group, left-hemispheric activation failed to increase during the nonverbal auditory task, whereas in the mixed group, this failure occurred during the phonemic task. Previ- ously, Lou and colleagues (1984) found that object naming failed to pro- duce increased blood flow in relevant cortical regions in dysphasic chil- dren during an active task.
Overall, the findings reviewed here seem to fit the general neuro- psychological view of language functions. Posterior perisylvian and tem- poro-parietal abnormalities could explain comprehension problems in children with receptive SLI or mixed language impairment. The fact that