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Hemodynamic brain imaging and lesion studies have suggested differential involvement of expressive language-related cortical regions based on the phonemic versus semantic characteristics of verbal cues. The aims of this study were: 1) to elucidate the relative timing of the activity of inferior frontal and anterior insular versus motor and supplementary motor cortex during a fluency task and 2) to assess potential differences in the location or timing of activity in anterior and posterior language areas based on letter versus category cues. Using magnetic source imaging (MSI), we found significantly earlier onset latencies and a greater number of activity sources in motor and supplementary motor compared with inferior frontal and anterior insular regions. We also observed greater left versus right hemispheric asymmetry of activation for letter compared with category cues. This study provides new insights into cortico-cortical interactions during expressive language tasks.

We used magnetic source imaging (MSI) to investigate the spatio-temporal patterns of brain activity associated with line bisection judgments and double simultaneous visual stimulation in 14 healthy adults. Consistent with lesion and hemodynamic neuroimaging studies, we found the greatest number of activity sources in right inferior parietal cortex. These sources were most prominent, on average, between 200 and 300 ms after the onset of single (left, right, or center) target stimuli. A greater number of significant activity sources were found in right inferior parietal, occipital, and prefrontal cortices during bilateral compared with unilateral stimulus presentation. Based on these observations, we suggest that a more parsimonious physiological explanation of visual extinction than the hemispheric rivalry account may be the additional neuronal excitation required in right occipital and parietal cortices for accurate bilateral visual perception.

Attentional control in children and adolescents with unilateral temporal-lobe excisions was examined using two experimental tasks, a lexical-decision and a spatial-cue task, and a standardized vigilance task. Participants with left temporal excisions took longer than controls to reorient their attention after invalid cues compared to neutral cues in the lexical task and they made more errors on all three of the tasks. Participants with right temporal excisions differed from controls in the number of errors made on the spatial task. No differences were found between the lesion and control groups on reaction-time measures of the spatial task. The results suggest that a material-specific inhibition impairment, as well as a more general sustained attention deficit, may result after a left temporal excision in childhood or adolescence. These deficits are considered in the context of neuroanatomical models of attentional control.