However, the specific ease with which particular participants or groups completed the task during scanning is unknown and may be variable. Variations in task difficulty can affect physiological responses, linearly increasing
neuronal firing with increasing difficulty (Chen et al., 2008) and increasing amplitude of electrical activity (Mulert et al., selleck kinase inhibitor 2007). However, using functional transcranial Doppler ultrasound, we have shown that difficulty in both an auditory naming and a word generation task does not affect lateralisation or the intensity of activation (Badcock, Nye, & Bishop, in press). There are a number of limitations of this research that relate to the small sample size and differences between the groups in terms of age ranges and distribution of handedness and sex. Although the group sizes are small, they are comparable with group sizes from other studies of brain structure and function in language-impaired populations
(e.g., Watkins et al., 2002b). To minimise the effects of differences on brain structure relating to factors such E7080 as age, sex and handedness, we implemented the use of a nonlinear registration of the functional images to standard space, which removes gross differences in size and shape among the brains. We also included an image of grey matter volume for each individual subject as a voxel-dependent covariate in the functional analysis; only functional differences over and above structural differences would remain, therefore. Finally, although our groups check details were small, we used a mixed-effects analysis to compare groups rather than a fixed-effects analysis, which is typically used in
small samples of special populations. By using a mixed-effects analysis, which combines between-subject and within-subject variance at the group level, our data are less likely to be influenced by outliers, such as the left-handed SLI subject whose LI is reliably right-lateralised. This approach allows us to generalise our results to the wider population rather than limit their inference to the study-population as with a fixed-effects analysis. In our experience, brain structure is minimally affected by handedness and sex (see Watkins et al., 2001), so the age differences among our participants is likely to be the main confound. It is well described that although white matter continues to increase linearly across the life span, grey matter increases to a peak during childhood or adolescence and then decreases during later years (Giedd et al., 1999 and Gogtay et al., 2004). A longitudinal analysis of grey matter volume collected on the same scanner with the same protocol as used here and analysed with the same tools, revealed reductions in grey matter from in a cohort aged 13 to 19 year olds over a 2–3 year period in mainly right hemisphere regions (Giorgio et al.