Is performance better when brain functions are typically lateralized?

Lateralization refers to the dominant involvement of one homologous region of the brain over the other in functional task performance. Direction and strength of lateralization depend on the functional task. It is well known that language is lateralized to the left hemisphere, even in most left-handers, and visuo-spatial capacities to the right hemisphere. Over the last few years several studies addressed the question of the prevalence of different patterns of brain lateralization in the population and in specific groups, such as left-and right handers. Most studies used fTCD, some fMRI, to determine the functional lateralization of the individual, usually for tasks that are known to be lateralized, such as word generation (WG) or visuo-spatial orientation. These studies show that apart from the typical pattern of lateralization , also ipsilateral dominance of these two functions and even mirrored dominance patterns exist. An interesting question is if the typical pattern of lateralization is associated with better performance. Lateralization measures are typically based on (indirect) measures of differences in brain activity between left and right functional areas, or in functional performance (dichotic listening, visual field stimulation, hand performance differences or hand preferences measures). The most simple measure than is R-L, with R and L representing respectively right or left brain activation or functional performance. More commonly, however, (R-L)/(R+L) is used to correct for individual differences in mean level of activation or performance. Another approach may be to use the mean activation or performance as a covariate in the analyses. For fTCD the standard AVERAGE software first subtracts a baseline level of bloodflow for each probe and then quantifies the maximal difference between left and right hemisphere during the task, basically R-L. A subject is assumed to be lateralized if the fTCD R-L difference is significant over repeated trials. I intend to discuss the advantages and disadvantages of these measures used to analyze direction and strength of lateralization