Cortical Thickness and Verbal Fluency
Recent studies have shown that the rate of increase in raw vocabulary fluency was positively correlated with the rate of cortical thinning. In other words, greater performance improvements were associated with greater thinning. This is more evident in left hemisphere regions, including the left lateral dorsal frontal and left lateral parietal regions: the usual locations of Broca's area and Wernicke's area, respectively.
After Sowell's studies, it was hypothesized that increased performance on the verbal fluency test would correlate with decreased cortical thickness in regions that have been associated with language: the middle and superior temporal cortex, the temporal–parietal junction, and inferior and middle frontal cortex. Additionally, other areas related to sustained attention for executive tasks were also expected to be affected by cortical thinning.
One theory for the relation between cortical thinning and improved language fluency is the effect that synaptic pruning has in signaling between neurons. If cortical thinning reflects synaptic pruning, then pruning may occur relatively early for language-based abilities. The functional benefit would be a tightly honed neural system that is impervious to "neural interference", avoiding undesired signals running through the neurons which could possibly worsen verbal fluency.
The strongest correlations between language fluency and cortical thicknesses were found in the temporal lobe and temporal–parietal junction. Significant correlations were also found in the auditory cortex, the somatosensory cortex related to the organs responsible for speech (lips, tongue and mouth), and frontal and parietal regions related to attention and performance monitoring. The frontal and parietal regions are also evident in the right hemisphere.
Read more about this topic: Language Processing
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