Voxel-based morphometry (VBM) enables semi-quantitative, unbiased analysis of anatomical data from magnetic resonance imaging. Automated segmentation partitions structural features from MRI into tissue categories of gray matter, white matter, skull, and ventricles. Normalization of the MRI data to neuroanatomical features allows pooling data from different individuals, and statistical analysis is then performed on a voxel-by-voxel manner to identify regional differences that vary significantly.
Significant differences in brain structure detected by MRI in individuals with specific highly developed skills provide evidence suggestive of use-dependent increases in gray matter volume in cortical regions known to be important in performing the task. One of the first studies to do so concerned professional taxi drivers in London, who were found to have significantly larger posterior hippocampal volume compared with control subjects. The anterior hippocampal gray matter volume in taxi drivers was significantly smaller than controls (Maguire and others 2000
). The posterior hippocampus in humans is preferentially involved with previously learned spatial information, whereas the anterior hippocampal region is more involved in encoding new environmental layouts. The left platum temporale in musicians with perfect pitch is larger than in nonmusicians (Schlaug and others 1995
), and professional keyboard musicians have larger gray matter volume in motor, auditory, and visual-spatial brain regions compared with age-matched nonmusicians (Gaser and Schlaug 2003
). A recent study reports that professional golfers have enlarged premotor and pariteal cortex, areas involved in sensory-motor control and cognitive processes (Jäncke and others 2009
Such differences in elite performers could represent use-dependent changes in the brain during the extensive practice required to achieve superiority in these skills, or they could reflect intrinsic differences in the brain that predisposed the individuals to excel as taxi drivers, musicians, or golfers. Retrospective analysis shows that the increase in posterior hippocampal volume in London taxi drivers (and the decrease in anterior hippocampal volume) correlates linearly with the amount of professional driving experience of individual drivers, suggesting that anatomical remodeling of the hippocampus in taxi drivers was generated by their learning experience (Maguire and others 2000
). Similar correlations between the hours practiced and the magnitude of changes in gray matter volume are also documented in golfers (Jäncke and others 2009
A definitive test of this interpretation requires longitudinal studies performed on the same individual during the process of learning. A comparison of subjects before and after learning to juggle found structural changes in gray matter (increased volume) in the left posterior intraparietal sulcus, an area involved in spatial analysis of moving objects (Draganski and colleagues 2004
). Three months later, the increase was still evident but was somewhat reduced from that seen soon after learning the skill. In another learned skill, increased gray matter density in the left occipitotemporal region was found in a study of 16 adult subjects examined by MRI before and after learning Morse code (Schmidt-Wilcke and others 2010
). A longitudinal study on adolescent girls found that the cerebral cortex had thickened in two areas after three months of practice on a visual-spatial problem-solving computer game called Tetris (Haier and colleagues 2009
An increase in gray matter volume after perfecting a new skill can occur much more rapidly than the several-month comparisons most experimental designs would suggest. In a longitudinal study of practice in mirror image reading, an increase in gray matter in the right dorsolateral occipital cortex was seen after only 15 minutes of practice/day for two weeks (Ilg and others 2008
). Repetitive transcranial magnetic stimulation can induce structural changes in gray matter of the auditory cortex within only 5 days (May and others 2007
Skill learning to perfect a specific sensory and motor function, such as that associated with elite performance in sports, musicianship, navigation, or computer games, might involve structural changes in the brain as the new process becomes increasingly automated, but whether more abstract learning would be associated with structural remodeling of gray matter is an important question.
This possibility was investigated in medical students studying for a major exam in Germany (Draganski and others 2006
). The students’ brains were imaged three months before the examination and on the day of the exam. The results were compared with an age-matched control group that had no exams in the last six months and who were not studying for an exam. Gray matter volume increased significantly over the three-month study period in the posterior and lateral cortex bilaterally, whereas there were no changes seen in students who had not studied for the exam. A third scan performed on 23 of the 38 students three months later revealed that these changes persisted. Interestingly, the posterior hippocampus showed a different pattern over time. Posterior hippocampal gray matter increase during the learning period and the increase became more pronounced at the third time point. The results suggest that the acquisition of highly abstract information is accompanied by structural changes in gray matter in specific brain regions involved in declarative memory and the analysis of visual information.
A more recent study performed by a different group of investigators has attempted to replicate these findings (Ceccarelli and others 2009
). In this study, Italian medical students were imaged only two weeks before classes involving anatomy, biology, and physiology and compared with a control group of students who were on vacation from school. No differences in gray matter were found between the experimental and control group on the initial scan, but after two weeks of study for classroom exams, there was a significant increase in gray matter volume in the left posterior medial frontal cortex and left precuneus, as well as in the right orbitofrontal cortex. These are cortical regions associated with reasoning and visualization, but they are not the same cortical regions that were found to increase in volume in the first study. The authors speculate that the differences in time points of analysis (15 days vs. 3 months) account for the different gray matter regions that changed in the two studies. (This concept will be explored later in this review.)
To summarize, the results of MRI studies show that learning a wide range of skills is accompanied by an increase in gray matter volume in appropriate areas of the brain (see also ). Structural changes in the cerebral cortex during learning are not limited to skills of increased sensory-motor coordination, but rather they can extend to abstract learning as in intellectual university study. The structural changes can be seen within two weeks of learning, and these changes can persist for weeks or months.
Human Brain Imaging Studies on Effects of Learning on Structure of Gray Matter