Trends in Cognitive Sciences
OpinionExpansion and Renormalization of Human Brain Structure During Skill Acquisition
Section snippets
Human Brain Plasticity: Expansion and Renormalization?
In 1894, Nobel Prize winner Santiago Ramón y Cajal, considered by many to be the father of modern neuroscience, proposed that mental activity might induce morphological changes in brain structure. Nearly 100 years later, studies using magnetic resonance imaging (MRI, see Glossary and Box 1) have reported experience-dependent increases in regional estimates of human brain volume and cortical thickness in adulthood. For example, it has been found that London taxi drivers’ gray matter in the
Expansion and Renormalization of Human Gray Matter Structure
Research has documented changes in human gray matter structure after a few months of training (e.g., 2, 5 as noted above) and has also indicated that such changes can emerge early during the learning process 7, 22, 23, 24. However, the fate and durability of these changes has not been tracked in the course of continuous learning in humans. Recently, we acquired up to 18 T1-weighted structural MR images over a 7-week period during which 15 right-handed adult participants practiced left-hand
Cellular Processes Potentially Underlying Gray Matter Changes
Increases and decreases of estimates of localized volume during learning as observed with MRI are most likely the result of a conglomerate of different cellular processes 14, 28. Figure 1 schematically illustrates these cellular processes. Several candidate mechanisms on the cellular and molecular level have been proposed that could account for MRI observations [14], including neurogenesis, synaptic changes, dendritic branching, and axon sprouting as well as changes in glial number and
Cortical Map Dynamics and Functional Changes
Similar to macroscopic changes in estimates of regional gray matter volume, cortical map plasticity follows a comparable pattern of expansion followed by renormalization during learning 16, 17, 18, 44, 45, 46, 47, 48, 49, 50. For example, it has been found that rats training to perform a skilled reaching task exhibit expanded cortical maps after 3 days of training [16]. After 8 days of training, however, these expansions had subsided, while behavioral performance remained stable. A similar
Formation and Elimination of Synapses
Research regarding learning-related changes in dendritic spines is consistent with the hypothesis that the memory trace serving skilled performance is often localized in rewired specific circuitry rather than in any large-scale expansion of tissue in the whole region 11, 36, 53. During motor skill acquisition or new sensory experiences, novel dendritic spines rapidly grow to form synapses in the sensory and motor cortices of rodents 19, 20, 21. In this process the dendrites are not merely
Plastic Changes: A Darwinian Learning Process
The pruning model of early development 56, 57, 58 posits the same general pattern of increase followed by decrease as described above, only on an ontogenetic timescale. The rapid increase of synapses after birth is followed by experience-dependent selective stabilization of behaviorally relevant connections and the elimination of those connections that prove to be non-functional [59], resulting in an overall trajectory of decrease from childhood to adolescence 60, 61 (but see also [62] for
Concluding Remarks and Future Perspectives
We have argued that the concepts of expansion, selection, and renormalization are consistent with animal models and theoretical accounts of skill acquisition and development and together contribute to a mechanistic understanding of human plasticity. Importantly, the expansion–renormalization model opens up several new research directions, informs predictions for work on experience-dependent and developmental changes in human brain structure (see Outstanding Questions), and calls for a critical
Acknowledgments
The authors thank Simone Kühn and Benjamín Garzón for valuable discussions, Ludmila Müller for rendering the figure, and Julia Delius und Lana Riccius for comments on the text. The work by M.L. on the manuscript was done in the context of research supported by the European Research Council under the EU’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement N° 617280–REBOOT and the Swedish Research Council (446-2013-7189). Work by U.L. profited from a research stay as a Fernand
Glossary
- Cortical maps
- the cortical organization of sensory and motor systems is often described in terms of maps. Cortical maps are collections (areas) of minicolumns in the cortex whose functional topography corresponds to graded variations on an underlying sensory or motor dimension. In the somatotopic map of the motor cortex, for example, stimulation of different areas evokes movement of distinct parts of the body. These cortical representations are not static. Cortical map expansion refers to the
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