Neuroscience 6e Web Topic 17.1 - Sensory Motor Talents and Cortical Space

Are special sensory motor talents, such as the exceptional speed and coordination displayed by talented athletes, ballet dancers, or concert musicians, visible in the structure of the nervous system? The widespread use of noninvasive brain imaging techniques (see Chapter 1 of the textbook) has generated a spate of studies that have tried to answer this and related questions. Most of these studies have sought to link particular sensory-motor skills to the amount of brain space devoted to such talents. For example, a study of professional violinists, cellists, and classical guitarists purported to show that representations of the “fingering” digits of the left hand in the right primary somatic sensory cortex are larger than the corresponding representations in the opposite hemisphere.

Although such studies in humans remain controversial (the techniques are only semi-quantitative), the idea that greater motor talents (or any other ability) will be reflected in a greater amount of brain space devoted to that task makes good sense. In particular, comparisons across species show that special talents are invariably based on commensurately sophisticated brain circuitry, which means more neurons, more synaptic contacts between neurons, and more supporting glial cells—all of which occupy more space within the brain. The size and proportion of bodily representations in the primary somatic sensory and motor cortices of various animals reflects species-specific nuances of mechanosensory discrimination and motor control. Thus, the representations of the paws are disproportionately large in the sensorimotor cortex of raccoons; rats and mice devote a great deal of cortical space to representations of their prominent facial whiskers; and a large fraction of the sensorimotor cortex of the star-nosed mole is given over to representing the elaborate nasal appendages that provide critical mechanosensory information for this burrowing species. The link between behavioral competence and the allocation of space is equally apparent in animals in which a particular ability has diminished, or has never developed fully, during the course of evolution. Nevertheless, it remains uncertain how—or if—this principle applies to variations in behavior among members of the same species, including humans.

One promising domain of human behavior that has been a fruitful paradigm for exploring the relationship between skill and brain structure is handedness. Investigators have reasoned that size asymmetry might exist in the somatic sensory and motor systems simply because 90% of humans prefer to use the right hand when they perform challenging manual tasks. Several in vivo and post mortem studies have examined the morphometry of the central sulcus in the two hemispheres and, despite some conflicting results, there is a degree of consensus favoring a deeper central sulcus on the left side of right-handers. This asymmetry in gyral size could reflect an underlying difference in the extent of the relevant Brodmann’s areas in the two hemispheres (areas 3, 1, and 2 in the postcentral gyrus; areas 6 and 4 in the precentral gyrus), but initial cytoarchitectonic studies have not shown a corresponding asymmetry in the overall size of areas 3 and 4. However, microanatomical studies have shown that more tissue volume is occupied by cellular processes and synapses in area 4 of the left hemisphere in comparison with the right, and that the precentral portion of the corticospinal/corticobulbar tract is larger on the left side than on the right. Although the neurobiological basis of human handedness remains unresolved, such studies suggest it may be possible to understand the relation between skilled behavior and cortical space if the relevant brain structures are identified and their functional connections assessed histologically.

It seems likely that individual sensory motor talents among humans will be reflected in the allocation of an appreciably different amount of space to those behaviors, but this issue is just beginning to be explored with quantitative methods.

References

Amunts, K., G. Schlaug, A. Schleicher, H. Steinmetz, A. Dabringhaus, P. E. Roland and K. Zilles (1996) Asymmetry in the human motor cortex and handedness. NeuroImage 4: 216–222.

Amunts, K., L. Jäncke, H. Mohlberg, H. Steinmetz and K. Zilles (2000) Interhemispheric asymmetry of the human motor cortex related to handedness and gender. Neuropsychologia 38: 304–312.

Catania, K. C. and J. H. Kaas (1995) Organization of the somatosensory cortex of the star-nosed mole. J. Comp. Neurol. 351: 549–567.

Elbert, T., C. Pantev, C. Wienbruch, B. Rockstroh and E. Taub (1995) Increased cortical representation of the fingers of the left hand in string players. Science 270: 305–307.

Hopkins, W. D., J. P. Taglialatela, J. L. Russell, T. M. Nir and J. Schaeffer (2010) Cortical representation of lateralized grasping in chimpanzees (Pan troglodytes): A combined MRI and PET study. PLoS ONE 5: e13383.

Rademacher, J., U. Bürgel, S. Geyer, T. Schormann, A. Schleicher, H.-J. Freund and K. Zilles (2001) Variability and asymmetry in the human precentral motor system. Brain 124: 2232–2258.

Welker, W. I. and S. Seidenstein (1959) Somatic sensory representation in the cerebral cortex of the raccoon (Procyon lotos). J. Comp. Neurol. 111: 469–501.

White, L. E., T. J. Andrews, C. Hulette, A. Richards, M. Groelle, J. Paydarfar and D. Purves (1997) Structure of the human sensorimotor system. II. Lateral symmetry. Cereb. Cortex 7: 31–47.

Woolsey, T. A. and H. Van der Loos (1970) The structural organization of layer IV in the somatosensory region (SI) of mouse cerebral cortex. The description of a cortical field composed of discrete cytoarchitectonic units. Brain Res. 17: 205–242.