Brain Gyrification and its Significance

Comparsion of Human, Macaque, and Mouse Brains and Extent of Gyrification. Credit to J. Horton.

Gyrification in the brain, also known as convolution, is process of cortical folding that leads to the wrinkle like appearance of mammal brains. It is the basis for the presence of gyri and sulci (hills and valleys) in cerebral cortex. The extent of gyrification of brains is highly implicated as being positively related to species intelligence. The basic idea is that gyrification allows for (or is a result of) greater surface area of cortical neurons within the same skull volume. However the exact mechanism by which this occurs, its true significance, and the implications of differences within species is not conclusively known or explored. With this in mind, this wiki seeks to explore the literature on the chemical or physical cause/mechanism of gyrification, differences in anatomy across species and within species, and possible theories of significance that could be derived based on previous findings.

Classification and Medical Terminology

General Terms

There are relativistic terms that describe the extent to which a brain is physically convoluted. These terms often have medical significance. Gyrencephaly refers to the condition in which the brain is highly convoluted. This term is really implies a loss function but rather a characterization of species with high gyrification compared to others. Lissencephaly, meaning smooth brain, can be described as the condition in which a brain is less gyrified than normal and has medical consequences. In most cases it is a result of a failure of neuronal migration in development ^[1] and results in mental retardation and severe developmental delays (^[2]. Causes may be viral or genetic (Ref 3). Its most extreme form is Argyria meaning no gyri. This condition may begin to hint at what the mechanism for the folding is and its role in intelligence.

Lissencephaly vs. Normal Brain. Source: http://www.hxbenefit.com/lissencephaly.html

Left to Right: normal, polymicrogyria, lissencephaly. Credit: Lefèvre and Mangin, 2010l

Relation to Species Intelligence

Brain Size to Body Weight Ratios Across Species

Figure 1: Relationship between brain size and body size in selected mammals. Credit: Roth & Dicke, 2005

Figure 2: EQ Values Across Species. Credit: Macphail

Gyrification Across Species

""Figure 3:"" Gyrification Across Species. Credit to Patricia Anne Kinser (2000). Source: http://serendip.brynmawr.edu/exchange/brains

References

1. Dobyns, W. B., and C. L. Truwit. "Lissencephaly and other malformations of cortical development: 1995 update." Neuropediatrics 26.03 (2007): 132-147.
2. Jones, Kenneth Lyons. Smith's recognizable patterns of human malformation. Philadelphia: Elsevier Saunders, 2006.
3. "Lissencephaly." Wikipedia. Wikimedia Foundation, 21 May 2013. Web. 05 June 2013.
4. Roth, Gerhard, and Ursula Dicke. "Evolution of the brain and intelligence."Trends in cognitive sciences 9.5 (2005): 250-257.
5. "Brain and Body Size... and Intelligence." Brain and Body Size... and Intelligence. N.p., n.d. Web. 05 June 2013. <http://serendip.brynmawr.edu/bb/kinser/Int3.html>.
6. Hof, Patrick R., Rebecca Chanis, and Lori Marino. "Cortical complexity in cetacean brains." The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology 287.1 (2005): 1142-1152.
7. Manger, Paul R., et al. "Quantitative analysis of neocortical gyrencephaly in African elephants (Loxodonta africana) and six species of cetaceans: Comparison with other mammals." Journal of Comparative Neurology 520.11 (2012): 2430-2439.
8. Van Essen, David C. "A tension-based theory of morphogenesis and compact wiring in the central nervous system." NATURE-LONDON- (1997): 313-318.
9. Smart, I. H., and G. M. McSherry. "Gyrus formation in the cerebral cortex in the ferret. I. Description of the external changes." Journal of anatomy 146 (1986): 141.
10. White, Tonya, et al. "The development of gyrification in childhood and adolescence." Brain and cognition 72.1 (2010): 36-45.
11. Luders, Eileen, et al. "Mapping the relationship between cortical convolution and intelligence: effects of gender." Cerebral Cortex 18.9 (2008): 2019-2026.
12. Luders, Eileen, et al. "The unique brain anatomy of meditation practitioners: alterations in cortical gyrification." Frontiers in Human Neuroscience 6 (2012).
13. Kippenhan, J. Shane, et al. "Genetic contributions to human gyrification: sulcal morphometry in Williams syndrome." The Journal of neuroscience 25.34 (2005): 7840-7846.
14. Palaniyappan, Lena, and Peter F. Liddle. "Differential effects of surface area, gyrification and cortical thickness on voxel based morphometric deficits in schizophrenia." Neuroimage 60.1 (2012): 693-699.
15. Wallace, Gregory L., et al. "Increased gyrification, but comparable surface area in adolescents with autism spectrum disorders." Brain (2013).