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 ^[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

Nuanced Convolutions

However, there are different types of gyrification phenotypes that seem to complicate the intuition that the magnitude of gyrification alone is related to intelligence; brains with reasonable amounts of gyrification but different gyri shapes have unfavorable consequences. Pachygyria for example is malformation involving unusually thick convolutions. Polymicrogyria is a malformation involving an excess number of smaller gyri. Both are associated with developmental delays.

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

Relevance to Species Intelligence

Brain Size to Body Weight Ratios Across Species

Before doing analyzing cross species comparisons of gyrification, it is important to note that the ratio of brain size to body weight was one of the original and more salient topics regarding how physiological differences in brains may predict differences in intelligence across species. Figure 1 shows a relationship between brain size and body size; as the weight/size of a species increases, brains size increases absolutely, but are relatively smaller ^[4]. The relative intelligence of species is considered related to how high above or below the best fit line a species is. For example, man and dolphin in the graph are “above” the best fit line (smart species for given body size) while pigs and hippos are below (dumb for body size).

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

A better of way of looking at this data is with what is known as the Encephalization Quotient (EQ), which a quantitative measure of the relative brain size based on brain mass vs. the predicted brain mass of an animal of a given size. Figure 2 features a table of species and their EQ values as shown by Macphail ^[5]. In general, these values agree with well established findings that primates, cetaceans (the family dolphins are in), and elephants have notably high cognitive abilities in the animal world ^[6].

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).