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

The same species we intuitively consider more intelligent that already were shown to a greater relative brain size to body weight ratio also show similar trends in gyrification. Figure 3 shows clear anatomical distinctions in the extent of convolution across species in a manner we would expect. This can also be looked at quantitatively; Figure 4 feature a relationship of Gyrencephalic Index to Brain mass; Cetaceans feature significantly higher ratios ^[7]. These findings suggest that gyrification has fair cause to be associated with intelligence.

""Figure 4:"" Gyrification to Brain Mass Comparison. Credit: Manger, 2012.

Mechanism of Folding

In order to better understand how gyrification can be related to intelligence, it is important to explore the mechanism by which convolutions develop. The exact details are not completely understood, however work in developmental neurobiology has made some progress. David Van Essen (1997) is often credited for providing the original theory of how gyri and sulci come about. He suggested the hypothesis that cortical convolution comes about from actual physical tension along axons running in the marginal zone within cortical layers that limit the lateral expansion of neocortex, thus causing folding ^[8].

The basic argument here is that a greater number of cortical neurons and axons causes crowding that builds up the physical forces. This concept was later supported by Smart & McSherry (1986) who showed that gyri are formed by longitudinal and radial expansion of collections of cortical neurons in tight areas that cause formation of sulcal floors ^[9].

Differences in Gyrification across Humans

Gyrification is a process that has predictable benchmarks in human fetal development. Often a phenotype of several mental disorders are abnormal phenotypes of cerebral convolution development ^[10]. Differences in gyrification across humans is often associated with such disorders, but there is also some literature on gyrification differences across age, gender, and other groups in normal and healthy humans.

Gyrification and Aging

It has been shown that as humans age, the cortex becomes progressively thinner, and the degree of gyri and sulci curvature changes such that cortex appears to become more atrophic ^[10].These features are associated with less cortical neurons which we may associate with a reduction in cognitive function or less ambitiously, the pruning of neurons.

Gyrification and Intelligence: Effects are Different Across Genders

Luders (2007) showed that intelligence scores were positively associated with the degree of folding in the temporo-occiptal lobe. Furthermore, greater gyrification in the frontal cortex of females was correlated with higher IQ, while less convolution in frontal cortex of males was associated with lower IQ ^[10]. See Figure 5. These mixed results might make any official statement about whether greater gyrification in normal humans can be positively correlated to intelligence due to the gender differences, however, it at least appears to be implicated in females and in the temporo-occiptal lobe.

""Figure 5:"" Correlations between Cortical Convolution and IQ. Credit: Luders, 2007

Gyrification in Meditation Practitioners

Luders later did a unique study looking at how gyrification differs in a unique population: practitioners of meditation. It was found that those who meditate have greater convolutions in several brain regions, most notably the insula. ^[10]. Luders argues that perhaps this suggests that the insula is a key structure in meditation.

Gyrification and Disorder

Abnormalities in gyrification have been implicated in autism spectrum disorders, schizophrenia, Williams syndrome, and severe disorders of neuronal migration (e.g. Reelin gene mutation) (Ref 13, 14). Uniquely though, the findings in autism spectrum disorder involve comparable surface area yet increased gyrification (Ref 15). One theory of autism argues that it involves atypical excessively early brain development, which negatively affects normal learning but sometimes involves savant like abilities in very specific cognitive tasks. Perhaps greater gyrification in specific regions of the brain account of savant like abilities.

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).