The image above depicts 12 functional regions of the brain clustered by genetic influence, either in terms of cortical thickness or by cortical surface area. The numbers identify specific regions. For cortical thickness: 1. Motor-premotor-supplementary area; 2. Superior parietal cortex; 3. Inferior parietal cortex; 4. Perisylvian region; 5. Occipital cortex; 6. Ventromedial occipital cortex; 7. Ventral frontal cortex; 8. Temporal pole; 9. Medial temporal cortex; 10. Middle temporal cortex; 11. Dorsolateral prefrontal cortex and 12. Medial prefrontal cortex. For surface area: 1. Motor-premotor cortex; 2. Dorsolateral prefrontal cortex; 3. Dorsomedial frontal cortex; 4. Orbitofrontal cortex; 5. Pars opercularis and subcentral region; 6. Superior temporal cortex; 7. Posterolateral temporal cortex; 8. Anteromedial temporal cortex; 9. Inferior parietal cortex; 10. Superior parietal cortex; 11. Precuneus; and 12. Occipital cortex.
Delving deeper into brain-building
Given its incredible structural complexity – 100 billion chattering neurons connected by a network of perhaps a quadrillion synaptic connections – it can hardly be surprising that the actual construction of a human brain is equally complex and marvelous.
A new paper by researchers at UC San Diego School of Medicine and VA San Diego Healthcare System underscores that point by plotting for the first time how genes influence the development and thickness of the cerebral cortex – the thin, outermost sheet of neural tissue, often dubbed “gray matter,” that plays a key role in high-level mental functions like memory, attention, perceptual awareness, thought, language and consciousness.
Writing in this week’s early edition of PNAS, William S. Kremen, PhD, professor of psychiatry, and colleagues describe mapping genetic influences and the varying thicknesses of the cortex.
The work adds a new dimension to a 2012 study by the same scientists that rendered the first atlas of the surface of the human brain based on genetic information.
“There are two major dimensions on the cortical ribbon or cortical sheet,” said first author Chi-Hua Chen, PhD. “One is the horizontal dimension for the expansion of surface area and the other is the vertical dimension for the size of cortical thickness. The 2012 work was to study the genetic topography of surface area; this study was to study cortical thickness.”
The researchers found that genetic influences work differently depending upon the direction taken during brain development. On the surface of the cortex, the effects are most striking horizontally, with maximum differences between those at the anterior or front of the brain compared to the posterior or back of the brain. With cortical thickness, the differences were dictated vertically, with maximum deviation measured dorsal (top) to ventral (bottom).
According to Chen, the findings confirm other evidence that cortical construction involves different mechanisms of brain development. “I think the importance and relevance of this study is that human cortical morphology is under genetic control that shows orderly spatial patterns on the cortical ribbon,” she said. Previous animal studies had suggested as much, but the concept of brain development through genetic gradation has not been well-studied in humans.
While there are no immediate clinical applications for the research, Chen and Kremen say the findings will further understanding of how complex genetic influences affect cortical morphology and aid future efforts to identify involved genes and associated neurological disorders.
“The genetic underpinnings of the brain are poorly understood, especially for the human brain,” said Kremen. “New genomic technologies are making it possible to explore this area and answer, from a genetic perspective, how genes affect brain development and how they are involved in neurological disorders like Alzheimer’s disease, mild cognitive impairment, schizophrenia and autism.