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Neural circuits in the brain are genetically determined

Max Planck Society : 19 August, 2002  (Technical Article)
How genetic factors and individual experience influence the structure of the brain and its cognitive capabilities is a key question in the cognitive neurosciences with far reaching implications for both education and society in general. Until now, the influence of genetic information on brain structure could only be quantitatively assessed for gross measures such as the volume of the brain or of its parts.
Researchers at the Max Planck Institute for Flow Research in Göttingen, at the Kavli Institute for Theoretical Physics in UC-Santa Barbara, USA, and at the Leibniz Institute for Neurobiology in Magdeburg have found evidence for the influence of genetic information on neuronal circuits which are responsible for information processing in the brain. According to their findings, patterns of neuronal activity in the visual cortex of genetically related animals resemble each other much more than patterns in unrelated animals. This breakthrough was achieved by developing a new mathematical technique that gives a quantitative description of the complex patterns and is thus able to distinguish subtle differences in the organisation of neural activity patterns.

Sensory information is represented and processed in the neuronal circuits of the brain by patterns of nerve cell activity. As you are reading these lines hundreds of millions of neurons throughout the cerebral cortex of your brain are actively representing and analyzing the visual information in complex spatial patterns. In the brains of different individuals, these patterns differ strongly in their detailed organization. In the visual cortex, activity patterns are composed of local domains of activated neurons - called functional cortical columns. The domains have a size range of a few hundred microns and contain nerve cells analyzing visual information from a small region of the field of view. Information from the entire field of view is represented in the ensemble of all activity domains in the visual cortex.

The spatial arrangements of these domains differ widely among individual brains. Previously, it was assumed that different experience is the main origin of inter-individual differences in the arrangement of activity patterns. Using a novel approach for the quantitative characterization of brain activity patterns, an interdisciplinary team of theoretical physicists at the Department of Nonlinear Dynamics in the Max Planck Institute for Flow Research in Göttingen, Germany and the Kavli Institute for Theoretical Physics in Santa Barbara (USA) and neuroscientists from the Research Group 'Visual Development and Plasticity' at the Leibniz Institute for Neurobiology in Magdeburg, Germany have now systematically compared patterns of neuronal activity domains in genetically related and unrelated animals. Using the new approach, they discovered a surprisingly strong influence of genetic factors on the layout of the activity domains.

In order to assess whether individual differences in cortical architecture are due to individual experience or are caused by genetic factors, the team investigated the inter-individual differences in one of the most thoroughly studied parts of the brain, namely the primary visual cortex, which is the first cortical stage of visual information processing. To objectively compare the arrangement of activity domains in this area in genetically related and unrelated animals, the researchers developed a new image analysis method which quantitatively characterizes the layout of the complex pattern by a few parameters, such as the average spacing of domains, their average shape, and how homogeneous the shapes and spacings of domains are across the entire primary visual cortex.

Whereas unrelated animals differed widely in these quantities, parameter values in littermates were strikingly similar. For instance, while the average spacing of adjacent domains ranged between 1000 and 1400 micrometers in different animals, spacings in siblings typically differed by less than 80 micrometers. Furthermore, the shape of domains and a measure of how homogeneous domain spacings are across visual cortex were found to be significantly clustered. These results indicate for the first time that a considerable part of the inter-individual variability in the layout of neuronal circuits is caused by genetic factors.

In the context of previous studies on the influence of genetic factors on the structural organization of the brain, these observations came as a big surprise. 'Previously, a substantial genetic influence of genetic factors on quantitative features of brain architecture had only been demonstrated at large scales such as the total grey matter volume or the overall volume of certain brain regions. When more specific features of brain morphology, such as the patterns of folds on the cerebral surface were analyzed, no significant genetic influence was found. This suggested that the tightness of genetic control decreases when finer scaled features of brain architecture are considered. These new results quantitatively demonstrate that fine scaled features of neocortical organization are influenced by genetic information. The previous interpretation is therefore rejected.' says Max Planck Researcher Dr. Fred Wolf who together with Dr. Siegrid Löwel of the Leibniz Institute in Magdeburg led the interdisciplinary team.

The new results also suggest how the genetic control of visual cortical architecture may mediate a genetic influence on an individual's visual abilities. The authors point out that, for instance, a genetic influence on the spacing of activity domains is likely to influence an area's information processing capabilities. In the visual cortex, the total number of domains varies from brain to brain and is larger if domains are more closely spaced. Each domain analyzes visual information from a small region of the field of view. In an individual with a larger number of domains, visual information can be independently assessed for more and finer scaled regions of the field of view, which should improve the visual systems capacity for the processing of complex images.

'If visual performance is indeed affected by the total number of activity domains in the visual cortex', Wolf argues, 'then our results predict a substantial degree of genetically induced inter-individual variability in visual abilities.' To test this prediction the researchers will attempt to further characterize the degree of inter-individual variability in visual abilities and their genetic control.
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