Brain: visual cortex less plastic than assumed

Long-term observation of the brain with surprising results

The primary visual cortex V1 © Logothetis / MPI for Biological Cybernetics
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How plastic is the brain? Can it still compensate for damage in adulthood by growing new neural connections or other functions? Previously, the visual cortex, the area where visual information is processed, was still considered plastic. But recent studies have now refuted this assumption.

Scientists at the Max Planck Institute for Biological Cybernetics in Tübingen have rekindled the debate on the potential of neuronal reorganization in the cerebral cortex with their recent studies on the plasticity of the visual cortex published in the journal Nature. In their investigations of macaques, the researchers were - contrary to previous ideas - after retinal damage over a period of several months no new neurons ingrowth and thus a corresponding neuronal reactivation discover.

Plasticity important for learning ability

The ability of the developing brain to adapt to damage is well documented. One example is children who have lost the left brain hemisphere at an early stage of development and still regain motion control of the right side of the body and achieve normal speech ability using the right hemisphere. However, this plasticity decreases as the brain matures. This is also evident when learning a second language: Up to the age of six - this is the critical time window or the sensitive phase - a child also learns a second language as their mother tongue. After that, normal language acquisition becomes increasingly difficult, as anyone who has learned to learn foreign languages ​​for many years can prove.

Scientists from the Max Planck Institute for Biological Cybernetics in Tübingen have now studied the plasticity of the visual system. So far, it has been assumed that in particular the interconnections in the cortex, ie the cerebral cortex, underlying the sensory systems remain plastic until adulthood. This will allow the neural circuit diagrams, referred to by scientists as cortical sensorimotor maps, to be continually changed by experience.

This dynamic nature of the circuitry in the cerebral cortex is important for learning, but also for post-injury repairs to the nervous system, for example as a result of stroke. Rehabilitation measures after a stroke are therefore aimed at the early promotion of brain plasticity. It hopes to be able to restore certain functions via the reactivation of functionally disturbed, but morphologically intact brain regions or by using alternative structures of the neural network. display

"Topographic map" not plastic

The visual cortex is divided into different areas. The region V1 mirrors the outside world so that each point of the external field of view is assigned a point in the V1 cortex. Looking at a simple pattern, such as a grid, the image is reflected in a matching pattern of neural activity on the surface of the brain. However, as the scientists from the Max Planck Institute for Biological Cybernetics have discovered, these "topographic maps" are not plastic in adult macaques.

The neurobiologists switched off a section of the retina and subsequently searched for changes in the cortical topography of the area V1 using magnetic resonance imaging (fMRI). In contrast to previous electrophysiological studies, V1 in adult macaques does not reach normal reactivity in the 7.5 months following retinal lesion, and its topography remains unchanged,

"Our data shows that V1 in adult macaques has only limited potential for significant reorganization in the months following retinal lesion, " explains Nikos Logothetis. "In addition, the work also demonstrates that magnetic resonance imaging can be effectively used to monitor cortical organization in anesthetized macaques over time, providing a viable model for research into the recovery of Post-stroke people. "The use of functional magnetic resonance imaging (fMRI) in macaques in conjunction with experimental pharmacological manipulations to promote plasticity is a promising method on the way to a better understanding of neuronal healing processes and reorganization.

(MPG, 27.05.2005 - NPO)