How the brain rewires after lesions

Scientists from Bochum investigate the reorganizational capacity of the adult brain

After retinal lesions brain nerve cells may become transiently juvenile again: up to one year processes related to a rewiring of the affected brain area can be observed. This is described in a new study by Dr. Dimitrios V. Giannikopoulos and Prof. Dr. Ulf T. Eysel (Neurophysiology, Faculty of Medicine, Ruhr-University Bochum, Germany). The scientists have recorded the activity of single nerve cells in the visual cortex of the adult brain and discovered new details regarding the time course, the dimensions, potentials and limits of brain reorganization. The results are published in the current issue of the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

Not clear: How plastic is the adult brain?

The high malleability of the early postnatal and juvenile brain is well known. "Young brains possess without doubt the ability for enhanced adaptation", says Prof. Eysel. "They display a relatively large capacity for rehabilitation by reorganization after lesions and in response to diseases of the nervous system. "It is, however, unclear how far this is possible in the adult brain. In spite of many results that showed such plastic events in the adult brain, new findings utilizing modern functional brain imaging suggest that the capacity to reorganize might be exceedingly limited in the adult.

New wiring spreads as a wave

To exactly determine the capability of reorganization in the adult brain, Dr. Dimitrios Giannikopoulos und Prof. Dr. Ulf Eysel recorded the activity of single cells in the visual cortex of adult cats. After circumscribed damage of the retina an extensive reorganization of the visual cortex is observed within weeks and up to one year. Affected or silenced regions of the brain are reactivated and the mapping of eye and brain is fundamentally rearranged. Dr. Giannikopoulos explains: "The rewiring process moves like a wave within weeks from the healthy border region progressively into the blind part of the brain". The affected nerve cells transiently remind in some properties of cells in early postnatal life. The brain cells that are "blind" immediately after the lesion are reconnected with undamaged parts of the retina. During this process they are hyperexcitable, with large unspecific receptive fields, and strongly reduced analytical performance.

Astonishingly plastic yet regional

Following rewiring the cells normalize, regain receptive fields of normal size and regain – although partially reduced – analytical specificity. The number of cells involved in reorganization in the affected area ranges in dependence on the distance from the normal border region between 10 and nearly 100 percent. Prof. Eysel summarizes: "The new long-term details from our work confirm the astonishing basic capacity of the adult brain for plasticity, however, with certain restrictions in extent and recovery of analytical specificity due to the structural prerequisites in the brain."

Late boost of plasticity

In addition to the new findings regarding time course and cell properties during rewiring, the discovery of a late component of functional reorganization that takes place between 3 months and 1 year is highly interesting for the scientists. In this late phase the number of cells participating in the reorganization far inside the affected cortical region increased once again from 10-20 percent to 40-50 percent. This finding emphasizes the possibility of additional improvement and stabilization in late phases after brain damage, in addition to the well-known notable achievements of early rehabilitation.