Sensory-Based Plasticity

by Tim Brunson, PhD

A major contribution regarding sensory-based plasticity came from the work of a team of neuroscientists in California. In 1996, Michael M. Merzenich, PhD, a professor emeritus neuroscientist at the University of California, San Francisco (Begley, 2007; Doidge, 2007) and his colleagues reported that when the arms and fingers of monkeys were exercised, the related areas of the motor cortex began to expanded radically, taking over areas that had previously controlled other parts of their bodies. This alone overturned the dogma that stated that the adult brain cannot change. He insisted that the experiences of life sculpt the brain and retain imprints of experiences and performed behaviors. He said, "These idiosyncratic features of cortical representation have been largely ignored by cortical electrophysiologists." (Merzenich et al., 1990).

If use stimulates the growth of neural networks, would the opposite also be true? Would disuse destroy networks? Certainly, this would follow the basic tenet of Hebbian Learning. The answer for this came from another non-neuroscientist. In 1981, Edward Taub, PhD, (Begley, 2008) a research psychologist in charge of the Institute for Behavior Research in Silver Spring, Maryland, was deafferenting (i.e. severing the sensory nerve) on monkeys' arms. This meant that the applicable section of the somatosensory cortex of a monkey's brain was not receiving input from their fingers, hands, or arms. The goal was to find out how this affected the corresponding part of the brain.

Due to legal problems initiated by People for the Ethical Treatment of Animals (PETA), it took until January 14, 1990 – when the last of first of the Silver Spring monkeys was euthanized – before anyone obtained any appreciable results from Taub's research. Neuroscientists led by Pons and Mishkin (Begley, 2007, p. 43) sought to discover how the monkey's brain had reorganized after twelve years of deafferentation. What they discovered was astounding. When they brushed the monkey's face, the deafferentation zone tingled with electrical activity. In fact, even a light touching of the camel hair brush produced vigorous neuronal responses in what was suspected to be a silent zone. What they discovered is that the face zone of the somatosensory cortex had intruded into the hand and arm zone.

These lessons were not lost on Taub, who eventually survived his legal ordeals with PETA and the Maryland court system and ended up at the University of Alabama, Birmingham (Begley, 2007). Meanwhile, however, his interest had shifted to concerns regarding how neuroscience discoveries can help stroke victims. Since portions of the brains of stroke victims die due to a failure of the vascular system to provide oxygen, Taub's concern was whether the lessons from the deafferentation research could be used to enhance the lives of people who survived. His subsequent research and clinical work sought to find out if unused (or under utilized) sectors could re-learn and take over servicing the functions of damaged sector(s).

The conclusion from Taub's efforts became controversial since it violated conventional wisdom related to the treatment of stroke survivors. Up to that point the typical treatment was to train them how to compensate for the lost capabilities. Since many neuroscientists still believed that the brain had no developmental potential after age 25, compensation was considered the only alternative. Regardless, Taub developed a new approach that he called "constraint-induced movement therapy. " For instance, if a patient's right arm function had been destroyed by a stroke, his clinicians would restrain the left arm. This forced the brain to attempt to re-activate the abilities for the brain to once again control and use the right arm. As the related somatosensory area was unavailable, the only option was for the brain to reorganize and re-install the right arm sensing and controlling function to another set of neurons somewhere else in the brain. The belief here is that as a result of the pruning or differentiation – which occur during the first 26 months of life – and/or the negative aspect of Hebbian Learning, dormant or under utilized neural networks exist and may be available to take over the relocated function. This is like saying that mentally, humans carry around neurological "spare tires", which are available when needed. Taub's further research and clinical experience showed that stroke victims can benefit from constraint therapy. This overturns the still ascribed to belief that the brain cannot restructure after the 25th year of life.

The International Hypnosis Research Institute is a member supported project involving integrative health care specialists from around the world. We provide information and educational resources to clinicians. Dr. Brunson is the author of over 150 self-help and clinical CD's and MP3's.

References: Begley, S. (2007). Train Your Mind, Change Your Brain: How a New Science Reveals Out Extraordinary Potential to Transform Ourselves. New Your: Ballantine Books.

Doidge, N. (2007). The Brain that Changes Itself: Stories of Personal Triumph form the Frontiers of Brain Science. London: Penguin Books Ltd.

Merzenich, M.M., Recanzone, G. H., Jenkins, W. M.., & Grajski, K.A. (1990). Adaptive Mechanisms in Corticol Networks Underlying Cortical Contributions to Learning and Non-declarative Memory. Cold Spring Harbor Symposia on Quantitative Biology 55, 873-87.