Posts Tagged ‘Donald Hebb’

A View of the Reading Brain

October 19, 2018

This post is taken from “READER COME HOME: The Reading Brain in the Digital World” by Maryanne Wolf. Please excuse the detail, but it is important to gain an appreciation of what is involved in reading. The brain’s design is with the principle of “plasticity within limits.” The brain is able to go beyond its original biological functions—like vision and language—to develop biologically unknown capacities such as reading and numeracy. To do so, it forms a new set of pathways by connecting and sometimes repurposing its older and more basic structures. Faced with something new to learn, the human brain not only rearranges its original parts, but is also able to refit some of its existing neuronal groups in those same areas to accommodate the particular needs of the new function. The brain recycles and even repurposes neuronal networks for skills that are cognitive or perceptually related to the new one, Wolf writes, “This ability to form newly recycled circuits enables us to learn all manner of genetically unplanned-for activities—from making the first wheel, to learning the alphabet, to surfing the net while listening to Coldplay and sending tweets. None of the activities is hardwired or has genes specifically dedicated to its development; they are cultural inventions that involved cortical takeovers.” As there is no genetic blueprint for reading, there is no one ideal reading circuit. There can be different ones.

In addition to neuroplasticity, there is the concept of cell assemblies formulated by the Canadian psychologist Donald Hebb. The concept is that cells that fire together wire together. These specialist groups build the networks that allow us to see the smallest features of a letter or hear the tiniest elements in the sounds of language, literally in milliseconds. Cell specialization enables each working group of neurons to become automatic in its specific region and to become virtually automatic in its connections to the other groups or networks in the reading circuit. For reading to occur, there must be sonic-speed automaticity for neuronal networks at a local level, which, in turn, allows for equally rapid connections across entire structural expanses of the brain. So, whenever we name even a single letter, we are activating entire networks of specific neuronal groups in the visual cortex, which correspond to entire networks of equally specific language-based cell groups, which correspond to networks of specific articulatory-motor cell groups—all with millisecond precision. Multiply this scenario a hundredfold when the task is to depict what you are doing when reading with complete (or even incomplete) attention and comprehension of the meanings involved.

“In essence, the combination of these principles forms the basis of what few of us would ever suspect: a reading circuit that incorporates input from the two hemispheres, four lobes in each hemisphere (frontal, temporal, parietal, and occipital) and all five layers of the brain (from the uppermost telencephalon and adjacent diencephalon below it; to the middle layers of the mesencephalon; to the lower levels of the mesencephalon and myelencephalon).” So anyone who still believes that we use only a tiny portion of our brains hasn’t yet become aware of what we do when we read.

Advertisements

The Loss of a Neuroscientist Who Should Have Been Awarded a Nobel Prize

September 6, 2017

And that neuroscientist is Marian Diamond who passed away on July 25, 2017 at the age of ninety. Her painstaking research showed that the body’s three-pound seat of consciousness was a dynamic structure of beautiful complexity, capable of development even in old age.

Prior to her research it was strongly believed the nervous system was fixed. We were stuck with the brain we were born with. And any damage to the brain was irreparable. The brain was a static and unchangeable entity that simply degenerated as we age.

Inspired by the research of psychologist Donald Hebb, she began studying the brains of lab rats. Rats that were raised alone, in small and desolate cages, had more trouble navigating a maze than did rats were raised in “enriched” cages, with toys and rat playmates. Through painstaking analyses of these rat brains she found that the cerebral cortices of rats in “enriched” cages were about 6% thicker than the rats in the “impoverished” cages.

Her findings, published in a 1964 paper with three colleagues, were a pivotal contribution to the long-running debate between nature and nurture, which seeks to determine the extent to which a person is shaped by their genes or by their life experiences.. UC-Berkely professor Robert Knight said “The idea that the brain could change based on environmental input and stimulation was felt to be silly, and that’s the boat she completely sank.

Further research generalized these conclusions to humans. Neuroplasticity was found to be ubiquitous. We continue to generate neurons until we die.

Dr. Diamond went on to develop a rich theory of brain plasticity summarized in the phrase use it or lose it. She outlined the following five factors crucial to brain development at any age: diet, exercise, challenge, newness, and love.

Later in her career she was given several sections of Albert Einstein’s brain. She found an unusually high amount of glial cells, which were thought to be a relatively unimportant part of the tissue that held the brain together. This discovery launched renewed interest in the role of glial cells, which are now believed to play a crucial role in cognitive processes.

This post is based in part on an obituary by Harrison Smith in the 31July 2017 Washington Post.

 

Neuroplasticity and Neurogenesis

June 8, 2016

Chapters 2 and 3 of Sharon Begley’s “Train Your MInd, Change Your Brain” cover neuroplasticity and neurogenesis.  Prior to discussing neuroplasticity, how learning takes place needs to be discussed.  To explain how learning takes place psychologist Donald Hebb conceived of cell assemblies.  He proposed that learning and memory were based on the strengthening of synapses.
Somehow either the neuron that fires first in the chain (the presynaptic neuron) or the neuron that fires next (the postsynaptic neuron), or both, change in such a way that the firing of the first is more likely to cause the firing of the second.  Learning and memory involve the firing of large assemblies of these cells.  Hence Hebb’s theory is called cell assembly theory.  Hebb’s maxim is that cells that fire together wire together.

Virtually all the research on neuroplasticity involved animals.  This is because surgery was almost always required. Sensory  or motor connections might be severed, and then observations would be made regarding the effects of these operations.  Sometimes connections were rewired so that animals would see sound or hear light. The late nineteenth psychologist William James had wondered , were scientists were able to alter neuron’s paths so that exciting the ear activates the visual cortex and exciting the eye the auditory cortex, we would be able to  “hear the lightning and see the thunder.”  So James was correct.  And all this research invalidated the longstanding dogma that the nervous system could not be rewired or rewire itself underscoring the reality that the nervous system can and does rewire itself.

The longstanding dogma that new neurons  could not be created, neurogenesis, was more difficult to disprove.   Before cells divide, they make a copy of their DNA.  As cells can’t conjure the double helix out of thin air, biochemicals snag the requisite ingredients from within the cell and assemble them.  One element of DNA, thymidine, lets a radioactive  molecules glom on to it.  When the thymidine becomes incorporated into the brand-new DNA, the DNA has a spot of radioactivity, which can be detected experimentally.  Old DNA does not have this glow.

Joseph Altman, a new neuroscientist at MIT, decided to try the new trick on brains.  By scanning neurons for tell tale glows he figured he would be able to detect newborn DNA, and newborn cells.  He found neurons of adult rats, cats,  and guinea pigs with thymidine—indicating that they had been born after Altman had injected them with the tracer.  He published these finding in three prestigious scientific journals in 1965, 1967, and 1970, yet his claims were ignored,   Altman was denied tenure at MIT and joined the faculty of Purdue University.

Research was done using nonhuman  animals with rich environments.  That is animals who lived in enriched environments with exercise wheels and novel features were compared to animals living in impoverished environments.  The formation and survival  of new neurons increased 15% in a part of the hippocampus called the dentate gyros, which is involved in learning and memory.

To this point humans had not been involved in the research, the reason being that noninvasive brain imaging could not address this issue.  Brains needed to be taken from   dead research participants.  Oncologists injected BrdU into cancer patients because is marks every newborn cell.  This allowed them to assess how many new cancer cells were developing.  The researchers were able to enlist the cooperation of oncologists and their patients.  After these patients succumbed to cancer, their brains could be examined to see if any new  noncancerous cells had been generated.  Thanks to these patients and their oncologists, new neurons, indicating neurogenesis, were found in the hippocampus.

An interesting find was that forced exercise does not promote neurogenesis.  The neuroscientist Gage explained to the Dalai Lama, “Running voluntarily increases neurogenesis and increases learning even in very, very old animals.  It seems like the effects of running on neurogenesis and on learning are dependent on volition.  It has to be a voluntary act.  It is not just the physical activity.

When the neuroscientist Fred Gage sat down with the Dalai Lama it was clear that new neurons arise from neural stem cells in the adult human brain, which persist and support ongoing neurogenesis.  This discovery expanded the possibilities for neuroplasticity.  The neural electrician is not restricted to working with existing wiring, he can run whole new cables through the brain.

In humans new neurons might do more than help with learning.  The hippocampus plays an important role in depression.  In many people suffering from depression, the dentate gyrus oaf the hippocampus  has drastically shrunk.  There is a question of cause and effect, whether another factor caused the hippocampus to shrink leading to depression, or whether depression caused the shrinkage.

New research suggests that people who are suffering from depression are unable to recognize novelty.  Gage said this to the Dalai Lama, “You hear this a lot with depressed people.  Things just look the same.  There is nothing exciting in life.”  “There is also evidence,” Gage said, “that if you can get someone with depression to exercise, his depression lifts.”  So neurogenesis might be the ultimate anti-depressant.  When it is impaired for any reason, the joy of seeing life with new eyes and finding surprises and novelty in the world vanishes.  But when it is restored the world is seen anew.

It is clear that chronic stress impairs neurogenesis, at least in mice.  Gage’s colleague, Peter Ericsson suspects that holds lessons for humans also.  “In lab animals, chronic stress dramatically decreases neurogenesis as well as spatial memory..  When people under stress experience severe memory problems—forgetting their way to work, going into the kitchen and then no remembering why they went in—it is likely that what they’re experiencing is the very negative of stress on the function of the hippocampus due to decreased neurogenesis.”