Plasticity

Stephan Anagnostaras stephan at nospam.ucla.edu
Wed Jun 3 05:46:15 EST 1998


Back to the original question;  although cortical plasticity has been
observed to be far greater than was originally assumed (see Frank's
post below), there is even in this work considerable evidence of
hardwiring. For example, in Merzenich's work,
when a finger is cut off, the sensory area
of the brain that used to represent that finger comes to represent
neighboring fingers.  Similar work has been found in visual cortex.
On the other hand, the areas are never found to represent completely
different things.  That is, the finger area doesn't come to respond to
stimulation of, say, the nose, nor does it ever come to represent
a different modality.  

So back to the original question, does the behavioral function of a 
neuron change as the result of plasticity, the answer is, probably
no... that is probably the case for neurons that are motor or
sensory in nature.  Probably not much has changed about these neurons
except their weighting of synaptic inputs that already existed
at the time of the trauma... a variety of simple Hebbian mechanisms
could handle this change, although, in these cases one has
not been demonstrated; it is largely an analogy to other
learning paradigms, where NMDA-receptor mediated changes in
synaptic efficacy are believed to mediate this kind of mechanism.

Now you should keep in mind that the adult brain must be very plastic,
because adults are still capable of learning and exhibiting changes in
behavior... and there has been ample evidence that memory has
structural substrates for almost 40 years. So the idea that the
brain was fixed was flawed for sometime. It was just accepted
dogma, which turned out to be untrue the first time anyone bothered
to look...

Cheers,
Stephan Anagnostaras

In article <6l2im3$d7r at sjx-ixn8.ix.netcom.com>, flefever at ix.netcom.com(F.
Frank LeFever) wrote:

> I'm replying off top of my head without access to proper references
> (they're at work), but want to encourage legitimate neuroscience
> discussion in a forum which has been much abused by other nonsense
> recently.
> 
> I don't think it is a matter of a neuron's changing its "behavioral
> function"; rather, changes of connections among neurons.  Doubt a 
> single neuron has a "behavioral function" (although of course there was
> the famous hyppothetical "grandmother neuron"!)(i.e. the endpoint of
> converging and increasingly specific visual stream inputs en route to
> the infratemporal region).
> 
> Denervation studies and "paired stimulation" studies in the past few
> years have shown reorganization of "cortical maps" with such
> astonishing rapidity as to suggest "reprograming" in the sense of
> altering synaptic biases in existing networks rather than formation of
> new "connections" of a "hardwired" sort.  (All right, I'll  try to
> recall the spelling without access to my files: Mezernich? One
> researcher who has done a lot of this rapid plasticity work).
> 
> On the other hand, Ed Taub's monkeys (kidnapped from his Silver Springs
> labs many years ago) did one last service: examination of their brains
> LONG after posterior root section (i.e. denervation) showed extensive
> reorganization, perhaps of the "hard-wired" sort--published in
> _Science_ several years ago.  These were mature monkeys.
> 
> ASIDE: Ed was a friend of mine in grad school; I visited his lab and
> saw monkeys used in his initial work.  It was impressive to watch them
> direct movements to targets (e.g. to pick up a raisin without visual
> guidance or somatosensory feedback!  And with very little deliberate
> attention, being busy challenging other monkeys--eye contact, jutting
> jaw, hoots, etc.).  I am happy that he has found a new start and that
> he has been belatedly recognized  (e.g. by American Psychological
> Assoociation) for his accomplishments.
> 
> Apparently some plasticity in adults is enhanced by noradrenergic
> activity.
> 
> All for now!
> 
> F. Frank LeFever, Ph.D.
> New York Neuropsychology Gropup
> 
> 
> 
> 
> 
> 
> In <357373D7.267FBEBD at interlog.com> Brian Scott <brians at interlog.com>
> writes: 
> >
> >Gernot S Doetsch wrote:
> >> 
> >>    Much has recently been written about the plasticity of adult 
> >>brains. Following peripheral denervation or sensorimotor training, 
> >>significant changes can occur in the response properties of cortical
> >> neurons and the details of somatosensory and motor cortical maps. Do
> >> such physiological changes mean that the sensory or motor function
> of
> >> the affected neurons has changed accordingly? Can the "behavioral 
> >>function" of adult mammalian neurons ever change? If so, under what 
> >>circumstances?
> >
> >I've read of neurons in the visual system becoming responsive to
> auditory
> >stimuli after denervation or sensory deprivation...details escape me. 
> Something
> >like synesthesia perhaps.  Does this count as a change in sensory
> function?  
> >
> >-- 
> >  Brian Scott          |  Bloorview Epilepsy Research Program
> >  brians at interlog.com  |  University of Toronto, Canada



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