Benjamin Godde Benjamin.Godde at
Tue Jun 9 10:31:45 EST 1998

Gernot S Doetsch wrote:
> 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...

You have to keep in mind the following:
When we talk about cortical plasticity, we talk about processes in the
very primary sensory or motor areas. Today, they are no (or only very
few) studies concerning higher cortical areas.
Neurons in the primary fields are characterized in their function by
their sensory input and this input generally comes from one type of
receptor surfaces (retina, cochlea, skin, etc.). So, if you want a
neuron in these areas to gain a new function (an auditory neuron gets
the possibility to "see", for example) it requires the growth of NEW
connections from subcortical to cortical areas (or the experimental
rewiring, as done by M.Sur et al.). This is (so far I new) not the case
in adult cortices. 
What happens in the known plasticity experiments is indeed the unmasking
of latent existing - subthreshold - connections. These subthreshold
connections can spread over several millimeters. So, in amputees, for
example, neurons representing the lost thumb often represent now the (in
the homunculus) neighbouring cheak.
Without further thinking about I know about only one experiment in which
sensory neurons gain another modality.
Icms in the somatosensory cortex of rats (hindpaw representation) has
the effect that neurons of the neighbouring primary motor area  show
some tactile responses (Look et Spengler & Dinse, Neuroreport 5:949,
1994). But the general opinion is, that there is a great amount of
subthreshold overlap of these areas.

For more information on cortical plasticity have a look of my homepage.


DR. BEN GODDE (Benjamin.Godde at
     Institut f. Neuroinformatik, Ruhr-Universitaet, D-44780 Bochum
     phone: + (0)234 7007975  fax: + (0)234 7094209

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