Gernot S Doetsch
GDOETSCH at mail.mcg.edu
Tue Jun 9 15:17:19 EST 1998
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.
Yes, there are significant changes in the physiological properties of neurons and therefore in cortical maps defined by those properties ... and various processes such as disinhibition, facilitation, Hebbian mechanisms, NMDA receptors, etc. are involved. But the issue is whether such changes produce corresponding changes in the sensory or motor FUNCTIONS of the affected neurons. If "hand neurons" were actually respecified as "arm neurons", would it not be necessary for similar respecification to occur in all higher-order areas connected to those neuronal populations? Just because neurons respond to new inputs or yield new outputs does not necessarily mean that their perceptual or motor function has changed.
I suggest that populations of neurons or cortical areas in the adult mammalian brain do not change their perceptual/motor function after deprivation or experience. However, populations of neurons can acquire new inputs/outputs by various unmasking mechanisms; their level of activity can increase or decrease. These changes modify normal patterns of activity across distributed networks that correspond to specific percepts or movements. Thus, a given pattern may be evoked by new inputs ("hand neurons" acquire receptive fields on the arm), or different patterns may be coactivated as in associative learning. In each case, the functional significance of a specific pattern does not change, although it can be modified to enhance or decrease sensory/motor performance. -- In short, stimulation of the arm of an amputee causes responses of "hand neurons" which still signify the now-phantom hand. Paired stimulation of whiskers or digits causes temporally-coincident responses of different but overlapping sets of neurons, resulting in associative learning. GSD
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