Benjamin Godde Benjamin.Godde at neuroinformatik.ruhr-uni-bochum.de
Wed Jun 10 05:13:03 EST 1998

Gernot S Doetsch wrote:

> 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


If I don't misunderstand You, You say that a "hand neuron", representing
the arm after hand amputation remains a "(phantom) hand neuron", because
the higher cortical neurons "looking"  on this primary sensory neuron
have learned so. Well, then the question would be, if these higher
cortical areas also have a plastic capability and how to tell these
neurones to re-learn their input. So far as I know, there are no studies
on plasticity in higher cortical areas and it could be the big challenge
for the next decade.

Another example:
Braille readers and string instrument players have enlarged finger
representations. This means, that neurons not responding to finger
stimulation at birth do so after some months or years of training. So I
would say, they have moved their function from not-finger to finger
representation and processing.

Cortical learning must have a structural or functional correlate. This
could be the intracortical interaction at higher levels as well as
changes in the properties of primary sensory/motor neurons. There are a
lot of perceptual learning studies in the visual system supposing that a
lot of learning is present at the very primary processing levels. So
changes in the properties of these neurons must have to do with the
change of function. 

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

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