> 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.
Unmasking of latent inputs certainly contibutes to some cases of plasticity
following injury, but I disagree with your assertion that growth of new
connections is only inferred from cross-modal plasticity. Some cases of
reorganization in the somatosensory system occur over larger expanses
of cortex than would be predicted by a simple unmasking of inputs
(Pons et al., 1991: Science). In addition, there has been evidence of
growth in the spinal cord of monkeys following peripheral nerve injury
(Florence et al., 1993: Brain Research). Unmasking and/or growth of new
connections could occur at multiple levels of the system and contribute to
reorganizational changes in somatotopic representations.